Bags of corn seed that mix genetically modified hybrids with and without Bt toxins that kill insects provide farmers easier compliance with federal regulations but could, over time, hasten insect resistance to Bt, a Purdue University entomologist says. Although "refuge-in-a-bag" seed technology has been approved for use by the U.S. Environmental Protection Agency, questions still remain over its long-term effect on corn rootworms, the main pest targeted by the technology, says Purdue's Christian Krupke.
"Is a guarantee of 100% grower compliance with refuge regulations for corn rootworm worth a bit of a risk in terms of resistance development?" he asks. "For many the answer is yes, as compliance has been declining in recent years."
Under EPA rules, farmers who plant Bt corn also must plant next to or around that corn with non-Bt hybrids equaling 20% of the Bt acreage. But with refuge-in-a-bag, farmers plant all the seed together. Refuge corn is interspersed in the field with Bt corn. Refuge-in-a-bag products contain 90% Bt corn seed with 10% non-Bt "refuge" seed.
Farmers this year still must plant a separate 20% refuge for corn borer
The new seed technology covers only below-ground feeding rootworms at this time, however. Farmers growing Bt corn still need to plant a separate 20% refuge for corn borers, but that will change next year with a new product offering refuge-in-a-bag for all corn pests.
Biologically speaking, refuge corn works in concert with Bt corn, which is genetically modified to express insecticide proteins from the Bacillus thuringiensis bacterium, to maintain the durability of the genetic traits in controlling many corn-feeding insects.
"In the absence of refuge corn, any insects that survive exposure to Bt corn would mate with one another and pass along the genetic traits that helped them survive," says Krupke. "Refuge corn dilutes these genes with susceptible ones from beetles that fed on non-Bt corn and, therefore, should be susceptible."
Refuge-in-a-bag doesn't completely remove risk of resistant insects
Refuge-in-a-bag does not completely remove the risk of Bt-immune insect populations.
"The concern with refuge-in-a-bag, or seed mixes, has always been sub-lethal exposure with toxic plants and non-toxic plants standing side-by-side," he says. "You could have a young corn rootworm beetle larva emerge, feed on a toxic plant but not die, and then move over to a non-toxic plant and feed until reaching adulthood. The larva now has sub-lethal exposure to Bt. That's one of the ways that resistance can develop in an insect population more rapidly."
He adds, "It's that old adage that whatever doesn't kill you makes you stronger. We could be giving those larvae selective advantage in the long term. That was one of the reasons this technology wasn't embraced initially."
Sub-lethal exposure works the other way, as well, Krupke says. Larvae could feed on a refuge corn plant and become larger, then move to a Bt plant to continue eating. Because the larvae are larger and it takes more Bt toxin to kill bigger insects, larvae might not ingest enough toxin to die, he says.
On the other hand, refuge-in-a-bag has definite advantages
On the other side of the issue, refuge-in-a-bag offers advantages to the traditional Bt/refuge planting system, Krupke says. "Compliance is unquestionably the main advantage," he points out. "There's no doubt about it. This way a grower can dump the seed into the planter and go. There's no changing out seed or calculating refuge acres.
"One of the other advantages with having Bt and refuge plants mixed together in a field is that you get the rootworm beetles closer together, which facilitates matings between beetles that might be Bt-resistant and those that are susceptible from the refuge."
By doing some of the matchmaking work for resistant and non-resistant insects, the ratio of Bt to non-Bt seed in refuge-in-a-bag products can be reduced to 9-to-1, says Krupke. Refuge-in-a-bag is available on a limited basis this crop season, with more seed brands expected to add the technology in coming years. About 65% of the corn grown in the United States is Bt hybrids.