Effects of Bacillus thuringiensis and Hd-73 Delta-Endotoxin on Growth, Behavior, and Fitness of Susceptible and Toxin-Adapted Strains of Heliothis virescens (Lepidoptera: Noctuidae)

Abstract

A commercial formulation of Bacillus thuringiensis var. kurstaki Berliner (Dipel 2X) and purified HD-73 delta-endotoxin were evaluated for effects on the growth, behavior, and fitness of susceptible and toxin-adapted strains of Heliothis virescens (F.) by incorporation of the materials into artificial diets at varying concentrations. Neonate larvae were placed in experimental arenas where they could choose a diet containing toxin and a control diet, or were placed in arenas where the only diet available contained a toxin. The difference between the strains in growth and survival was much more pronounced when larvae could not choose to feed on the control diet. Both strains avoided moderate and high concentrations of the toxins. At the lowest concentration of HD-73, neither strain avoided the diet that contained the endotoxin. At the two lowest concentrations of Dipel 2X, only the susceptible strain avoided the diet containing Dipel. Data on growth rate, survival, and fecundity of the two strains reared on HD-73 delta-endotoxin were used to set parameters of a Single-locus genetic model to predict the rate at which H. virescens populations would adapt to B. thuringiensis var. kurstaki or its endotoxin when exposed to it in choice and no-choice situations. Results from the model generally predict much more rapid adaptation in no-choice situations.