Biochemical and Physiological Mechanisms Involved in Insecticide Toxicity and Host Plant Resistance to the Soybean Looper, Pseudoplusia Includens (Walker).

Abstract

Permethrin, fenvalerate, methomyl, acephate and methyl parathion were bioassayed on larvae of the soybean looper, Pseudoplusia includens (Walker) and the velvetbean caterpillar, Anticarsia gemmatalis (Hubner) fed on susceptible (Bragg) versus resistant (PI 227687) soybean, Glycine max (L.) Merr., cultivars. Soybean loopers reared on resistant leaves were significantly more susceptible to acephate (2-fold) than when reared on susceptible leaves. Velvetbean caterpillars fed resistant leaves were more susceptible to both fenvalerate (1.5-fold) and acephate (1.6-fold) than when fed susceptible leaves. Differences in toxicity of other insecticides due to host plant were not observed. Midgut homogenates from last instar soybean loopers reared on resistant leaves throughout larval life had significantly greater glutathione S-transferase (1.5-fold) and monooxygenase (2-fold) activity than those reared on susceptible leaves. Minor metabolic differences were observed for NADPH cytochrome c reductase and hydrolysis of p-nitrophenyl acetate, alpha-naphthyl acetate and acephate. Soybean loopers fed resistant versus susceptible foliage in short term feeding assays had smaller increases in monooxygenase activity (1.5-fold) than were observed in larvae provided leaves throughout larval life. No increase in glutathione S-transferase activity was observed. Significant decreases in hydrolysis of alpha naphthyl acetate and p-nitrophenyl acetate were also observed for larvae fed resistant versus susceptible leaves. Coumestrol, an isoflavonoid associated with resistant leaves, amended into artificial diet resulted in weight reductions of soybean looper larvae. Soybean loopers fed coumestrol-amended diet were less tolerant of fenvalerate (1.5-fold) and more tolerant to methomyl (2-fold). Midgut homogenates of larvae fed coumestrol-amended diet had significant increases in glutathione S-transferase activity. These results suggest that coumestrol is involved in PI 227687 resistance, but also indicates involvement of other factors. Soybean loopers suspected of permethrin resistance were less susceptible to knockdown by permethrin than a laboratory population. They also had increased levels of glutathione S-transferase (2.7-fold), monooxygenase (1.8-fold), and hydrolase (alpha naphthyl acetate (1.5-fold), p-nitrophenyl acetate (1.5-fold), permethrin (1.5-fold)) activities than susceptible larvae. The observed resistance is therefore due to a combination of knockdown resistance and increased metabolic activity.