BASIS Project:

Comparisons between Metaldehyde and Ferric Phosphate Slug Pellets used at Different Application Rates

Robert Holmes

1

Contents

PageSummary 3Introduction 4Slugs Grey Field Slug 5

Garden Slug 6Keeled Slug 6Spanish Slug 7

Slug Identification 8Life Cycle 8Cultural Controls 9Monitoring and Thresholds 10Chemical Control Metaldehyde 12

Ferric Phosphate 13Case Study 14Conclusion 20References 21

2

Comparisons between Metaldehyde and Ferric Phosphate

Slug Pellets Used at Different Application Rates.

Summary

"It has been estimated that slugs can cause £43.5 million of damage in wheat and oilseed rape alone" (Source: AHDB, 2016). So it is critical to keep populations to a manageable, economic level.

With the withdrawal of Methiocarb in 2014 and the continuing pressure to keep metaldehyde out of drinking water, it is more important now to keep this active ingredient for as long as possible through responsible use. Ferric Phosphate is a relatively new active ingredient and whilst the manufacturer states that it is as effective as metaldehyde, there is a general perception that it is inferior and more expensive than metaldehyde.

The aim of the trial is to prove whether the perception is true (or not), and whether lower rates can produce satisfactory results. The trial field was identified as having a slug population above threshold and despite there being an established winter wheat crop growing, it was felt that with an abundant food source, the products would be tested harder.

The field had 5 traps set out, population counts were done then various applications were made, these were:

Full rate (7 kg/ha) metaldehyde Half rate (3.5 kg/ha) metaldehyde Full rate (7 kg/ha) ferric phosphate Half rate (3.5 kg/ha) ferric phosphate Control (no treatment)

Both products at full rate had the greatest result, the ferric phosphate product achieved the quickest reduction in population and was only marginally more expensive than the metaldehyde, disproving the myths surrounding it.

3

Introduction

Slugs are one of natures 'natural recyclers' by consuming decaying plant matter, fungus's and in

some cases decaying animal matter, however, if the populations aren't controlled to

manageable levels then slugs can cause thousands of pounds worth of damage to crop,

especially salad and vegetable crops, and if left unchecked, can completely wipe fields out

(Source AHDB, 2016).

With the continual loss of key active ingredients it is becoming more important to use current

ones more effectively and safely, with minimal impact to the environment.

Slugs have always been a problem and with the more recent trend of milder winters and wetter

summers, the problems are becoming exacerbated, the trend for minimal cultivations and no-

till are also adding to the problem. "An acre of land can typically sustain 250,000 slugs."

(Source: www.slugwatch.co.uk, 2016)

The recently introduced 'Three Crop Rule' could also be a factor in the battle to control slugs,

this has been an EU law to discourage mono-cropping and to encourage biodiversity, but in

some instances, farmers have been forced to grow crops that are not suited to that particular

farms climate and topography, for example, a continuous wheat grower could be forced to

grow a brassica crop which would naturally be a preferred habitat to slugs and would propagate

their numbers. There is also a current trend for plant breeders to cultivate oilseed rape varieties

with lower glucosinolates to open more markets for oilseed rape but this has reduced the

'bitterness' of the plant and therefore, more palatable to slugs.

The loss of methiocarb which was banned for sale in September 2014 has left only two

economic active ingredients on the market, and with the possible likelihood that metaldehyde

could also be withdrawn, it is especially important that the industry uses these products

responsibly.

The project involves monitoring slug populations in a growing crop of winter wheat using

metaldehyde and ferric phosphate products used at full and half rates.

4

Slugs – Species

Grey Field Slug (Deroceras reticulatum)

Fig 1. Grey Field Slug (Source: www.fwi.co.uk)

The grey field slug is one of the most common and most damaging. It's appearance is brown or

light grey, and grows to about 5 cm's long. The slime is a milky white colour.

The reason that they are the most damaging is that in favourable conditions, these can breed

throughout the year and will start to breed after 16 weeks of hatching, so populations can

multiply very rapidly. They are active in wet conditions and to near freezing point, they have

been seen actively foraging at 1oC. The Grey Field Slug is the most active and is the most

common to be seen on the soil surface.

The grey field slug feeds on seeds and plant material above ground as is particularly the main

threat to cereal crops.

5

Garden Slug/Round backed Slug (Arion hortensis & Arion distinctus)

Fig 2. Garden Slug

The Garden Slug is slightly smaller than the grey field slug at around 3 cm's long. It has a dark

body and a foot which can look orange or yellow. It's produces a yellowy/orange slime.

Egg hatching occurs in late spring/early summer and juveniles grow quickly and can produce

eggs in the same year.

They are less active on the surface than the grey field slug and when temperatures are over 5oC.

The Garden Slug prefers leafy and root crops and is a particular pest of potatoes

Keeled Slug (Milax, Tandonia & Boettgerilla spp)

Fig 3. Keeled Slug (Source: www.slugwatch.co.uk)

These have a dark appearance and grow to around 6 cm's in length, they tend to have a

pronounced ridge (keel) along the back. They produce a clear, colourless slime.

They have an annual lifecycle and will produce eggs from autumn to spring.

6

The keeled slug can be difficult to control as they spend most of their life underground, only to

be on the surface for breeding.

Being subterranean, the Keeled Slug is a problem with newly sown seeds and root crops.

Spanish Slug (Arion vulgaris)

Fig 4. Spanish Slug (Source: www.slugwatch.co.uk)

This is a non-native species to the UK but has established itself due to it's size and that it

produces twice as many eggs as native slug species.

These can come in a multitude of colours of black, brown, fawn or yellow, and can grow up to

15 cm's long.

It is an omnivore and has a diet of dead animals, excrement and plant material not normally

susceptible to slug damage.

7

Slug Identification

Fig 5. Slug Identification Guide. (Source: www.slugwatch.co.uk)

Life Cycle

Slug are hermaphrodite's, so don't require a partner to reproduce. A single slug can lay a batch

10-50 eggs at a time, and up to 500 eggs over several weeks, apart from Spanish slugs which

lays twice as many (Source: AHDB, 2016). Egg hatching is very dependent on temperature,

warm and moist conditions are suitable for a quick hatch and population growth.

Slugs are generally more active in mild and damp conditions, and night time tends to favour

these.

8

Cultural Control

Minimising Weeds, Volunteers and TrashBy controlling weeds and volunteers in particular, brassica type weeds such as volunteer oilseed

rape (Brassica napus) and Charlock (Sinapis arvensis) will cause a reduction of food source and

shelter for the slugs. The removal or incorporation of the trash disturbs the habitat of the slugs

as they prefer to occupy the soil surface underneath the layer of trash, which is cool, damp and

provides an abundant food source.

CultivationsIncorporation of the trash, destroys the slugs habitat and forces them to surface to forage for

food, this makes them vulnerable to natural predators and if pellets have been applied to the

surface then the slugs are more likely to encounter them.

The physical action of the cultivation would cause a significant and immediate decline in the

population. After any cultivation, a rolling sound be done to hinder remaining slug movements.

A fine and consolidated seedbed would also promote quick germination of the seeds enabling

them to 'grow out of danger' from the slugs. Rolling also helps to trap the slugs which are

between clods, as a slug cannot burrow through soil like an earthworm can, if a slug is sealed

between clods it will either wait until the frosts will open a fissure wide enough for it to move

or die of starvation.

9

Promoting Natural PredatorsThe creation of beetle banks or field margins can significantly reduce slug populations as the

Common Ground Beetle (Pterostichus melanarius) consumes slugs. Field margins would also

promote the populations of hedgehogs, toads and thrushes whom also eat slugs. Although this

can also be detrimental as these beetle banks would be of the ideal habitat for a slugs lifecycle

and crops adjacent to a margin or beetle banks may see increased slug damage.

Fig 6. Ground beetles consuming a slug (Source: AHDB)

Monitoring Methods and Thresholds

The most consistent way of measuring slug populations it to use refuge traps, traps should be

25 cm across and can be made from a variety of materials such as a wooden board, carpet,

plant pot tray or a specially made refuge trap.

Monitoring should be done whilst slugs are active, and is to be avoided in dry, hot conditions. A

cereals based bait should be used and not slug pellets because of the risk to non target species

and exceeding the maximum active ingredient dose rate, the bait should be placed underneath

a trap onto moist soil, the traps should be checked in the following morning whilst conditions

are still moist and the numbers of slugs should be recorded. 9 traps should be used per field (13

if over 20 ha) in a W pattern across the field (Source: AHDB, 2016)

10

Trapping should continue until thresholds have been reached or the vulnerable stage of the

crop has passed.

Crop Threshold (av. Slugs/trap) Monitoring period

Winter cereal 4 Sowing to first tillering(G.S. 21)

Oilseed rape (cereal stubble) 1 Before sowing to 4 true leaves

Potatoes 12 critical control periods:

1st @ 50% canopy closure 2nd @ 75% canopy closure

Field Vegetables 1 Jan/Feb & March/AprFig 7. Slug thresholds in various crops (Source: AHDB)

11

Chemical ControlThere are now only 2 main types of chemical control available to farmers, metaldehyde and

ferric phosphate.

Until 2014 methiocarb (Draza) was also available, but this has been withdrawn due to evidence

that it was causing harm to grain feeding birds such as sparrows and finches.

MetaldehydeMetaldehyde works by damaging the mucus producing cells and cause them to produce an

excessive amounts of slime and dehydrate, if the slug doesn't ingest a lethal dose, the

dehydrated slug may be immobilised and not able to retreat to a safe location during periods of

high temperatures and die due to exposure to sunlight or predators.

Metaldehyde is currently under pressure due to residue being found in drinking water,

although it is not dangerous to health or the environment, excessive amounts with regard to

the European drinking water limits (Council Directive 98/83/EC of 3 November 1998 on the

quality of water intended for human consumption) have been continually detected which is 1

part per billion, especially during wet winters, water authorities say that it is impossible to get

metaldehyde out of water and the only way to maintain satisfactory levels is to blend

contaminated water with clean water to dilute the metaldehyde. It is therefore important to

use metaldehyde responsibly, there is an initiative promoting the safe use of metaldehyde

(www.getpelletwise.co.uk) and have a seven point programme to minimise the risk of

metaldehyde getting into watercourses:

1. Use minimum active per hectare to avoid drainage and runoff losses

2. Maximum application rate of 210 grams metaldehyde/hectare

3. 210 grams Metaldehyde as a total dose between 1st August and 31st December

4. Maximum annual does of 700 grams metaldehyde per calendar year

5. 6 metre buffer zone against a watercourse (LERAP)

6. Do not apply if heavy rainfall is forecast

7. If field drains are flowing then do not apply metaldehyde based slug pellets.

12

Metaldehyde has low toxicity to non target species. There is a 21 days harvest interval for

potatoes, 7 days for vegetables and non for cereals and oilseeds, but care must be observed it

applying close to harvest that pellets don't get harvested with the grain (Source: Adama Enzo®

MAPP label).

Ferric PhosphateThis is available in products Sluxx® and Derrex®. The active ingredient is ferric phosphate, a

natural occurring substance that is found in most soils and is approved for use in organic

systems. It's mode of action is a stomach poison, once a lethal dose has been ingested it

causes irreversible pathological changes to the slug's stomach and hepatopancreas (the

digestive organ), this causes it to quickly stop feeding, the slug will die 3-6 days later. The slug

tends to go underground to die so dead slugs are not always found on the surface. There is no

buffer zone required and a zero day harvest interval and can be used on edible and non-edible

crops. If not ingested, the product degrades into plant nutrients and has low toxicity to non

target organisms. Local water authorities are now offering farmers in a Metaldehyde Sensitive

Areas subsidised ferric phosphate to encourage it's use.

13

Case Study

This was conducted on a field of winter wheat, at approximately growth stage 31. The field is

located near Ashbourne, Derbyshire, is a clay loam with an average annual rainfall of 760 mm

(30 inches). The field was previously winter wheat, the variety was Dickens which yielded in

excess of 14 tons per hectare and approximately 4 tons per hectare of straw, which was baled

and removed.

Cultivations consisted of two separate passes which a Claydon stubble rake, one 10 days prior

to drilling before the drill to create a shallow tilth to encourage a germination of weeds prior to

a glyphosate spray and the second pass was just before the drill to disturb any slug habitats and

to spread any remaining straw which was left by the balers to aid trash flow through the drill.

Fig 8. Claydon Straw Harrow

The field was drilled with J.B Diego with Redigo and Latitude seed dressing, there was no

dressing for slug control used on the 30th September at a seed rate of 185 kg/ha and with 60

kg/ha of Triple Super Phosphate also applied at drilling. This was done with a Claydon strip til

drill

The crop proceeded into the winter with very little sign of slug damage with no slug control

methods implemented as the crop was sown into a good, firm seedbed and was rolled to

14

encourage swift germination, the crop grew well during the wet winter and then on a routine

crop inspection, it was found that there was a large population of slugs crawling up the plants

at around growth stage 31, it was decided that this should be where the trial should be

conducted.

The trial was based on which would be the best product to use with regard slugs controlled,

price of application and whether lower rates would produce adequate control.

The trial consisted of five refuge traps placed along several tramlines of the field with chicken

layers pellets used as the attractant bait. The refuge traps used were a 25 cm x 25 cm mat with

a dark underside and a shiny, reflective upper layer to reflect heat away, there were weighed

down with stones to stop them blowing away. The weather at time of placement was mild and

warm but overcast.

Fig 9. Slug refuge trap.

15

Slug counts were carried out the following morning to determine the base population levels,

then an application of slug pellets was carried out using 2 different products:

Enzo® made by Adama. This is a wet processed pellet containing 3% Metaldehyde with

zero harvest interval for non edible crops (7 days for Cauliflower and 21 days for

potatoes) £40 per 15 kg bag (£2.66/kg)*

Sluxx® made by Certis. This is a wet processed pasta based pellet containing 3% Ferric

Phosphate with zero harvest interval on all crops, £60 per 20 kg bag (£3/kg)*

*Prices from M.G. Simisters and Sons

The applicator used was a Kuhn Axis 30.1 fertiliser spreader, using the calibration charts from

Kuhn (https://streutabellen.rauch-community.de/kuhntabelle/), the fertiliser spreader has also

been inspected and checked by SCS (www.spreadcheck.com). Coveralls, dust mask and gloves

were worn at all times during the filling and application of the pellets.

Fig 10. Field map and trap locations

16

Trap 1, Enzo® at 7 kg/ha, 90 pellets/m2, £18.60/ha

Trap 2, Enzo® at 3.5 kg/ha, 45 pellets/m2, £9.30/ha

Trap 3, Control, no treatment

Trap 4, Sluxx® at 7 kg/ha, 66 pellets/m2, £21/ha

Trap 5, Sluxx® at 3.5 kg/ha, 33 pellets/m2, £10.50/ha

The number of slugs in the traps were recorded over a period of a month.

Date Trap 1 Trap 2 Trap 3 Trap 4 Trap 59/5/16 6 17 13 10 59/5/16 APPLICATION APPLICATION APPLICATION APPLICATION

10/5/16 17 20 16 4 912/5/16 6 8 12 3 1919/5/16 7 9 5 1 1530/5/16 2 8 9 3 4% Reduction 66% 52% 30% 70% 20%Fig 11. Table to show recorded slug numbers per trap.

Fig 11. Graph to show slug numbers over trial period.

17

5/1/2

016

5/3/2

016

5/5/2

016

5/7/2

016

5/9/2

016

5/11/2

016

5/13/2

016

5/15/2

016

5/17/2

016

5/19/2

016

5/21/2

016

5/23/2

016

5/25/2

016

5/27/2

016

5/29/2

016

5/31/2

0160

5

10

15

20

25

0

5

10

15

20

25

30

0

20.2 0 0 0

3.4

0 0.2

12.2

3

0 0 0 0 00.8

5.6

0.8 1.2

7.6

0 0.201.8

0 0 0 0 0 0.2

Slug Populations with Temperatures and Rainfall

Trap 1 Trap 2 Trap 3 (Control) Trap 4 Trap 5 Rainfall (mm)

Threshold Av. Daytme Temp Av. Night time Temp

Date

No. o

f Slu

gs

Tem

p (C

elsiu

s)

Fig 12. Graph to show slug populations with regard air temperature and rainfall.

All traps apart from the full rate Sluxx® showed an increase in slugs after a few days of

treatment, this is most likely because the slug pellets (and also the layers pellets)

contain attractants, this drew the slugs from their shelters and made them more active

on the soil surface. The time of peak activity was observed when the air temperature

was lightly cooler and the moisture levels were high (from 9/5 to 12/05), during the end

of the trial, the slug numbers were reduced significantly, this is mainly due to the slug

pellets applied but the weather conditions (warm and dry) are conducive to reduced

slug activity, although the lush canopy of the crop would negate some of these effects

and have a moist cooler micro-climate.

18

Trap 1 (7 kg/ha Metaldehyde)

The initial population was quite low on this trap, and the slug numbers took a few more days to

peak that the other traps. There was a noticeable population of ground beetles in this trap

which did not appear in any other trap. The decline on the slug population was surprisingly slow

and only just brought the populations below threshold. It was the second most expensive

application in the trial.

Trap 2 (3.5 kg/ha Metaldehyde)

This had a rapid knockdown of the population but maintained it at around 8-9 slugs, probably

due to that with the reduced dose that all the pellets were eaten early after application,

resulting in the rapid knock down, then there was insufficient pellets to reduce populations

further.

Trap 3 (Control)

This was to show the maintained populations without applications, it was surprising that the

slug populations reduced towards the end of the trial, this maybe down to the fact that the

slugs were migrating to areas which had lower populations due to the application of pellets.

There was possibly natural reduction in numbers due to natural predators present, although

this was not noticed in the traps.

Trap 4 (7 kg/ha Ferric Phosphate)

The full rate Ferric Phosphate had the biggest and quickest impact on the populations, with it

rapidly bringing the numbers down to below threshold over a few days which is surprising as

the manufacturer claims that a kill could take 3-6 days although this could also be due that the

affected slugs migrate underground to die. This plot had a 70% reduction but was the most

expensive application, yet it could be argued that this was the best value for money application.

19

Trap 5 (3.5 kg/ha Ferric Phosphate)

This was the only plot that showed an increase in numbers, this is most likely down to there

was simply not enough pellets to have enough baiting points for the population of slugs to

consume a lethal dose. This plot is very important as it does show that reduced rates of ferric

phosphates don't work as a reduced rate of metaldehyde would.

Conclusion

Out of the 4 treated plots, 3 managed to get the populations below threshold after a month,

this was particularly impressive as there was an abundant food source and it could have been

entirely possible that none of the applications would have worked. The two full rate trials had

the greatest success but it was surprising that the metaldehyde took longer to bring

populations down and the ferric phosphate had a rapid knock down contrary to the

manufacturers claim.

Half rates do not work if there is a high population or a huge food source, if the trial was

performed on a newly sown seedbed then the results would be very different. The full rate

plots benefited from a greater number of baiting points, so the slugs were more likely to

encounter them.

There is a myth that ferric phosphate pellets are expensive, the full rate ferric phosphate plot

was only £2.40/ha more expensive than it's metaldehyde counterpart and due to the fact it had

a rapid decrease in population, this would give best value for money and would be the

preferred choice in that situation, also, the ferric phosphate pellets are a safer product in

general.

If more trials were to be done, it would be interesting to see how the different active

ingredients performed in a particularly wet autumn in a late drilled, slow crop, as there is a

theory that slugs can rehydrate after ingesting a lethal dose of metaldehyde in extremely wet

conditions, unfortunately, due to the timeliness of this project it was impossible to do this trial.

20

References

Adama Enzo (viewed July 2016) Enzo Product Information Guide, Available from URL: http://www.adama.com/uk/en/Images/enzo-product-information-sheet_tcm105-85834.pdf

AHDB (viewed June 2016) Implications of not controlling slugs in oilseed rape and wheat in the UK, Available from URL: http://www.adhb.org.uk/projects/documents/AHDBResearchReview-Slugs.pdf

Certis (viewed July 2016) Sluxx HP Factsheet, Available from URL: http://www.sluxxhp.co.uk/Sluxx%20HP%20Factsheet%20APP%20HR.pdf

European Commission (viewed July 2016) Drinking Water Directive, Available from URL: http://ec.europa.eu/environment/water/water-drink/legislation_en.html

Metaldehyde Stewardship Guide (viewed June 2016), Available from URL: http://www.getpelletwise.co.uk/msg-guidelines/

Severn Trent Water (viewed July 2016) Metaldehyde, Available from URL: https://www.stwater.co.uk/environment/catchment-management/metaldehyde

Slugwatch (viewed June 2016) All About Slugs, Available from URL: http://www.slugwatch.co.uk/?page_id=13

(Spring 2016) Integrated Slug Control, Information Sheet 4, AHDB

Slugwatch (viewed June 2016) Slug Identification Guide, Available from URL: http://www.slugwatch.co.uk/?page_id=21

21