Factors Inducing Resurgence in the Diamondback Moth After Application of Methomyl

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.