Nature's own insecticides_natural bacteria that are toxic to insects, for example_have long been touted as superior to chemical insecticides because it was assumed that bugs would not develop resistance to them.

A scientist at the Department of Agriculture has recently proven that assumption wrong. He has shown that a major pest of stored grain, the Indian meal moth, can become resistant to BT, the most widely used biological insecticide and the main one used to protect stored grain. The moth can become resistant to BT after only four generations of exposure.

BT is the common name for the bacterial species bacillus thuringiensis, a natural species that produces substances toxic to many insects but not to other animals. BT is grown in factories and sold as a mixture of dried toxin and dormant bacterial spores that begin reproducing if sprayed into a moist environment.

Previous tests of BT resistance relied on inbred laboratory strains of insects, which did not become resistant. William H. McGaughey of USDA's Grain Marketing Research Laboratory in Manhattan, Kan., tested wild strains of insects and found the opposite.

He fed a colony of the moths' larvae food laced with BT. Only 19 percent survived. When they reproduced and ate the same diet, 44 percent survived. In the third generation, survival went up to 63 percent. Of the fourth generation, 82 percent survived -- roughly the survival rate among meal moths given no insecticide at all.