Abstract Applications of sublethal concentrations of methomyl to 4th instar larvae and pupae of the diamondback moth resulted in increased fecundity of the adults which emerged. The adult females derived from the treated pupae laid more eggs with a higher rate of fertilization as compared with the untreated check. However, the adult females thus treated had a shorter life span than the untreated ones. These effects of methomyl enhanced the intrinsic rate of natural increase ( r ) or the finite rate of increase per month Applications of methomyl decreased lycosid spiders which were important predators of 3rd to 4th instar larvae of the diamondback moth in the cabbage fields. It is suggested !hat the application of methomyl might cause a resurgence of the moth population through the stimulation of the reproductive potential and differential mortality between predators and prey

Hisashi Nemoto

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37 
Factors Inducing Resurgence in the Diamondback 
Moth After Application of Methomyl 
Hisashi Nemoto 
Saitama Horticultural Experiment Station, Rokumanbu, Kuki-shi, Saitama 346, Japan 
Abstract 
Applications of sublethal concentrations of methomyl to 4th instar larvae and pupae of 
the diamondback moth resulted in increased fecundity of the adults which emerged. The adult 
females derived from the treated pupae laid more eggs with a higher rate of fertilization as 
compared with the untreated check. However, the adult females thus treated had a shorter 
life span than the untreated ones. These effects of methomyl enhanced the intrinsic rate of 
natural increase ( r )  or the finite rate of increase per month Applications of methomyl 
decreased lycosid spiders which were important predators of 3rd to 4th instar larvae of the 
diamondback moth in the cabbage fields. It is suggested !hat the application of methomyl might 
cause a resurgence of the moth population through the stimulation of the reproductive potential 
and differential mortality between predators and prey. 
Introduction 
The major caterpillar pests of crucifers in Japan are common cabbageworm, Pieris 
rapae crucivora Boisduval (Lepidoptera: Pieridae), diamondback moth (DBM), Plutella 
xylostella (L) (Lepidoptera: Yponomeutidae), and beet semi-looper, Autographa 
nigrisigna (Walker) (Lepidoptera: Noctuidae). Insecticides are the most commonly used 
control agents against these pests in Japan. 
In 1981, at Saitama Horticultural Experiment Station (SHES), DBM became 
abundant after applications of methomyl against common cabbageworm in the 
cauliflower field but not in the untreated field (Figure 1). Ripper (1956), who reviewed 
resurgences of pest populations after insecticide treatment, suggested the following as 
the causes for insect pest resurgence: (a) reduction of natural enemies along with the 
pest by pesticides, (b) favorable influence of pesticides on phytophagous arthropods 
(stimulating influence on the pest directly or via the plant), and (c) the removal of 
competitive species. 
This paper describes results of two experiments conducted to analyze the process 
of DBM resurgence. In the first experiment direct stimulation of methomyl on the 
reproductive potential was examined and in the second, predators of DBM were identified 
and the toxicity of methomyl against on these predators evaluated. 
Stimulation of Reproductive Potential 
Larval treatment with methomyl 
Effects of sublethal concentrations of methomyl applied to 4th instar larvae on 
the fecundity of DBM are shown in Table 1. The adult females derived from the treated 
larvae deposited more eggs than the control irrespective of concentrations of methomyl. 
388 Nemoto 
Figure 1. 
Effects of methomyl treatments on 
the diamondback moth popula- 
tions captured in pheromone traps 
from cauliflower fields of SHES. 
Takeda Chemical Ind Ltd) baited 
with “Px” (a synthetic sex phero- 
mixture of 0.05 mg of (Z)-11 
Three dry type traps (made by 
mone for P. xylostella, which is a 
treatment 
25 30 10 30 10 20 30 10 
Aug Sep Oct Nov 
hexadecenal with 0.05 mg of (Z)-11 
hexadecenyl acetate and 0.001 
Table 1. Effects of the treatment of the fourth instar larvae with sublethal concentrations of 
methomyl on the fecundity of DBM 
Methomyl No. eggs 
concentrations per mg of 
per female 
(ppm) pupal weight 
No. 
examined 
100 180.0 ± 44.8d (1.49)* 42.8 ± 8.0 (1.40) ** 4 
50 139.5±16.7 (1.15)* 36.3 ± 2.1 (1.1 9) **  6 
10 157.2±18.4 (1.30)* 38.0±3.3 (1.25)** 6 
Control 121.2±25.2 30.5 ± 6.4 6 
a Figures in parentheses indicate the rate of relative increase as compared with control. b Figures with 
asterisks differ significantly from control at 5% level ( * )  and 1 % level ( * * )  by t-test. c No eggs 
deposited per mg of fresh body weight of pupa. d Mean ± standard deviation. 
In particular, at concentrations of 10 and 100 ppm, methomyl significantly increased 
reproduction. A 49% increase in the number of eggs per female was observed at 100 
ppm followed by 30% at 10 ppm and 15% at 50 ppm. Based on pupal weight differences 
were highly significant. 
Pupal treatment with methomyl 
Dipping pupae (5.5 to 6.5 mg/each) in sublethal concentrations of methomyl 
resulted in increased fecundity of the adults. The adult females derived from these treated 
pupae laid more eggs with higher rates of fertilization, as compared with the untreated 
ones, irrespective of the concentrations of methomyl (Table 2 ) .  At 10 ppm, there was 
a 17% increase in number of eggs per female followed by a 12% increase at 100 ppm, 
a 10% increase at 500 ppm, and an 8% increase at 50 ppm. The number of fertilized 
eggs per female increased 1.3 times at 10 ppm and 1.15 times at 500 ppm compared 
to the untreated ones. The number of fertilized eggs per female per day increased 1.68 
times at 10 ppm and 1.33 times at 500 ppm. The percentage of fertilized eggs also 
increased by methomyl treatment as compared to the untreated check. However, the 
adult females thus  treated had a shorter life span than the untreated ones (Table 2). 
Methomyl Induced Resurgence in DBM 389 
Table 2. Longevity and fecundity of female DBM adults treated with methomyl in pupal stage 
Methomyl concentrations (ppm) 
10 50 100 500 
Adult emergence (%) 98.3 100 95.5 98.5 97.0 
7.1 f 2.0 Longevity of female 7.7± 1.1ª 6.1 ± 0.7 6.5 f 0.7 
Oviposition period (days) 5.1 f 0.9 4.4 f 1 .0 4.6 f 0.7 4.6 f 1 .0 5.6 f 2.0 
Fecundity (eggs/female) 164.2±37.9 191.9±44.4 176.9±28.4 183.2±40.2 180.6±41.4 
No. of eggs/female/day 24.9±6.6 37.5±8.3 32.3±6.1 33.0±8.1 31.9±11.7 
No. of fertilized 139.3±37.0 182.0±43.3 170.1 ±28.2 175.1 ±35.3 160.6±37.2 
eggs/female (1.31) (1.22) (1.26) (1.15) 
No. of fertilized 21.2±6.5 35.7±8.4 31.1 ±6.1 31.6±7.4 28.3±9.9 
eggs/female/day (1.68) (1.47) (1.49) (1.33) 
Control 
No. of pairs examined 10 7 13 13 1 1  
6.7 f 1.2 
adults (days) 
(1.1 (1 .08) (1.12) (1 .10) 
Fertilized eggs (%) 84.4 94.7 96.2 95.6 88.9 
a Mean ± standard deviation. 
compared to the control. 
Figures in the parentheses indicate the rate of relative increase as 
Figure 2 shows survivorship and oviposition curves of female adults treated with 
methomyl in the pupal stage. The adults derived from the treated pupae deposited greater 
numbers of eggs for the first two days than the untreated ones. Figure 3 shows a typical 
comparison of cumulative oviposition curves between control and treated individuals at 
10 ppm of methomyl in the pupal stage. The treated females laid not only more eggs 
but also a greater proportion of fertilized eggs as compared with the check. 
Control 
0 5 10 
Age of adults ( days after emergence ) 
70 
35 
0 
70 
35 
35 Figure 2. 
Survival and oviposition curves of female 
adults treated with different concentrations 
70 of methomyl in the pupal stage. pairing 
35 
35 
0 
3 90 Nemoto 
200 
I50 
l o o -  
No of eggs 
Control 
Figure 3. 
Comparison of cumulative oviposition 
curves between untreated and methomyl 
treated individuals. pairing 
0 5 
Age of adults days a f t e r  emergence)  
Rate of population increase 
We obtained a survival rate (1x (E-L)) value of 0.45 from egg to pupa by rearing 
the insects on radish Raphanus sativus (L) seedlings in the laboratory. Substituting the 
values of 0.5 for sex ratio, age-specific survival rate (1x) is expressed by the following 
equation (Nemoto et al 1984): 
1x= 1x(E-L) x (sex ratio) x 1x(P) x 1x(A) 
= 0.225 x 1x(P) x 1x(A) 
where 1x(P) is the rate of adult emergence and 1x(A) is the age-specific survival rate 
of female adults. 
The value of r (intrinsic rate of natural increase) was estimated by using the age- 
specific survival rate and fecundity tables based on the equation given by Birch (1948): 
where is net reproductive rate, is age-specific fecundity, x is age (days), T is 
mean generation time and is finite rate of natural increase after 30 days. 
Methomyl Induced Resurgence in DBM 391 
The population doubling time under given conditions can be calculated as: 
log, 2 doubling time = 
The population parameters of DBM treated with methomyl in the pupal stage were 
calculated by using the number of fertilized eggs shown in Table 2. The values of T 
for the treated ones were smaller than in the untreated check (Table 3). Also, the values 
of increased as compared with the untreated check. As a result there was an increase 
in the value of r by 6% to 13% in the treated one. Consequently the values of (after 
30 days) increased 2.03, 1.59, 1.63, and 1.37 times for 10, 50, 100, and 500 ppm 
methomyl, respectively. 
Table 3. Population parameters of DBM treated with different concentrations of methomyl 
in the pupal stage 
Methomyl concentrations (ppm)ª 
10 50 100 500 
Control 
Mean length of one 
generation (T) (days) 
Net reproductive 
rate 
Intrinsic rate of 
natural increase 
(r)/female/day 
Finite rate of 
increase 
Doubling time (days) 
18.42 17.71 
30.81 40.95 
(1 33) 
0.186 0.210 
(1.13) 
265.75 538.46 
(2.03) 
3.73 3.31 
17.85 
36.55 
(1.19) 
0.202 
(1.08) 
423.39 
3.44 
(1.59) 
17.95 
37.78 
(1.23) 
0.202 
(1.09) 
432.38 
(1.63) 
3.43 
18.10 
35.06 
(1.14) 
0.197 
(1.06) 
364.02 
3.53 
(1 37)  
a Figures in the parentheses indicate the rate of relative increase as compared with control. 
Effectiveness of Predators as Biotic Mortality Agents 
Identification of DBM predators 
Many species of predatory arthropods are collected by pitfall traps in the cabbage 
field in SHES (Table 4). In order to identify the predators of DBM, we used an 
immunological test. This method was sensitive enough to detect the whole-body extracts 
of 3rd to 4th instar larvae, pupae, and adults of DBM (Nemoto et al 1985). Field-collected 
lycosid spiders exhibited a positive reaction in the test, as did some Misumenops 
tricuspidatus (Feb) (Aranea: Thomisidae) (Figure 4). But few of Labidura riparia 
japonica (de Hann) (Dermaptera: Labiduridae) reacted positively and neither did any 
other suspected predators. 
Around 10% of the lycosids collected in July showed a positive reaction. No 
individuals showed a positive reaction in August when the density of DBM larvae was 
low (Table 5). 
Effect of methomyl on predator population 
The effect of methomyl on predator populations was studied in the cabbage field. 
The numbers of lycosid spiders, particularly Pardosa astrigera L Koch and Lycosa 
pseudannulata (Bos et Str), were reduced by the applications of methomyl but certain 
predatory insects were not affected (Figure 5). 
3 92 Nemoto 
Table 4. Predatory arthropods captured from the cabbage fields at SHES 
Family Species 
ARANEA 
Atypidae 
Micryphantidae 
Tetragnathidae 
Hahniidae 
Lycosidae 
Thomisidae 
Salticidae 
Gnaphosidae 
Ctenidae 
DERMAPTERA 
Atypus karschi Donitz 
Gnathnarium deutatum (Wider) 
Dyschiriognatha quadrimaculata Bos et Str 
Hahnia corticicola Bos et Str 
Arctosa subamylacea (Bos et Str) 
Lycosa Pseudoannulata (Bos et Str) 
Pardosa laura Karsch 
Pardosa astrigera L Koch 
Misumenops tricuspidatus (Feb) 
Gen et sp 
Gnaphosa kompirensis (Bos et Str) 
Anahita fauna Karsch 
Labiduridae 
COLEOPTERA 
Harpalidae 
Labidura riparia japonica (de Hann) 
Epomis nigricans W i ed e m a n n 
Chlaenius sp Gen et sp 
Cicindelidae Cicindela japana Thunberg 
Table 5. Record of lycosid spiders captured in pitfall traps in the cabbage field at SHES 
and level of their predation 
July 1983 Aug. 1983 
8 to 12 22 to 26 5 to 9 19 to 23 
Date of collection 
No. of individuals tested. c 25 27 28 6 
Positive reaction (%) 8.0 11.1 0 0 
a Micro-Ouchterlony precipitation test. 
collected every day. 
No. of spiders/30 pitfall traps Trapped spiders were 
Figure 4. Precipitation test (Micro-Ouchterlony method) with anti-DBM 
serum against extract of Pardosa astrigera L Koch. The anti- 
DBM serum was placed in the central well and the extracts 
of P. astrigera diluted with saline was placed in the wells 
around the center. a: feeding on DBM, b: control 
Methomyl Induced Resurgence in DBM 3 93 
5 
Treated 
0 
Figure 5. 
Effect of methomyl treatments on 
predatory arthropod populations cap- 
tured in pitfall traps from cabbage field 
in SHES. date of treatment, A: lycosid 
spiders, B: Cicindela japana, C:  Harpa- 
lidae, D: Labidura riparia japonics. 
20 30 10 20 
June June July 
Dates (1983 ) 
Susceptibility of DBM and Pardosa astrigera to methomyl 
P.  astrigera is the most common lycosid in the cabbage field. The LC50 value of 
methomyl was greater to DBM larvae than to P. astrigera (Figure 6) .  The LC50 to 
lycosids was around 10 ppm as against about 7500 ppm for 4th instar DBM larvae using 
the dipping method and about 20,000 ppm for 3rd instar larvae by feeding methomyl- 
contaminated cabbage leaves. Methomyl is generally sprayed at 450 ppm to control 
crucifer pests. 
6 
Log concentrat ion (ppm) 
Figure 6. Concentration-mortality curve of methomyl for the spider, Pardosa 
astrigera and DBM. 
3 94 Nemoto 
Discussion 
One of Ripper’s hypotheses (1 956) that insecticide treatment directly stimulates 
reproductive potential of insects was supported by our data with DBM. It is apparent 
that several factors such as larval and pupal survival rate, adult longevity, oviposition 
period, and fecundity affected the intrinsic rate of natural increase (r  ). Results of our 
precipitation test showed that lycosid spiders played an important role as a biotic 
mortality agent of 3rd to 4th instar larvae of DBM. These predators were also more 
susceptible to methomyl than the DBM larvae. 
Resurgense of DBM is indeed induced by application of methomyl in the cabbage 
fields. This is due in part to stimulation of reproductive potential of DBM and also 
to differential mortality between predators and DBM larvae. The involvement of 
additional factors contributing to resurgence and the relative importance of these factors 
in the induction of resurgence of DBM needs to be investigated. 
Acknowledgement I am indebted to K. Kiritani, Division Head, National Institute 
of Agro-Environmental Sciences, for his valuable suggestions during the course of this 
work and for critical reading of the manuscript. My thanks are also due to Y. Sekijima, 
Professor, Saitama College of Health, for his helpful advice on immunological method. 
I am especially grateful to S. Shibukawa, section chief and H .  Udagawa, Director, SHES, 
for their encouragement. 
Literature Cited 
Birch, L. C.  1948. The intrinsic rate of natural increase of an insect population. J .  Anim. 
Nemoto, H.,  K. Kiritani, and H. Ono. 1984. Enhancement of the intrinsic rate of natural 
increase induced by treatment of the diamondback moth (Plutella xylostella (L.)) with 
sublethal concentrations of methomyl. Jpn. J. Appl. Entomol. Zool. 28: 150-155. 
Nemoto, H., Y.  Sekijima, Y. Fujikura, K .  Kiritani, and S. Shibukawa. 1985. The 
utilization of immunological method for identification of predators of the diamondback 
moth, Plutella xylostella (L.) (Lepidoptera: Yponomeutidae), in the cabbage fields. 
I .  Evaluation of the precipitation reaction, especially the Micro-Ouchterlony method. 
Jpn. J. Appl. Entomol. Zool. 29:61-66. 
Ripper, W. E. 1956. Effect of pesticides on balance of arthropod populations. Annu. 
Rev. Entomol. 6:333-346. 
EcoI. 17:15-26. 


Factors inducing resurgence in the diamondback moth after application of methomyl

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Factors inducing resurgence in the diamondback moth after application of methomyl

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