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

Abstract

Developmental time and behavior of Spodoptera exigua (Hübner) larvae and their pupal weight were investigated in dual-choice arenas containing Bacillus thuringiensis Berliner ( Bt ) toxins. In both artificial diet and cotton leaf-choice tests with Bt formulation MVP (containing delta endotoxins var. Kurstaki), the mean proportion of larvae on Bt -free diets was higher than on Bt -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 the Bt transgenic 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 on Bt -free than on Bt -containing leaves in MVP and ‘C 1076’ choice tests. Leaf-choice tests with the Bt formulations Dipel ES (containing B. thuringiensis var. Kurstaki) and Xentari (containing B. thuringiensis var. Aizawai) and the transgenic Bt line ‘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 only Bt -free diets. However, lower pupal weight and longer developmental time were observed. Experiments with neonate S. exigua larvae 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 to Bt in pest management are discussed.