Yield Loss & Action Thresholds of Chronic Insect Pests

8/2/2019 Yield Loss & Action Thresholds of Chronic Insect Pests

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Evaluation of action thresholds for chronic rice insect pests in the

Philippines. I. Less frequently occurring pests and overall assessment

J. A. LITSINGER1, J. P. BANDONG2, B. L. CANAPI3, C. G. DELA CRUZ2, P. C. PANTUA2,

A. L. ALVIOLA2, & E. H. BATAY-AN III4

11365 Jacobs Place, Dixon, CA, USA,

2International Rice Research Institute, Metro Manila, Philippines,

3Monsanto

Philippines, Makati, Metro Manila, Philippines, and4

Philippine Department of Agriculture

Abstract

Action thresholds as decision tools for insecticide application were developed and tested against the major insect pests of rice

at four sites in the Philippines over a 13-year period. Action threshold treatments were compared to the farmers practice,prophylactic insecticide usage, and an untreated check. Yield loss data using the insecticide check method partitioned yieldlosses over three crop growth stages in the same test fields. Chronic pests that exceeded action thresholds in 79% of fieldswere whorl maggot Hydrellia philippina Ferino (Diptera: Ephydridae), defoliators Naranga aenescens Moore and Rivulaatimeta (Swinhoe) (Lepidoptera: Noctuidae), leaffolders Cnaphalocrocis medinalis (Guenee) and Marasmia patnalis Bradley(Lepidoptera: Pyralidae), and stemborers Scirpophaga incertulas (Walker) and S. innotata (Walker) (Lepidoptera: Pyralidae).Minor chronic pests reached threshold levels in only one site each: rice bug Leptocorisa oratorius (F.) (Koronadal),whitebacked planthopper Sogatella furcifera (Horvath) (Zaragoza) and green leafhopper Nephotettix virescens (Distant)(Guimba); brown planthopper Nilaparvata lugens (Stal) did not exceed a threshold in any field. Stemborers were the mostimportant pest group in terms of yield loss. Despite the insecticide check method underestimating losses, a mean crop loss of0.62 t/ha was measured which showed ample scope for corrective action. But loss was evenly distributed across crop growthstages (0.15 0.24 t/ha) reducing the impact of insecticides. Action threshold treatments overall outyielded the untreatedcheck, more so in the two sites with highest pest density. The benefit of thresholds was to reduce insecticide usage, as a costsaving. However all the practices showed poor economic returns including the farmers practice. Farmers practice employedlow insecticide dosages and timing was not consistent with pest damage, but yields were often similar to threshold treatments.

Farmers appear to use insecticide more for risk aversion than for profit. The best threshold characters when evaluated againstresulting pest density and yield loss criteria showed accuracies 490% correct decisions. Future work is needed to improvethe insecticide response rather than monitoring tools. Thresholds need to be incorporated into improved crop management,which was often found suboptimal by farmers, to take advantage of the high levels of tolerance in modern high tilleringcultivars. Crop husbandry practices which improve yield potential such as selection of longer maturing varieties and nitrogenfertilizer may be a more effective pest management strategy than insecticides.

Keywords: Pest control, irrigated rice, insecticides, decision making, yield loss, plant tolerance, planting date, colonisation

pattern, nitrogen fertilisation

1. Introduction

Modern rice varieties possessing genetic resistance

against brown planthopper Nilaparvata lugens (Stal)

and green leafhopper Nephotettix virescens (Distant),

the major epidemic insect pests, have been widely

grown by farmers since the mid 1960s (IRRI 1985).

Genetic resistance against chronic insect pests has

been less successful (Heinrichs 1986). Whorl maggot

Hydrellia philippina Ferino, defoliators Naranga

aenescens Moore and Rivula atimeta (Swinhoe),

leaffolders Cnaphalocrocis medinalis Guenee and

Marasmia patnalis (Bradley), stemborers Scirpophaga

incertulas (Walker) and S. innotata (Walker), rice bug

Leptocorisa oratorius (F.), and whitebacked planthop-

per Sogatella furcifera (Horvath) are recurring pestscausing significant yield losses in the Philippines as

determined by the insecticide check method (Lit-

singer et al. 1987). Shortages in rice production in

the 1970s and 1980s due to epidemic pests spurred

the development of integrated pest management

(IPM) strategies with the focus on minimising

insecticide usage, which had been found to upset

natural enemy populations leading to pest resurgence

and secondary pest outbreaks. It is ironical that

farmers now often overuse insecticide from fear of

losses caused by a past history of crop failures from

epidemic pests (Kenmore et al. 1987; Litsinger

1989). Farmers also overestimate losses from chronic

pests based on their experience from epidemic pests

prompting further overuse (Heong and Escalada

1997).

Chronic insect pests are the main targets for IPMtraining programmes now widespread in Asia where

Correspondence: J. A. Litsinger, 1365 Jacobs Place, Dixon, CA 95620, USA. Tel: 1 707678 9068. Fax: 1 707678 9069. E-mail: [email protected]

International Journal of Pest Management, January March 2005; 51(1): 45 61

ISSN 0967-0874 print/ISSN 1366-5863 online # 2005 Taylor & Francis Group Ltd

DOI: 10.1080/09670870400028284


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farmers learn crop monitoring and management

decision skills (Matteson 2000). One of the basic

tenets of IPM is to minimize insecticide usage, thus

pest populations are tolerated until pest density has

reached an economic threshold level or when other

control methods are impractical or uneconomical(Norton and Mumford 1993). The main tools for

insecticide decisions are economic thresholds which

are pest densities that trigger a corrective action

before the damage reaches the critical economic

injury level (Morse and Buhler 1997). Economic

thresholds are based on damage relationships be-

tween abundance and yield which along with

economic parameters can predict the most optimal

new thresholds as costs change. Damage functions

have not been worked out for chronic rice insect

pests, thus empirically derived action thresholds

(ATs) have been utilised along with surveillance

methods (Dyck et al. 1981; Reissig et al. 1986; Wayet al. 1991).

In the developed world, farmers often contract

surveillance activities to professional scouts to make

pest control decisions. In the developing world,

small-scale farmers cannot afford to hire scouts, thus

the use of ATs must be able to be mastered by

farmers. Thresholds have often been said to be too

difficult for farmers to master (Goodell 1984;

Matteson et al. 1994; Morse and Buhler 1997), as

those developed by researchers usually are not in a

form that farmers understand. Goodell et al. (1982)

advocated that for IPM technology to be easilyassimilated, research should start with the farmers

and not from research stations. Studies have shown

that most farmers develop their own thresholds and

monitoring techniques (Bandong et al. 2002). A

number of the threshold characters being evaluated

in this study were ideas that came from farmers

(Bandong et al. 2002). A reciprocal relationship

between the farmers and researchers perception of

ATs needs to be fostered as both stand much to gain

from each others perspectives and individual skills

(Goodell 1984; Heong and Escalada 1997).

This study represents the field testing of ATs

against chronic insect pests of rice following on from

the early work reported in Heinrichs et al. (1978),

Waibel (1986), and Smith et al. (1988). ATs are

composed of a character to measure, a level for that

character, a monitoring protocol, and a corrective

response, normally an insecticide that would entail a

recommended dosage and timing. ATs were com-

pared to the farmers practice, a prophylactic best

insecticide practice, and an untreated check.

The resilience of rice crops to pest damage was

evaluated by site and season using yield loss data.

There are reports of exceptional examples of modern

rices ability to compensate from abnormally highinsect infestation: crops suffering 50 and 82% rice

whorl maggot damaged leaves (Viajante and Hein-

richs 1986; Shepard et al. 1990), 67% damaged

leaves from leaffolder (Miyashita 1985), 30% dead-

hearts and 10% whiteheads (Rubia et al. 1996), and

three whiteheads/hill (Litsinger 1993). Nitrogen (N)

contributes to this resilience and was tested as a

contrasting strategy to insecticide in response to ATs.

This paper is the first of a four-part series. The

succeeding papers focus on the development for ATsfor whorl maggot and defoliators (Litsinger et al.

2005) while those on leaffolders and stemborers will

follow. This first part reports on the ATs for the less

common pests and gives an overview.

2. Materials and methods

2.1. Study sites

ATs were tested on-farm over a 13-year period in

four irrigated, double-crop rice areas typical of the

Philippines major rice bowls (Pingali et al. 1997)

23 crops in Zaragoza (141 fields), 15 crops inKoronadal (109 fields), 13 crops in Guimba (88

fields), and 17 crops in Calauan (81 fields). Farm

sizes ranged from 51 to 4 ha with land preparation

done by rotary tillers. Two sites Zaragoza (mean

2.1-ha farms) and Guimba (mean 0.9-ha farms)

were in Nueva Ecija province, Central Luzon, while

Calauan (mean 2.1-ha farms) lies in Southern

Luzon, Laguna province. A fourth site, Koronadal

(mean 1.0-ha farms), South Cotabato province is in

Mindanao southern Philippines.

Zaragoza comprised the villages of Marawa,

Malabon Kaingin, Imbunia, Rajal Norte, and Bati-tang in portions of Zaragoza, Jaen, and Santa Rosa

towns. Zaragoza farmers fields lie at the lower end of

the Upper Pampanga River Irrigation System and

consequently were planted late in the wet season due

to delayed water delivery. Crops often reach maturity

during the time of the strongest typhoons (between

October 15 and November 15) and as a result suffer

severe damage from flooding or lodging, or indirectly

by diseases spreading from wind-whipped foliage or

due to harvested grain that cannot be dried. Further

description of farmers and the area is given by

Goodell et al. (1982).

The Guimba site was located in the village of

Bantug next to the International Rice Research

Institute (IRRI) and Philippine Regional Department

of Agriculture Cropping Systems Program research

site in nearby San Roque and Macatcatwit. Deep

well pumps, one per village, each irrigated 60 100

ha. Due to the small irrigation systems farmers plant

their wet season crop earlier than in Zaragoza,

avoiding the severe typhoons. Tungro disease is

endemic, occurring sporadically. The electric pumps

are expensive to operate and break down often

leading to drought stress. Further site description is

given in Entomology Department (1985).Calauan is located 10 km south of the IRRI

research centre in the villages of Pulong and Dayap.

The rice area is irrigated by river diversion from small

streams feeding Laguna de Bay lake from the

46 J. A. Litsinger et al.


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watershed of Mt. Makiling volcano. The villages are

located along the lake edge but away from the

perennial flood zone. Calauan farmers experienced

large scale tungro outbreaks in the early 1970s. In

Koronadal the villages of Namnama, Magsaysay,

Barrio 1, Avancenia, and Morales were selected inthe Marbel River Irrigation System. Koronadal lies

outside of the typhoon belt but has been the site of

outbreaks of tungro, grassy stunt, and brown

planthopper hopperburn in the early 1980s. With

its longer rainy season, the double crops are referred

to as first and second rather than wet season and

dry season. Its volcanic soil is still fertile and high

yields are obtained with lower amounts of inorganic

fertilizer. Further site description is given in En-

tomology Department (1985, 1988).

2.2. Research teamsATs were developed and tested in farm commu-

nities by resident research teams for each site. Field

workers were recruited from the surrounding villages

representing major ethnic groups. A field office was

rented and staffed by a laboratory assistant trained in

ricefield arthropod identification and equipped with a

dissecting microscope. The resident entomologist

and team were encouraged to improve on current

thresholds with farmers as co-partners. Thus there

was constant revision of threshold characters, levels,

insecticides, and methods of application.

2.3. Experimental design

Thresholds were tested under farmers agronomic

conditions as much as possible including their

currently used cultivars and seedbed methods. The

only stipulation made was not to include direct

seeded crops as this has been found to affect pests

(e.g., whorl maggot Litsinger 1994). The purpose

was to conduct the trials under farmers field

conditions and to only vary the insect control

variable. The many cultivars used, for example,

possessed genetic resistance against a similar com-

plex of insect pests and diseases. The dapog seedbed

method adds 2 weeks to the crop in the field.

Farmers vary their management practice as they see

fit throughout the season more commonly than

following a standard set of practices. We did not

attempt to dictate crop management practices for

them. The results of the research will yield technol-

ogy that is more robust and adaptive.

Each season four to nine new farmers (replica-

tions) were identified from each site to be co-

operators. These farmers were dispersed geographi-

cally within each community and grew the most

popular variety at the time. Planting dates werestaggered over the planting season to allow expres-

sion of the full range of pest densities. The most

popular varieties changed over time but included

IR36, IR42, IR62, IR64, IR74 (maturities ranging

from 90 to 110 days) in most sites. Locally farmers

had other selections: Koronadal with IR60 (released

only in Mindanao) and an unofficial line #90;

Calauan with C1 and Malagkit (both 120 days) and

IR70; Guimba with IR58; Zaragoza with IR52 and

IR56. To be released, Philippine varieties must beresistant to green leafhopper and brown planthopper

which caused severe epidemics in the 1970s before

improved multiple pest-resistant varieties were devel-

oped. As resistance is not durable, new varieties must

be developed continually.

Crops were transplanted from wet seedbeds or

dapog. The latter was popular in Calauan and

Koronadal where seeds are sown on banana leaves

thus roots do not enter soil to minimize transplanting

shock (from roots torn off while pulling seedlings),

field establishment occurs 2 weeks earlier than a

wetbed. Each season 30 40 farmers, including the

current co-operators, were interviewed every fewweeks to record their cultural practices (e.g.,

Department of Entomology 1985, 1988).

Within each co-operators field, a 0.2-ha research

area was demarcated with plastic string tied to

bamboo stakes (Litsinger et al. 1980a). Typically

eight to 10 treatments were included in the experi-

mental design each season combining yield loss

assessment along with two thresholds compared to

the farmers practice, a prophylactic insecticide

regime, and an untreated check. With the exception

of the farmers practice (1000 m2

), plot sizes were

100200 m2

. This design and plot size arrangementhave been determined to have unbiased effects on

pest abundance (Litsinger et al. 1987). Five of the

treatments were designed to measure yield loss using

the insecticide check method which had been tested

earlier in other sites (Litsinger 1991). Three crop

stages were recognised following Yoshida (1981)

vegetative (transplanting to panicle initiation), repro-

ductive (panicle initiation to flowering), and ripening

(flowering to maturity 10 days before harvest). In a

typical 110-day variety, the reproductive stage would

begin about 40 d.a.t. and end about 30 days later.

The seedbed had been included in experiments prior

to the current study, but as no significant yield loss

was ever measured, the seedbed portion of the

vegetative growth stage was eliminated.

The first treatment termed full protection at-

tempted to show the yield potential with the least

possible insect damage. Weekly insecticide sprays at

the manufacturers recommended dosages were

applied to each plot with interplot spray drift

minimised by a mosquito cloth on a 1 6 3-m wood

frame held downwind by two assistants. Insecticides

were selected both for their efficacy as well as proven

neutrality regarding phytotoxic or phytotonic effects

on rice (Venugopal and Litsinger 1984). Insecticideswere applied as foliar sprays with 19-l, lever-operated,

knapsack sprayers fitted with hollow cone nozzles

using a 200 300-l/ha spray volume (increased with

crop growth). From the second to fourth treatments,

Action thresholds for chronic rice insect pests 47


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insecticide was withheld from each successive growth

stage, with the fifth treatment being the untreated

check. The prophylactic treatment involved soil

incorporation of 0.5 kg a.i. carbofuran granules/ha

before transplanting followed by two foliar sprays of

0.4 kg a.i. chlorpyrifos/ha 10 days apart during thelate reproductive stage to prevent stemborer white-

heads. All treatments were in randomised complete

block design with replications as fields. Yield was

taken from five 5-m2

crop cuts in a stratified sample

per treatment and dried to 14% moisture.

Total yield loss was calculated both in terms of grain

weight and percentage. Total yield loss was the

difference between the full protection and untreated.

To calculate percentage, total loss was divided by the

untreated yield and multiplied by 100. Loss in each of

the three growth stages was calculated in separate

treatments where protection was omitted from each

stage sequentially. Yield was subtracted from that ofthe full protection treatment. The loss in each of the

growth stages was summed and adjusted upwards or

downwards proportionally so that the total of the three

stages equalled the total yield loss. Cropcompensation

was measured by regression analysis using the yield

loss dataset for each of the two seasons per location

using crop averages that varied over a range of yield

potentials. Compensation would occur if the rate of

yield loss did not rise proportionally with increasing

yield (i.e., slope of the linear regression equation was

insignificant).

2.4. Sampling methods

Pest incidence was sampled weekly in the thresh-

old treatments and untreated check, but only once

per growth stage in the yield loss treatments. Pest

monitoring for AT decision making was carried out

in the respective threshold plots. Pest or damage

densities were usually measured on a per-hill basis

with the sample size of 20 hills selected individually

in a stratified pattern exclusive of a 1-m border zone.

Mechanical hand counters were used to record the

number of tillers and leaves per hill with pest damage

recorded on those plant parts as appropriate. One

staff scored the hill while another recorded.

2.5. Action thresholds

Thresholds are multifaceted involving a number of

variables, any one of which can affect efficacy. The

first variable is a character such as an insect stage

(adult, nymph) or its damage symptom (damaged

leaves, deadhearts). Second is the sampling unit and

number of samples that express the character

(planthoppers = 20 hills, green leafhopper = 25 net

sweeps, rice bug = per hill). Third is the density of thecharacter per sampling unit (e.g., one planthopper or

rice bug per hill, one leafhopper per sweep).

Normally a single character with two threshold levels

was tested each season per site, termed low level

(e.g., one planthopper per hill) and high level (e.g.,

two planthoppers per hill), in separate treatments.

New characters were continually being developed in

an effort to improve performance.

Another set of variables is associated with the

corrective response triggered by a threshold, usuallyan insecticide or N. As development of ATs was

iterative there was no balanced design to test the

many characters and response variables in a given

field. Most characters were tested in multiple sites.

Data analysis after each season entailed comparing

yield in the threshold treatments to that in the

untreated check, farmers practice, and prophylactic

treatment. An economic analysis was performed on

each practice where marginal returns and benefit:cost

ratios were calculated. Included in the analyses were

crop monitoring and insecticide application labour,

cost of labour, and interest. The farmers practice

was what each individual farmer collaborator carriedout in the trial field. Results were scrutinised to

determine if yield loss occurred in each growth stage

where thresholds were reached. If no yield loss were

recorded but thresholds were reached, levels were

raised the following season and vice versa. Figures

were drawn to illustrate the weekly pest abundance

and degree of control obtained field by field (e.g.,

Department of Entomology 1985, 1988).

2.5.1. Planthopper thresholds. The whitebacked and

brown planthoppers were monitored weekly in the

vegetative and reproductive stages by recording thenumber of adults and nymphs as well as their predators

from 20 hills along a zigzag transect in each plot

(Matteson et al. 1994). Each hill was bent over the

water with one hand and slapped with the other open

hand to dislodge arthropods including predators onto

the water surface for easier detection. The average

number of tillers per hill was calculated. Only the

brownish mature nymphs (stadia 4 5) and adults

were counted. Densities of immature white nymphs

are particularly prone to predation. Predator density

was incorporated into the ATs, as for each found, five

planthoppers were subtracted from the total. The AT

levels ranged from 0.5 to 1 planthoppers (adults and

nymphs) per tiller. Once half of the AT level were

reached, monitoring was increased to twice a week.

Fenobucarb (BPMC) insecticide was applied at 0.4 kg

a.i./ha timed to when the planthopper population was

in the late nymphal stage. Timing with mature nymphs

was to minimize the egg load inside the tillers, as the

insecticide has only minimal activity against eggs. If

the crop were to be sprayed when adults were

dominant, adults would be killed but eggs would

survive inside the tillers to emerge under very low

predator densities, often leading to resurgence.

2.5.2. Green leafhopper threshold. Green leafhopper

was only monitored if tungro virus was observed in

the community. The orange yellow discolouration,

diagnostic of tungro, is masked in mature plants but



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becomes particularly noticeable in regrowth after

harvest. A sweepnet was used in the seedbed through

the vegetative stage, taking 25 strokes while walking.

The AT level was set at 0.5 adults + nymphs/sweep

with 0.4 kg a.i. fenobucarb/ha as the response.

2.5.3. Ricebug threshold. Twenty hills were inspected

during the milk stage of grain development for rice

bug adults and nymphs. The range of AT levels tested

was four to 10 bugs/20 hills with a response of 0.4 kg

a.i. endosulfan/ha as a single spray.

2.6. Insecticide response

When a threshold were reached and insecticide

was the given response, it was applied within a day of

monitoring. Insecticide technology was developed in

an iterative process as well, thus if efficacy was low,

adjustments were made, normally changing thechemical but in certain instances involved research

on other application methods. Tests to determine the

minimum effective dosage were undertaken with a

view to cost savings. Foliar sprays were applied as

described in the yield loss trials. Percentage control

of each threshold character was based on the

untreated check.

2.7. Nitrogen substitution

Application of N has been shown to increase the

rice crops tolerance of pest damage (Litsinger 1993,1994; Rubia et al. 1996) and was tested in one of the

threshold treatments in all four sites during the final

2 years as a substitute for insecticide. The motive was

derived from the often discouraging results with

insecticides. Urea was broadcast at 25 kg N/ha,

equivalent in cost to an average insecticide applica-

tion, in response to surpassing an AT for whorl

maggot (eggs per hill from a neighbouring field),

defoliators (larvae per hill), leaffolders (larvae per

hill), and stemborers (egg masses per m2

).

2.8. Statistical analysis

All statistical analyses were performed by SAS and,

unless otherwise stated, we use P 4 0.05 as the

criterion for significance. Results were subjected to

ANOVA and regression/ correlation analysis where

appropriate. Treatment means were separated using

the paired t-test for two variables or least significant

difference (LSD) test for more than two variables.

Means are shown with standard errors of the mean

(SEM) using a pooled estimate of error variance.

3. Results

3.1. Insect pests and densities

The chronic pests that regularly exceeded thresh-

olds were whorl maggot, defoliators, leaffolders, and

stemborers (Scirpophaga incertulas was prevalent in

all sites except Koronadal where S. innotata was

dominant). Less common pests that reached thresh-

old levels only in single sites were rice bug in three

crops in Koronadal (5.8% of all test fields in the

site), whitebacked planthopper in two crops inZaragoza causing patches of hopperburn (9.1% of

fields), and green leafhopper in one crop in Guimba

with tungro symptoms occurring sporadically (0.8%

of fields). The latter three pests occurred too

infrequently for different threshold characters to be

tested. Brown planthopper affected 510 fields

nearby test fields causing isolated patches of

hopperburn on susceptible cultivars in all sites

except Calauan, but never in the test fields

themselves.

3.2. Yield lossMeasurement of yield loss is an essential element

in evaluating thresholds. With use of the insecticide

check method it was expected that the full protec-

tion treatment would provide 480% control for

each pest based on damage. This goal was only

achieved with leaffolders averaging 82.5+ 4.2%

damaged leaves. Control of damage by defoliators

(71.3+5.0% damaged leaves) and stemborers

(67.0+3.2% control based on deadhearts and

whiteheads) nearly reached the goal but the greatest

disappointment came with whorl maggot. Despite

weekly applications of high dosage foliar sprays, only55.2+5.3% control was achieved based on damaged

leaves over the four sites. Stemborers were the only

pest to show significant differences by season

(62.8+6.4 vs. 71.2+5.6% in the wet and dry

seasons, respectively, by paired t-test with P= 0.04,

df = 126).

Yield loss, despite the suboptimal insecticide

protection, showed considerable scope for IPM when

viewed as a total (0.62 t/ha or 12.7%) (Table I).

Yields across sites and seasons in the full protection

treatment averaged 4.99 t/ha. Highest seasonal yields

were recorded in Zaragoza dry season (6.23 t/ha) and

lowest in the Guimba wet season (4.39 t/ha), the two

closest sites. Untreated crops averaged 4.37 t/ha

overall with the highest and lowest site yields

occurring in the Zaragoza dry season (5.50 t/ha)

and in the Guimba wet season (3.67 t/ha). Lowest

yield per field was in Guimba 0.77 t/ha in the 1984

wet season. Highest yield loss per crop was also in

Guimba dry season (0.77 t/ha); lowest was in

Calauan wet season (0.30 t/ha).

Losses during the vegetative stage (0.23 t/ha) were

significant in all sites and seasons except the dry

seasons in Guimba and Calauan. No one site or crop

had significantly higher losses than another duringthis stage. The reproductive stage loss (0.24 t/ha loss)

was significant in all site season combinations

except Koronadal second crop and Calauan wet

season. Least loss occurred in Calauan wet season



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Table I. Measurement of yield loss by season in four irrigated rice sites by the insecticide check method, Philippin

Yield (t/ha) Yield lossa/

Crops Fields Full

Total Vegetative stage Reproductive s

Site Season3 (no.) (no.) protection Untreated t/ha % P t/ha % P t/ha %

Zaragoza WS 12 72 5.09+0.28 b 4.42+0.18 b 0.70+0.19 ab 12.8+2.8 50.0001 0.24+0.11 4.5+1.8 0.03 0.27+0.07 a 5.3+1.DS 11 69 6.23+0.21 a 5.50+0.20 a 0.63+0.11 bc 10.2+1.6 50.0001 0.25+0.06 4.0+0.9 0.005 0.23+0.06 ab 3.8+0.

Koronadal 1st 7 52 5.16+0.25 b 4.55+0.20 b 0.60+0.11 bc 11.3+1.8 50.0001 0.24+0.07 4.4+1.2 0.004 0.21+0.08 ab 4.1+1.2nd 8 57 4.85+0.19 b 4.10+0.15 b 0.75+0.11 ab 15.3+1.9 50.0001 0.32+0.05 6.8+1.3 0.0004 0.26+0.07 a 4.1+1.

Guimba WS 7 44 4.39+0.58 b 3.67+0.58 b 0.72+0.09 ab 21.9+7.9 50.0001 0.21+0.06 8.6+5.0 0.001 0.29+0.03 a 7.4+1.DS

6 44 4.80+

0.53 b 4.03+

0.53 b 0.77+

0.16 a 18.1+

4.85

0.0001 0.20+

0.06 5.0+

1.7ns

0.34+

0.07 a 8.1+

0Calauan WS 9 44 4.61+0.22 b 4.27+0.25 b 0.30+0.10 c 6.3+2.1 50.0001 0.18+0.08 3.9+1.7 0.03 0.06+0.02 b 1.4+0.DS 8 37 4.79+0.24 b 4.38+0.25 b 0.39+0.08 bc 8.4+1.8 50.0001 0.13+0.06 2.4+1.2 ns 0.16+0.06 ab 3.7+1.

total 68 419

avg 4.99+0. 12 4 .3 7+0. 12 0 .6 2+0.08 12.7+0.4 50.0001 0.23+0.07 4.8+0.3 50.0001 0.24+0.07 4.9+0.P 0.003 0.003 0.05 ns 0.02

F 3.5 3.64 2.13 0.49 2.62

df 67 67 67 67 67

1In a column, means+SEM followed by a common letter are not significantly different (P40.05) by LSD test. 2Probabilities show significant differences of losse

protection in that stage compared to the insecticide protected treatment. Total yield loss was measured as the difference between the untreated and insecticid

vegetative, reproductive, and ripening stages. 3WS= wet season, DS = dry season.


7/17

0.06 t/ha. The ripening stage loss (0.15 t/ha) was

significant in only half the site-crop combinations.

No one site or crop combination was significantly

different.

A wide range of yield losses and yield potentials

occurred within each site and season. With yield lossas the dependent variable, regressions were made for

each site and season (Figure 1). In Zaragoza a

distinct seasonal difference in compensation ability

was seen (Figure 1a). The wet season data showed an

increasing rate of yield loss with rising yield levels

leading to a significant regression showing a lack of

compensation. But in the dry season a high degree of

compensation was evident as crops with rising yield

potentials from 1.5 to 7 t/ha showed an insignificant

rise in yield loss.

Data from Koronadal (Figure 1b), however,

showed a lack of compensation in either the first

or second crops as the regressions were bothsignificant. Whereas in both wet and dry season

crops in the low pest density sites of Guimba and

Calauan (Figure 1c d) high rates of crop compen-

sation were evident.


The results of substituting insecticide with fertiliser

as a response to thresholds showed significant yield

gains over the untreated plots in three of the eight

pest season combinations (Table II). Due to the fewseasons of testing, the results were pooled over

locations. Only responses to stemborers showed a

yield gain in the wet season, while both whorl maggot

and defoliators did in the dry season. There were no

significant yield gains with leaffolder thresholds in

any season.

3.4. Threshold treatments compared to other practices

Thresholds, both high and low levels, were com-

pared to three other pest control strategies: (1) doing

nothing (the untreated check), (2) farmers practice

(among the farmer co-operators), and (3) prophylacticinsecticide scheduling. The full protection treatment

served as a check showing yield potential with

maximum insecticide protection. The variables ana-

lysed for each treatment were percentage of fields

Table II. Yield gain from pooled fields across sites where nitrogen fertiliser replaced the insecticide response to action thresholds 1.

Wet season Dry season

Pest Yield gain (kg/ha)2 P df Yield gain (kg/ha)2 P df

Whorl maggot 52+42 ns 23 389+43 0.001 24

Defoliators 121+57 ns 15 274+87 0.02 18Leaffolders 220+132 ns 25 250+112 ns 24

Stemborers 261+117 0.03 30 240+186 ns 19

1Nitrogen was applied as urea at 25 kg N/ha when a threshold was reached. 2Difference between threshold treatment and untreated by paired

t-test at P40.05, mean+SEM.

Figure 1a d. Relationship between yield and insect pest-caused yield loss in four sites showing compensatory capacity in those crops with

significant slopes in the regression equation: (a) Zaragoza, (b) Koronadal, (c) Guimba, and (d) Calauan, Philippines.



8/17

treated with insecticide, application frequency of

treated fields, dosage load per crop, efficacy against

target pest groups, yield gain, total yield, marginal

returns from insect control, and benefit cost ratio.

Over all sites, ca. twice the percentage of fields were

treated with the low value than the high valuethresholds (68 vs. 36%) (Table III). This was much

less than the average for farmers (90%) and the

standardised prophylactic regime (100%). As deter-

mined from interviews, farmers in three of the sites

treated 492% of fields for an average crop, with

Guimba farmers treating the fewest (76%). Guimba

farmers may have tailored applications to the rela-

tively low pest densities recorded there, but, more

likely, they were more strapped for cash to purchase

insecticide than farmers in other sites due to the high

cost of running the electrical pump. Highest treat-

ment frequency occurred with Calauan farmers

(4 98%), also a low pest density site. Significantdifferences occurred within and between each thresh-

old treatment with regard to pest density. The

frequency of fields treated in the high pest density

sites with the low threshold level was equivalent to

farmers (90% or more) but was557% from the high

threshold treatment. But in the low pest density sites,

the low threshold resulted in significantly fewer (39

53%) fields being treated, and even fewer (525%)

with the high threshold level.

The same general trends were evident in terms of

the mean number of applications per field, but the

differences were less distinct. This statistic was,however, based on treated fields and not all fields.

Farmers averaged one more application (2.3 times)

than the high threshold treatment (1.3 times), with

the low threshold treatment closer to the high

threshold (1.6 times). This indicated that the thresh-

old levels may have been set too close together, and

thus were more likely to be exceeded in both

treatments in each field. Zaragoza was unusual as

there was no difference between the farmers practice

and the threshold treatments, all ranging between 1.9

and 2.0 times. This was due both to the greater whorl

maggot pressure and the response of double spray-

ings, as well as more than one pest exceeding

thresholds per field.

Greatest frequency occurred with Koronadal farm-

ers (3.1 applications per crop), probably due to

farmers responding to recent history of pest out-

breaks (Waibel 1986). In Guimba where only 76% of

farmers had treated, the number of applications was

equal to all sites but Koronadal. This indicated that

of the Guimba farmers who sprayed, they applied

more frequently than farmers in other sites. The

threshold treatments resulted in lower application

frequencies than the farmers practice in three sites

for the high thresholds and two sites in the lowthresholds. There was no statistical difference be-

tween threshold treatments in any site.

The most popular insecticides of farmers, account-

ing for 74% of total applications, were mono-

TableIII.Comparisonofcorrectiveactionstakenbetweenth

resholdsandfarmerspracticeinsecticideapplicationfrequency

1.

Fieldstreated(%)

Insectic

ideapplications(no./field)3

P

est

CropsFields

Low

High

Farmers

Low

High

Farmers

Site

de

nsity

(no.)

(no.)

threshold2

threshold2

practice

P

F

df

threshold

threshold

practice

P

F

df

Zaragoza

H

igh

22

126

89.5+6.9Aa

50.8+7.8Ab

92.5+3.2Aa

0.003

4.06

65

2.0+0.4Aa

1.9+0.3Aa

1.9+0.1Ba

ns

1.38

52

KoronadalH

igh

15

125

91.1+5.1Aa

56.1+5.4Ab

94.5+6.7Aa

0.001

3.95

61

1.7+0.2ABb

1.4+0.2ABb

3.1+0.2Aa

50.0001

16.91

40

Guimba

L

ow

13

81

38.8+3.7Bb

14.0+3.7Bc

76.0+13.4Ba

50.0001

6.78

54

1.4+0.2Bab

1.0+0.2Bb

1.7+0.2Ba

0.003

7.17

38

Calauan

L

ow

16

89

52.5+4.6Bb

22.2+4.9Bc

98.4+2.9Aa

50.0001

5.01

58

1.4+0.3Bb

1.0+0.1Bb

2.3+0.2Aa

50.0001

12.32

42

avg

68.0

35.8

90.4

1.6

1.3

2.3

P

50.0001

0.0001

0.006

0.006

0.008

50.0001

F

5.78

4.74

3.96

4.55

3.28

14.41

df

66

54

48

66

54

48

1Applicationsnotcountedintheseedbed.Inacolumn,means+SEMfollowedbyacommonupperca

seletterarenotsignificantlydifferent(P40

.05)byLSDtest.Inarow,means+SEMfo

llowedbyacommon

lowercaseletterarenotsignificantlydifferent(P40.05)byLSDtest.

2Fertiliserassubstituteforinsecticidewasincluded.

3Averageofinsecticide

usersonly.Doublespraysinresponsetoathresholdwascounted

astwoapplications.



9/17

crotophos (33%), chlorpyrifos + fenobucarb (15%),

endosulfan (9%), methyl-parathion (8%), and cyper-

methrin (7%). All are broad spectrum materials and,

with the exception of methyl-parathion, did not differ

from those used in the threshold treatments. The

mean dosage per application of farmers ranged from0.20 to 0.23 kg a.i./ha for organophosphate, organo-

chlorine, and carbamate insecticides (Table IV). This

is half the recommended dosage which was set at the

minimum effective dosage level. Insecticide timing,

ranged among farmers from 20 to 26, 36 to 42, and

43 to 51 d.a.t. for the first through third applications

per crop. The late timing of the first application

showed the farmers practice likely would have

minimal effect against whorl maggot.

The calculation of mean dosage load is based on

multiplying the mean frequency of treated fields,

mean number of applications per treated field, and

mean dosage per application. Highest to lowestloads per crop over all sites were ranked from the

low value threshold 0.46 kg a.i./ha, farmers practice

0.45 kg a.i./ha, with the high value threshold at half

the load (0.21 kg a.i./ha) of the other two treatments

(Table IV). These data can be compared to the

prophylactic which was standardised at 1.3 kg a.i./

ha. Highest loads (40.6 kg a.i./ha) occurred in the

low threshold treatments of the high pest density

sites and in the farmers practice in Koronadal. In

order for the dosage loads of the threshold treat-

ments to be equal to those of farmers, Koronadal

farmers had to have sprayed twice as frequently percrop. This did not happen in Zaragoza where spray

frequency was similar, thus the low threshold load

was twice the farmers practice (0.35 kg a.i./ha).

Among the threshold treatments, significantly high-

er loads occurred in the high pest density than low

density sites reflecting the higher pest pressure. This

trend did not occur for the farmers practice. In

Guimba, because of the low frequency of fields

treated by farmers, there was no difference between

the farmers practice and the low value threshold. In

all sites, there were higher loads in the low threshold

level than high level (0.46 vs. 0.21 kg a.i.). Only in

Calauan was the farmers practice significantly

higher than the low threshold.

The pest control strategies were compared for

overall efficacy in controlling each pest group withthe full protection serving as a reference (Table V).

Prophylactic scored the highest overall control (41%)

among the practices which was significantly lower

than the full protection (60%) for all pests except

defoliators. Low and high thresholds were statisti-

cally similar (31 34%) to each other and the

farmers practice (24%) with only slight numerical

superiority in regard to whorl maggot and stem-

borers. Low threshold was similar to prophylactic for

all pests but leaffolders, while high threshold was

similar but for leaffolders and whorl maggot. Farm-

ers practice was only similar to prophylactic

regarding defoliator control.In terms of yield gain from control of each pest

group (comparing only those fields where thresholds

were reached), all practices were statistically lower

(206 342 kg/ha) than the full protection (729 kg/

ha) and, with the exception of stemborers, were

statistically undifferentiated. Yield gain from stem-

borer control by prophylactic was superior to that of

the farmers practice.

Further comparisons were made regarding total

yield over all fields. Comparing all sites and seasons,

the prophylactic was the only treatment equal to the

full protection (4.99 vs. 4.78 t/ha) (Table VI). Totalyield loss was 0.62 t/ha as the difference between the

full protection and the untreated. The prophylactic

treatment closed 66% of that yield gap. There was no

significant difference in yield between the prophy-

lactic, both threshold treatments, and farmers

practice (4.49 4.78 t/ha), but all were higher than

the untreated (4.37 t/ha).

The results from Calauan and Guimba mirrored

those of the overall analysis, but more treatment

separation occurred in the high pest density sites.

Table IV. Comparison of mean insecticide load per field between thresholds and farmers practice in four test sites, Philippines.

Dosage load per field (kg a.i./ha)1 Farmers insecticide

spray dosage2

Pest Low High Farmers (kg a.i./ha)

Site density Threshold threshold practice P F df (A)

Zaragoza High 0.72+0.25 A a 0.39+0.13 A b 0.35+0.17 B b 0.005 3.11 52 0.20

Koronadal High 0.62+0.21 A a 0.31+0.15 A b 0.67+0.27 A a 0.001 4.69 40 0.23

Guimba Low 0.22+0.08 B a 0.06+0.07 B b 0.28+0.09 B a 50.0001 5.09 38 0.22

Calauan Low 0.29+0.04 B b 0.09+0.06 B c 0.50+0.20 AB a 50.0001 7.28 42 0.23

avg 0.46 0.21 0.45 0.22

P 50.0001 50.0001 0.006

F 6.13 5.23 4.96

df 66 54 48

1Data averaged over all fields, derived from Table III by multiplying percentage fields treated by insecticide application frequency by the

dosage (0.4 kg a.i./ha used in thresholds and data in column (A) used for farmers. In a column, means+SEM followed by a common upper

case letter are not significantly different (P5 0.05) by LSD test. In a row, means+SEM followed by a common lower case letter are not

significantly different (P40.05) by LSD test. 2Dosage per application, synthetic pyrethroid insecticides not included.



10/17

Averaging Zaragoza wet season yield data, all

treatments were significantly lower than the full

protection. Yields in both the threshold treatments

were equivalent to the prophylactic (all closing the

yield gap 47 50%), but the farmers practice did not

raise yield above the untreated. Dry season averageshad higher yield potential and in this case both the

prophylactic and low threshold were equal to the full

protection, closing the yield gap 90 and 69%,

respectively. The high threshold and farmers prac-

tice closed the yield gap 43 and 57%, respectively,

and were significantly higher yielding than the

untreated. With the first crop in Koronadal, only

the prophylactic equalled the full protection closing

the yield gap 81%. All the other treatments closed the

yield gap 20 29% but were statistically similar to the

untreated. The lower yielding second crop registered

a higher yield gap, and all treatments were superior to

the untreated but indistinguishable among them-selves closing the yield gap 23 49%.

Among sites Calauan recorded the most positive

marginal returns across treatments followed by

Zaragoza (Table VII). There was no trend based on

pest density. In Calauan the $48.10 marginal return

in the high threshold treatment was worth 376 kg of

unhusked rice. The two other sites had mostly

negative results. Among treatments the results were

highly varied by site with no clear best treatment

between the threshold treatments and farmers

practice (Table VII). The farmers practice had the

greatest mean benefit ($5.80) but only slightly higherthan the two threshold treatments. Low threshold

had the highest returns in Zaragoza but the results

were not significantly different than the high thresh-

old or farmers practice. Benefit cost ratios should be

2.0 or above to be attractive to farmers (Smith et al.

1989) which was only achieved in Calauan for both

threshold treatments. In no other treatment-site

combination did the benefit cost ratio exceed 1.5.

4. Discussion

4.1. Insect pests and densities

In this 13-year study in four prominent rice

growing areas in the Philippines, pest census and

yield loss measurement determined that whorl

maggot, defoliators, leaffolders, and stemborers were

the key chronic pests. Thresholds were surpassed for

at least one of the four pests in over three-quarters

(79%) of fields for each crop on average. These

figures include all the threshold characters tested but

would have been less if only the best characters had

been employed, lowering the should not have

treated error rate. With the exception of defoliators

in Calauan, all pests surpassed thresholds in eachstudy site at least once. Other rice pests rice bug,

whitebacked planthopper, and green leafhopper

occurred minimally; each surpassed thresholds in

only one site. The brown planthopper never reached

TableV.Comparisonof

correctiveactionstakenbetweenthreshold

sandotherpracticesdegreeofinsectcontrolandresultingyieldgain1.

Control(%)14WAT2

Damagedleaves

Stemborers

Yieldgain(kg/ha)3

Treatment

Whorlmaggot

Defoliators

Leaffolders

(deadhearts)

A

verage

Whorlmaggot

Defoliators

Leaffolders

Stemborers

Average

Fullprotection

51.7+4.0a

57.7+5.1a

80.1+4.4a

50.2+5.2a

59.9

821+68a

747+50a

742+55a

606+65a

729

Prophylactic

34.5+4.6b

48.1+5.1ab

57.5+4.4b

23.3+5.4b

40.9

397+78b

246+62b

489+57b

237+65b

342

Lowthresholds

26.4+3.9bc

43.7+5.0b

44.9+4.3c

18.9+5.3bc

33.5

215+66b

227+51b

327+55b

133+61bc

226

Highthresholds

22.1+2.8c

37.7+4.3b

43.9+3.5c

20.9+6.6bc

31.2

232+74b

221+35b

318+42b

115+51bc

222

Farmerspractice

16.0+4.1c

40.9+5.1b

31.2+5.0c

6.8+5.6c

23.7

254+78b

235+63b

322+66b

12+68c

206

P

50.0001

0.05

50.0001

50.0001

50.0001

50.0001

50.0001

50.0001

F

14.36

2.68

22.39

11.80

15.73

24.10

12.44

16.15

df

285

562

410

400

261

472

443

439

1Inacolumn,m

eans+SEM

followedbyacommonletter

arenotsignificantlydifferent(P40.05)byLSDtest.WAT=weeksaftertreatment.

2B

asedondamage.

3Dataonlyincludefields

wherethresholdsfor

eachrespective

pestwasexceeded.



11/17

threshold levels in a single study field, mainly due to

resistant varieties adopted by most farmers.

The most ubiquitous pests were stemborers, whose

densities (2 3% damaged tillers over the entire

crop) and mode of damage, contributed more

significantly to yield loss than any of the other pest

groups. The same conclusion was reached by Savary

et al. (1994) but particularly in association with

weeds. Stemborer damage was measured in units of

the more important tillers rather than leaves. Eachmature tiller bears 10 18 leaves (Yoshida 1981),

thus for the same percentage, its damage is more

profound. Also a deadheart or whitehead is a severed

non-bearing tiller, while damaged leaves were rarely

entirely defoliated. In addition stemborer larvae

infest more tillers than are manifested as withered

tillers (unpublished data) further raising the percen-

tage of injury.

All four pest groups have been favoured by

inorganic fertilizer and irrigation, cultural practices

associated with modern varieties (Litsinger 1989).

Stemborers reputably have been the most important

insect pest of long-maturing, low-tillering, traditional

rices (Cendana and Calora 1967) but their impact

has been lessened by the earlier maturing, high-

tillering and narrow-stemmed modern rices. It was

surprising, therefore, that in Calauan, where many

farmers grow longer maturing varieties, that higher

stemborer damage did not occur. In Calauan the

120-day varieties would allow an extra (third)

stemborer generation to occur than is the case for

most modern rices. Natural enemies may have been

able to cope with this increase, whereas van der Goot

(1925) reported that with 6 9-month traditional

rices they usually could not.Whorl maggot and defoliators, not mentioned in

the pre-Green Revolution literature, emerged as new

pests. Whorl maggot, along with stemborers, were

the only pests that averaged damage levels above the

benchmark standards on a per-crop basis. Defoliators

and leaffolders are probably not normally a yield

threat in their own right due to the nature of the

damage (Heong 1990), but there is evidence that

defoliator injury can be significant when associated

with whorl maggot (Litsinger 1993). Both Zaragoza

and Koronadal had the highest combination of whorl

maggot and defoliators, and highest pest abundance

in general. Such densities may be related to extensive

irrigated rice areas (Loevinsohn et al. 1988), as bothsites lie in large irrigation systems.

4.2. Insecticide check method

Accurate yield loss estimates formed the basis of

threshold character evaluation. Unfortunately the

full protection treatment, which is the core of the

insecticide check method, only achieved the desired

480% control with leaffolders. Control was espe-

cially low in wet seasons due to monsoonal rainfall

which reduced insecticide residual activity. The

difference in control by season was most noticeable

in stemborers (least control in the wet season).

Typhoons were common in Luzon and when they

struck near harvest, the most vigorous growing

treatments (usually the full protection) were most

prone to lodge, biasing yield loss estimates down-

wards. Significant yield gain from thresholds, despite

high frequencies of correct decisions not to treat

scores, indicated that the insecticide check method is

not a good tool to match yield loss with a particular

pest in multipest crops such as rice.

A further confounding effect in trying to associate

losses with a single pest comes from synergistic losses

from multiple pests, each at subeconomic densities,that attack jointly causing significantly higher losses

than those caused by each acting singly (IRRI 1983,

1984; Wu et al. 1995). Furthermore the yield loss

contribution of each pest is significantly influenced

Table VI. Comparison of threshold treatments to other practices in terms of yield.

Yield (t/ha)1

Over all sites andZaragoza3 Koronadal3

Treatment Seasons2 Wet season Dry season First crop Second crop

Full protection 4.99+0.13 a 5.03+0.11 a 6.17+0.13 a 5.09+0.16 a 4.83+0.11 a

Prophylactic 4.78+0.13 ab 4.68+0.12 b 6.07+0.14 ab 4.98+0.16 ab 4.43+0.11 b

Low thresholds4 4.61+0.14 b 4.69+0.11 b 5.87+0.16 abc 4.67+0.16 bc 4.34+0.12 b

High thresholds5 4.49+0.14 b 4.70+0.11 b 5.62+0.13 c 4.62+0.16 bc 4.23+0.11 b

Farmers practice 4.53+0.15 b 4.28+0.13 c 5.76+0.14 bc 4.66+0.19 bc 4.46+0.11 b

Untreated 4.37+0.13 c 4.33+0.11 c 5.21+0.13 d 4.50+0.15 c 4.05+0.11 c

P 0.008 0.0002 0.003 0.007 50.0001

F 3.20 5.07 3.67 2.97 5.66

df 359 354 371 288 380

1In a column, means+SEM are not significantly different (P40.05) by LSD test. 2Data from 68 crops in Zaragoza, Guimba, Koronadal, and

Calauan sites. 3Data by fields, not crop averages. 4Low level characters, thus lower threshold values for a given character. 5High level

characters, thus higher threshold values for a given character.



12/17

by associations of non-insect pest crop stresses

(Savary et al. 1994, 2000; Willocquet et al. 2000).

Correlations of insect pest densities to yield (damage

functions) therefore have been difficult to achieve in

rice (Litsinger et al. 1987; Litsinger 1991).

Large-scale field trials with 100-m

2

plot sizesemployed in the current study were dependent on

natural infestations and did not result in sufficiently

wide ranges of pest densities to derive damage

functions. Although total yield loss was high, it was

evenly distributed among growth stages, making

statistical distinctions between treatments difficult.

Therefore in future work, other yield loss assessment

methods should be considered. Artificial infestation

in units of individual hills can be readily achieved but

has had little success due to extreme yield variation

(Rubia et al. 1996). As soil fertility is highly variable

hill to hill in transplanted rice (Dobermann et al.

1995), larger 1 5-m2 units should tried (Litsinger1991).

4.3. Yield loss

Typhoon damage was prominent in Zaragoza and

the 1978 wet season (WS) crop was totally lost and

the data were not included as there was no way to

measure losses from insect pests. Included, however,

were years when significant typhoon damage oc-

curred (1980, 1985, 1988, 1989, 1990). Some of the

highest crop yields occurred, however, in dry seasons

following such losses (7.18 t/ha after the 1978 WSand 7.47 t/ha after the 1988 WS), the latter year

registering the highest yield per field of 9.47 t/ha.

Yield was enhanced as the naturally occurring

fertility that accumulates over time in flooded rice

soils (Cassman et al. 1996) was not removed in the

harvested wet season crop.

The most severe losses occurred when insect pest

damage was coupled with drought. Droughts

occurred in Guimba from the frequent shut down

of the deep well pumps as farmers could not pay

their electricity bills. Loss was particularly exacer-

bated in the very early maturing variety IR58

(maturity 585 d.a.t.) which severely constrained

crop compensation. Low losses in Calauan were

probably due to longer maturing varieties which

allowed greater compensation (Litsinger et al. 1987;

Litsinger 1993). Average field maturity for Calauan

cultivars was 119 days compared to 86 91 days at

other sites.

An important indirect outcome of this study was

the contribution of on-farm yield data for the

Philippines. As reported by Pingali et al. (1990),

yields have continued to increase relative to those on

research stations. Farmers yields, as determined

from crop cuts in socioeconomic surveys in CentralLuzon and Laguna, representing three of the study

areas (except Koronadal), increased from a mean of

2.19 t/ha in 1966 to 3.37 t/ha in 1979, a 53% increase

(Cordova et al. 1981). Farmers averaged 4.53 t/ha in

TableVII.Economicanalysisofthresholdtreatmentscomparedtootherpractices

1.

Marginalreturnfrominsecticide($

/ha)

2

Benefit:costratio

P

est

Low

High

Farmers

Site

density

Prophylactic

Lowthre

shold

Highthreshold

Farm

erspractice

P

F

df

Prophylactic

threshold

thres

hold

practice

Zaragoza

H

igh

711.30+9.80Bb

11.40+10

.20Ba

71.70+3.20Bab

3.50

+5.70Bab

0.007

4.23

52

0.9

1.2

1.0

1.1

KoronadalH

igh

733.70+29.60Cb

717.60+13

.50Cab

720.90+24.90Cab

79.10

+5.50Ba

0.005

3.29

40

0.6

0.6

0.5

0.8

Guimba

Low

734.40+41.40Cb

722.90+12

.00Cb

77.80+9.60Ba

0.50

+0.40Ba

50.0001

2.99

38

0.6

0.4

0.7

1.0

Calauan

Low

24.10+20.50Ab

43.30+42

.60Aa

48.10+27.10Aa

28.10

+23.40Ab

50.0001

5.12

42

1.3

2.1

2.6

1.5

avg

713.80

3.58

4.41

5.76

0.8

1.1

1.2

1.1

P

50.0001

0.004

50.0001

0.003

F

4.36

6.34

5.99

3.58

Df

66

66

54

48

1Inacolumn,m

eans+SEM

followedbyacommonuppercaseletterarenotsignificantlydifferent(P40.05)byLSDtest.Inarow,means+SE

M

followedbyacommonlowercaseletter

arenotsignificantly

different(P40.05)byLSDtest.

2Costbasisbasedonpricesin1986forinsecticide,unmilledrice$0.128/kgfarmgate,interestonmaterials60%perseason,labour8htospray1ha,labour

$0.10/h,interestfor

labour33%perseason,pestmonitoring60hperseasonforthresholdsand4hperseasonforfarme

rsmonitoring.



13/17

the current study, a further 34% increase and double

the 1966 pre-Green Revolution average per crop.

Dry season yields were 12% higher than the wet

season (excluding Koronadal which lies south of

monsoonal influence) based on increased solar

radiation (Yoshida 1981). Monsoon clouds particu-larly at grain filling stages can severely limit yield

potential. Farmers responded to the higher potential

in the dry season by increasing N rates, particularly in

Luzon, where farmers N averaged 68 kg/ha in the

wet season and 101 kg/ha in the dry season, a 49%

increase (average from Guimba, Zaragoza, and

Calauan sites). Added N has less effect in Mindanao

where soil fertility is naturally high (farmers averaged

35 kg N/ha with no difference between seasons).

Differences in crop management, yield potential,

and insect pest incidence can explain the differences

between sites in the crops ability to compensate from

pest injury. High compensation was observed in bothGuimba and Calauan where pest incidence was

generally low and N inputs high. In Zaragoza under

high pest pressure, high compensation occurred

during the dry season, whereas in the wet season,

the crop could not outgrow damage. In Koronadal

pest incidence was high and compensation was not

recorded in any crop probably as added N levels were

too low.


The best threshold characters predicted both yieldloss and damage benchmarks 490% of occasions

(Litsinger et al. 2005). The constraint with AT

performance was not in choice of characters but was

due to the poor result of the insecticide response. N

substitution attempted a new strategy to tap the

crops resilience to pest damage. The property of

modern rices to compensate for high levels of pest

damage has been overshadowed by the accomplish-

ments in genetic resistance (Khush 1989), but both

act in a complementary fashion. The key to resilience

in modern rices over that of traditional rices is their

high tillering ability. Thus if a tiller is severed by

stemborers, neighbouring ones can compensate

(Rubia et al. 1996). Modern rices also produce

many more spikelets per area than traditional types

and thus have a larger physiological sink. When a

whitehead occurs the photosynthate can transfer to

unfilled spikelets of an adjacent tiller. Trials showed

compensation is enhanced by optimal crop manage-

ment with N application at the top of the list

(Litsinger 1993). Modern rices also do not lodge as

easily as the tall traditional types in response to

higher N rates.

N substitution supplements a technology farmers

already use (but not necessarily optimally) and doesnot have the negative effects of insecticides in terms

of safety, environmental hazards, and impacting

beneficial arthropods (Pingali and Roger 1995).

The N response, however, produced mixed results.

The applied N rate of 25 kg/ha has the potential to

increase yield 500 kg/ha (Yoshida 1981), but sig-

nificant yield gains only ranged from 220 to 389 kg/

ha, less than the potential. The additional N was

applied when a pest threshold was reached which

may not always have been the most physiologicallyappropriate time.

N use is a double-edged sword as overuse

promotes pest abundance, particularly diseases (Sa-

vary et al. 1995). Most farmers apply 70% of total N

11 21 d.a.t., and thus overstimulate plant to plant

competition and disease susceptibility. N broadcast

into paddy water is lost within several weeks of

application, and as a result only about 40% enters the

crop (Cassman et al. 1996). Thus more frequent

applications are generally required. Applications in

later stages favour compensation from leaffolder and

stemborer damage by delaying leaf senescence (Peng

et al. 1996) but also prolongs pest attack.

4.5. Threshold treatments compared to other practices

The prophylactic treatment of three applications

was designed to prevent damage from early season

whorl maggot and defoliators as well as stemborer

whiteheads. The total insecticide dosage of 1.3 kg

a.i./ha was three times that of the low threshold and

farmers practice. Due to its higher cost and only a

slight yield benefit compared to other treatments, it

was the lowest performing treatment economically.

Low threshold resulted in about two-thirds of thefields being treated with an average of 1.6 applica-

tions per crop (half that of the prophylactic). Its

efficacy levels were statistically similar to the prophy-

lactic against all pests but leaffolders. Yield gain was

similar to prophylactic but equal to the other

practices as well. Total yield and efficiency in

insecticide usage were equal in all crops to the

prophylactic giving better economic returns.

The high threshold treatment resulted in only one-

third of all fields receiving insecticide with an average

of 1.3 applications each for a low dosage load of

0.21 kg a.i./ha, half that of the low threshold

treatment and one-sixth that of the prophylactic.

Efficacy against pests suffered as a result, as its

percentage control was equal to the prophylactic in

only two pest groups (defoliators and stemborers).

But yield gain was similar to the prophylactic. Total

yield was equal to the prophylactic in most crops. Its

economic returns were 22% better than the low

threshold and four times better than the prophylactic.

Overall the economic returns were similar to the low

threshold in being acceptable in only one site.

The farmers practice applied insecticide to a

similar percentage of fields as the low threshold

treatment in the high pest sites (490%). The dosageload (0.45 kg a.i./ha) was equal to the low threshold

because of the high application frequency over all

sites (2.3 applications) despite the lower dosage per

application. Filipino farmers chronically underdose



14/17

(Litsinger et al. 1980b, 1982; Marciano et al. 1981;

Pineda et al. 1984; Waibel 1986), and resist

extension efforts to increase as they experienced

dizziness and headaches upon doing so (Goodell et

al. 1982). They resist increasing spray volume as they

believe the crop is damaged from excessive walkingthrough the field, although research shows otherwise

(Arida and Shepard 1987). By increasing only the

concentration in the spray tank and spraying directly

in front while walking, they have increased exposure.

They believe insecticides act in the same way as

fertilizers in that a small amount will produce some

response. Thus they do not understand the nonlinear

dosage mortality relationship (Litsinger et al. 1980b).

As a result of low dosages by farmers, the quantity of

insecticide applied per crop was similar between the

threshold treatments and the farmers practice.

The threshold method of weekly crop monitoring

is designed to respond to variations in pest density,whereas farmers were often guided by a combination

of prophylactic insecticide usage (timed to fertilizer

application) and use of very low threshold levels such

as the mere presence of flying moths (Bandong et al.

2002). In Calauan, farmers applied at frequencies

more consistent with a high pest density site. That

site is urbanised and consequently visited more

frequently by chemical company representatives.

Compared to the high threshold treatment, farm-

ers applied twice the dosage load per field with

similar efficacy, yield gain, and total yield. The

reason farmers insecticide practices of frequent,albeit low dosage applications, gave relative good

yield responses is unknown. The average 0.22 kg a.i./

ha dosage is just above threshold of the dosage

mortality curves of most insecticides, generally

causing 20 30% mortality (unpublished dosage trial

data). This meant that about half of their applications

were sublethal, but testing these dosages in the

laboratory, Tevapunchom and Heong (1991) noted

subtle negative effects on leaffolder development.

Sublethal doses may have affected the adult stages of

pests, which were not examined.

Farmers practice outgained the high threshold

treatment by 24% in economic returns on average

with the exception of Calauan. The benefit cost ratio

was similar to both threshold treatments but did not

meet the economic standards in any one site. The

fact that the farmers practice was uneconomical

supports the evidence that Filipino farmers use

insecticide to reduce risk of crop failure rather than

optimising profit (Waibel 1986).

The largest differences between threshold treat-

ments and the farmers practice, were timing of

application, dosage per application, and in the low

pest density sites, percentage fields treated within a

crop. On the other hand, similarities existed betweenthresholds and farmers practice in choice of in-

secticides, and in the high pest density sites with low

level thresholds, percentage of fields treated and

number of applications per field.

4.6. The future of action thresholds

It was hypothesised that threshold characters based

on life stages (eggs, larvae, adult moths) rather than

damage would improve insecticide timing from

earlier warning (Litsinger et al. 2005). This turnedout not to be true as the most effective characters,

with few exceptions, were based on insect damage.

The most likely reason for the poorer showing of

insect stages was the profound effect of natural

enemies (Ooi and Shepard 1991). Those characters

further removed in development time, such as moths,

from damaging larval stages proved least reliable

probably because of the greater time for natural

enemy activity. Stemborer egg mass characters had

incorporated the contribution of egg parasitoids into

the threshold but did not account for egg predators

which can be equally effective. Whorl maggot eggs

were the exception, producing good results, as itscolonisation generally precedes that of natural

enemies (van den Berg et al. 1988).

Both farmer-inspired threshold characters: (1)

flushed moths and (2) earlier-planted fields produced

mixed results. Moths turned out to be poor

monitoring tools for both leaffolders and stemborers,

for the reason just mentioned, resulting in low

frequencies of correct decisions and high error rates.

Characters employing monitoring of earlier-planted

fields showed promise with whorl maggot and

defoliators but not with the other two chronic pests.

In the case of whorl maggot better control was relatedto application early in the crop.

Because of the high degree of crop compensation

inherent in high tillering and longer-maturing vari-

eties, crop management should play a central role in

IPM along with conservation of natural enemies.

Despite great adoption of modern varieties, there lies

great challenges ahead to improve crop management

as illustrated by normally wide yield ranges in a given

community, from 51 to 4 9 t/ha; measured in this

as well as other studies (Pingali et al. 1990). In the

latter study, farmers were grouped by yield, with the

top one-third achieving yields on a par with research

stations. Thus the current yield gap and rationale for

extension efforts is not between farmers and

researchers, but between the top one-third and lower

two-thirds of farmers. The top one-third is achieving

maximum pest compensation benefits. But the

relationship of IPM vis-a-vis crop management

practices is complex due to two opposing forces:

(1) the great capacity of high tillering and longer-

maturing rices that bolster compensation from

damage counterbalanced by (2) the synergistic effect

of multiple stresses in reducing yield, with each pest

being just one stress (Litsinger 1991).

In the monsoon season in Zaragoza (Figure 1,Table VI), due to later planting compared to

Guimba, the crop is usually in the grain filling stage

during the main typhoon season. The effect is to

reduce the effectiveness of insecticide due to frequent



15/17

rains but farmers select longer maturing varieties

which have longer periods of compensation. Yield is

limited due to monsoonal clouds. Thus it is doubtful

that improved crop management, other than select-

ing longer maturing varieties, will bring higher

returns. Risk of crop failure is too high for farmersto take advantage of applying higher N rates. Farm-

ers, therefore, should strive for earlier planting

(Savary et al. 1994). The lower yields in the wet

season can be made up in the dry season. This is

contrasted with the dry season with greater yield

potential where the yield gap was wider and

compensation was not lacking from inadequate N

usage but from the farmers selection on earlier

maturing varieties due to the limited water supply.

In Koronadal it may be worthwhile for farmers to

grow longer-maturing varieties, apply higher N rates,

or practice better weed management. In contrast

Calauan, a low pest density site, which has fewer cropstresses appeared to offer the best conditions for use

of thresholds. Farmers already use high levels of N

and good crop management in general. The agro-

nomic feature that distinguishes Calauan is that

farmers cultivate long maturing varieties and use of

the dapog seedbed method. N usage averaged 73 and

82 kg /ha in the wet and dry seasons. These compare

well to Guimba 77 and 112 days. The benefit in

compensation from longer-maturing varieties has

already been shown (Litsinger et al. 1987; Litsinger

1993). Dapog seedbed minimizes transplanting

shock and may have helped compensate for whorlmaggot damage.

Threshold levels will need to be locally fine tuned

based on experience and current risk assessment: the

better the management, the higher the thresholds.

Risk may change from season to season even for the

same farmer (Litsinger 1993). Leaf colour charts

could be used to determine a crops N demand

(Singh et al. 2002). If there are several stresses, say

insect pests, in a given growth stage, then only one of

them may need to be confronted, while leaving the

more difficult-to-control to crop compensation.

Acknowledgements

We are duly appreciative of the generous coopera-

tion provided by over 400 farmers in the study sites.

Their willingness to become experimenters with the

research teams and devote at times a tenth of their

ricelands to trials is a testament to their desire to seek

improvements in rice production technology. Many

locally hired project staff were responsible for

conducting the trials and their invaluable contribu-

tions are acknowledged. Those assisting in Zaragoza

were Catalino Andrion and Rodolfo Gabriel, in

Guimba George Romero, in Calauan MarianoLeron, Eduardo Micosa, and Carlos de Castro, and

in Koronadal Hector Corpuz, Joseph Siazon, Beatriz

Velasco, and Anita Labarinto. Cooperation of the

staff in the Central Luzon and Mindanao regions of

the Philippine Department of Agriculture is highly

appreciated.

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Yield Loss & Action Thresholds of Chronic Insect Pests