Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 14, 1999
Publication Date: February 20, 1999
Citation: James, R.R., Croft, B.A., Strauss, S.H. 1999. Susceptibility of the cottonwood leaf beetle (coleoptera: chrysomelidae) todifferent strains and transgenic toxins of bacillus thuringiensis. Environmental Entomology 28:108-115.
Interpretive Summary: Hybrid poplar trees are grown in plantations as a source of wood, fiber (pulp) and fuel. Trees are harvested and replanted every 5-7 years. The cottonwood leaf beetle feeds on poplar leaves and is a serious pest. In a search for an alternative to conventional pesticides for controlling this insect we tested a biological pesticide. Bacillus thuringiensis (Bt) is a bacterium that produces toxins which affect only certain insects and have little or no toxicity to birds and other animals. In laboratory bioassays, we tested the effect of different strains of Bt on the survival and development of all stages of the cottonwood leaf beetle. We also tested toxins produced by genes that had been isolated from Bt and transferred to non-pathogenic bacteria. Three toxins were found to be highly toxic to immature insects, but none were effective against adults. Plans are now underway to transfer to Bt toxin gene into poplar to produce an insect-resistant tree. Resistant trees would greatly reduce the need for pesticide applications in poplar plantations, which means reducing growers' production costs and the environmental and health risks associated with pesticides. Cropping with trees that continuously produce a toxin could result in pest populations that are no longer susceptible to the toxin, that is, the insects become resistant. In laboratory assays, we tested the effect of toxin on immature beetles from three different regions of the U.S. Immatures from one region were less sensitive than ones from other regions. This variation means that insecticidal trees may not be equally effective in all regions, and that the potential for this insect to become resistant may already exist. It is very important that crop management strategies be developed and implemented to prevent resistance to Bt.
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 Fabricus, Coleoptera: Chrysomelidae) is a major pest of Populas throughout N. America. Insect-resistant poplars could be developed through genetic engineering. Toxins from Bacillus thuringiensis genes have negligible non-target effects and are amenable to genetic engineering. We tested the toxicity of 16 B. thuringiensis (Berliner) preparations to identify genes which produce toxins effective against the cottonwood leaf beetle. B. thuringiensis preparations which contained spores were found to be only moderately more virulent than isolated toxins. Strains which produced Cry3A, Cry3B and Cry8A caused 97-100% mortality in 1st and 2nd instars, with a mean time to death of 1-5 d. Mature larvae were eless susceptible to these toxins than were neonates; however, the toxicity of one moderately active strain was not affected by larval age. Adults were not as sensitive as larvae to any of these toxins. Cropping with trees that continuously produce B. thuringiensis toxins could select for pest populations with toxin-resistance. We found beetles from three 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 the cottonwood leaf beetle. It is very important that management strategies be developed and implemented to prevent the evolution of resistance to B. thuringiensis in the cottonwood leaf beetle.