Digestive and detoxification enzyme activity and nutrient composition were examined in the body of fourth instar beet armyworms,Spodoptera exigua(Hübner),fed on transgenic Bacillus thuringiensis(Bt) and non-Bt cotton for different time periods.Nutrient composition and specific enzyme activities differed significantly between the S.exigua fed Bt vs.non-Bt cotton.At 1,6 and 24 h,free fatty acid and glucose levels were significantly lower in S.exigua fed on Bt cotton than those fed on non-Bt cotton.S.exigua fed on Bt cotton had significantly higher trypsin and total superoxide dismutase(T-SOD) activities and significantly lower lipase,carboxylesterase and acetylcholinesterase activities than non-Bt fed worms for all feeding time periods.Differences were also observed among feeding times within each cotton variety group.Significantly lower free fatty acid and total amino acid were observed in S.exigua fed on Bt cotton for 24 h than in those fed for 1 h.Significantly lower activities of lipase and trypsin were detected in S.exigua fed on Bt cotton for 24 h than those for 1 and 4 h.However,carboxylesterase and acetylcholinesterase activities in S.exigua fed on Bt cotton for 24 h were significantly higher than those for 1,4 and 6 h.The interaction between cotton variety and feeding time significantly affected the activities of lipase,trypsin,acetylcholinesterase and T-SOD enzymes in S.exigua.Measuring the temporal allocation of protection and detoxification enzyme activities in the body of S.exigua in response to B.thuringiensis can provide a meaningful evaluation on the metabolic tolerance of herbivorous insects under the continuous selection pressure of a toxic protein.
The nutrient composition and enzyme activities in larvae of the beet armyworm, Spodoptera exigua (Hbner), fed on high, medium or low gossypol cotton cultivars were examined at different time intervals. Significantly lower free fatty acid was observed in larvae fed for 6 h on high gossypol 'M9101' compared to larvae fed on the low (ZMS13) and intermediate (HZ401) gossypol cultivars. Significantly higher trypsin activity was observed in larvae fed on high gossypol 'M9101' for 24 h compared to those fed for 1, 4 and 6 h. Significantly higher catalase and total superoxide dismutase enzyme activities were observed in larvae of S. exigua fed on high gossypol 'M9101' compared with low gossypol cultivars 'ZMS13' and 'HZ401' for 1, 4, 6 and 24 h. However, significantly lower carboxylesterase and acetylcholinesterase enzyme activities were found in larvae fed on high gossypol 'M9101' compared with the other cultivars for 1, 4, 6 and 24 h. The interaction between cotton variety and beet armyworm infestation time significantly affected the carboxylesterase enzyme activity in S. exigua. The characterization of the effects of plant allelochemicals on herbivorous larvae is important for aiding understanding of plant-insect interaction as well as in devising solutions to pest problems by breeding plant resistance, identifying metabolic targets for insecticide development, etc.
