Variation in Susceptibility of Noctuid (Lepidoptera) Larvae Attacking Cotton and Soybean to Purified Endotoxin Proteins and Commercial Formulations of Bacillus thuringiensis

Abstract

Susceptibility of Heliothis virescens (F.), Helicoverpa zea (Boddie), Pseudoplusia includens (Walker), Spodoptera exigua (Hübner), and Spodoptera frugiperda (J. E. Smith) to purified endotoxins and commercial formulations of Bacillus thuringiensis Berliner was measured in a wide range of colonies collected from 8 states in the U.S. Cotton Belt during 1992 and 1993. Results indicated that field-collected colonies of H. virescens, H. zea, P. includens, and S. frugiperda were as susceptible as laboratory-reared colonies and those reported in the literature to the purified endotoxin proteins Cry1Ac and Cry1Ab or the commercial formulations Javelin WG, Dipel ES, and Condor OF in diet-treated assays. Colonies of S. exigua collected from transgenic cotton expressing endotoxin protein had elevated median lethal concentrations (LC₅₀s) compared with a colony collected from nontransgenic cotton or those from laboratory colonies. Ranges of LC₅₀s for field-collected colonies of H. virescens generally were similar to the ranges observed for laboratory colonies and similar to those reported in the literature. Wider ranges of variation in LC₅₀s were observed among populations of H. zea and S. exigua than among populations of the other species. However, the highest LC₅₀ observed for H. zea was no higher than those reported in the literature. Only a few colonies of P. includens and S. exigua were tested. P. includens susceptibility was generally greater than that of H. virescens and less than that of H. zea. S. frugiperda was the least susceptible species studied. Variability in LC₅₀s obtained with Cry1Ac (r = 0.702) correlated with variability in LC₅₀s for Cry1Ab across 13 colonies of H. virescens exposed to both proteins. Colonies of H. zea and H. virescens were pooled into single colonies and selected with the insecticidal proteins to produce endotoxin-resistant strains. Selection for resistance in H. virescens was not successful, but a strain of H. zea was selected that had elevated LC₅₀s (10 times) after 2 generations and very high LC₅₀s compared with a susceptible laboratory strain (100 times) after 8 generations, suggesting that H. zea has the genetic capacity to develop resistance to endotoxin proteins.