Study finds options to slow resistance to Bt corn

One of the biggest fears associated with genetically engineered crops is the prospect that insect pests will evolve resistance to Bacillus thuringiensis (Bt) toxin, the highly effective natural insecticide built into such crop staples as corn and cotton, and widely used in organic-based agriculture and gardening.

To slow resistance to the Bt toxin and preserve the utility of the most potent natural insecticide available, the federal government imposed a tactic known as the high-dose refuge strategy. Farmers planting engineered Bt corn or cotton must plant a certain percentage of their fields with non-engineered varieties, typically 20 percent for Bt corn. The idea is to provide a refuge for some insect pests and the genes that make them susceptible to the Bt toxin.

But according to a study published in the current issue (Nov. 1, 2002) of the journal Ecology Letters, the high-dose refuge strategy could be altered to permit the normal application of pesticides to crop refuges without risking the overall resistance prevention strategy. Further-more, clarifying the consequences of where refuges are located relative to Bt fields may, according to UW–Madison Zoologist Anthony R. Ives, lead to better resistance control than under the current strategy.

“The high-dose refuge strategy works,” says Ives, a co-author of the study. “Our suggestion is that it can be made more attractive by allowing farmers to spray some insecticides in the refuges and providing farmers with more flexibility.”

The study, which focuses on the effects of the strategy on the European corn borer, was funded by the U.S. Department of Agriculture and co-authored by David A. Andow, an entomologist and affiliate of the Center for Community Genetics at the University of Minnesota.

Because crops engineered to express the Bt toxin are highly effective, killing almost all of the corn borers in a field, there is a general fear with the advent of the technology that insect pests, which have shown a remarkable ability to evolve resistance to chemical and natural pesticides, would quickly render Bt useless — unless steps were taken to preserve the genes that make insect pests vulnerable to the Bt toxin.

The upshot has been a mandatory refuge strategy, which requires farmers to grow crops for insect pests in the hope of delaying genetic resistance to the toxin by preserving insects that retain their genetic vulnerability to Bt. Insects from the refuges, it was believed, will mate with survivors from the fields containing Bt corn, pass along the genes that make them vulnerable to the toxin, and thereby help ensure future generations’ vulnerability to the Bt insecticide.

The refuge strategy was thought to depend on large numbers of insects feeding in the refuges to ensure that insects with the genes that make them vulnerable to Bt would mate with the surviving — and possibly resistant — insects in Bt fields.

But, the study found, very few insects survive in fields where engineered corn is planted, and large numbers of insects from untreated fields are not necessary to help keep resistance to Bt at bay.

“What’s important is the fraction of the insect population killed by Bt,” says Ives, “not the total number of insects killed. The refuge has to be there, but you don’t necessarily need large numbers of animals to delay resistance.”

In fact, very high dispersal of insect pests from untreated fields could contribute to faster evolution of resistance as the genes that may confer resistance are present in any population and can be carried from a nearby refuge to fields where Bt is deployed in the crop, according to Ives and Andow.

“While it is important for refuges to be close enough that some pests disperse to Bt fields, closer is not always better,” Andow says.

The authors suggest that current restriction on spraying the refuge with insecticides is based on an incomplete understanding of the interplay of insects in the wild and how the refuge system actually works. Moreover, an improved strategy that permits farmers more flexibility in treating refuges with insecticides and provides clearer guidance for locating refuges on a farm will bolster the refuge strategy by making it more convenient for farmers.

“The high-dose refuge strategy can be implemented in a more effective way and a more friendly way,” Ives says. “If you can make it easier for farmers, they are more likely to use it.”

A better basic understanding of the refuge strategy and the key factors that underpin it, Ives argued, are critical to preserving the effectiveness of Bt. Corn with the Bt toxin engineered into it is now planted on 19 million acres in the United States, representing 24 percent of the corn crop. Maintaining the effectiveness of Bt, Ives said, is necessary as effective natural alternatives have yet to be developed.