Missing Enzyme Linked to Bt-Resistance in Insects

The absence of an enzyme from the gut of certain Indianmeal moths makes those stored grain pests resistant to the powerful biological control agent Bacillus thuringiensis (Bt), Agricultural Research Service scientists report.

ARS researchers at the Grain Marketing and Production Research Center in Manhattan, Kan., have found that two strains of Indianmeal moths lack an enzyme that converts Bt protoxins to activated toxins that kill insects.

In laboratory studies, scientists demonstrated that insects lacking the enzyme survive when fed Bt diets. The scientists also showed that loss of the enzyme is genetically linked to insect resistance to Bt.

Bt, a soil bacterium, has been used for 35 years as a safe and effective alternative to chemical insecticides for controlling moth pests. Its use either as a spray or through genetic insertion into crops doesn't harm humans, animals, beneficial insects or other crops. But insect resistance to Bt is sometimes reported in areas where farmers use Bt extensively for insect control.

Bt transgenic plants were first used by farmers in 1996 and crop yields were excellent. This success may lead to increased planting of transgenic crops, which will lead to increased insect exposure to Bt. Understanding how insects cease responding to Bt toxins may help to prolong the use of this safe insecticide.

The researchers are now studying 35 other strains of the Indianmeal moth for the presence or absence of the enzyme. They are seeking industry collaborators to help monitor for resistance in insects exposed to Bt transgenic crops and to search for a similar "missing enzyme" phenomenon in other insect pests, such as corn earworms and cotton bollworms.

Scientific Contact: Brenda Oppert, ARS-USDA, Grain Marketing and Production Research Center, 1515 College Avenue, Manhattan, KS 66502. Telephone: (785) 776-2780; fax: (785) 537-5584, bso@usgmrl.ksu.edu.