Development and Behavior ofSpodoptera exigua(Lepidoptera: Noctuidae) Larvae in Choice Tests with Food Substrates Containing Toxins ofBacillus thuringiensis

Developmental time and behavior ofSpodoptera exigua(Hübner) larvae and their pupal weight were investigated in dual-choice arenas containingBacillus thuringiensisBerliner (Bt) toxins. In both artificial diet and cotton leaf-choice tests withBtformulation MVP (containing delta endotoxins var. Kurstaki), the mean proportion of larvae onBt-free diets was higher than onBt-containing diets. Artificial diet tests further showed that larvae were more often found on diets where MVP was applied on the diet surface than on diets where the formulation was mixed in the diet. In leaf-choice tests with theBttransgenic cotton line ‘C 1076’ and the nontransgenic ‘C 312’, more larvae were found on nontransgenic cotton leaves. Also, when feeding damage was measured in the leaf experiments, feeding damage was more frequently observed onBt-free than onBt-containing leaves in MVP and ‘C 1076’ choice tests. Leaf-choice tests with theBtformulations Dipel ES (containingB. thuringiensisvar. Kurstaki) and Xentari (containingB. thuringiensisvar. Aizawai) and the transgenicBtline ‘C 531’ showed patterns of larval behavior and feeding damage different than those obtained in the other leaf tests. The survival rate of larvae on MVP was comparable to the larval survival on control tests containing onlyBt-free diets. However, lower pupal weight and longer developmental time were observed. Experiments with neonateS. exigualarvae on MVP-treated cotton plants demonstrated that the number of larvae remaining on the plant was negatively correlated with concentration and exposure time. The consequences and opportunities for behavior adaptation toBtin pest management are discussed.