The influence of resistant soybean (PI 227687) foliage and coumestrol on the metabolism of xenobiotics by the soybean looper, Pseudoplusia includens (Walker)

Abstract

The metabolic activity toward several xenobiotics was examined in larvae of the soybean looper, Pseudoplusia includens (Walker), reared on susceptible (Bragg) or resistant (PI 227687) soybean cultivars. Midgut homogenates from last instar soybean loopers reared on PI 227687 leaves throughout larval life had greater glutathione transferase (1.5-fold) and monooxygenase (2-fold) activity than those reared on Bragg leaves. Minor metabolic differences between larvae fed the two soybean cultivars were observed for NADPH cytochrome c reductase activity and the hydrolysis of p-nitrophenyl acetate, α-naphthyl acetate, and acephate. In contrast, there was no increase in the glutathione transferase activity in last instar larvae fed PI 227687 foliage in a short term (48 hr) feeding regime. Monooxygenase induction was also less than previously observed. In addition, small decreases in p-nitrophenyl acetate and α-naphthyl acetate activities were associated with short term feeding on PI 227687 foliage. The incorporation of coumestrol, an allelochemical associated with insect resistance in PI 227687 foliage, into the larval artificial diet significantly increased glutathione transferase activity. None of the other enzymes assayed were affected by the coumestrol-amended diet. The increase in glutathione transferase activity for larvae fed the coumestrol-amended diet relative to the control diet was similar to that observed when larvae were fed PI 227687 foliage versus Bragg foliage. Results of this study provide further evidence of coumestrol involvement in PI 227687 resistance as well as suggesting that other factors are also involved.