A RESEARCHER at Iowa State University is urging Iowa farmers to adopt a diverse range of pest management tactics to suppress the resistance of western corn rootworm to Bt corn.
Bt corn derives its name from Bacillus thuringiensis, a bacterium that produces toxins that kill insects. Those toxin-producing genes are genetically engineered into Bt corn, which allows the corn to fight pests such as the rootworm.
However, starting in 2009, farmers in northeastern Iowa began to report rootworm problems despite the use of Bt corn, according to Aaron Gassmann, an Iowa State assistant professor of entomology.
That year, Gassmann said he confirmed four fields in which rootworms had developed resistance to Bt corn, and resistance problems have persisted in Iowa ever since then.
Between 2010 and 2013, Gassmann said he visited 9-25 fields each year where either Bt resistance was confirmed or where western corn rootworm imposed severe feeding injury to Bt corn.
Gassmann, the author of a recently published article on rootworm resistance in Proceedings of the National Academy of Science, said it's hard to grasp the full extent of the problem because there has been no systematic random sampling of Iowa corn fields for resistance. However, states like Nebraska, Illinois and Minnesota have confirmed cases of resistance to Bt corn, meaning the issue is of growing concern to U.S. corn producers.
Gassmann said Bt corn has become an important tool for pest management because it is more precise and has a smaller environmental impact than pesticides. It also targets other pests besides rootworm, such as the European corn borer.
"It's a convenient and safe technology that should remain part of an integrated pest management system, but the future of the technology depends on how farmers use it," Gassmann said.
He said planting corn on the same acreage season after season — a practice encouraged by high corn prices in recent years — likely accelerated the rootworm's resistance to Bt corn. Around three-quarters of U.S. corn is Bt, and if a rootworm carries the genetic resistance to the toxin, it can pass on that gene through reproduction, according to Gassmann.
Crop rotation, however, breaks the life cycle of rootworms between growing seasons and helps delay the development of resistance, he explained.
In addition, the use of refuges — or sections of fields in which non-Bt corn is planted — helps delay the development of Bt resistance. Gassmann also recommended that farmers make use of "pyramided" Bt corn, which produces more than one toxin poisonous to pests.
Combining those tactics will result in an integrated approach that slows rootworm resistance and allows farmers to maintain the advantages that Bt corn provides, Gassmann said.