Cornell University researchers have identified how cabbage looper caterpillars in the field develop resistance to the bacterium Bacillus thuringiensis (Bt), which naturally occurs in the soil and on plants and has been developed into a successful and widely used biological insecticide.

Organic farmers use Bt as a key weapon against insects, and crops genetically engineered with insecticidal Bt genes are now sown on more than 145 million acres worldwide. When ingested, the insecticidal toxins in Bt kill insects by destroying their guts. Some insects such as cabbage loopers develop resistance to Bt via a genetic mechanism that alters a toxin receptor in the insect's gut, two Cornell University researchers have discovered.

Under normal circumstances, the Bt toxin Cry1Ac, which is a caterpillar-specific toxin, binds to an enzyme called APN 1 along the wall of the insect's gut, where the toxin destroys the gut lining. But when cabbage loopers develop resistance, APN 1 significantly decreases, allowing the insect to properly digest food and Bt without harm.

Ping Wang is an associate professor of entomology at Cornell and senior author of the paper published online in the Aug. 15 issue of the Proceedings of the National Academy of Sciences. Kasorn Tiewsiri, a postdoctoral associate in Wang's lab, is the paper's lead author.

Farmers first reported Bt resistance in the field 20 years ago. Since then, researchers have uncovered a number of mechanisms for resistance in insects in the lab, but then learned that lab insects, which don't face the same stressors as field insects, develop different tactics for overcoming Bt. The researchers hope their studies will lead to new management strategies for Bt-resistant insects.

For more information about this research visit  http://www.news.cornell.edu/stories/Aug11/BtLooper.html.