rice stemborer action thresholds

8/2/2019 Rice Stemborer Action Thresholds

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Evaluation of action thresholds for chronic rice insect pests in the

Philippines. IV. Stemborers

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

P. C. PANTUA2, A. L. ALVIOLA2, & E. H. BATAY-AN III4

1Dixon, CA, USA,

2International Rice Research Institute, Metro Manila, Philippines,

3 Monsanto Philippines, Makati,

Metro Manila, Philippines, and4

Philippine Department of Agriculture, Philippines

Abstract

Action thresholds (ATs) as insecticide application decision tools were developed and tested against mainly yellow

Scirpophaga incertulas (Walker) and white S. innotata (Walker) rice stemborers in four sites in the Philippines over 68 crops.Damage incidence was low with a mean over all crops and sites of 2% deadhearts (DH) and 3% whiteheads (WH) based onweekly sampling. Highest incidence reached 19% WH as a mean of one crop and 31% WH in an individual field in 1 week.AT characters were based on deadhearts, egg masses, or flushed moths. A mean of 9% fields exceeded ATs in the vegetative,5% in the reproductive, and 4% in the ripening stages. The most effective AT character in each of the three growth stages waspercentage DH with 5% being optimal in the vegetative, 25% in the reproductive, and 10% in the ripening stages. These ATsresulted in 96 99% correct decisions based on criteria involving DH and yield loss benchmarks set from earlier studieson economic loss. Insecticide response to the highly accurate ATs was a disappointing 540% control, with chlorpy-rifosBPMC mixture being superior to chlorpyrifos or endosulfan alone. Despite this low level of control and the noted hightolerance of modern rices to stemborer damage, the AT treatments resulted in significant 0.2 0.3 t/ha yield gains over theuntreated check in each growth stage. This yield response is explained in part by control of coterminous nontarget chronicpests and in part to a synergistic compensatory yield response when a crop under multiple stress is even partially released. It ishypothesized that under such conditions, even low levels of control allow greater compensatory capacity to tolerate stemborerinjury but also from other causes, thus accentuating yield responses, particularly if the nutrient supply is adequate and thevariety is longer maturing. Stemborer IPM is seen as a two-pronged strategy, the first is couched in integrated crop

management as a preventative measure to bolster the crops ability to compensate from stemborer injury or other crop stressand the second to regularly monitor the crop using ATs. Crop monitoring protocols were seen to be improved if adjustmentswere made for crop maturity and damage pattern. The typical damage pattern over a crop showed DH initially increasing inthe vegetative stage, then leveling off during the reproductive stage (indicating a period of natural plant resistance) before alate peak of WH. Vigilance can be relaxed during the mid-growth stages and heightened during periods of tiller elongationand panicle exsertion. AT levels need to be adjusted for each location, but as a rule of thumb percentage DH could follow theratio 1:5:3 and 2:3:1 in the three growth stages for longer and shorter maturing varieties, respectively.

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

pattern

1. Introduction

Yellow Scirpophaga incertulas (Walker) and white

S. innotata (Walker) rice stemborers and are recurringpests causing significant yield losses in the Philippines

as determined by the insecticide check method

(Litsinger et al. 2005). The former species is most

important in Luzon and the latter in Mindanao.

Moths lay egg masses on rice foliage of all stages and

the dispersing neonate larvae tunnel into rice

tillers within a few hours to feed on internal tissues.

The rice crop is most susceptible when it is actively

elongating, either during tillering or panicle exsertion,

where injury occurs as deadhearts (DH) when the

base of tillers are severed or as whiteheads (WH) when

the vascular tissue at the base of panicles is cut offcausing the grains to wither (Bandong and Litsinger

2005).

High tillering modern rices can tolerate stemborer

damage best particularly if they are not suffering stress

from other causes such as drought or nitrogen

deficiency (Litsinger et al. 2005). Longer maturingrices exhibit greater tolerances to insect pest losses

(Litsinger et al. 1987). Greatest natural mortality of

stemborers occurs from generalist egg predators and

specialized egg parasitoids (Ooi and Shepard 1991).

The internal feeding habits of the larvae normally

result in poor control with foliar insecticides.

Action thresholds (ATs) have been used to improve

upon insecticide timing for rice stemborers (Dyck

et al. 1981; Bandong and Litsinger 1988). This is the

fourth paper in a series that reports on the develop-

ment of decision tools for insecticide application for

the chronic rice insect pests in the Philippines. Themost commonly tested AT character has been DH.

The purpose of this study was to test other characters

Correspondence: James A. Litsinger, 1365 Jacobs Place, Dixon, CA 95620, USA. Tel: 1 707 678 9068. Fax: 1 707 678 9069. E-mail: [email protected]

International Journal of Pest Management, JulySeptember 2006; 52(3): 195 207

ISSN 0967-0874 print/ISSN 1366-5863 online 2006 Taylor & Francis

DOI: 10.1080/09670870600659797


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including egg and moth densities with a view to

increase performance. ATs are composed of a

character to measure, a level for that character, a

monitoring plan, and a corrective response, normally

an insecticide that would entail a recommended

dosage and timing. All of the AT components weretested in a series of trials spanning 13 years, in 68

crops, and in four locations in Luzon and Mindanao,

Philippines.

2. Materials and methods

The four study sites, research teams, and experi-

mental design were discussed in Litsinger et al.

(2005).

2.1. Action thresholds

Thresholds comprise a series of variables, any oneof which can affect efficacy. The first variable is a

character such as an insect stage (egg, moth) or

damage symptom (DH). Second is the character

density (percentage DH or number of flushed moths

per m). Third is the sampling unit (e.g., percentage

DH in 20 hills or number of moths flushed per 20 m

walked). Rice seedlings are planted in clumps called

hills with a range of normally three to eight seedlings

per hill. Higher densities tend to occur if transplant-

ing is contracted to crews who transplant more

hurriedly and are not paid per hour. Normally, each

character was tested at two threshold levels eachseason per site, termed low level (e.g., 5%DH) and

high level (e.g., 15%DH).

The levels of each of these thresholds were adjusted

season to season as deemed necessary in an iterative

process based on performance (e.g., tested variously

as 3, 5, 8, 10, 15% DH). Thus, sites with higher pest

pressure and more rapid colonization rates evolved

lower threshold levels in both the low and high level

treatments and vice versa for sites with lower pest

pressure and less rapid rates. Lower levels were

needed to respond to high infestations as the damage

curves were steeper and earlier warning was required.

Having two or more levels tested per crop enabled

more reliable adjustments to be made. New char-

acters were continually being developed in an effort to

improve performance. WH have been used as an AT

character (Way et al. 1991) but are considered to be

too late in the crop cycle to be useful.

As development of ATs was iterative, there was no

balanced design to test many characters and response

variables in a given field. Most characters were tested

in the four study sites providing further replication.

Data analysis after each season entailed comparing

yield in the threshold treatments to that in the

untreated check. The design of the yield loss trialsincluded treatments that partitioned losses by the

three major growth stages. Yield loss results were

scrutinized to determine if yield loss occurred in each

growth stage where thresholds were reached. If no

yield loss was recorded but thresholds were reached,

levels were raised the following season and vice versa

if yield loss did occur and no threshold was reached.

Data were graphed to illustrate the dynamic weekly

pest abundance as noted in Litsinger et al. (2006a).

The earliest threshold character was DH percen-tage. Two additional characters were comparedegg

masses and flushed moths. As it was noticed that

earlier planted fields had low infestations whereas

late planted fields had highest levels, earlier-planted,

neighboring fields might serve as an early warning to

impending economic densities. Neighboring fields

(NF) were defined as the nearest two fields planted

1 2 weeks earlier than the target field. Monitoring

NF was done weekly beginning 1 week after trans-

planting (WAT). Two NF were monitored per target

field with results averaged. As farmers in irrigated

areas tend to plant within 2 months of one another,

such fields were readily available. NF were selectedwithin 100 m of each other. Sampling was stopped

after panicle exsertion thus WH were not used as a

character. Levels of each character were adjusted by

growth stage to account for periods of crop suscept-

ibility in a ratio of 2:3:1 for each of the three growth

stages (vegetative, reproductive, and ripening)

(Yoshida 1981). The crop is moderately susceptible

during the vegetative stage as damage can be

compensated for the most part by tillering (Viajante

and Heinrichs 1987; Rubia et al. 1996) thus thresh-

old ranges tested were, e.g., 8 10%DH and NF8

10%DH. As the reproductive stage is the leastsusceptible to damage (Shiraki 1917; Lin 1980),

levels were made highest, e.g., 15 25%DH and

NF15 25%DH. Lowest levels were indicated for the

ripening stage, e.g., 3 5%DH and NF3 5%DH.

Egg masses (EM) from Scirpophaga stemborers are

deposited on rice leaves near the tips and are covered

by a mat of pale yellow or brown hair scraped by the

female from her abdomen. Masses are ovoid in shape

(5 10 mm diameter), but as the leaves may curl,

manipulation of the foliage is necessary to detect the

masses. EM densities were tested with a range of

0.25 4 per 100 hills. The same levels may have

been tested in each growth stage from time to time

but the ratio 2:3:1 was maintained. Levels ranged

from 0.5EM (range 0.25 0.5EM/100 hills), 1EM,

2EM, and 3EM (range 3 4). A sample size of 100

hills (ca. 4 m2 at 20 cm spacing between hills) was

inspected each week in a stratified sampling pattern.

ATs were designed to account for egg parasitism

which at times can be very high. Egg masses were

clipped from the foliage and pooled from the various

plots on a site basis and held in screw-capped glass

jars until hatch. The ratio of parasitoids:larvae was

assessed, and only if parasitism was 550%, was a

response given. The third character tested came fromthe farmers themselves (Bandong et al. 2002), to

monitor flushed moths. Stemborer moths are easily

distinguishable by morphology and flight behavior

from other lepidopterous pests such as defoliators and

196 J. A. Litsinger et al.


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leaffolders. Levels tested ranged from 2 to 5 moths

per 20 linear m in a transect as two characters: 2 and 4

moths/20 m (range 4 5 moths), termed 2Moths and

4Moths.

2.2. Corrective response

Another set of variables was associated with the

corrective insecticide response triggered by a thresh-

old as explained in Litsinger et al. (2006). DH, the

most indicative character associated with yield loss,

were assessed for a period of 1 4 weeks after

treatment (WAT), allowing ample time for the crop

to recover by generating new tillers. In the tropics,

tillering has mostly terminated by the end of the

vegetative stage (Yoshida 1981). Percentage control

of each threshold character was based on the

untreated check.

Three insecticide products were evaluated chlorpyrifos, endosulfan, and a mixture of chlorpy-

rifosBPMC. All were applied as single sprays at

0.4 kg a.i./ha and represented the most effective

materials (Litsinger et al. 1980).

2.3. Sampling methods

Stemborer incidence was sampled weekly in the

threshold 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 rather than theuntreated check. Pest or damage levels were mea-

sured on a per-hill basis with the sample size of 20

hills taken in a stratified pattern. Mechanical hand

counters were used to tally the number of tillers per

hill with pest damage recorded as percentage of tillers

as DH. Moths were flushed using a wooden

improvised hockey stick the length of the standard

sweep net brushed side and side in a pendulum swing

ahead of the person while walking. The number of

steps was calibrated for a 20-m distance.

2.4. Threshold assessment

In order to assess the outcome of each AT character

and in the absence of adequate damage functions, the

pest infestation and yield loss were scored against

benchmark infestation and yield loss levels in each

growth stage. Combining pest damage with yield loss

was necessary to assess the economic impact in a given

crop growth stage. The benchmarks were based on

average insect pest infestation levels that were

associated with the losses based on completed

economic analyses of the basic threshold characters

(Smith et al. 1988). The standardized infestation

levels for stemborers were set at 10%DH in thevegetative stage but raised to 15% in the reproductive

stage and lowered to 5% in the ripening stage. ATs

were then scored on a per field basis. The benchmark

for yield loss was set at 250 kg/ha in each growth stage.

Each combination of pest infestation and yield loss

was scored and considered justified on the basis of

the infestation exceeding both the damage and yield

loss benchmarks during that growth stage. Four

outcomes were possible: (1) if the AT were surpassed

and was justified based on the benchmarks, it wasscored correct to treat, (2) if the AT were not

surpassed and was not justified it was scored correct

not to treat, (3) if the AT were surpassed but was not

justified (false positive) it was scored should not

have treated, and (4) if the AT were not surpassed

but was justified (false negative) it was scored should

have treated. The frequencies of these four out-

comes add to 100%.

Five criteria were developed to judge each char-

acter: (1) correlation to the damage yield loss bench-

mark, (2) most correct decisions, (3) least errors,

(4) ratio of errors per correct decision to treat 51,

and (5) correlation to yield gain. The fourth criterionrewards characters that triggered at least moderate

numbers of responses, and in doing so made

proportionally fewer errors than correct decisions as

distinguished from characters which had predomi-

nantly correct decisions based on correct not to treat

errors.

2.5. Response assessment

Stemborer control from insecticide treatments was

measured as the percentage difference in pest dam-

age level between the treated and the untreated plotsdivided by the level in the untreated plot multiplied

by 100. Because stemborers were monitored weekly

in the threshold plots, there was an opportunity to

measure the effect of applying insecticide against non-

target pests. These chronic pests were whorl maggot

Hydrellia philippina Ferino (Diptera: Ephydridae),

defoliators Naranga aenescens Moore and Rivula

atimeta (Swinhoe) (Lepidoptera: Noctuidae), and

leaffolders Cnaphalocrocis medinalis (Guenee) and

Marasmia patnalis Bradley (Lepidoptera: Pyralidae).

The data were analysed in the same way as for target

pest control.

2.6. Crop age and seasonal damage patterns

Stemborer damage patterns were graphed in time

series by site to describe the rates of damage as the

crop aged from an expected low point early in the

crop cycle to a peak sometime later. Knowledge of

such patterns could indicate the optimal AT moni-

toring requirements in terms of timing and fre-

quency. The crop-age damage pattern was described

for stemborer damage level by site using the aver-

ages of each of the weekly sampling dates in the

untreated plots. The results were then averaged overeach crop.

Using the same dataset, a second analysis was

made on the effect of seasonal planting date. The

hypothesis supporting monitoring earlier planted

Evaluation of action thresholds for rice stemborers 197


4/13

fields assumes progressively increasing pest damage

levels from earlier to later fields over the season. The

dataset comprised the untreated plots in the trials

with fields being selected randomly from the set of

fields to be planted every 1 2 weeks in a stratified

manner spanning the breadth of dates each season.The number of elapsed days between the date the

earliest field was transplanted and date for each

succeeding field was calculated over all crops by site.

The number of elapsed days (seasonal age) from the

date the first field was planted to that for each

successive field was regressed against the mean

damage (averaging the mean weekly counts, and

then calculated as the percentage change from the

earliest field). Regression was carried out for each site

separately on a per field basis and a significant

positive correlation would indicate a rising popula-

tion over the season.

2.7. Statistical analysis

Results were subjected to one-way 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. Stemborer damage incidence

Stemborer damage was generally low with only 3%

of fields 15% DH/WH including 1% 20%DH/

WH (highest level per field was 31% WH at 10 weeks

after transplanting) over the study. Mean stemborer

infestation on a per crop basis averaged over sites,

cultivars, and seasons increased during the cropping

seasons from 2% DH in the vegetative stage to 3%

WH in the ripening stage (Table I). This relationship

generally held true except for the dry season crop in

Calauan. The longer maturing varieties used by

Calauan farmers probably allowed a greater carryover

between wet and dry season crops in that instance.

Statistical analyses of the differences in stemborer

damage incidence between sites and crops were

insignificant for all three crop stages.Aside from Scirpophaga species, infestations in-

cluded low numbers of striped stemborer Chilo

suppressalis (Walker) and pink stemborer Sesamia

inferens (Walker) as determined from light trap

data and stem dissections (Jahn et al. 2006). Infes-

tation levels were remarkably similar between sites

and crops showing the ubiquity of this pest group.

Highest mean damage occurred in Guimba wet

season with 5% WH in the critical ripening stage.

Highest recorded damage levels in any site per season

were all in the ripening stage (Guimba 19% in 1984

WS under severe drought stress, Zaragoza 9% 1986

WS, Calauan 5% 1983 WS, Koronadal 4% 2ndcrops of 1986 and 1990). Highest DH incidence in

the vegetative stage was 8% also in the 1984 WS

Guimba crop but in the other three sites the range

was 3 5%.

3.2. Crop age and seasonal damage patterns

Stemborer damage patterns by crop age over each

of the four sites averaged over seasons were exp-

ressed in different ways (Figure 1). In Guimba and

Koronadal DH rose to a peak in the late vegetative

stage, leveling off in the reproductive stage, andsteeply climbing again in the ripening stage. In

Zaragoza this same pattern emerged with the excep-

tion that WH incidence sharply declined at the end of

the ripening stage. In Calauan no WH peak materia-

lized and it was the only site where WH damage

was less than that of DH. Guimba was distinguished

by having the lowest levels of DH but highest levels

of WH.

Stemborer damage levels also showed only a

minimal seasonal increase from earlier to later

planted fields. The results of regressing the number

Table I. Comparison of stemborer pest density by season for three crop growth stages in four sites, Philippines. a

Deadhearts/whiteheads (%)

Site Seasonb Crops (no.) Fields (no.) Vegetative Reproductive Ripening P F df

Zaragoza WS 12 72 1.7+0.4 b 2.9+0.4 a 2.7+0.7 a 0.02 3.48 71

DS 11 69 1.1+0.5 b 2.1+0.5 a 2.0+0.8 a 0.003 3.11 68

Koronadal 1st 7 52 1.5+0.5 b 1.6+0.5 b 2.5+1.0 a 0.004 2.98 52

2nd 8 57 1.8+0.5 b 1.4+0.5 b 2.4+0.9 a 50.0001 5.67 56

Guimba WS 7 44 1.7+0.5 b 1.9+0.5 b 4.7+1.0 a 50.0001 7.21 43

DS 6 44 1.0+0.6 b 1.3+0.6 b 2.4+1.1 a 0.04 2.19 43

Calauan WS 9 44 1.6+0.5 1.6+0.5 2.3+0.9 ns 1.6 43

DS 8 37 2.9+0.5 a 2.2+0.5 ab 1.6+0.9 b 0.03 2.37 36

total 68 419

average 1.7+0.2 b 1.9+0.2 ab 2.6+0.3 a 0.002 4.62 134

aIn a row, means+SEM followed by a common letter are not significantly different ( P0.05) by LSD test. Mean differences between

sites and seasons (column-wise comparisons) were insignificant for each growth stage.b

WS, wet season; DS, dry season.



5/13

of elapsed days from the first field planted with the

mean percentage change in stemborer damage levels

were non-significant for all sites except in Calauan

where they were negatively significant (Table II).

Therefore monitoring earlier planted fields would

not be justified for stemborers.

3.3. Stemborer thresholds

3.3.1. Decision threshold characters. Results are pre-

sented for each of the three crop growth stages in

separate tables (Tables III V). Results by site are in

the top half of each table and by AT character in the

bottom half. The most commonly tested AT char-

acter was egg mass density followed by DH and

flushed moths monitored in the field itself, and in

turn by DH monitored in neighboring fields.

3.3.1.1. Vegetative stage. In the vegetative stage

(Table III), the greatest frequency of ATs surpassed

occurred in Koronadal (21% of fields) followed by

Calauan (13%) (column 2 top). In Zaragoza and

Guimba, not a single field reached a threshold, being

justified by the lack of instances where the dam-

age yield loss benchmark was reached (column 4),and consequently had the highest score for correct

decisions, 98 100% correct not to treat (column 6).

The frequency of fields in Koronadal and Calauan

justified by the damage yield loss benchmark was

many-fold less than the frequency of decision triggers

to treat leading to significant frequencies of should

not have treated errors (13 19%). The reason for

the errors in Koronadal may have been from the heavy

use (45% of fields) of ATs based on flushed moths,

while in Calauan it was probable that AT levels were

set too low. All sites registered significant yield gains

in fields where vegetative stage stemborer ATs were

reached (column 12) compared to all other fields, but

there was no difference among sites.In total nine characters were evaluated with the

2Moths and 4Moths characters most frequently

surpassing AT levels (28 51% of fields) in the

vegetative stage (Table III, bottom of column 2). It is

Figure 1. Stemborer infestation pattern expressed as resulting damage as the crop aged in each of four locations, Philippines.

Table II. Regression correlations between planting date and

percentage change in mean weekly stemborer damage on a per-

field basis in four sites, Philippines.a

Site Crops Linear regression

Zaragoza 22 ns, df 112

Guimba 15 ns, df 67

Koronadal 16 ns, df 67

Calauan 13 y78.03.7 x, r0.371, P0.02,

df41

aPercentage change in mean stemborer damage is the dependent

variable (y), planting date is the independent variable (x) based

on the number of elapsed days after the first planted field, level

of significance (P0.05).



6/13

TableIII.Stemboreraction

thresholdanalysisofthevegetativestagebysiteandcharacterfromfourPhilippinericebowlsovera13-yearperiod.

Frequency(%fields)a

D

ecisions(%)a

Justified

Correlations

(ATvsdamage

yieldloss)d

Correctdecision

Incorrectdecision

Crops

Field

s

Pest

From

From

damageb

Correct

notto

Correct

to

Tota

l

Should

have

Should

nothave

Ratio

(9)(10)

Yieldgain(ATvsuntreated)e

(no.)

(no.)

AT

damagea,

b

yieldlossa,c

r

P

treat

treat

(67

)

treated

treated

(7)

k

g/ha

P

df

Site

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

Zaragoza

22

141

0+

2.9b

0+

1.6b

0+

0.9

100+

2.9a

0+

0.4

100+2

.9a

0+

1.2

0+

2.8a

f

Guimba

15

88

0+

3.5b

1.9+

1.1

0+

1.9b

98.1+

3.5a

0+

0.5

98.1+3

.5a

1.9+

1.4

0+

3.4a

f

Calauan

13

81

13.2+

3.2a

2.1+

1.0

1.8+

1.7b

85.7+

3.2b

0.6+

0.5

86.4+3

.2b

1.0+

1.3

12.6+

3.1b

156+

50

0.004

31

Koronadal

16

109

21.1+

3.3a

10.7+

1.8a

3.5+

1.0

76.4+

3.3b

1.4+

0.5

77.7+3

.3b

3.8+

1.3

18.5+

3.2b

158+

45

0.002

30

avg

8.

6

3.

1

1.

9

90.

0

0.

5

90.5

1.7

7.

8

157

P

50.0001

ns

50.0001

50.0001

ns

50.0001

ns

50.0001

ns

F

10.62

2.31

8.40

12.15

1.80

10.84

1.60

8.76

0

.28

df

126

126

126

126

126

126

126

126

121

Character

Level

Samplingsite

Abbreviation

Eggmass

0.250.5

Fielditself

0.5EM

22

133

0+

5.0c

0+

2.7b

0+

1.7b

0.000

ns

100+

5.1a

0+

0.8

100+5

.1a

0+

1.7a

0+

5.0a

0

155+

68

0.03

58

(no./hill)

1

Fielditself

1EM

29

186

8.6+

4.3c

4.3+

2.3b

0+

1.4b

0.888

50.0001

91.4+

4.3a

0+

0.7

91.4+4

.3a

0+

1.5a

8.6+

4.3a

0

164+

56

0.005

80

2

Fielditself

2EM

27

172

6.1+

3.0c

1.2+

1.0b

0.4+

1.6b

0.307

ns

93.3+

3.0a

0+

0.5

93.3+3

.0a

0.6+

1.1a

6.1+

3.0a

0

224+

46

50.0001

154

Deadhearts

3

5

Fielditself

5%DH

15

86

3.0+

4.1c

1.7+

2.2b

0.8+

1.4b

0.601

0.01

97.0+

4.2a

2.2+

0.7

99.2+4

.2a

0+

1.4a

0.8+

4.1a

0.4

299+

68

50.0001

81

(%)

8

15

Fielditself

10%DH

16

94

0+

4.0c

0.8+

2.1b

0+

1.3b

0.000

ns

98.4+

4.0a

0+

0.7

98.4+4

.0a

0.8+

1.4a

0.8+

3.9a

0

130+

67

ns

73

5

15

Neighboring

NF10%DH

6

36

0+

6.5c

8.3+

2.1a

0+

3.3b

0.000

ns

91.7+

6.6a

0+

1.1

91.7+6

.6a

8.3+

2.3b

0+

6.5a

0

7196+

138

ns

15

Moths(no./

2

Fielditself

2Moths

7

44

51.2+

6.0a

22.7+

3.1a

5.6+

2.0a

0.875

0.01

48.4+

6.1b

2.0+

1.0

50.5+6

.1c

0+

2.1a

49.5+

6.0c

24.8

65+

105

ns

36

20linearm)

4

5

Fielditself

4Moths

8

49

28.2+

5.6b

15.2+

2.2a

4.9+

1.9ab

0.758

0.02

65.5+

6.7b

1.8+

0.9

67.3+5

.7b

6.3+

1.9b

26.4+

5.6b

18.2

17+

109

ns

41

P

50.0001

50.0001

0.01

50.0001

ns

50.0001

0.02

50.0001

ns

F

9.56

8.31

2.44

9.95

1.7

9.28

2.39

8.81

1

.28

df

127

127

127

127

127

127

127

127

121

aAT,actionthreshold.

Columns6

7

9

10

100%.In

acolumn,means+

SEMfollowedbyacommonletterarenotsignificantlydifferent(P

0.0

5)byLSDtest.

bStandardbenchmarkof10%deadhearts.

cStandardbenc

hmarkof250kg/hayieldlossinvegetative

stage.dSignificant(P

0.0

5)ANOVAregressioncorrelations.eYieldcomparisonby

pairedt-test(P

0.0

5).fIn

sufficientdataforanalysis.



7/13

TableIV.

StemboreractionthresholdanalysisforthereproductivestagebysiteandcharacterfromfourPhilippinericebowlsovera13-yearperiod.

Frequency(%fields)a

De

cisions(%)a

Justified

C

orrelations

(ATvsdamage

yieldloss)d

Correctdecision

Incorrectdecision

Crops

Fields

Pest

From

From

damageb

Correct

notto

Correct

to

Total

Should

have

Should

nothave

Ratio

(9)(10)


(no.)

(no.)

AT

damagea,

b

yieldlossa,c

r

P

treat

treat

(67)

treated

treated

(7)

kg/ha

P

df

Site

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(

12)

Zaragoza

22

141

5.8+

2.4

6.7+

2.3

2.1+

1.8

94.2+

2.5

0.4+

0.9

94.6+2.3

1.3+

1.2

4.1+

2.0

205+

59

0.001

34

Guimba

15

88

1.9+

2.9

3.8+

2.8

0+

2.2

97.1+

3.1

1.0+

1.1

98.1+2.8

1.9+

1.4

0+

2.5

154+

48

0.005

19

Calauan

13

81

6.9+

2.6

8.8+

2.5

6.7+

1.9

90.0+

2.8

3.5+

1.0

93.5+2.5

3.1+

1.3

3.3+

2.2

157+

51

0.004

31

Koronadal

16

109

7.2+

2.7

5.2+

2.6

4.4+

2.0

89.7+

2.8

0+

1.1

89.7+2.6

1.9+

1.3

8.4+

2.3

158+

45

0.002

30

avg

5.

4

6.

1

3.

3

92.

8

1.

2

94.0

2.

1

4.

0

169

P

ns

ns

ns

ns

ns

ns

ns

ns

ns

F

0.73

0.66

2.00

2.97

1.51

1.69

0.39

2.12

0

.18

df

126

126

126

126

126

126

126

126

121

Character

L

evel

Samplingsite

Abbreviation

Eggmass

0

.5

Fielditself

0.5EM

10

59

7.6+

3.1

6.1+

2.8b

3.8+

2.4

0.90

5

50.0001

90.9+

3.5

0+

1.3

90.9+3.1ab

1.5+

1.5

7.6+

2.9ab

0

143+

47

0.003

122

(no./hill)

1

Fielditself

1EM

85

186

3.5+

2.7

4.4+

2.5b

2.8+

2.1

0.95

7

50.0001

92.2+

3.1

2.0+

1.1

94.3+2.7a

2.4+

1.3

3.4+

2.6ab

2.9

238+

47

50.0001

153

2

Fielditself

2EM

187

172

1.5+

2.8

11.2+

5.0b

3.3+

2.2

0.60

5

0.0008

96.5+

3.2

1.5+

1.2

98.1+2.9a

1.9+

1.4

0+

2.7a

1.3

149+

42

50.0001

149

3

4

Fielditself

3EM

25

160

4.2+

3.3

1.4+

1.1b

0.5+

1.7

0.33

2

ns

95.1+

3.3a

0+

0.5

95.1+3.3a

0.7+

1.1

4.2+

3.2ab

0

204+

47

50.0001

131

Deadhearts

5

10

Fielditself

10%DH

13

78

9.5+

4.0

5.7+

3.7b

3.4+

3.1

0.45

2

ns

90.5+

4.5

1.3+1.7

91.8+4.1ab

0+

1.9

8.2+

3.7b

6.3

258+

68

0.003

66

(%)

1

525

Fielditself

25%DH

18

102

1.7+

3.4

3.1+

3.1b

0.9+

2.6

0.74

2

0.0003

98.3+

3.8

0.9+

1.4

99.2+3.5a

0+

1.7

0.8+

3.2a

0.9

189+

67

0.006

88

1

025

Neighboring

NF15%DH

6

36

8.3+

5.9

27.8+

5.4a

0+

4.2

0.00

0

ns

87.5+

6.6

4.2+

2.5

91.7+6.0ab

8.3+

2.9

0+

5.5a

2.0

7197+

138

ns

15

Moths(no./

2

Fielditself

2Moths

7

44

14.5+

5.5

11.2+

5.0b

8.9+

4.0

0.97

5

50.0001

81.4+

6.1

0+

2.3

81.4+5.6b

4.1+

2.7

14.5+

5.1b

0

65+

106

ns

36

20linearm)4

Fielditself

4Moths

8

49

12.7+

5.1

9.8+

4.7b

7.8+

3.7

0.97

7

50.0001

83.7+

5.7

0+

2.1

83.7+5.2b

3.6+

2.5

12.7+

4.8b

0

17+

109

ns

41

P

ns

0.009

ns

ns

ns

0.05

ns

0.05

ns

F

1.44

2.86

0.71

1.49

0.51

2.07

1.19

2.01

1

.00

df

127

127

127

127

127

127

127

127

121

aAT,actionthreshold.

Columns6

7

9

10

100%.In

acolumn,means+

SEMfollowedbyacommonletterarenotsignificantlydifferent(P

0.0

5)byLSDtest.

bStandardbenchmarkof15%deadhearts.

cStandardbenc

hmarkof250kg/hayieldlossinvegetative

stage.

dSignificant(P

0.0

5)ANOVAreg

ressioncorrelations.eYieldcomparisonby

pairedt-test(P

0.0

5).



8/13

TableV.

Stemboreraction

thresholdanalysisfortheripeningstageb

ysiteandcharacterfromfourPhilippinericebowlsovera13-yearperiod.

Frequency(%fields)a

D

ecisions(%)a

Justified

Correlations

(ATvsdamage

yieldloss)d

Correctdecision

Incorrectdecision

Crops

Fields

Pest

From

From

damageb

Correct

notto

Correct

to

Total

Should

have

Should

nothave

Ratio

(9)(10)


(no.)

(no.)

AT

damagea,

b

yieldlossa,c

r

P

treat

treat

(67)

treated

treated

(7)

k

g/ha

P

df

Site

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

Zaragoza

22

141

9.3+

2.2a

30.2+

4.2a

3.0+

1.2

84.0+

3.4

6.4+

1.9

90.4+

2.7

a

4.6+

2.5a

5.0+

1.3b

20

3+

45

0.001

34

Guimba

15

88

1.4+

2.7b

32.8+

5.1a

1.4+

1.5

78.1+

4.2

1.4+

2.3

79.6+

3.4

b

20.4+

3.0c

0+

1.6a

15

4+

63

0.005

19

Calauan

13

81

5.2+

2.4ab

11.5+

4.6b

1.0+

1.3

90.5+

3.8

0+

2.1

90.5+

3.0

a

6.9+

2.7ab

2.6+

1.4ab

17

6+

50

0.004

31

Koronadal

16

109

0+

2.5b

24.9+

4.8ab

0+

1.4

87.1+

3.9

0+

2.2

87.1+

3.1

ab

12.9+

2.8bc

0+

1.4a

15

8+

45

0.002

30

avg

4.

0

24.

8

1.

4

84.

9

2.

0

86.

9

11.

2

1.

9

17.

3

P

0.03

0.008

ns

ns

ns

0.05

0.0006

0.03

ns

F

3.10

4.14

0.93

1.75

2.35

2.55

6.25

3.11

0.18

df

126

126

126

126

126

126

126

126

121

Character

Level

Samplingsite

Abbreviation

Eggmass

0.250.5

Fielditself

0.5EM

22

133

4.2+

2.7

34.1+

5.7

1.9+

1.6

0.460

ns

89.6+

4.5

0.6+

2.4

90.2+

3.6

5.7+

3.3a

4.2+

1.7

16.5

14

3+

47

0.003

122

(no./hill)

1

Fielditself

1EM

29

186

2.4+

2.4

26.4+

5.1

0.4+

1.5

0.251

ns

82.8+

4.0

0.9+

2.1

83.7+

3.2

14.8+

2.9b

1.5+

1.5

18.1

23

8+

47

50.0001

153

2

Fielditself

2EM

27

172

0.6+

2.5

21.7+

5.3

0.6+

0.5

0.045

ns

84.0+

4.2

0+

2.2

84.0+

3.4

15.4+

3.0b

0.6+

1.6

0

14

9+

42

50.0001

149

Deadhearts

35

Fielditself

5%DH

13

78

12.5+

3.5

17.1+

7.5

3.2+

2.1

0.534

0.05

87.9+

5.9

1.9+

3.1

89.8+

4.7

2.2+

4.3a

8.0+

2.2

5.4

25

8+

68

0.003

66

(%)

815

Fielditself

10%DH

18

102

8.3+

2.9

12.6+

6.3

4.2+

1.8

0.937

50.0001

86.6+

4.9

9.7+

2.6

96.3+

4.0

1.4+

3.7a

2.3+

1.9

0.4

18

9+

67

0.006

88

310

Neighboring

NF5%DH

6

36

0+

5.2

33.3+

10.9

0+

3.2

0.000

ns

75.0+

8.7

0+

4.6

75.0+

7.0

25.0+

6.3b

0+

3.3

0

719

7+

138

ns

15

Moths(no./

2

Fielditself

2Moths

7

44

0+

4.8

28.6+

10.1

0+

2.7

0.000

ns

90.3+

8.0

0+

4.2

90.3+

6.5

9.7+

5.9ab

0+

3.1

0

6

5+

106

ns

36

20linearm)

4

Fielditself

4Moths

8

49

0+

4.5

30.0+

9.5

0+

2.6

0.000

ns

89.0+

7.5

0+

3.9

89.0+

6.1

10.9+

5.5ab

0+

2.9

0

1

7+

109

ns

41

P

ns

ns

ns

ns

ns

ns

0.005

ns

ns

F

1.78

1.27

0.65

0.52

1.54

1.63

3.13

1.54

1.00

df

127

127

127

127

127

127

127

127

121

aAT,actionthr

eshold.

Columns6

7

9

10

100%.In

acolumn,means+

SEM

followedbyacommonletterarenotsignificantlydifferent

(P

0.0

5)byLSDtest.

bStandardthresholdof5%deadhearts.

cStandardthresholdof250kg/hayieldlossinripeningsta

ge.

dSignificant(P

0.0

5)ANOVAregressioncorrelations.eYieldcomparisonbypairedt-test(P

0.0

5).



9/13

noteworthy that with these two characters the most

fields attained the 10% DH benchmark criterion of

damage (15 23%) (bottom of column 3), but when

the benchmark also included 250 kg/ha yield loss

component, in only 5 6% of fields was corrective

action justified. As a result these two AT charactersscored highest levels of should not have treated

error (26 50%). All other characters registered

levels of correct decisions 490%, based on egg

mass characters (0.5EM, 1EM, 2EM) as well as

two DH characters in the field itself (5%DH and

10%DH) and one monitoring neighboring fields

(NF15%DH). It is noteworthy that none of the egg

mass characters nor 10%DH or NF15%DH scored

any correct to treat decisions. It was surprising that

0.5EM, being so low a level, did not trigger an AT

response in any field. Records showed that egg

parasitism levels tended to be high in those crops

thus allowing natural control to occur. Those char-acters most correlated with the damage yield loss

results were 1EM, 5%DH, 2Moths, and 4Moths

(column 5). The DH character 5%DH had the most

favorable ratio of incorrect decision errors to correct

decisions to treat (0.4) (column 11), while 0.5EM,

1EM, 2EM, and 5%DH had significant yield gains

over the untreated. Overall 5%DH scored highest in

all five categories (seen in columns 5, 8, 910, 11,

12) with 1EM second lacking only a good ratio.

3.3.1.2. Reproductive stage. In the reproductive stage

(Table IV) an average of 5% of fields surpassed ATlevels with no significant differences evident between

sites. The relationship between frequencies of fields

surpassing ATs and surpassing the damage bench-

mark of 15% DH250 kg/ha yield loss were

similar among all sites. There were no differences

among sites regarding frequencies of total correct

decisions ranging from 90 to 98% with only an

average of 1% correct decisions to treat. Error

frequencies were similar among the sites. All sites

registered significant yield gains from treatments

where ATs were reached over the untreated with no

differences between sites.

Among the nine characters tested there was no

difference in frequencies in those reaching ATs

which ranged from 2 to 15% of fields. Six of the

characters had significant correlations between fre-

quencies surpassing ATs and the damage yield

loss benchmark. Characters that failed were 3EM,

10%DH, and NF10%DH. Most of the decisions

were correct not to treat ranging from 81 to 98%

and all characters scored similarly. Characters with

the highest total correct scores were 1EM, 2EM and

10%DH, while the lowest were 2Moths and 4Moths.

Highest levels of should not have treated errors

were affiliated with 2Moths and 4Moths as well as10%DH. The character 25%DH had the most

favorable ratio of errors to correct to treat decisions

(0.9). The best characters based on the fewest errors

were 2EM, 25%DH, and NF15%DH. The highest

scoring character was 25%DH which had high

rankings in all five categories. The next best character

was 2EM which only lacked a better ratio.

3.3.1.3. Ripening stage. The ripening stage (Table V)

had the lowest rate of surpassing AT decisionsaveraging 4% fields over the three growth stages.

Zaragoza surpassed AT levels most (9% fields) where

EM characters dominated, with Koronadal least

(0%) in which flushed moth characters prevailed.

Benchmark damage levels were high but were not

matched by significant yield loss as perhaps the 5%

DH benchmark was set too low. Most decisions were

correct not to treat, and Zaragoza and Calauan had

the highest score of correct decisions reaching 90%.

A high of 20% of fields with the should have treated

error occurred in Guimba, while 5% of fields in

Zaragoza had the opposite error. As with the earlier

growth stages, significant yield gains occurred in thefields where ATs were reached compared to the

untreated but with no differentiation between sites.

In Guimba the low AT frequency in the ripening

stage seems at odds with the high WH damage levels

registered in Figure 1. This is explained by the high

should have treated error and the low DH levels

preceding the ripening stage. ATs were based on DH

and not WH as it was assumed that once WH were

formed it was too late for control to have an effect.

Guimba was under moisture stress in a number of

crops due to failure of the electric irrigation pump

which accentuated WH expression.There were no differences among the eight

characters in terms of frequencies of fields surpassing

ATs (range 0 13%). Likewise correct decisions that

ranged from 75 to 96% were statistically similar

between characters. Only 5%DH and 10%DH

characters had high correlations between surpassing

ATs and the 5% DH damage250 kg/ha yield loss

benchmark. The characters 1EM, 2EM, and

NF10%DH had the highest levels of should have

treated errors (15 25%) with 2Moths and 4Moths

following (10 11%). Significant yield gains were

registered by all the egg mass characters as well as

5%DH and 10%DH. Only 10%DH came out with

the best ratio of errors to correct decisions to treat

(0.04) and based on the five criteria was the superior

character overall with 5%DH coming in second with

only a poor ratio.

3.3.2. Insecticide response. Overall the level of control

achieved by insecticides was low for single spray res-

ponses (536% control) to stemborer thresholds in

the four test sites based on DH damage (Table VI).

The combination chlorpyrifosBPMC (34% con-

trol of DH) was superior to both chlorpyrifos alone

(13%) and endosulfan (9%) 1 4 weeks after treat-ment (WT). Data from the 3 WT sampling date

revealed the combination (36%) was statistically

equal to endosulfan (18%) but higher than that of

chlorpyrifos alone (75%).



10/13

When insecticide control data was aggregated by

the four AT main characters, egg masses and DH in

the field itself were superior (26 29% control) to

DH from a neighboring field (5% control) 1 4

WT when measured as DH damage (Table VII).

Flushed moths from the field itself was in between(11%). On the 3 WT sampling date, DH sampled

from a neighboring field showed significantly lower

control than when the other three characters were

used.

The control of nontarget pests (whorl maggot,

defoliators, leaffolders) ranged from 23 to 40%

(Table VIII). These would be the same pests in the

other articles in the series that were sampled from the

same fields where ATs were tested.

4. Discussion

4.1. Stemborer action thresholds

ATs based on egg masses or moths would appear

to have been a more logical choice as these stages

preceded the period of damage, and neonate larvae

would be more vulnerable to sprayed insecticide. But

both characters had high levels of false positive

triggers in the vegetative and reproductive stages and

high levels of false-negative responses in the ripening

stage. The least accurate character was flushed moths

which had the lowest correct decision rates. In

addition insecticides performed equally well when

either egg mass or DH characters were monitored in

the field itself. It is most probable that natural

enemies and natural resistance of rice plants were

responsible for the low performance of these char-

acters by severely limiting the number of larvae

successfully infesting tillers. The egg mass AT even

had a provision that included preventing a correctiveresponse if egg parasitism reached 50%. Egg and

larval predators are also abundant in rice fields (Ooi

and Shepard 1991) reducing infestation rates when

egg parasitism levels were low particularly in later

growth stages.

The DH character monitored in the field itself

outperformed all others in each of the three growth

stages averaging 92 99% correct decisions and gave

the most outstanding score among AT characters.

DH measured in neighboring fields registered con-

sistently high should have treated false-negative

errors showing this character lacked accuracy. This is

reinforced by the fact that only in Calauan wasplanting date correlated with increasing seasonal

stemborer abundance (Table II). Rising stemborer

incidence in the field itself is the reason that the DH

character worked. This was particularly evident in

Guimba and Koronadal in the late reproductive and

ripening stage and in Zaragoza in the late vegetative

stage and early reproductive stages.

Comparing pest densities with yield loss suggested

that the damage benchmark could be improved. The

best performing levels in each growth stage were

35, 1525, and 815% DH from vegetative to

ripening stages. These represented a ratio of ca.

Table VI. Insecticide efficacy against stemborers in response to thresholds.

Insecticide dosage Control (%)a (deadhearts/whiteheads)

Insecticide (kg a.i./ha) n 1 4 WT 3 WT Yield gaina

(kg/ha)

Chlorpyrifos 0.4 47 8.5+5.8 b 75.4+9.9 b 91+ 92

ChlorpyrifosBPMCb

0.4 52 33.8+5.5 a 35.5+9.7 a 157+ 87

Endosulfan 0.4 18 13.3+14.0 b 18.4+13.5 ab 210+ 82

P 0.007 0.02 ns

F 5.28 4.34 0.54

df 106 84 112

aIn a column, means+SEM followed by a common letter are not significantly different (P0.05) by LSD test. WT, week after treatment.b

Mixture of 20% chlorpyrifos and 11% BPMC.

Table VII. Insecticide efficacy and yield as affected by stemborer threshold character.

Control (%)a (deadhearts/whiteheads)

Character Monitoring site n 1 4 WT 3 WT Yield gaina

(kg/ha)

Egg mass Field itself 51 26.0+5.7 a 17.9+8.1 a 266+ 88

Flushed moths Field itself 37 11.3+6.7 ab 21.7+10.2 a 126+ 94

Deadhearts Field itself 18 28.8+7.6 a 25.0+12.1 a 406+ 146

Deadhearts Neighboring field 17 5.2+7.9 b 717.9+6.9 b 230+ 63

P 0.03 0.002 ns

F 2.58 5.25 0.94

df 106 90 90

aIn a column, means+SEM followed by a common letter are not significantly different ( P0.5) by LSD test. WT, weeks after treatment.



11/13

1:5:3, whereas the original ratio was set at 2:3:1.

Based on damage pattern and yield loss data from

Bandong and Litsinger (2005) the original ratio fits

the data best for a 110-day variety. Differences in

crop maturity not only affect the damage pattern

but also tolerance to damage with longer matu-ring varieties having greater compensatory ability

(Litsinger et al. 1987). The crop compensates from

DH during tiller elongation by producing more

productive tillers (Rubia et al. 1996). Compensation

from WH occurs as increased grain weight in

undamaged panicles. Longer maturing varieties

(120 135 days) such as those commonly used in

Calauan (IR70, C1, Malagkit) have a prolonged

vegetative stage and consequently higher DH den-

sities from the same egg mass infestation level

and with relatively fewer WH. This can be seen in

Figure 1 for Calauan and for IR70 in Bandong andLitsinger (2005).

Foliar spraying is the application method of choice

by farmers, with only a few using granular formula-

tions. Systemic granulars such as carbofuran are

most economical only if soil incorporated before

planting (Bandong and Litsinger 1979). To achieve

an equal level of control when applied to a standing

crop, a 4-fold increase in dosage is required when

broadcast into paddy water, as much of the chemical

becomes bound to the soil before entering roots

(Seiber et al. 1978; dela Cruz et al. 1981). As farmers

prefer to respond to damage only when observed,

preplant application is not popular.

Chlorpyrifos has been the best performing insecti-

cide spray for stemborer control but the results

showed there was an enhanced synergy even at a

lower active ingredient level with the addition of

BPMC (Table VI). ChlorpyrifosBPMC is sold as a

mixture for broad spectrum control of chronic rice

pests. BPMC is effective against leafhoppers, plan-

thoppers, and leaffolders but is not recommended for

control of stemborers alone (PCARR 1977; Litsinger

et al. 1980).

4.2. Pest pressure-crop tolerance paradox

Research has shown up to 30% stemborer dead-

hearts and 10% whiteheads (Rubia et al. 1996) and 3

whiteheads/hill (Litsinger 1993) can be tolerated

by modern rices without yield loss. Despite this com-

pensatory ability, significant yield increases were

associated with the best performing ATs against

stemborers in this study (Tables III V, column 12)

where damage did not exceed the above levels in

any of the four sites. The recorded chemical con-trol of nontarget pests may only be a partial expla-

nation, because these pests were not at economic

densities.

A more profound explanation is sought. Such a

reason for this seeming paradox were put forth in a

companion paper on leaffolder (Litsinger et al.

2006b) based on concept of synergistic yield losses

from multiple stresses in a given growth stage. The

basis for this hypothesis is 2-fold: (1) low yields have

been found to be associated with multiple stresses by

Savary et al. (1994) such as the combination of

stemborer damage and weeds and (2) synergistic

losses have been documented by IRRI (1984) fromthe combined action of rice insect pests at low

densities. The opposite effect of small stresses jointly

causing high losses would be that upon releasing

even one stress such as stemborer, large yield gains

would occur under favorable conditions. If true this

response should occur even upon low level of

insecticide control, just as low pest densities con-

tributed to synergistic losses, particularly when the

crop is managed under optimal agronomic condi-

tions. This hypothesis is supported by trials which

showed crop tolerance to stemborers is enhanced

through increasing seeding rate or nitrogen fertilizeras well as via selecting a longer maturing variety

(Litsinger 1993). As a corollary, high tolerance

would be expected to occur in a crop where

agronomic requirements are met in an otherwise

stress free environment. Such results have a bearing

on developing IPM strategies.

4.3. Input to IPM programs

The first step in IPM for irrigated rice advocated in

farmer field schools is to grow a good crop following

integrated crop management principles (Matteson

2000). The rationale behind this conviction has been

to bolster modern rices with greater capacity for pest

tolerance including stemborers. Interpretation of the

pest pressure-crop tolerance paradox supports this

approach. Crop management then becomes a two

pronged strategy. The first thrust is for the farmer to

undertake steps to increase the crops inherent yield

potential commensurate with the magnitude of the

complex of stresses present a given season and

expected favorable weather. This can be best done

by aping the practices of the highest yielding farmers

in the area who have found the best crop manage-

ment practices often by trial and error. Someexamples of such practices often include selecting

a longer maturing variety (within the range of

135 days as much longer durations will favor

stemborer buildup), utilizing healthy seeds to

Table VIII. Control of nontarget pests by insecticides used in

response to action thresholds against stemborers.

Control of nontarget pests

(% damaged leaves)a

Target pest

Whorl

maggot n Defoliators n Leaffolder n

Stemborers 35.0+5.2 41 39.9+5.7 25 22.5+4.1 154

aAverage of four sites and 66 crops, n number of fields, mean+

SEM.



12/13

minimize diseases, increasing the seeding rate,

optimizing the nutrient regime, providing adequate

water management, and removing weeds from both

the seedbed as well as crop before canopy enclosure.

Early planting is also a good practice to avoid pests

(Savary et al. 1994). In the current study we showedthat there is not a progressive increase in stemborer

infestation levels over the planting season among

neighbors as would be required to support monitoring

earlier planted fields, but stemborer populations tend

to rise and fall, often dynamically, over the season.

Earliest plantings tend to escape damaging infestation.

The second thrust is to monitor the crop on a regular

basis; trouble shooting not only to detect stemborers

but also other biotic and abiotic factors that restrain

yield and then respond via corrective action.

Bandong and Litsinger (2005) showed that the rice

plant has natural resistance to stemborer damage

during mid-growth, evidence of which can be seen asa plateau in DH density in each of the four study sites

(Figure 1). DH did not decrease during this mid-

growth period as it takes 3 4 weeks for a dead tiller to

rot and fall away. Also that incidence did not increase

is a sign of reduced infestation. The resistant period

for early maturing varieties, typical of Koronadal and

Guimba, runs from late vegetative to pre-booting but

with longer maturing varieties it is extended to

booting (Bandong and Litsinger 2005). Zaragoza

represents a mixture of early and medium maturing

varieties as farmers tend to plant early maturing rices

in the dry season due to insecure water supply andlonger maturing, taller rices in the wet season to

tolerate expected monsoon flooding. The fact that

DH/WH rose again in only some sites can be

explained by differences in crop maturity (Bandong

and Litsinger 2005). In the Calauan site, typical for

longer maturing rices under similar pest infestation

levels, higher DH densities occurred in the late

vegetative stage along with lower WH levels than for

earlier maturing rices. The monitoring periods should

be tailored to the maturity class of rice.

Responses from ATs were justified in 7% of fields

in the course of the study based on benchmarks,

about equally divided between the two susceptible

stages. A program of regular monitoring should

involve heightened vigilance during tiller elongation

and panicle exsertion when the crop is most

susceptible to stemborer damage. This is seen as the

reason for the higher rates of ATs being justified in the

reproductive (Table IV) than the ripening (Table V)

stages. AT levels will need to be adjusted based on

experience at the site for each of the three growth

stages but a ratio of 1:5:3 for percentage deadhearts

would be a good place to begin for longer maturing

and 2:3:1 for early maturing varieties. During the

tillering stage, farmers would have the option of usingan insecticide and/or reducing stresses from other

causes (particularly weeds), but during panicle

exsertion there are not many other options than

insecticide, which would be particularly favored

(1) with early maturing varieties, (2) crops are under

multiple stresses, and/or (3) crops have otherwise

been well managed and favorable weather is expected.

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 Mariano

Leron, Eduardo Micosa, and Carlos de Castro, and

in Koronadal Hector Corpuz, Joseph Siazon, BeatrizVelasco, 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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