Bacillus thuringiensis' effectiveness as a natural pesticide against crop-damaging caterpillars is the aim of USDA research studies in Stoneville, Miss.

There, at the Agricultural Research Service's Southern Insect Management Research Unit, Dick Hardee and Doug Sumerford are monitoring the ability of tobacco budworms and bollworms to feed and grow on artificial diets containing insecticidal Bt proteins used in transgenic cotton. In U.S. cotton crops, these caterpillar pests cause up to $300 million annually in damages and control costs.

Since 1996, cotton varieties engineered with Bt genes for making the protein have offered growers a way to reduce insecticides. Bt cotton is now grown on 2 million-plus U.S. acres. Because of such extensive plantings of cotton and other Bt crops, there is concern that natural selection will favor insects having traits for Bt resistance.

The researchers have sought to quantify this in budworms and bollworms by obtaining caterpillar specimens from cotton fields across the nation and rearing them to adulthood on Bt-free diets. Once they've mated, the insects' offspring are reared on Bt diets and compared with the lab's control colonies. This allows scientists to check the insects' Bt tolerance levels. The more tolerant a caterpillar is, for example, the faster it grows on the Bt diet and the bigger it gets. A sensitive assay also enables scientists to compare genetic differences among Bt-fed insects, based on these physiological cues.

More information can be found in USDA's online magazine at: Agricultural Research.