Some caterpillars are cottoning to transgenic cotton.

Genetically engineered cotton and corn produce a toxin that kills caterpillar larvae and other pests, but a new study shows that resistance to this toxin could be spreading among one species of caterpillar.

Farmers worldwide plant more than 400 million acres of these transgenic crops each year. A bacterial gene inserted into the plants' DNA enables the crops—called Bt crops—to kill insects without sprayed pesticides.

But killing vulnerable caterpillars can drive the evolution of resistance to the toxin, since only the survivors reproduce.

To keep resistance in check, farmers plant refuges of unaltered crops for the pests to eat. That way, caterpillars susceptible to the toxin may mate with the few individuals that have developed resistance. Offspring from these mixed matings are usually vulnerable to the Bt crops.

The strategy is likely working with five caterpillar species observed between 1992 and 2004 in Spain, Australia, China, and the United States, according to a paper in the February Nature Biotechnology. "There's lots of solid evidence that resistance is not evolving in those pests," says study leader Bruce E. Tabashnik of the University of Arizona in Tucson. But for one species (Helicoverpa zea), resistance has become more widespread. The reason, in part, is that mixed matings among H. zea produce toxin-resistant offspring—progeny that can pass that resistance on.

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