Variation in susceptibility of noctuid (Lepidoptera) larvae attacking cotton and soybean to purified endotoxin proteins and commercial formulations of Bacillus thuringiensis

Susceptibility of Heliothis virescens (F.), Helicoverpa tea (Boddie), Pseudoplusia includens (Walker), Spodoptera exigua (Hubner), 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 a virescens, a tea, 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 (LC(50)s) compared with a colony collected from nontransgenic cotton or those from laboratory colonies. Ranges of LC(50)s for field-collected colonies of a virescens generally were similar to the ranges observed for laboratory colonies and similar to those reported in the literature. Wider ranges of variation in LC(50)s were observed among populations of a tea and S. exigua than among populations of the other species. However, the highest LC50 observed for a tea 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 a virescens and less than that of a zea. S. frugiperda was the least susceptible species studied. Variability in LC(50)s obtained with Cry1Ac (r = 0.702) correlated with variability in LC(50)s for Cry1Ab across 13 colonies of a virescens exposed to both proteins. Colonies of a tea and a 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 a ten was selected that had elevated LC(50)s (10 times) after 2 generations and very high LC(50)s compared with a susceptible laboratory strain (100 times) after 8 generations, suggesting that a tea has the genetic capacity to develop resistance to endotoxin proteins.