Insecticide Toxicity to the Soybean Looper and the Velvetbean Caterpillar (Lepidoptera: Noctuidae) as Influenced by Feeding on Resistant Soybean (PI 227687) Leaves and Coumestrol

Abstract

Permethrin, fenvalerate, acephate, methyl parathion, and methomyl were tested on larvae of the velvetbean caterpillar, Anticarsia gemmatalis (Hubner), and the soybean looper, Pseudoplusia includens (Walker), fed on a susceptible (‘Bragg’) or a resistant (PI 227687) soybean cultivar. At LD50, susceptibility of A. gemmatalis larvae to fenvalerate and acephate was significantly enhanced by feeding on the resistant cultivar (1.5 and 1.6 times, respectively). P. includens reared on resistant leaves were also significantly more susceptible to acephate (2 times) than when they were reared on susceptible leaves. The toxicity of the other insecticides examined for each insect was not affected. Incorporation of coumestrol, an isoflavonoid associated with PI 227687 resistance, into a modified artificial diet resulted in significant reductions in weight gain for P. includens larvae. Feeding on a diet amended with coumestrol significantly enhanced the toxicity of fenvalerate (1.5 times) while reducing toxicity of methomyl (2 times) to larvae of P. includens .