Pests unexpectedly resist biocontrol.

Because there have been few solid reports of insect resistance to microbial insecticides, a presumption has developed that bugs are less likely to become resistant to them than to chemical pesticides, explains William McGaughey, an entomologist at the U.S. Grain Marketing Research Laboratory in Manhattan, Kan. But that presumption is ill founded, his research now indicates. Working with the bacterium Bacillus thuringiensis (BT), the most widely used and intensively studied microbial pesticide, he found that high rates of resistance could develop in less than a year.

He was tipped off to the potential problem last year. As a final stage in research to get BT registered as an approved pesticide for stored grains, McGaughey and his colleagues examined field test results. They found a small but statistically significant decrease in the pesticide's efficacy among populations of Indian meal moth larvae collected from grain stores where BT had been applied.

McGaughey confirmed the finding in the lab: Increased resistance to BT developed in the larvae in just two or three generations -- roughly two or three months. Succeeding generations became increasingly tolerant until resistance hit a plateau of about 100 times the original level at about the 15th generation.

That's about the same magnitude of resistance one might expect to see develop in insects exposed to a chemical pesticide, McGaughey told SCIENCE NEWS, "although similar experiments with chemicals typically take 30 or 40 generations."

Resistance normally develops only after an insect population has continuous contact with a pesticide for many generations, McGaughey says. Since neither BT nor the toxins it produces are stable in sunlight, field pests have, to date, received only intermittent exposure to BT. But with BT use increasing and with genetic engineers attempting to transfer BT's insecticidal traits into plants and field-stable microbes (SN: 12/15/84, p. 373), McGaughey believes a similar resistance to the one he uncovered in dark storage bins could begin surfacing outdoors.