Bollworm beats the odds against Bt.

A pest insect known as bollworm is the first to evolve resistance in the field to plants modified to produce an insecticide called Bt, according to a research report from the University of Arizona, Tucson. Bt-resistant populations of the bollworm Helicoverpa zea were found in more than a dozen crop fields in Mississippi and Arkansas over the last few years.

"What we're seeing is evolution in action," lead researcher Bruce Tabashnik indicates. 'This is the first documented case of field-evolved resistance to a Bt crop."

Bt crops are so named because they have been genetically altered to produce Bt toxins, which kill some insects. The toxins are produced in nature by the widespread bacterium Bacillus thuringiensis, hence the abbreviation Bt. The bollworm resistance to Bt cotton was discovered when a team of University of Arizona entomologists analyzed published data from monitoring studies of six major caterpillar pests of Bt crops in Australia, China, Spain, and the U.S. The data documenting bollworm resistance first was collected seven years after Bt cotton was introduced in 1996.

"Resistance is a decrease in pest susceptibility that can be measured over human experience," explains Tabashnik, professor and head of the Entomology Department and a specialist on insect resistance to insecticides. "When you use an insecticide to control a pest, some populations eventually evolve resistance."

The researchers note that Bt cot ton and corn have been grown on more than 400,000,000 acres worldwide, "generating one of the largest selections for insect resistance ever known."