Susceptibility of the cottonwood leaf beetle (Coleoptera : Chrysomelidae) to different strains and transgenic toxins of Bacillus thuringiensis

Populus spp. (which include cottonwoods, aspens, and poplars) are important sources of wood, wood filer (pulp), and biofuels throughout the world, and are often intensively managed in short rotation stands. The cottonwood lea f beetle, Chrysomela scripta F., is a major pest of Populus throughout Nort h America. It would be difficult to breed insect resistance into these tree s using traditional plant breeding techniques because of their long generat ion time (4-8 yr); however, insect resistance could be produced through gen etic engineering. Toxins from Bacillus thuringiensis (Berliner) genes have negligible nontarget effects and are amenable to genetic engineering. We te sted the toxicity of 16 B. thuringiensis preparations to identify genes tha t produce toxins effective against the cottonwood leaf beetle. B. thuringie nsis preparations that contained spores were found to be only moderately mo re virulent than isolated toxins. Strains that produced Cry3A, Cry3B, and C ry8B caused 97-100% mortality in 1st and 2nd instars, with a mean time to d eath of 1-5 d. Mature larvae were less susceptible to these toxins than wer e neonates; however, the toxicity of 1 moderately active strain was not aff ected Ly larval age. Adults were not as sensitive as larvae to any of these toxins. Cropping with trees genetically engineered to produce B. thuringie nsis toxins could lead to the evolution of toxin resistance in cottonwood l eaf beetles, We found beetles from 3 different U.S. states varied significa ntly in their susceptibility to Cry3A. This variation means that regional d ifferences in control levels could occur in the field, and that the potenti al for evolution of B. thuringiensis-resistance may already exist in some p opulations of this insect. Therefore, it is very important that before gene tically engineered poplars are used extensively, management strategies be d eveloped and implemented to prevent the evolution of resistance to B, thuri ngiensis in the cottonwood leaf beetle.