Susceptibility of the Cottonwood Leaf Beetle (Coleoptera: Chrysomelidae) to Different Strains and Transgenic Toxins ofBacillus thuringiensis

Abstract

Populus spp. (which include cottonwoods, aspens, and poplars) are important sources of wood, wood fiber (pulp), and biofuels throughout the world, and are often intensively managed in short rotation stands. The cottonwood leaf beetle, Chrysomela scripta F., is a major pest of Populus throughout North America. It would be difficult to breed insect resistance into these trees using traditional plant breeding techniques because of their long generation time (4–8 yr); however, insect resistance could be produced through genetic engineering. Toxins from Bacillus thuringiensis (Berliner) genes have negligible nontarget effects and are amenable to genetic engineering. We tested the toxicity of 16 B. thuringiensis preparations to identify genes that produce toxins effective against the cottonwood leaf beetle. B. thuringiensis preparations that contained spores were found to be only moderately more virulent than isolated toxins. Strains that produced Cry3A, Cry3B, and Cry8B caused 97–100% mortality in 1st and 2nd instars, with a mean time to death of 1–5 d. Mature larvae were less susceptible to these toxins than were neonates; however, the toxicity of 1 moderately active strain was not affected by larval age. Adults were not as sensitive as larvae to any of these toxins. Cropping with trees genetically engineered to produce B. thuringiensis toxins could lead to the evolution of toxin resistance in cottonwood leaf beetles. We found beetles from 3 different U.S. states varied significantly in their susceptibility to Cry3A. This variation means that regional differences in control levels could occur in the field, and that the potential for evolution of B. thuringiensis- resistance may already exist in some populations of this insect. Therefore, it is very important that before genetically engineered poplars are used extensively, management strategies be developed and implemented to prevent the evolution of resistance to B. thuringiensis in the cottonwood leaf beetle.