The induction by dietary nicotine of a series of cytochrome P-450 enzyme activities was investigated in early fifth-instarManduca sexta larvae. At a low nicotine concentration in the diet (0.1 %), three of 12 midgut microsomal enzyme activities were significantly increased. At a higher concentration (0.75%) commonly found in plants of the genusNicotiana, nine of 12 activities were induced by 1.4- to 10.0-fold. Total cytochrome P-450, P-450 reductase activity, and midgut microsomal metabolism of nicotine were also increased by feeding 0.75% nicotine. Nicotine was metabolized by midgut microsomes to nicotine-1-N-oxide and cotinine-N-oxide. Fat body microsomal nicotine metabolism was low and unaffected by dietary nicotine. Isolated nerve cords were able to metabolize nicotine in vitro but this metabolism was not inducible by dietary nicotine. Nicotine-fed fifth-instarM. sexta larvae showed an increased tolerance to subsequent nicotine injection when compared to larvae fed a control diet. These results support the idea that induction of midgut cytochrome P-450-related metabolism is an adaptation ofManduca sexta to dietary nicotine.

Five insecticide synergists, all of which were either methylenedioxyphenyl compounds or analogs, were compared as to their effect on cytochrome P450 monooxygenase induction caused by an allelochemical in fall armyworm larvae. Feeding the synergists (piperonyl butoxide, safrole, isosafrole, MCK 264, and myristicin) individually to the larvae caused decreases in the microsomal aldrin epoxidase activities ranging from 38% to 74% when compared with controls. Feeding indole-3-carbinol resulted in a 4-fold increase in the rnicrosomal epoxidase activity. However, cotreatment of any of the synergists and the inducer completely eliminated the induction. Sixth instar larvae were more inducible than second instar larvae with respect to microsomal epoxidase and glutathione transferase in the fall armyworm. Enzyme inducibility varied widely among the seven phytophagous Lepidoptera examined. When indole-3-carbinol was used as an inducer of microsomal epoxidase, the extent of inducibility of the enzyme was fall armyworm > velvetbean caterpillar > corn earworm > beet armyworm > tobacco budworm > cabbage looper > diamondback moth. When indole-3-acetonitrile was used as an inducer, the inducibility of glutathione transferase was fall armyworm > beet armyworm > corn earworm > cabbage looper > velvetbean caterpillar > tobacco budworm > diamondback moth. lnducibilityoffive microsomal oxidase systems also varied considerably in the corn earworm, indicating the multiplicity of cytochrome P450 in this species. Microsomal epoxidase and glutathione transferase were induced by cruciferous host plants such as cabbage and their allelochemicals in diamondback moth larvae. o 1993 WiIey-Liss, Inr

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