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SID 5 Research Project Final Report

SID 5 (Rev. 3/06) Page 1 of 31

NoteIn line with the Freedom of Information Act 2000, Defra aims to place the results of its completed research projects in the public domain wherever possible. The SID 5 (Research Project Final Report) is designed to capture the information on the results and outputs of Defra-funded research in a format that is easily publishable through the Defra website. A SID 5 must be completed for all projects.

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Project identification

1. Defra Project code PS2803

2. Project title

A review of Slug Control in Winter cereal and Oilseed Rape

3. Contractororganisation(s)

     ADAS UK Ltd                    

54. Total Defra project costs £ 19,123(agreed fixed price)

5. Project: start date................ 01 January 2010

end date................. 30 April 2010

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6. It is Defra’s intention to publish this form. Please confirm your agreement to do so...................................................................................YES NO (a) When preparing SID 5s contractors should bear in mind that Defra intends that they be made public. They

should be written in a clear and concise manner and represent a full account of the research project which someone not closely associated with the project can follow.Defra recognises that in a small minority of cases there may be information, such as intellectual property or commercially confidential data, used in or generated by the research project, which should not be disclosed. In these cases, such information should be detailed in a separate annex (not to be published) so that the SID 5 can be placed in the public domain. Where it is impossible to complete the Final Report without including references to any sensitive or confidential data, the information should be included and section (b) completed. NB: only in exceptional circumstances will Defra expect contractors to give a "No" answer.In all cases, reasons for withholding information must be fully in line with exemptions under the Environmental Information Regulations or the Freedom of Information Act 2000.

(b) If you have answered NO, please explain why the Final report should not be released into public domain

Executive Summary7. The executive summary must not exceed 2 sides in total of A4 and should be understandable to the

intelligent non-scientist. It should cover the main objectives, methods and findings of the research, together with any other significant events and options for new work.

Summary

Slugs are important pests of autumn sown cereals, particularly winter wheat, and oilseed rape and can cause severe damage and loss of yield. This project reviews pesticide usage data, results from ADAS field experiments and the wider scientific literature to evaluate current strategies for control of slugs in cereals and oilseed rape crops, and to consider potential alternatives for the future. The need for the review has been stimulated by the frequent detection of metaldehyde in surface water and the need to minimise the risk of detection so that water quality continues to meet the requirements of the Water Framework Directive WFD (2000/60/EC). There has also been an apparent increase in damage to crops by slugs despite continued significant expenditure on slug pellets. Therefore it is opportune to review slug control with the aim of maximising efficacy whilst minimising environmental impact and complying with legislation.

This review had the following objectives:

1. To examine trends in molluscicide usage in winter cereals and oilseed rape. 2. To determine the relative importance of grain/seed hollowing and leaf grazing in wheat and oilseed

rape.3. To examine historic data and estimate the overall impact of slugs on crop yield.4. To consider how tolerant cereal and rape crops are to slug attack.5. To consider future control options for slugs in cereal and oilseed rape.

Molluscicide usage is dominated in terms of area treated and weight of active ingredient by applications of metaldehyde to wheat and oilseed rape. Farmers’ choice of metaldehyde over methiocarb is probably because it is less expensive. The Pesticide Usage Survey indicates that in 2008 few crops treated with metaldehyde received more than the 700 g a.i./ha/year limit recommended by the industry’s Metaldehyde Stewardship Group (MSG), but some crops received more than 250 g a.i./ha in a single application, which exceeds the MSG recommendation. Molluscicide usage in wheat crops has fluctuated over recent years while the area of oilseed rape treated has increased significantly. Overall, usage is influenced most by autumn rainfall. However during 2008 applications to both cereals and oilseed rape were greater than would have been predicted by rainfall figures. This is likely to be due to changing agronomic practice, particularly increased adoption of minimum cultivation.

Grain hollowing rather than leaf shredding has the greatest potential impact on the yield of wheat crops.

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In oilseed rape the seed is rarely damaged and leaf grazing has greater effect. Once both cereal and oilseed rape plants are beyond the four leaf stage they are considered to be less vulnerable to slugs. The impact of slugs on yield remains a paradox. There is a general assumption that crops damaged by slugs will suffer consequent reduction in yield but there is limited empirical evidence of yield responses to molluscicide treatment in either cereals or oilseed rape. Expert opinion suggests the average yield loss in cereals and oilseed rape from slugs is only 1.1% and 2.4% of national production respectively. These observations present challenges in determining the most appropriate data required for product approval and constitute critically important components of future risk assessment strategies.

Slug problems in autumn-sown crops generally occur early in the growing season and can result in the total loss of plants. However, there is potential for both crops to tolerate significant damage without adverse effect on yield. Winter wheat can be established at significantly lower seed rates than those currently used in practice suggesting that it is possible to lose a proportion of the sown crop to slugs without affecting gross margins. There is no evidence to suggest that wheat varieties differ in their susceptibility to slug attack. In oilseed rape, slug damage to seedlings has been found to be inversely related to their glucosinolate concentration but breeding programmes in recent years have sought to reduce these levels which may account for observed increases in slug damage. Limited data on oilseed rape suggest that optimum seed rates may be lower than those used in practice.

Potential alternatives to slug pellets include seed treatments, pellets based on metal salts, particularly ferric phosphate, parasitic nematodes, controlling slugs with natural products and cultural control.

Seed treatment offers potential advantages through improved targeting and reduced quantities of active ingredient. However, whilst products tested to-date were effective in the laboratory their efficacy was reduced in the field. Research suggests that iron phosphate can be as effective as conventional pellets. The price is comparable with methiocarb but more expensive than some metaldehyde products. Parasitic nematodes (Phasmarhabditis hermaphrodita) have been used effectively in high value horticultural crops but are unlikely to be cost effective in cereals or oilseed rape. There is additional potential for the exploitation of innate plant defences and natural plant extracts for control of slugs. The FERA LINK project on orally-delivered fusion proteins containing specific molluscicidal toxins may also provide an alternative to conventional pellets. However, these will not be short-term solutions and further research is required to develop potential control strategies

The Metaldehyde Stewardship Group is attempting to address industry concern over the use of molluscicides by proposing limits on the amount of product applied in a single application and over a calendar year. However more fundamental studies are urgently needed on pest/crop interactions and crop responses to reduce, the need for treatment. This would lead to a significant reduction in the amount of product used.

The review proposes future research with particular emphasis on understanding the potential of both cereal and oilseed rape crops to tolerate pest attack and the development of more robust risk assessment strategies.

Project Report to Defra8. As a guide this report should be no longer than 20 sides of A4. This report is to provide Defra with

details of the outputs of the research project for internal purposes; to meet the terms of the contract; and to allow Defra to publish details of the outputs to meet Environmental Information Regulation or Freedom of Information obligations. This short report to Defra does not preclude contractors from also seeking to publish a full, formal scientific report/paper in an appropriate scientific or other journal/publication. Indeed, Defra actively encourages such publications as part of the contract terms. The report to Defra should include: the scientific objectives as set out in the contract; the extent to which the objectives set out in the contract have been met; details of methods used and the results obtained, including statistical analysis (if appropriate); a discussion of the results and their reliability; the main implications of the findings; possible future work; and any action resulting from the research (e.g. IP, Knowledge Transfer).

A review of slug control in winter cereals and oilseed rape

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Introduction

Slugs are important pests of autumn sown cereals, particularly winter wheat, and oilseed rape and can cause severe damage and loss of yield. This project reviews pesticide usage data, results from ADAS field experiments and the wider scientific literature to evaluate current strategies for control of slugs in cereals and oilseed rape crops, and to consider potential alternatives for the future.

The need for the review has been stimulated by the Water Framework Directive WFD (2000/60/EC) which commits European Union member states to achieve good chemical and ecological status of all water bodies by 2015. To ensure compliance with drinking water standards, water companies routinely analyse their supplies for pesticides and have recently detected the presence of metaldehyde. Although amounts were well below health impact level they exceeded the EU limit of 0.1 parts per billion for any individual pesticide in drinking water. This has cast doubts over the future availability for widespread use of metaldehyde as a molluscicide for both arable and horticultural crops. Water companies have made it clear that unless metaldehyde levels in water are reduced they will seek restrictions on its use. At present over 90% of farmers apply metaldehyde-based pellets for slug control in preference to products based on other active ingredients (data provided by the Pesticide Usage Survey Group at FERA).

There has also been an apparent increase in damage to crops by slugs despite continued, significant expenditure on slug pellets. Over the last decade there has been an almost fourfold increase in pellet use (Garthwaite & Thomas, 2003) yet despite this, losses from slug damage in winter wheat have risen from an estimated £2.69 million in 1985 to £4 million per annum in 2001 (Shirley, et al., 2001). The Pesticide Usage Survey Report 224 for Arable Crops in Great Britain 2008 indicates that this trend is continuing, with the weight of molluscicides applied to wheat and oilseed rape doubling between 2006 and 2008. Clarke et al. (2009) suggest that, in the absence of slug pellets, losses due to slugs could increase to £22.2 million. With metaldehyde pellets costing from £10 to £15 per hectare and 583,205 hectares of wheat treated annually, each application costs the industry between £5.8m and £8.7m. Similarly, for oilseed rape, where 323,956 hectares are treated, each application costs £3.2m to £4,9m. Clearly, on regulatory and economic grounds, there is a continuing need to improve the efficacy of slug control and to minimise the unnecessary number of pellet applications.

In addition, questions remain on the efficacy of surface treatment with pellets if slugs are feeding below the soil surface or if there are alternative food sources. Improved understanding of the inherent tolerance of crops to pests suggests that they are able to compensate for a degree of pest damage and this should also be taken into account when assessing slug risk.

Taking all these factors into account it is opportune to review slug control with the aim of maximising efficacy whilst minimising environmental impact and complying with legislation. The potential for alternative slug control strategies not reliant on pellets is also considered.

Ultimately this report aims to improve understanding of the response of crops to slug damage, refine risk assessment methods for slugs in cereals and oilseed rape and reduce the unnecessary use of molluscicides. .

Aims and objectives

This review has the following objectives:

1 To examine trends in molluscicide usage in winter cereals and oilseed rape.

This section relies heavily on pesticide usage survey data. Recent trends in molluscicide usage are examined and potential reasons/drivers for these changes proposed.

2 To determine the relative importance of grain/seed hollowing and leaf grazing in wheat and oilseed rape.

Slugs can significantly reduce the number of viable wheat seeds by hollowing grains below the soil surface and reducing crop emergence but this does not seem to be a significant problem in oilseed rape. Leaf grazing is common in both cereals and oilseed rape but it is unclear whether this always has an impact on crop yield, e.g. it is known that the crop becomes progressively more tolerant to attack as it develops. This section examines the relative importance of both types of damage by reviewing evidence from previous studies (e.g. Glen et al., 1992, Port & Young 1992, unpublished ADAS data) and considers whether this justifies different approaches to slug control in cereals or oilseed rape.

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3 To examine historic data and estimate the overall impact of slugs on crop yield.

Historic data (e.g. Spink et al., 2004, Glen et al., 1992, Port & Young 1992) from field experiments with slugs are re-examined to determine the likely impact of these pests on crop yield. In particular, this section considers whether control of slugs necessarily results in yield benefits, and investigates whether any specific factors increase the likelihood of yield increases.

4 To consider how tolerant cereal and rape crops are to slug attack.

There have been significant advances in our understanding of how environmental factors affect the establishment of wheat and oilseed rape plants (Blake et al., 2003; McWilliam et al., 1998), and how the minimum number of wheat plants required to achieve potential yield varies with sowing date and latitude (Spink et al., 2000; Spink et al., 2004). There is less understanding about the minimum plant number required to achieve yield potential in barley and oilseed rape, but some information is given in McWilliam et al. (1995) and in unpublished datasets (e.g. HGCA RD-2005-3242, Re-evaluating thresholds for pollen beetle in oilseed rape, HGCA/LINK RD-2004-3116 spring barley, HGCA/LINK RD-2004-3017 winter barley physiology and pathology). For wheat and oilseed rape there is evidence that the minimum plant density required for potential yield is significantly less than the number of plants that are commonly established, and therefore many crops have the capacity to lose seeds or plants without reducing subsequent yield potential. This information should allow improved assessment of the potential for slugs to reduce yield and the likely need for control measures. The literature is also examined for evidence of varietal susceptibility to attack by slugs in cereals and oilseed rape which could be used in breeding programmes.

In the main body of this review, objectives 2 and 3 are addressed in the combined Section 2 entitled ‘The impact of slugs on crop yield’

5 To consider future control options for slugs in cereal and oilseed rape.

The viability of different control strategies is considered in relation to findings on the relative susceptibility of cereals and oilseed rape to slug attack.

Overall, the project aims to improve risk assessment for slugs in cereal and oilseed rape crops with the aim of rationalising molluscicide usage. This will help farmer/agronomist understanding of which crops are likely to benefit from molluscicide treatment which in turn will help to reduce unnecessary molluscicide applications and minimise the risk of contamination of water courses. The project should also benefit Defra/CRD by helping to guide future research/policy on slug control and in particular whether there are viable control options worthy of investigation which do not involve molluscicide pellets.

REVIEW

1. Molluscicide usageMolluscicide usage is dominated by applications to arable crops. The most recent Pesticide Usage Survey Report 224 - Arable Crops in Great Britain 2008, recorded 1,686,383 hectares treated with 486 tonnes of active ingredient. By comparison, the most recent Pesticide Usage Survey Report – Outdoor Vegetable Crops in Great Britain 2007, recorded 33,850 hectares treated with 11 tonnes of active ingredient.

Of the molluscicide applications made to arable crops, the majority of usage (approx. 90% of active ingredient) is to wheat and oilseed rape. Table 1 summarises data taken from the Pesticide Usage Survey Report 224 – Arable Crops in Great Britain 2008. In addition, an estimate of the average weight of active ingredient applied per hectare has been calculated from total weight of active ingredient applied and the percent of the total crop area receiving a molluscicide treatment.

Table 1. Molluscicide usage in arable crops in 2008 (data from: Pesticide Usage Survey Report 224 – Arable Crops in Great Britain 2008).

Crop Crop area (ha)

% crop area

treated

Area treated with

molluscicide (ha)

Active ingredient applied (t)

Active ingredient

applied /ha (g)

Wheat 2,068,104 28.2 872,347 269.2 462Winter barley 410,236 9.8 54,867 14.5 361Spring barley 596,077 0.1 1,206 0.5 839Oats 132,604 4.1 6,842 2.0 364Oilseed rape 597,706 54.2 555,537 166.5 514

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From the data summarised in the above table it can be seen that over half the oilseed rape crop and more than a quarter of the wheat crop received at least one molluscicide application in 2008. By comparison, only small proportions of the winter barley, spring barley or oat crops were treated. The average weight of molluscicide active ingredient applied per hectare was also higher in wheat and oilseed rape than in winter barley and oats. The apparently higher rate of molluscicide application in spring barley is likely to be distorted by the fact that only a very small area was treated.

Molluscicides applied to wheat and oilseed rape crops were dominated by the use of products containing metaldehyde with only relatively small amounts of methiocarb and thiodicarb used (Table 2). This is probably due to price. A single full rate of metaldehyde costs £10-15/ha whereas a single full rate of methiocarb is £20-25/ha. The full label rate of metaldehyde product was used only 46% of the time on both wheat and oilseed rape crops. This would further increase the price differential between metaldehyde and methiocarb. Some agronomists opt for half rate applications of small metaldehyde pellets as it is perceived that they provide as many baiting points as higher rates of more expensive products such as methiocarb. In contrast, thiodicarb was typically used at full label rate, although the area treated was small. Thiodicarb was withdrawn from use on 25 November 2008.

Table 2. Molluscicide active ingredients applied to wheat and oilseed rape crops in 2008 (Data provided by the Pesticide Usage Survey team at FERA).

Wheat Oilseed Rapemetaldehyde methiocarb thiodicarb metaldehyde methiocarb thiodicarb

Area treated with molluscicide (ha)

806,660 59,221 6,466 518,166 27,499 9,871

Active ingredient applied (kg)

262,536 5,400 1,224 161,502 2,838 2,168

Proportion of treated area (%)

91 7 1 93 5 2

Proportion of total crop area (%)

26 3 <1 49 4 2

Number of applications

1.43 1.13 2.08 1.72 1.21 1.02

Proportion of full label rate (%)

46 61 95 46 69 110

The above data should be set against limits set by the Metaldehyde Stewardship Group (MSG), which has been set-up to establish best practice for the use of slug pellets and to minimise contamination of surface water. Although the MSG guidelines were set in June 2009 in readiness for autumn 2009 they provide a useful guideline against which to compare molluscicide usage in 2008. The MSG has set suggested application limits of 700 g metaldehyde per hectare per calendar year with a maximum of 250 g metaldehyde per hectare per application. Based on the data presented above it is possible to estimate the average weight of each active ingredient applied per hectare using the total weight of active ingredient applied and the percent of the total crop area receiving that active ingredient. Furthermore it is possible to estimate the average weight of active ingredient applied at each application by dividing the total weight applied by the average number of applications (Table 3).

Table 3. Molluscicide active ingredients applied per hectare and per application to wheat and oilseed rape crops in 2008 (Calculated from data provided by the Pesticide Usage Survey team at FERA).

Wheat Oilseed Rapemetaldehyde methiocarb metaldehyde methiocarb

Active ingredient applied per hectare (g)

488 87 551 118

Active ingredient applied per application (g)

341 77 321 98

The average weight of metaldehyde applied per hectare in 2008 was approximately five times the average weight of methiocarb applied per hectare. Despite this, the rate of metaldehyde application was within the application limit of 700 g metaldehyde applied in the calendar year set by the MSG. Indeed, closer inspection of the data indicates that less than 2% of the wheat or oilseed rape crops exceeded the 700 g limit. However, the average weight of metaldehyde applied per application during 2008 did exceed the 250 g limit for a single application set by the MSG. This may have been due to the availability in 2008 of metaldehyde products with maximum

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application rates that, if followed, would have exceeded the 250 g single application limit, e.g. 8 kg/ha of product containing 5% metaldehyde.

Changes in molluscicide usage in wheat, winter barley, spring barley, oats and oilseed rape crops between 1998 and 2008 are summarised in Tables 4 and 5.

Table 4. Cereals and oilseed rape crop areas and percent areas treated with molluscicides between 1998 and 2008 (data from: Pesticide usage survey reports).

1998 2000 2002 2004 2006 2008Crop Crop area

(ha)%

treated area

Crop area (ha)

% treated

area

Crop area (ha)

% treated area

Crop area (ha)

% treated

area

Crop area (ha)

% treated area

Crop area (ha)

% treated

areaWheat 2,035,686 10.1 2,078,908 25.8 1,989,417 22.4 1,981,661 8.0 1,824,181 19.3 2,068,104 28.2Winter barley

760,497 2.7 583,105 9.4 541,769 7.7 416,204 2.6 382,941 4.3 410,236 9.8

Spring barley

455,594 0.1 510,550 0.8 530,777 0.1 566,307 0.9 475,324 1.0 596,077 0.1

Oats 94,714 1.3 105,663 4.4 123,205 6.8 105,495 0.2 119,607 0.8 132,604 4.1Oilseed rape

505,424 20.9 332,104 37.4 356,780 43.7 498,155 16.7 499,069 41.2 597,706 54.2

Table 5. Changes in molluscicide usage in cereals and oilseed rape crops between 1998 and 2008 (data from: Pesticide usage survey reports).

1998 2000 2002 2004 2006 2008Crop Area

treated (ha)

Weight applied

(t)

Area treated

(ha)

Weight applied

(t)

Area treated

(ha)

Weight applied

(t)

Area treated

(ha)

Weight applied

(t)

Area treated

(ha)

Weight applied

(t)

Area treated

(ha)

Weight applied

(t)Wheat 259,447 86.8 807,813 253.1 628,078 209.1 204,484 64.4 399,758 132.3 882,555 269.2Winter barley

22,481 6.7 59,016 16.8 44,641 12.7 16,539 5.1 15,808 3.6 54,867 14.5

Spring barley

667 0.16 5,403 1.3 303 <1 5,087 1.0 4,177 1.4 1,206 0.5

Oats 2,419 1.4 5,558 1.3 7,675 3.7 347 0.1 914 0.2 6842 2.0Oilseed rape

134,048 45.7 187,283 63.7 198,206 74.5 126,930 38.4 290,818 97.3 555,633 166.5

Molluscicide usage fluctuated widely between 2000 and 2008. The area of wheat treated during this period varied between 8% and 28.2% of the total crop area. Highest levels of use were in 2000, 2002 and 2008 when at least 22% of the crop area was treated. During the same period, between 16.7% and 54.2% of the total oilseed rape crop area was treated. As with wheat there were high levels of molluscicide use in 2000, 2002 and 2006, but the greatest use was recorded in 2008 when 54.2% of the crop area was treated. Not surprisingly the total weight of active ingredient applied mirrored closely the overall area of both rape and wheat treated with molluscicides (table 5)

Relating higher usage of molluscicides in these years to risk factors associated with slug damage is difficult. However, Figure 1 shows the relationship between autumn rainfall and the proportion of wheat and oilseed rape crops treated during crop establishment. Although the rainfall data are taken from a single meteorological station in Cambridge, they broadly reflect national rainfall patterns. This analysis is consistent with observations that slug activity is greatest during wet conditions at establishment and prompts increased frequency of treatment. However, in 2008 the area of wheat and oilseed rape treated was the highest recorded over the last 10 years. Approximately 28% of the wheat area and 54% of the oilseed rape area was treated. This represented a 46% increase in usage on wheat since the previous pesticide usage survey in 2006 and a 32% increase for rape. These anomalous results reflect the importance of other risk factors such as changes in agronomic practices. In particular, there has been a trend over recent years towards minimal cultivation for both cereals and oilseed rape. This has involved the use of a cultivator (eg Sumo trio) equipped with subsoilers, discs and a soil packer; a seed rig is often attached before the soil packer. In practice, the seed is not buried as efficiently as with a conventional drill with the result that a higher proportion of seed remains on the soil surface. This is perceived to be at greater risk of slug attack and routine pellet applications often follow crops that are established in this way. In addition, minimal cultivation leaves more trash and organic matter on the soil surface which is also likely to encourage slug activity.

SID 5 (Rev. 3/06) Page 8 of 31

0

10

20

30

40

50

60

150 200 250 300 350

Autumn rainfall (mm) at Boxworth, Cambridgeshire

% o

f cro

p tre

ated

with

mol

lusc

icid

es

Wheat

Oilseed Rape

Figure 1. Relationship between autumn rainfall and molluscicide usage in wheat and oilseed rape crops between 1998 and 2008. (Points circled in red indicate 2008 data).

Molluscicide usage between 1998 and 2008 should also be considered in relation to changes in cropping area. During this period, crop areas increased by 2, 18, 31 and 40% for wheat, oilseed rape, spring barley, and oats respectively.

An estimate can be made of the average weight of molluscicide active ingredient applied per hectare from the crop area grown, percent area treated and total weight of active ingredient applied (Figure 2). For both wheat and oilseed rape crops, the weight of active ingredient applied per hectare has remained relatively constant over the last 10 years. This is despite considerable fluctuations in the area of both crops treated.

It is noticeable that in 2004, 2006 and 2008 average weight of molluscicide applied per hectare was markedly greater in oilseed rape crops than in wheat crops. It is possible that this is in part due to the trend for establishing rape by minimal cultivations as previously discussed. It is apparent that similar weights of molluscicide were applied per hectare for wheat and rape in 1998 and 2002 while the difference in 2000 was less than that seen in the later years. By 2008 the average application rate in treated wheat and oilseed rape crops was 462 and 514 g a.i./ha respectively. In comparison, winter barley, spring barley and oat crops typically received lower application rates in the range of 350-450 g a.i./ha.

0

0.2

0.4

0.6

1998 2000 2002 2004 2006 2008

Year

Aver

age

wei

ght o

f mol

lusc

icid

e ap

plie

d pe

r hec

tare

(kg

a.i./

ha)

WheatOilseed rape

Figure 2. Average weight of molluscicide applied per hectare to treated wheat and oilseed rape crops between 1998 and 2008.

SID 5 (Rev. 3/06) Page 9 of 31

A similar estimate can be made of the average number of applications per year (Figure 3). As expected, the average number of applications follows a pattern broadly similar to that of the average weight of molluscicide applied (Figure 2), indicating that weight of molluscicide applied is determined by the number of applications. Indeed, if the total weight of molluscicide applied per hectare is divided by the number of applications (Figure 4) it can be seen that the weight of molluscicide applied per hectare per application has remained relatively constant throughout the period and in recent years has even shown a slight reduction. In 2008 average weight of molluscicide applied per application was approximately 300 g a.i./ha for both wheat and oilseed rape which exceeds the MSG limit for a single application.

0

0.5

1

1.5

2

1998 2000 2002 2004 2006 2008

Year

Aver

age

num

ber o

f app

licat

ions

WheatOilseed rape

Figure 3. Average number of molluscicide applications made to treated wheat and oilseed rape crops between 1998 and 2008.

0

0.1

0.2

0.3

0.4

1998 2000 2002 2004 2006 2008

Year

Aver

age

wei

ght o

f mol

lusi

cicd

e pe

r ap

plic

atio

n (k

g a.

i./ha

)

WheatOilseed rape

Figure 4. Average weight of molluscicide active ingredient applied per application per hectare to treated wheat and oilseed rape crops between 1998 and 2008.

Data from 2006 and 2008 provided by the Pesticide Usage Survey team at FERA reveals further insight into the timing of molluscicide applications (Figure 5). It is notable that for wheat and oilseed rape the majority (60 and 63% respectively in 2008, 59 and 61% respectively in 2006) of molluscicides are applied to the growing crop (post emergence treatment). This is contrary to HGCA advice published in 2005 for integrated slug control in winter

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wheat and winter oilseed rape which recommends that the greatest benefit is derived from applications made immediately after drilling. The data therefore suggest that farmers either prefer to wait until after crop emergence before applying molluscicides or wait and treat in response to a perceived problem. This is despite the fact that Spink et al. (2004) noted that good crop establishment is critical to achieving potential yield.

0

10

20

30

40

50

60

70

2006 2008 2006 2008

Wheat OSR

% o

f mol

lusc

icid

e ac

tive

ingr

edie

nt a

pplie

d

Before planting

At sowing

Pre-emergence

To growing crop

Figure 5. Timing of molluscicide applications to wheat and oilseed rape in 2006 and 2008.

Conclusions

Molluscicide usage is dominated in terms of area treated and weight of active ingredient used by applications to arable crops.

Within arable crops, use of molluscicides in wheat and oilseed rape accounts for 90% of the active ingredient used.

In 2008 treated wheat crops received on average of 462 g a.i./ha and oilseed rape 514 g a.i./ha.

The majority of molluscicides used on wheat and oilseed rape crops are products based on metaldehyde with only small amounts of methiocarb and thiodicarb used.

In 2008 few crops treated with metaldehyde received more than the 700 g a.i./ha/year limit recommended by the MSG, but some crops received more than 250 g a.i./ha in a single application, which is above the MSG recommendation.

Molluscicide usage in wheat crops has fluctuated over recent years while the area of oilseed rape treated has increased significantly. This increase is likely to be due to changing agronomic practice, particularly increased adoption of minimum cultivation.

2. The impact of slugs on crop yield

Attacks by slugs on cereals are usually most severe in crops that follow oilseed rape, grassland, beans or peas and previous cereal crops. In addition, damp summer and autumn conditions favour attacks (Gratwick, 1992). Damage to winter wheat occurs as the germ of the seed is scraped out, often hollowing the grain. Similar damage may occur in rye and barley but rarely in oats. Shoots may be cut through just above the seed or at ground level, but once the shoots have thickened they are rarely severed but may still be holed on one side. Shredding of leaves is generally less damaging than grain hollowing or severing of stems, but can continue throughout the life of the crop in mild damp weather. The flag leaf may also be damaged during wet weather and slugs may feed directly on the ear, especially if the crop lodges. Glen & Moens (2002) considered that winter wheat is highly vulnerable to slug attack during establishment. Wheat seeds are especially at risk and severe damage can result in complete crop failure or such severe loss of stand that the crop will not recover. In general it is thought that cereal crops are no longer susceptible to slug damage once they are beyond the four leaf stage (GS 14) (Oakley, 2003).

SID 5 (Rev. 3/06) Page 11 of 31

In contrast, oilseed rape seeds are not damaged by slugs, but the seedlings are attacked and they are most vulnerable to slug damage in the early stages of germination (Glen & Moens, 2002). Glen & Moens (2002) suggested that modern cultivars of oilseed rape, with their low concentrations of glucosinolates, are more susceptible than winter wheat to slug damage. Indeed Glen et al. (1990) found a strong inverse relationship between the number of oilseed rape seedlings damaged by slugs and total glucosinolate concentration. A similar relationship was found for the extent of slug damage. However, Glen & Moens (2002) noted that there is little information on the relationship between slug population densities in soil and the severity of damage to oilseed rape at establishment. Oilseed rape is vulnerable to slug damage until the four true-leaf stage (GS 1.4) after which grazing does not normally threaten plant survival (Oakley, 2003)

Duthoit (1964) showed that all species of slugs found in cereal fields were capable of damaging cereal seeds and seedlings in the laboratory and that all species tended to eat both the embryo and endosperm of wheat seeds. When given the choice of seeds or seedlings, Deroceras reticulatum, Arion ater. and Arion fasciatus agg. all caused equal damage to both, whereas Arion hortensis agg. and Tandonia budapestensis were more likely to damage seeds than seedlings. Duthoit (1964) concluded that Deroceras reticulatum and Arion ater were potentially the most damaging species present in cereal crops and, given that D. reticulatum is often prevalent in arable fields with cereal-dominated crop rotations (Glen & Wiltshire, 1986), it is generally considered to be the most important pest species.

Review of the literature revealed relatively little published information on the impact of slugs on the yield of cereals and oilseed rape. Therefore two alternative sources of data were examined:

a) ADAS field experiments

b) Expert opinion

a). ADAS field experiments

Estimates of yield loss from field experiments must be treated with caution because sites with high pest populations were usually chosen to demonstrate differences between treatments. Furthermore, as pest numbers vary significantly between years, data from one year cannot be used to represent the average potential yield loss. Where possible, series of experiments have been used as these provided yield loss data over a number of years and therefore give a more representative estimate of the average yield impact of slugs.

Surprisingly few studies have investigated the effect of applications of slug pellets on crop yield. In most cases the impact of molluscicide treatment on crop damage was assessed but this was rarely related to effects on crop yield. Exceptions to this were two series of trials undertaken by ADAS between 1987 and 1992.

The first series of 17 experiments on winter wheat was done by ADAS between 1987 and 1989 with 14 of these taken to harvest. The experiments were designed to investigate the efficacy of three treatments: methiocarb (as Draza) at 5.5 kg/ha, metaldehyde (as Mini Pellets) at 15.0 kg/ha and methiocarb plus metaldehyde at 2.75+15.0 kg/ha respectively. All treatments were broadcast post-drilling and compared with an untreated control. Crop damage was assessed in terms of percentage plants grazed. The results of these experiments are summarised in Table 5.

Table 5. Mean yield (t/ha) and responses to molluscicide treatments in 14 ADAS trials undertaken between 1987 and 1989 compared with untreated control.

Treatment Application rate (kg/ha)

Mean yield (t/ha) % yield response (cf untreated)

Untreated - 5.82 -Methiocarb 5.5 6.20 6.5Metaldehyde 15.0 6.14 5.5Methiocarb + metaldehyde

2.75 + 15.0 6.24 7.2

At 1991 prices for feed wheat of £117/tonne, a yield response of 3% would have repaid the cost of methiocarb treatment suggesting that all treatments were profitable. However, these mean values are misleading. Overall yield responses were typically small and significant responses to treatment were demonstrated at only two of the 14 sites. At some sites, little or no slug damage was recorded.

The results from these two sites contributed disproportionately to the mean yield responses over the series of 14 experiments. For example, one site in the south east of England showed a 21.2% increase in yield following

SID 5 (Rev. 3/06) Page 12 of 31

methiocarb application, although metaldehyde was less effective. At this site, plant populations in the untreated control were reduced from 294/m2 in December to 99/m2 in early March; although recent research (Spink et al., 2000) suggests that this should still be sufficient to provide optimum yield. At another site in the north east of England, methiocarb apparently increased yield by 40.9% and metaldehyde by 23.8% although these treatments were not significantly better than the untreated control due to large variability between plots.

In the second series of wheat experiments conducted by ADAS between 1990 and1992, treatments of methiocarb and metaldehyde admixed with the seed, were compared with broadcast application of pellets post drilling at eight sites. Assessments were made of slug activity pre- and post-drilling, plant populations in autumn and spring, percentages of plants damaged by slugs and crop yields. Significant reductions in the percentage of plants attacked were obtained at three of eight sites in Avon, Warwickshire and West Yorkshire. At these sites 11.4%, 27.8% and 76.6% of plants were damaged in the untreated controls respectively. This series of experiments is summarised in Table 6

Table 6. Responses to molluscicide treatments compared with untreated mean in eight ADAS trials undertaken between 1990 and 1992.

Treatment Application rate (kg/ha)

Mean yield (t/ha)

Mean % plants damaged

% reduction in damage (cf untreated)

% yield response (cf untreated)

Untreated - 5.74 40.7 - -Methiocarb 5.5 6.18 29.5 7.7 27.5Metaldehyde 15.0 6.06 34.6 5.6 15.0Methiocarb admixed with seed

5.5 6.13 36.3 6.8 10.8

Metaldehyde admixed with seed

15.0 5.87 38.2 2.3 6.1

Although there was a positive yield response to all treatments, these were not reported as being statistically significant suggesting that there was considerable variation between experimental plots and/or responses to treatment varied markedly between sites.

Further work on the potential impact of slugs on yield of winter wheat sown following cereals or oilseed rape was provided by Glen et al. (1993). In this study the percentage of winter wheat seeds killed by slugs was measured at 93 sites in the UK from 1987 to 1990. Slug damage to seeds was generally slight with 83% of sites showing fewer than 5% of seeds hollowed by slugs (Figure 6). Damage to wheat seeds sown after oilseed rape was generally worse than those following cereals but at fewer than 20% of sites were 10% or more of seeds killed. Bearing in mind that by the end of February, the benchmark for establishment is 70% of sown seed (Sylvester-Bradley et al., 2008) it seems unlikely that the seed damage due to slugs reported by Glen et al. (1993) would significantly impact on yield. Indeed, Sylvester-Bradley et al. (2008) suggest that poor establishment or low plant population does not reduce yield unless significant areas of the field have few or no plants, and/or that conditions are unsuitable for compensatory tillering and root growth.

SID 5 (Rev. 3/06) Page 13 of 31

0

20

40

60

80

100

0-5 5-10 >10

% seeds killed by slugs

% fi

eld

with

eac

h le

vel o

f dam

age

Previous crop cereals

Previous crop OSR

Figure 6. The effect of previous crop (cereals or oilseed rape) on the percentage of winter wheat seeds killed by slugs in fields monitored throughout the UK.

Spink et al. (2004) suggested that the percentage of seeds lost to slugs remains fairly conservative across a range of seed rates. This implies that the proportion of seed accessible to slugs remained relatively constant across seed rates. As slugs can only access seed in soil cavities large enough for them to enter this should be expected if seedbed conditions remains constant. It also contradicts the widely-held belief that a given slug population will eat a predetermined amount of seed and that as seed rate is reduced, this amount of damage will remain constant, resulting in an increased proportion of cropped area with no plants. Whilst yield loss at very low seed rates was greater when no slug control was used, the optimum number of plants or the number of plants needed to get on the shoulder of the response curve remained relatively stable. This implies that in these experiments any slug grazing of leaf material was not affecting the compensatory ability of the crop. However, if the slug population was higher or leaf grazing was particularly severe, the loss of green area may be expected to have an effect on yield. Establishing the importance of slugs in determining the seed rate to drill appears therefore to be largely a consideration of the likely percentage plant establishment of the site. Whilst there is no need to increase the target plant number, given a high risk of slug damage, there is a need to increase the seed number drilled to achieve that population. It also seems prudent that given a risk of poor establishment it is wiser from both an economic and environmental point of view to increase seed rate rather than increase slug pellet use, although greater attention to detail in seed bed formation and consolidation may have a greater effect than either.

Glen et al. (1989) demonstrated an empirical relationship between the square root of the biomass of slugs in soil and the percentage of wheat seeds and seedlings killed, although there were no comments on the impact on crop yield. Small, immature individuals often make up the bulk of slug populations (e.g. Glen & Wiltshire, 1986) and are often under represented in refuge traps, so it was important to know whether small slugs are capable of killing wheat seeds and to understand the capacity of slugs to kill seeds in relation to their body weight. This question was addressed by Glen et al. (2006) who investigated the capacity of individuals of three common pest slug species, Deroceras reticulatum, Arion distinctus and Milax gagates, to kill winter wheat seeds under laboratory conditions. This was done by measuring the numbers of wheat seeds killed in the first week after sowing in relation to slug body weight. Individual slugs killed up to about 50 seeds during this period. For D. reticulatum and M. gagates, the number of seeds killed increased with slug body weight but at a declining rate towards an asymptote. Thus, weight-for-weight, smaller individuals of these species killed more wheat seeds than older individuals. One reason for this was that although juveniles ate less of each seed they always took the embryo resulting in seed death. For A. distinctus, the number of wheat seeds killed also increased with slug body weight but, weight-for-weight, this species killed fewer seeds than the others, partly because it consumed more of each individual seed than did the others.

SID 5 (Rev. 3/06) Page 14 of 31

If slugs are able to destroy 50 seeds per week as suggested by Glen et al. (2006) this would represent a significant threat to a wheat crop. However, these studies were done under laboratory conditions so in the field, levels of consumption are likely to be significantly lower. This is supported by the work of Glen et al. (1993) in which, at 83% of monitored sites, fewer than 5% of seeds were lost.

b). Expert opinion

Collating expert opinion from a number of sources formed part of a study undertaken by Clarke et al. (2009) to estimate the impact of different scenarios that could affect the availability of pesticides for use in cereals and oilseeds. The effects of the potential losses of pesticides as a result of revision of 91/414/EEC and the implementation of the Water Framework Directive were predicted. Estimates were made of the current economic impact in the UK of the most important pests if crops were untreated with pesticides. Generation of this information confirmed that there is little, if any, data on the average national yield loss of crops due to slugs. Therefore an alternative approach was used to collate expert opinion from a range of sources.

Firstly, pests were ranked in terms of their perceived importance on crop yield. The ranking was determined using anecdotal evidence from the industry, including entomologists, agronomists, farmers and the agricultural press. Slugs are potentially one of the most important pests of cereals and oilseed rape and although they can destroy sufficient plants to justify re-drilling (ie 100% of the crop is lost), the average yield loss is significantly less than this. On the basis that an average yield loss of 10% of the annual wheat crop due to slugs is unlikely in practice, this pest was ascribed a subjective yield loss value relative to their perceived ranking. Not surprisingly this estimate is significantly less than has been measured in trials.

The consensus of estimates of yield loss from slugs in a range of cereal crops and oilseed rape are summarised in Table 7. Opinion on area of the wheat crop affected by slugs each year (20-25%) was consistent with published pesticide usage data on area treated. Experience confirmed that the worst cases of damage required re-drilling and therefore yield losses could theoretically be as high as 100%. However, overall yield losses in wheat were estimated at 5% if crops were left untreated. In general, field experience confirmed that the impact on other cereal crops is significantly less than for winter wheat. Expert opinion confirmed the view that oilseed rape was susceptible to the highest yield losses with potentially 2.4% of production being lost if fields were left untreated.

Table 7. Subjective estimates of the effect of slugs on the production and yield of a range of cereal and oilseed rape crops assuming no molluscicide treatment.

Crop % area affected

% yield loss untreated

Loss of production

untreated (t)

% loss of production untreated

Estimated financial

loss for UK (£million)

Winter wheat

22 5.0 188,000 1.1 15.8

Winter barley

22 2.0 11,000 <1 <2.4

Spring barley

2 0.5 320 <1 <3.7

Oats 2 0.5 85 <1 <0.7Oilseed rape

59 4.0 46,000 2.4 <4.5

In practice, over the last 10 years the proportion of the total crop area treated against slugs has ranged from 8% to 28% for wheat and from 17% to 54% for oilseed rape. Therefore the figures for loss of production in Table 7 may be an overestimate.

Overall, there are limited data to confirm that molluscicide treatment has a beneficial effect on yield of cereals or oilseed rape. Numerous trials and field observations have shown that molluscicides do indeed reduce crop damage but there is limited empirical evidence of consequent yield responses.

Conclusions

Grain hollowing rather than leaf shredding has the greatest potential impact on the yield of wheat crops. In oilseed rape the seed is rarely damaged and leaf grazing has greater impact.

SID 5 (Rev. 3/06) Page 15 of 31

Once cereal plants are beyond the four leaf stage and oilseed rape beyond the four true-leaf stage they are considered to be less vulnerable to slugs.

There are limited data to show that application of molluscicide pellets has a beneficial effect on yield of cereals or oilseed rape.

Expert opinion suggests the average yield loss in cereals and oilseed rape from slugs is 1.1% and 2.4% of national production respectively.

There is a general assumption that crops damaged by slugs will suffer consequent reduction in yield but there is limited empirical evidence of yield responses to molluscicide treatment.

3. The tolerance of cereal and oilseed rape crops to pest attack

Slug problems in autumn-sown crops generally occur early in the growing season and can result in the total loss of plants. However, there are means by which crops can tolerate levels of pest attack and these are discussed below.

Affect of crop type

In a recent review of pest thresholds (Ellis et al., 2009), pests were categorised as either affecting a crop’s sink size, i.e. the number of seeds per m2, or it’s source size, i.e. the amount of resource available to fill the grains. Due to its affect on the number of plants per m2, and potentially the number of seeds per m2, the grey field slug was categorised as affecting a crop’s sink size. This means that crops whose yields are generally limited by the number of seeds per m2 (sink limited) are likely to be less tolerant to damage by slugs.

Crops known as being sink-limited are barley (Bingham et al., 2007) and oilseed rape (Berry & Spink, 2006). However, in both cases, any reductions in yield would not be due to a decrease in plant numbers per se. Barley has fewer fertile spikelets per ear (Kirby & Appleyard, 1984) than wheat and so has a lower capacity to compensate for lower tiller numbers through increases in grains per ear. In oilseed rape, the number of seeds per m2 is determined during a phase lasting about 300 day degrees (2-3 weeks) after mid-flowering, where pod and seed abortion occurs (Mendham et al., 1981).

Wheat differs to barley and oilseed rape in that it is generally thought of as a source-limited crop (insufficient resource to fill grains), or co-limited by both source and sink (Shearman, 2005). Therefore, it is more likely to be able to tolerate damage caused by pests that affect sink capacity such as slugs.

Varietal susceptibility to slugs

There was no evidence in the literature to suggest that wheat or barley varieties differ in their susceptibility to slug attack. In oilseed rape, slug damage to seedlings has been found to be, inversely related to glucosinolate concentration (e.g. Glen et al., 1990). However, glucosinolates and their breakdown products are also toxic and anti-nutritive to livestock (Chew, 1988). Therefore, breeding programmes in recent years have sought to reduce glucosinolate levels in oilseed rape, so that currently, widely-grown varieties have relatively low and uniform levels (Bellostas et al., 2007) which may account for the perceived increase in susceptibility to slug damage.

Optimising plant populations for tolerance

A summary of the tolerance of different cereal species and oilseed rape to pest damage is shown in Table 8.

SID 5 (Rev. 3/06) Page 16 of 31

Table 8. Tolerance of cereals and oilseed rape to pest damage. Taken from Ellis et al. (2009).

Crop parameter Minimum to achieve potential yield

Range in practice Degree of tolerance to loss in a typical crop

Winter wheatPlants/m2 <100 early sown

200 late sown50 to 600 High

Ears/m2 400 400 to >2000 shoots/m2 HighGAI at flowering 5 to 7 5 to 10 ModeratePost-flowering photo-assimilate Unknown Unknown Low Winter barleyPlants/m2 unknown Up to 600 ModerateEars/m2 unknown 400 to >2000 shoots/m2 LowGAI at flowering 5 to 7 4 to 9 LowPost-flowering photo-assimilate Unknown Unknown ModerateSpring barleyPlants/m2 unknown Up to 600 LowEars/m2 unknown unknown LowGAI at flowering unknown unknown LowPost-flowering photo-assimilate unknown unknown ModerateWinter oilseed rapePlants/m2 Unknown 20 to 120 ModeratePods/m2 7000-8000 5000 - 12000 ModerateGAI at flowering 3 to 4 3 to 6 ModeratePost-flowering photo-assimilate Unknown Unknown ModerateSpring oilseed rapePlants/m2 Unknown 20 to 120 LowPods/m2 7000-8000 Unknown LowGAI at flowering 3 to 4 Unknown LowPost-flowering photo-assimilate Unknown Unknown Moderate

.

Wheat

There have been a number of studies to investigate the economic optimum plant population required to achieve potential yields (Spink et al., 2000, 2004, Gooding et al., 2002). In all studies, the general relationship between seed rate and yield was similar – an initial steep increase in yield with increasing seed rates, followed by a levelling off (Figure 7). In some circumstances, yield decreased at high seed rates because, for example, of lodging (Spink et al., 2004).

7

8

9

10

11

12

0 100 200 300 400 500 600 700

Seed rate (seeds/m2)

Gra

in y

ield

(t/h

a @

85%

DM

)

Figure 7. Effect of seed rate on yield of winter wheat grown at Sutton Bonington, Nottinghamshire in 2002 using either an early ( ) or late ( ) nitrogen fertiliser regime. From Spink et al. (2004).

Experiments by Spink et al. (2000) at two sites (Herefordshire and Nottinghamshire) over three growing seasons (1996-1999) tested wheat grown at five seed rates from 40 to 640 seeds per m2. Linear plus exponential curves were used to determine the economic optimum plant population assuming a grain price of £80 /t and a seed price of £300/t, although it was found that the optima were relatively insensitive to changes in prices. From these calculations the optimum were determined as 62 plants/m2 when the crop was drilled by the end of September,

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and increased by 1.6 plants per day of delay in drilling (Spink et al., 2000). This is considerably lower than commercial practice. In the HGCA’s Wheat Growth Guide (Anon., 2008), the benchmark for number of plants established is 260 per m2.

When Gooding et al. (2002) carried out a comparable experiment, again using five seed rates (50 to 600 seeds/m2) similar effects of seed rate on yield were found. However, the optimum plant populations did differ significantly with both year and with the choice of curve to fit to the data. When the same approach was taken to that of Spink et al. (2000), optimum plant populations in the 1999 experiments were similar to those of the authors at 100 and 90 plants per m2. However, in the 1998 experiment, the optimum plant population was significantly higher at 210 plants/m2 (Gooding et al., 2002). It was suggested that this result was due to the shallower response in 1998 and related to nitrogen availability (Gooding et al., 2000). Indeed, when a more extensive study of seed rates was carried out (Spink et al., 2004), optimum plant population was also found to relate to latitude (seed rate should increase by 7 or 8% per degree increase in latitude), and rotational position (lower optimum population where take-all infection occurred) which interacted with seed treatment.

In both the Spink et al. (2000) and Gooding et al. (2002) studies, the lower plant populations maintained yields through increased numbers of tillers. It was found that these increases were due to a longer duration of tillering rather than increased rate of tillering (Spink et al., 2000). It should be noted, though, that too many tillers from too low a plant population can lead to variable and delayed maturity and problems with harvest (Gooding and Davies, 1997).

It can be seen from these studies that seed rates much lower than those used commercially can be tolerated by wheat crops because plants can compensate by producing extra tillers. This suggests that wheat may cope with a certain amount of damage by slugs without adverse effect on yield. Interestingly, Spink et al. (2004) found that the proportion of wheat seeds accessible to, and eaten by, slugs remained relatively constant over a range of seed rates. This implies that a given slug population will not eat a predetermined amount of seed, as was often thought, so a lower seed rate will not necessarily lead to an increased proportion of the field with no plants (Spink et al., 2004).

The optimum seed rate was also determined with and without slug treatments (Spink et al., 2004) and it was found that when a slug treatment had been applied, the optimum was 30-40 seeds per m2 lower than with no molluscicide application.

Barley

There is no evidence in the literature that similar seed rate studies have been carried out in winter or spring barley in the UK (Table 8). The benchmark for the number of plants required per m2 quoted in HGCA’s Barley Growth Guide (Spink, 2005) is higher than for wheat at 305 plants/m2, which is assumed to be 85% of the number of seeds sown. Different figures are used as benchmarks for crops grown in the south (277 plants/m 2) and the north (327 plants/m2) of the country. The reason for these higher seed rates may be related to the smaller number of spikelets per ear found in barley than in wheat.

Oilseed rape

In oilseed rape, typical seed rates used by growers range from 60 to 120 seeds/m2. However, there is little work in the literature to investigate the effect of seed rate on potential yield and it appears that no work has been done to determine the economic optimum seed rate in this crop. A current HGCA project (RD-2005-3242 Re-evaluating pollen beetle thresholds) investigated the effect of seed rate on yield and flower numbers (Ellis and Berry, 2010). Five seed rates, from 20 to 160 seeds/m2 were tested on three winter rape varieties – an open pollinated variety Castille, the hybrid variety Excalibur and the hybrid semi-dwarf variety PR45D03 in North Yorkshire. For all varieties, yields were highest at a seed rate of 40 seeds/m 2 (the second lowest tested) and lowest at the highest seed rate although differences were not statistically significant (Ellis and Berry, 2010). It is thought that a high plant population may lead to an excess number of flowers which absorb and reflects a lot of light, rather than allowing it to reach the photosynthetic part of the plant. This hypothesis was not proven by the flower number results in this experiment (Ellis & Berry, 2010) but further experiments are underway.

These results, however, do indicate that current seed rates used by growers are likely to be higher than the optimum for yield, and so some loss through slug damage would not be detrimental. This is consistent with work by MacWilliam et al. (1995) who found that a low plant population of oilseed rape in the region of 20-30 plants/m 2

benefitted yield, although it was stressed that an even distribution of plants was important (MacWilliam et al., 1995) and slug damage is rarely uniform across fields.

SID 5 (Rev. 3/06) Page 18 of 31

Optimising establishment for tolerance

In addition to optimising the number of plants, tolerance to slugs can be improved by ensuring good crop establishment. Work has been done on factors affecting the establishment of both cereals (Blake et al., 2003) and oilseed rape (MacWilliam et al., 1998). In terms of tolerance to slugs, it has been found that consolidation (but not compaction) of the soil is important (Blake et al., 2003) since the pests can only access seed in cavities that are big enough for them to enter. It has also been found that cultivation method is important, with more slug damage as a result of direct drilling than plough-based establishment (Sievert et al., 1999).

Conclusions

Winter wheat can be established at significantly lower seed rates than those currently used commercially without adverse impact on yield.

Limited data on oilseed rape suggest that optimum seed rates may be lower than those used in practice.

There is potential for both wheat and oilseed rape crops to tolerate significant slug damage.

Both weather and crop agronomy will affect crop tolerance to slug damage.

More work is required to clarify optimum seed rates in barley and oilseed rape. In addition, minimum plant number per m2, GAI at flowering and post flowering assimilates in winter and spring barley are worthy of further investigation to improve understanding of crop tolerance.

4. Alternative control measures

Slug control has been reliant on molluscicide pellets for decades and there have been few successful alternatives introduced. Pellets based on metal salts are one option as is the use of parasitic nematodes. Both of these are discussed below along with other options that did not become commercially available or are still at early stages of development.

Seed treatments

Simms et al. (2002) noted that slug pellets often fail to give adequate protection against slugs as the active ingredient is repellent to slugs, and so slugs may stop feeding prior to ingesting a lethal dose. In addition, pellets break down in wet weather, when slugs are most active, pellets may become covered with soil, and pellets on the soil surface may pose a hazard to wildlife. Seed treatment was therefore investigated as an alternative to pellets as it allows better targeting of the active ingredient, reduces the amount of the active ingredient required, and removes need for a separate pellet application. As the most important damage to wheat is to the seed, seed treatment is potentially the most efficient way of enhancing establishment (Ester & Nijenstein, 1995).

A range of seed treatments including metaldehyde and methiocarb have been found to be effective in protecting seeds from slug damage in laboratory-based experiments (Ester & Nijenstein, 1995; Simms & Wilson, 2005). Although seed damage caused by slugs is of less importance in oilseed rape crops, it has been found that treatment may protect the emerging seedling as well as the seed (Simms et a., 2002).

Metaldehyde and methiocarb seed-treatments were found to be as effective at protecting oilseed rape seedlings from damage by field slugs, keeled slugs and round back slugs, as baited pellets (Simms et al., 2002; 2003). In outdoor pot experiments, protection of seedlings was recorded for up to eight weeks after planting, by which time plants had reached the six true leaf and so were less susceptible to slug damage. However, although metaldehyde and methiocarb seed treatments were as effective, or more effective at protecting oilseed rape seedlings from slugs, than pellets in laboratory or semi-field experiments they did not offer effective control under field conditions (Simms et al., 2003). It is thought that the reduced efficacy of these seed treatments under field conditions may be due to loss of active ingredient due to microbial degradation or precipitation (Simms & Wilson 2005).

In addition to trials using metaldehyde and methiocarb, the broad spectrum insecticide, imidacloprid, has been found to reduce slug damage significantly in field experiments of winter wheat when applied as a seed treatment (Simms et al., 2005). In contrast, imidacloprid seed treatments did not protect oilseed rape plants from slug damage. It was concluded that the inconsistent and short-lived activity of imidacloprid would preclude its development as a seed treatment for control of slugs.

Pellets based on iron (ferric) phosphate

Iron phosphate/ferric phosphate marketed as Ferramol and Sluxx by Certis and Sluggo® by Omex is now approved for slug control in the UK in all edible and non-edible outdoor crops. Ferramol and Sluggo contain 1%

SID 5 (Rev. 3/06) Page 19 of 31

w/w ferric phosphate and Sluxx contains 2.97% ferric phosphate. Company literature suggests that it has an advantage over other molluscicides in that it is broken down by microorganisms to iron and phosphate which can be used as plant nutrients. The price ranges from £17-24/ha depending on the rate of use. This compares with £20-25/ha for methiocarb and £4-19/ha for metaldehyde.

There is little evidence available in the public domain on the efficacy of iron phosphate for slug control. Rae et al., (2009) suggested that it can significantly reduce slug damage caused by the grey field slug. In the United States iron phosphate as Sluggo® was considered to be as effective as metaldehyde formulated as Deadline M-P and Bullet in artichokes. Iron phosphate was also considered to be environmentally safer than metaldehyde (Bari, 2004). Koch et al., (2000) demonstrated that iron phosphate as Ferramol gave good control of slugs of the families Arionidae and Agriolimacidae in both the laboratory and the field. However, Lehmannia valentianaI, a major pest in glasshouses in Germany, could not be controlled although this species is not native to the UK. In Switzerland, Speiser and Kistler (2002) showed that iron phosphate reduced leaf loss of lettuce, increased the number of marketable heads and reduced numbers of the slug Arion lusitanicas. However, the reference treatment metaldehyde was more effective in preventing slug damage and reduced numbers of A. lusitanicas, A. hortensis and D. reticulatum. In oilseed rape, iron phosphate reduced the percentage of seedlings with slug damage but did not affect the total number of seedlings per unit area.

Edwards et al. (2009) used mesocosms to compare an untreated pellet with metaldehyde, Sluggo®, iron phosphate and two chelating agents ethylene diamine triacetic acid (EDTA) and ethylene diamine succinic acid (EDDS) at the recommended and five times recommended rate. There was virtually no earthworm mortality over the 14 days of the experiment, but there were considerable differences in earthworm weights, although none of them differed significantly from the control. The earthworms that were exposed to the recommended and five times recommended application rate of Sluggo® gained significantly less weight than those exposed to iron phosphate only.

Parasitic nematodes

The nematode Phasmarhabditis hermaphrodita is a biological control agent for slugs (Glen & Wilson, 1997). It is produced in liquid fermenters, harvested and formulated as a spray. In the UK it is available as Nemaslug for use in vegetable crops from Becker Underwood. The nematode responds to slug-associated cues such as slug mucus and faeces in order to locate potential hosts (Rae et al., 2009).

The formulated product contains the nematode in its aggressive juvenile form which infects slugs through the breathing hole (Pennell, 2009). Once inside the slug the nematodes release symbiotic bacteria, which initially stop the slug feeding and then quickly kills it and releases a new generation of infective juveniles which disperse in search of further prey. The ability to stop slugs feeding quickly and the absence of residues is particularly valuable in horticultural crops where crop quality is of primary importance.

The nematode is usually applied by spraying but it is also possible to use in most forms of irrigation equipment. Trickle irrigation has been used for some time and the nematode can now be applied through rain guns.

In Holland parasitic nematodes gave better control of slugs than two applications of traditional pellets in Brussels sprouts (Pennell, 2009). Similarly good results in this crop were reported from Lincolnshire.. Parasitic nematodes have also been effective in asparagus and in lettuce where they were again better than slug pellets with no slugs found in the crop at harvest (Pennell, 2009). Work has also investigated their potential in potatoes where the ability of the nematode to move through the soil and reduce tuber damage can maximise yields of high quality potatoes.

However, successful use of parasitic nematodes for slug control in the UK to-date has been confined to high value horticultural crops such as Brussels sprouts. There have been no reports of use in cereal and oilseed rape where it is likely to less cost effective. An estimated cost for parasitic nematodes is £110/ha which is significantly more expensive than molluscicide pellets. Other factors such as low efficacy under unfavourable conditions, limited shelf life and application timing for optimum efficacy will also need to be addressed for parasitic nematodes to gain a higher proportion of the slug control market in the UK.

Controlling slugs with natural products

Secondary metabolites are produced by many organisms, mainly as attractants or as a means of defence. For plants, this strategy is particularly important, and it is commonly accepted and supported by scientific research that plant pests and pathogens have driven the evolution of complex biosynthetic pathways of chemicals that can control feeding or infection. However, the cultivation of food crops has meant that these chemical defences in seeds, fruits foliage and roots have been lost in the pursuit of palatability for humans and livestock.

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Plant defence chemistry can potentially be exploited for protection against specific pests either by optimising the chemistry of target plant species in situ (by selection and/or breeding), or by producing extracts, fractions or single chemical entities from non-host plants for external application. Plants have provided some of the most widely-used plant protection chemicals in the form of nicotine and pyrethroids, but there is potentially much more chemical diversity to exploit in the estimated 350,000 plant species around the globe.

Innate chemical defences in plants

The importance of innate chemistry in defence relative to plant age was indicated in a study of glucosinolate concentrations in relation to generalist herbivory by woodpigeons and the slug Deroceras reticulum in oilseed brassicas (Lambdon, et. al., 2003). Glucosinolates were maximal in young foliage, declining significantly with age, and mature leaves were 1.7 times more likely to be damaged than younger leaves. In a further study, age-related differences in palatability to D. reticulum were reported for 29 British herbaceous plant species (Fenner, et. al., 1999). Highly significant differences in the palatability of seedling and mature plants were observed. In most cases seedlings were more palatable than mature plants, but in certain cases the latter were more palatable. A wide range of defensive attributes will have been involved in the relative attraction of younger or older foliage, but chemistry must be an important factor, and worthy of further investigation.

Leaf-discs of the leaves and seedlings of Arnica montana and 20 other plant species were compared (Scheidel & Bruelheide, 1999). Arnica was one of the most preferred plants by the three slug species tested (Arion lusitanicus, A. subfucus and D. agreste) with seedlings being most susceptible. Epidermal cell wall thickness and leaf hairiness were shown to influence acceptability, but it was also found that undamaged leaves were preferred to damaged leaves, suggesting the induction of chemical defences. Whilst constitutive chemical defences may provide the most effective barriers to generalist herbivore damage, in order to meet the required standards of palatability as food crops, inducible chemical defences could play an important role in limiting damage to foliage.

An interesting strategy for influencing the antifeedant chemical profile of plants has been suggested by Barker (2008). Fungi of the genus Neotyphodium are endophytes of festucoid grasses, and infection is associated with species-specific secondary metabolite profiles. Feeding by Deroceras was affected by endophytes conferring different chemical profiles, and feeding with artificial diets incorporating specific chemical components generally supported the observed slug preferences for Neotyphodium-infected grasses. For example, whereas the indole diterpenoid lolitrem B was shown to reduce feeding, ergotamine was phagostimulatory. The significance of the secondary metabolite profile in endophyte-infected grasses in Deroceras herbivory suggests a potential novel approach to pest management.

Finally, not only the concentration but the diversity of secondary metabolite profile might be an important factor in relative palatability. Dayan and Romagni (2001) reported that when offered a choice of potential lichens to feed on, the slug Pallifera varia avoided those that produced a wide variety of compounds and preferred those with the lowest diversity of secondary metabolites. As a general principle, chemical diversity within a single species is logically more likely to confer effective defence, by increasing the chances of affecting multiple pest targets (enzyme, receptors). The breadth of defence to different pests may be increased, and synergism due to the hitting of multiple targets within specific pests may result.

Natural product extracts

Once extracted, natural plant products that could be applied to crops possess a number of potentially advantageous features. For example, they generally degrade rapidly as a result of exposure to sunlight, oxygen and moisture, or by the action of microbial detoxification enzymes. Whilst rapid degradation reduces the risk to non-target organisms, more frequent applications may be necessary so there may be trade-offs between costs and selectivity. Another advantage is a history of safe use. There are thousands of plant species that have been used safely for centuries as foods, medicines and personal care products, and records of safe use can become an important first step in discovering those with potential for pest control. As with any synthetic chemical, natural products can be toxic to non-target organisms, including beneficial insects and mammals, and to plants.

Active ingredients extracted from plants can potentially be deployed in the same way as any synthetic chemical, for example in seed dressings or as soil drenches, depending upon the specific chemical characteristics (e.g. solubility, lability). A few examples of plant constituents that have been reported to exhibit slug antifeedant or molluscicidal activity are discussed below. It should be emphasised that this is not a comprehensive list. Extracts from 15 different lichen species were screened for antifeedant activity at Rothamsted Research and the most effective came from the species Letharia vulpine (Clark et. al., 2009). The main active ingredient was identified as vulpinic acid, which protected against slug feeding when applied as a foliar spray to turnip plants and as a dressing to wheat seeds in laboratory experiments.

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The toxicity of a number of natural products to the eggs of Deroceras reticulatum was tested by Iglesias et al. (2002). Carvone, saponin, extract of Pongamia pinnata and azadirachtin all killed eggs after periods of exposure ranging between two and 14 days. One of the most active products was azadirachtin. Carvone, a terpenoid derived from caraway seeds, has also been reported as a feeding deterrent in a study with the slug Arion lusitanicus (Frank et al., 2002). The highest concentration used (0.75ml litre -1 mulch) also caused 50% morbidity in a laboratory choice experiment.

Saponins have been widely studied in the context of controlling aquatic molluscs. In a US Patent (United States Patent 5290557), the use of saponins extracted from a number of plant sources, particularly Yucca schidigera and Hedera helix, was also described as an antifeedant for terrestrial molluscs, and at higher concentrations as a molluscicide.

Foliage extracts of 33 species of umbellifers (Apiaceae) were evaluated for their effects on the feeding behaviour of Deroceras reticulatum using an electrophysiological recording assay (Dodds et al., 1999). Extracts of Petroselinum crispum, Conium maculatum, and Coriandrum sativum were identified as being the most neuroactive as well as the most antifeedant. A more recent paper identified the active principles from these extracts using coupled GC-MS and neurophysiological assay, and reported that one of the most potent actives was the toxic alkaloid -coniceine from Conium maculatum (hemlock) (Birkett et al., 2004). However almost all the isolated neuroactive chemicals from the three species showed similar antifeedant activity, e.g. -coniceine from hemlock and myricetin from parsley reduced feeding by 70 5.4% and 69 4.3% respectively.

Hemlock is a well-known poison and the isolation of an active ingredient with mammalian toxicity is not surprising, but intuitively, herbs such as parsley and coriander should be a good place to look for “safe” and selective actives. Accordingly, other food plants have received attention as potential slug control agents. Tarragon (Artemisia dracunculus) herb extract incorporated into wheatflour pellets had antifeedant activity in starved D. reticulatum (Clark et al., 1999). Schüder et al., (2003) reported that garlic applied as a 2.5% or 5% solution showed promising activity for the control of slugs (D. panormitanum) and snails (Oxyloma pfeifferi) in UK nurseries. This built upon previous reports of the molluscicidal activity of garlic, where allicin was isolated as the active ingredient (Singh & Singh, 1995). A few years ago, caffeine as a slug repellent was reported in a brief communication in the journal Nature (Hollingsworth et al., 2002). Caffeine solutions (1%-2%) caused 100% of slugs (Veronicella) to exit drenched soil, with the majority of these subsequently dying. Using leaf-dip bioassays, caffeine solutions of just 0.01% significantly reduced feeding.

Differences in the responses between different species of pest were evident from a study into the effects of a semi-purified extract from Agropyron repens (couch grass) on two slug species, three freshwater snail species and a garden earthworm. The extract, containing phenolic glycosides, showed dermal and intestinal toxicity only in the slug species D. reticulatum and D. laeve and had no apparent effect on the other test species (Hagin & Bobnick, 1991). The active compound was identified as 6-hydroxy-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid, and its LD50 for the more resistant D. laeve was estimated at c. 5mg kg-1.

In summary, plants can accumulate secondary metabolites with important pest-repellent properties. These chemicals are extremely diverse in structure, and different mechanisms of action are apparent. Given the economic impact of slug damage, there would appear to be enormous opportunity for research into new antifeedants and molluscicides from plants to be developed either as (standardised) extracts, or as single entities. However, it is critical to gather information on toxicities (phyto-, mammalian, and beneficial insect) at an early stage of the development process and this information is often lacking.

With respect to innate chemical defences, a number of aspects deserve attention: the relative importance of constitutive and inducible metabolites; the importance of chemical concentration and profile (and the significance of the complexity of profile) and the relevance of chemistry at different developmental stages. Furthermore, current crops may have much to gain from examining the chemistry of ancestral species.

Novel control methods

An ongoing FERA LINK project is investigating new environmentally-friendly technologies for slug control based on orally-delivered fusion proteins containing specific molluscicidal toxins. Expression of functional mollusc-specific toxins has proved to be more problematic than anticipated and further work will be undertaken to address this issue. However, insecticidal fusion proteins have shown activity against Deroceras reticulatum in injection and feeding assays suggesting potential for use in the control of slugs. This work will not provide a short-term solution to slug control but may provide an alternative to conventional molluscicidal pellets in the future.

Cultural control

Good seedbed consolidation reduces damage by slugs (Emson & Gair, 1982). Often in fields of winter wheat that have failed, plants growing on the headlands and in wheelings where the ground is firmer, often escape more

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serious attack. Less damage occurs in seasons when conditions are favourable for cultivations and the production of a good tilth. Autumn sowings should also be made in good time when conditions favour rapid germination and establishment. Glen et al. (1992) showed that seeds sown at a depth of 25mm were less damaged than those drilled at 10mm probably because the pest found it more difficult to locate the seed at the deeper drilling.

Conclusions

There are a range of potential alternatives to slug pellets including seed treatments, pellets based on ferric phosphate, parasitic nematodes, controlling slugs with natural products and cultural control.

Seed treatment is potentially a good alternative to pellets as it allows better targeting of the active ingredient, reduces the amount of the active ingredient required, and removes need for a separate pellet. However, whilst products tested to date were effective in the laboratory their efficacy was reduced in the field.

Iron phosphate/ferric phosphate marketed as Ferramol and Sluxx by Certis and Sluggo by Omex is now approved for slug control in the UK in all non-edible outdoor crops crops. Research suggests that iron phosphate can be as effective as conventional pellets. The cost of the product is similar to that of methiocarb (£20-25/ha).

Parasitic nematodes (Phasmarhabditis hermaphrodita) have been used effectively in high value horticultural crops but are unlikely to be cost effective in cereals or oilseed rape (estimated cost £110/ha).

There is significant potential for the use of innate plant defences and natural plant extracts for control of slugs. However, this will not be a short term solution and further research is required to develop potential control strategies.

Cultural control is an important component of any potential IPM strategy against slugs. However, soil conditions, the weather and time constraints can often combine to inhibit the creation of seedbeds that encourage rapid growth and establishment.

Discussion

1. Molluscicide usage

Molluscicide usage in the UK is dominated in terms of area treated and weight of active ingredient applied by arable crops. Within arable crops applications to wheat and oilseed rape account for 90% of the active ingredient used. Applications to other cereal crops such as winter and spring barley and oats account for only a small proportion of the total usage. Therefore any attempt to achieve more rational use of pellets should concentrate on applications to wheat and oilseed rape.

The vast majority of applications to wheat and oilseed rape are of metaldehyde with only small amounts of methiocarb and thiodicarb being used. This is probably because metaldehyde products are generally less expensive (a single full dose of metaldehyde is £10-15/ha and of methiocarb £20-25/ha) and also because more than one application of metaldehyde is permitted per season.

Patterns of molluscicide use have fluctuated over the period 1998 to 2008. Usage in wheat has varied between 8% and 28.2% of the total crop area treated and for oilseed rape the comparable figures are 16.7% and 54.2%. These patterns are reasonably well explained by rainfall over the autumn. Molluscicide use tends to increase during wet autumns as slugs will be active and so potentially have a greater impact on crops. However, in 2008 molluscicide usage was higher than might have been predicted by autumn rainfall. It is possible that this was due to an increase in the proportion of crops that were established by minimal cultivation techniques. In general, crops established by minimal cultivations will be at greater risk from slugs than those established conventionally. This is simply because fewer slugs will be killed by cultivation and there will be more trash/organic matter on the soil surface which will encourage pest activity. If the trend towards minimal cultivation continues then it could have a significant impact on molluscicide usage. The justification for routine applications of slug pellets following the use of minimal cultivation is debatable and the proportion of seeds that establish with or without slug pellets deserves further investigation. If minimal cultivation inherently leads to poor plant populations in the presence of slugs, the potential to modify machinery to improve establishment should be considered.

Concern over the detection of metaldehyde residues in water has led to the establishment of the MSG. The MSG advocates that in order to prevent contamination of water applications of metaldehyde should be limited to 700 g a.i. per calendar year and 250g a.i. for a single application. Examination of molluscicide usage statistics suggests

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that currently UK farmers and growers are within the threshold for a calendar year with 488 g a.i. being applied to wheat and 551 g a.i. being applied to rape. However, the threshold for a single application was exceeded in both wheat and oilseed rape in 2008 when, on average, 341 g a.i. and 321 g a.i. were applied respectively. It seems likely that this was because metaldehyde products were available in 2008 with maximum application rates that if followed would have exceeded the 250 g a.i. limit suggested by MSG.

The timing of pellet applications in 2006 and 2008 to both wheat and rape is interesting in that the majority of applications (approximately 60%) were applied to the growing crop. This contradicts most advice suggesting that applications to the prepared seed bed are most effective as slugs have no alternative food source. It is also counterintuitive when it is generally accepted that in wheat, grain hollowing is known to have greatest potential impact on the crop in comparison with leaf grazing. It seems unlikely that pellets applied to the growing crop will have a significant impact on reducing grain hollowing. In oilseed rape the situation is different in that leaf grazing and severing of plants as they emerge are thought to have greater effect on yield than damage to the seed. However, crops are thought to be tolerant of slug attack beyond the four true- leaf stage and would be expected to have some tolerance even before this. Even in oilseed rape the best time to apply pellets is to the prepared seedbed so the reliance on applications to the growing crop, although more understandable than in wheat, is probably due to subjective decisions taken in response to visible damage.

Surveys suggest that a small proportion of molluscicide applications are also being made at sowing with pellets being soil incorporated. It is unclear whether pellets are being drilled with the seed but soil incorporation is likely to give poor control of slugs. In general, slug pellets rely on the pest contacting a pellet at random before ingesting it. This is the rationale for applying pellets to a prepared seedbed when there is limited alternative food available. If pellets are soil incorporated it would be expected that slugs are less likely to encounter them when active on the soil surface. Where pellets are drilled with the seed the ratio of pellets to seed is so low that it is likely that many grains could be hollowed before a slug is killed. A review of the recommendations for treatment is therefore warranted to re-assess the optimum strategy.

Potential research areas

a. Evaluation of slug risk posed by adoption of minimal cultivation

Use of minimal cultivation, particularly for the establishment of oilseed rape, is perceived to increase the risk of slug damage as many seeds are left on the soil surface prompting routine application of molluscicide pellets. Use of this establishment method should be investigated in more detail to quantify the extent to which seeds are left on the soil surface and to investigate whether this increases the vulnerability to slug damage. The potential to modify the machinery in order to achieve better seed incorporation could also be studied.

b. Evaluation of the efficacy and persistence of the range of molluscicidal pellet options.

To make more informed decisions on product choice, farmers and growers need further information on the relative efficacy and persistence of molluscicidal pellets. Currently there are numerous products and formulations on the market that make informal claims about persistence and efficacy that are difficult to verify. The work should focus on the range of formulations in commercial use to evaluate efficacy and persistence with specific attention given to risks of contamination of water.

2. The impact of slugs on yield

Slugs pose a different threat to cereals than to oilseed rape. In cereals, grain hollowing is believed to have the greatest potential impact on yield with leaf grazing considered much less damaging. In contrast, oilseed rape seeds are rarely damaged but leaf grazing and the severing of the plant soon after emergence are thought to be much more important. Certainly, slugs can have a dramatic impact on crop establishment such that in some instances re-drilling becomes necessary. Whilst this is costly it is often a practical option in the south of the UK but less so further north. If an oilseed rape crop is lost in the north of England there is usually insufficient time to re-drill necessitating a change to cropping plans. The potential risk of crop loss from slugs no doubt has a significant influence on decisions regarding molluscicide application. The distribution of damage is also likely to have an impact on the need for molluscicides. If damage is distributed randomly throughout the crop it is likely to have less of an impact on crop yield than if it is patchy. Patches of damage can provide areas for pigeons to land in rape crops and inflict further damage. Large areas with few plants also provide the opportunity for colonisation by weeds which could create problems for existing and future crops.

Despite this perceived risk of significant damage from slugs it seems that actual damage sustained is considerably less than might be expected. In years of severe slug damage some crops need to be re-drilled but these tend to be the exception rather than the norm. Plant populations may be reduced but are often still within the limits necessary to achieve potential yield. In addition, if slugs are such an important pest of arable cropping it is surprising that there is such limited information on the impact of molluscicide pellets on yield of both wheat and oilseed rape. Although there are some empirical data for wheat, information is lacking for oilseed rape. The

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wheat data are mainly from ADAS experiments undertaken between 1987 and 1992. Yield responses to treatment were as high as 41% but were very variable and often not significantly different from the untreated control. Experimental data often report that molluscicide pellets reduce damage due to slugs but this is rarely followed through to assessments of their impact on yield. Indeed, the approval process requires companies to show a reduction in crop damage due to slugs rather than demonstrate a consequent effect on crop yield. It is possible that this is in part due to the practical difficulty in demonstrating yield effects from the use of molluscide pellets. Slug damage is often very patchy and so yield responses can be extremely variable across an experimental site. Assumptions appear to be made that by reducing visible slug damage there will be an associated yield benefit. This takes insufficient account of the potential of the crop to compensate for pest attack and further research is required.

Further evidence that the impact of slugs on the national production of wheat and oilseed rape is overestimated is provided by the review undertaken by Clarke et al. (2009) in which it was suggested that if crops were left untreated, slugs would account for loss of 1.1% of the national production of wheat and 2.4% of oilseed rape. These values were derived from a consensus of expert opinion and provide a valid estimate of the risk from slugs based on practical observation across the industry.

In summary, the impact of slugs on the yield of wheat and oilseed rape is a paradox. There is little doubt that slugs can have a dramatic effect on yield but it is very difficult to demonstrate this consistently on an experimental basis. It is less difficult to demonstrate a reduction in crop damage by treatment against slugs but it cannot be assumed that this will necessarily lead to a yield response. These observations present challenges in determining the most appropriate data required for product approval and constitute critically important components of future risk assessment strategies for slugs.

Further research

a. Relationship between crop damage and yield

In view of the difficulty in demonstrating positive yield responses to molluscicide use, the relationship between crop damage and yield should be investigated further. This work should to take account of the crop’s ability to tolerate pest attack and also indicate whether crop damage can be used as an indicator of effects on yield. Work on the impact of leaf grazing to oilseed rape in particular needs further study and could be done in the absence of slugs by simply removing different proportions of the total leaf area at a range of growth stages and measuring the crop’s ability to compensate. Similar work could be done to measure the impact of loss of plants on yield and this is discussed in the section on crop tolerance. If cereals do not suffer yield loss from leaf grazing the use of pellets could be restricted to applications before or at drilling only which could prevent frequent repeat treatment to the growing crop.

b. Investigating the distribution of slug damage in cereals and oilseed rape

As previously discussed the distribution of damage can affect decisions on the need for molluscicide pellets. The circumstances under which patches of damage justify treating an entire field with pellets are poorly understood. Precision farming technology may provide an opportunity to manage treatment of specific areas of the field and so reduce total molluscicide usage.

c. A probabilistic approach to estimating the potential impact of slugs on crop yield

Estimating slug populations in the field is notoriously difficult. Also there is the added complication of an interaction with pest activity because a large slug population is only a risk if individuals are active. A probabilistic approach to estimating the impact of slugs on yield potentially side steps the need to assess populations. It would involve generating estimates of the likely risk of significant slug damage under different cropping scenarios taking into account such factors as soil type, previous cropping and soil moisture. The impact of slugs on crop yield in plots treated with pellets or left untreated would be measured at multiple sites reflecting the various risk factors.. Statistical analysis of these data would lead to a ‘risk matrix’ for use in an IPM strategy.

3. The tolerance of cereal and oilseed rape crops to pest attack

Winter wheat can be established at significantly lower seed rates than are currently used commercially without impacting on yield. Similarly, limited data on oilseed rape suggests that optimum seed rates may be lower than those used in practice. Therefore there is clearly potential for both crops to tolerate some level of slug damage.

In winter wheat the minimum plant number to achieve potential yield is less than 100 plants/m 2 in early sown crops and 200 plants/m2 in late sown crops. In practice actual plant numbers can vary between 50 and 600 plants/m2. Whilst at the bottom end of this range slug damage could be expected to impact directly on yield, at the top end there is the potential to lose 400-550 plants/m2 and still achieve potential yield. This represents a loss

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of 83% of plants for an early sown crop and a loss of 67% of plants for a late sown crop. It would need a very high population of slugs to account for this number of plants in the field.

In winter oilseed rape, results from one year of data suggests that 40 plants/m2 might be sufficient for potential yield. Further work is underway to help clarify this figure.

Winter wheat and oilseed rape are said to be able to tolerate slug grazing once they are beyond the four leaf stage. However, it seems likely that crops can tolerate damage even before this stage depending on the degree of leaf grazing. This information is not currently available but would help to determine where molluscicide application is justified.

Ultimately the impact of slug attack is to reduce plant number. Therefore decisions on the need to control slugs should begin with an assessment of plant populations. It is recommended that this should be done before assessing pest numbers because it is likely that some crops will have such a high tolerance that the likelihood of experiencing sufficient pest numbers to reduce yield is negligible, thus making an assessment of pest number unnecessary.

Further research

a. Minimum plant numbers required for potential yield

Whilst this figure is well-established for wheat there is much less data available for oilseed rape. Early indications from HGCA project RD-2005-3242, Re-evaluating thresholds for pollen beetle in oilseed rape, suggests that 40 plants/m2 may be sufficient to achieve potential yield but further work is required. In particular, how the minimum plant number is influenced by sowing date and variety requires investigation. Using the minimum plant number as an indicator of potential yield will help to identify those crops potentially at risk from slug attack and also the plant survival rate required before significant yield loss. This information would contribute significantly to an improved risk assessment model for slugs in both wheat and oilseed rape.

b. Tolerance of crops to leaf grazing

This has already been discussed under the heading of the relationship between crop damage and yield.

4. Alternative control measures

Seed treatments in principle offer an effective way of protecting wheat seeds from grain hollowing. Theoretically, they would also have environmental benefits over pellets through better targeting of the active ingredient, reducing the amount of the active ingredient required, and removal of the need for a separate pellet application. Previous work with metaldehyde and methiocarb suggests that whilst seed treatments are effective in laboratory test they are less so in the field. The reasons for lack of efficacy in the field are unclear but in general seed treatments are worthy of further investigation. Deter (Clothianidin, Bayer CropScience) is approved for control of slugs in durum wheat, triticale, winter barley winter oats winter rye and winter wheat. There is potential to formulate other active ingredients as seed treatments, possibly including some natural extracts of plants. The potential risks to birds from all active ingredients formulated as seed treatments would need careful assessment.

Pellets based on iron phosphate are an alternative to metaldehyde. The efficacy of this product and others based on metal salts should be compared with conventional pellets to determine whether they offer any significant advantages. There also needs to be consideration of cost implications.

Parasitic nematodes have given effective control of slugs in horticultural crops but are unlikely to be cost effective in cereals and oilseed rape. Brussels sprouts growers in East Yorkshire have attempted to reduce the cost of parasitic nematodes by limiting application to the headlands on the basis that most slugs migrate into the crop from these areas. Whether such an approach would be effective or cost effective in arable crops is an area worthy of further study.

The use of natural plant extracts is an alternative approach to control of slugs and other invertebrate pests warranting further study. It is likely that such products will have limited environmental impact in that they are derived from indigenous plant species. They have potential to be used as antifeedants and could be formulated as seed treatments. However, this is unlikely to be a short term solution and further research is required to isolate the most promising products and develop potential control strategies. Similarly, understanding innate plant defences offers scope for pest control more broadly, for example by incorporation in crop breeding programmes. The FERA LINK project which is using fusion proteins to deliver toxins to slugs also has the potential to provide an alternative to conventional pellets.

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Cultural control should not be ignored as part of any strategy to control slugs and can reduce reliance on slug pellets. Providing conditions that encourage rapid establishment and growth of the crop is fundamental to control of any pest. However, it should be recognised that factors beyond the farmers/growers control such as the weather can often conspire against their best intentions.

Further research

a. Evaluation of the efficacy and persistence of the range of molluscicidal pellet options.

This is covered earlier in the discussion in the section on pesticide usage. Pellets based on metal salts should be included in any product comparisons.

b. Investigating application patterns for parasitic nematodes

The potential to apply parasitic nematodes to field margins/headlands as a means of reducing the cost of treatment should be considered. This technique has proved effective in Brussels sprouts crops in East Yorkshire.

c. Studies on natural plant products and innate plant defences

Further research is required into new antifeedants and molluscicides from plants. These could be developed either as (standardised) extracts, or as single entities. The toxicity of these products (phyto-, mammalian, and beneficial insect) should also be considered.

With respect to innate chemical defences, a number of aspects deserve attention: the relative importance of constitutive and inducible metabolites; the importance of chemical concentration and profile (and the significance of the complexity of profile) and the relevance of chemistry at different developmental stages. Furthermore, current crops may have much to gain from examining the chemistry of ancestral species.

Concluding remarks

Control of slugs in cereals and oilseed rape has not changed fundamentally for decades with molluscicide pellets being the preferred method of preventing damage. Metaldehyde is the product of choice probably because it is cheaper than the alternatives. However, detection of metaldehyde residues in water casts some doubt over the future of this product. In addition, there has also been an apparent increase in damage to crops by slugs despite continued, significant expenditure on slug pellets. This review has highlighted a number of issues in relation to slug control. There appears to be an assumption that slug damage necessarily has a consequent impact on the yield of the crop. However, this fails to take account of the crops inherent ability to tolerate some level of pest attack. Improved understanding of the mechanism of yield development has indicated that both wheat and oilseed rape can be sown at seed rates much lower than those that are currently used and still realise potential yield. This implies that crops can afford to lose a proportion of the sown crop without any significant impact on yield. It is therefore suggested that risk assessment for slugs should begin with an assessment of plant populations. This will help to categorise crops into those at high, moderate or low risk of attack. In general, plant populations should be the starting point when assessing the risk from any pest that has the potential to reduce plant numbers. This is a radically different approach to current risk assessment which advocates counting pest numbers and how these relate to published thresholds. However, many of these thresholds are over thirty years old and others are based on limited experimental data. Varieties and crop production methods have changed and it is vital that risk assessment also evolves and take into account the latest developments in crop physiology. Crop tolerance is an essential component of any pest control strategy and could have a significant impact on the level of pesticide use.

Slug control strategies specific to cereals and oilseed rape crop should be developed. This is primarily because grain hollowing is most important in cereals whereas leaf grazing and the severing of seedlings causes most damage in oilseed rape. It may be that once a wheat crop has emerged, the risk of slug damage is greatly reduced. In contrast, the degree to which slugs remove leaf area in a rape crop could be crucial in determining ultimate yield. This in turn will have implications for the timing of control measures against slugs in both crops. There is no doubt that some applications of pellets are made when the crop is no longer susceptible to slug attack. This is wasteful from both an economic and environmental point of view.

There have been a few attempts to introduce alternative control measures for slugs but in general these have not been widely adopted. Natural plant products and parasitic nematodes show some potential for the future but in the short term, consideration of crop tolerance and recent advances in crop physiology offer the best opportunity to minimise unnecessary molluscicide applications.

By establishing the Metaldehyde Stewardship Group, the industry is attempting to address current concern over the use molluscicides, and in particular metaldehyde, for slug control. However, it is also an opportune time to

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undertake more fundamental studies on slugs taking much greater account of the role of the crop in determining the need for pesticide treatment. This will help to shape control strategies for the future and inform policy whilst minimising environmental impact.

References to published material9. This section should be used to record links (hypertext links where possible) or references to other

published material generated by, or relating to this project.

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Bari, M.A. (2004). Comparative efficacy of mollusk baits containing metaldehyde (Slugfest and Deadline) and iron phosphate (Sluggo) against the gray garden slug, Deroceras reticulatum occurring on artichokes. Acta Hort., 660, pp. 39-45.

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