Resistance confirmed in Illinois, feared elsewhere
What Mike Gray suspected for more than a year he has now confirmed: Field-evolved western corn rootworm resistance to the YieldGard Bt technology is present in some corn farmers’ fields in Henry and Whiteside counties in northwest Illinois. Gray, a University of Illinois Extension entomologist, collaborated with Aaron Gassmann of Iowa State University to make the determination.
"If you’re in an area where a resistant population exists to the Cry3Bb1 protein, expressed in some Bt hybrids, there are good odds that single protein won’t provide adequate rootworm control in 2013," Gray says.
Tough-to-control rootworm populations are also present in parts of Minnesota, according to Bruce Potter, University of Minnesota pest management specialist. Janet Knodel, North Dakota State University Extension entomologist, is also concerned about some rootworm populations in her state. Neither scientist, though, has confirmed resistant populations in their respective states.
Gray recommends that farmers who now have resistant rootworm populations—or fear that they might—rotate to soybeans in 2013.
"I’m telling growers to mix up their control options next spring—pick hybrids with a different trait or series of traits [pyramid], or use a non-Bt hybrid and protect that investment with soil insecticide," Gray says.
Farmers who opted to plant corn hybrids with two Bt proteins this past spring saw good rootworm protection in their fields throughout the season, according to Monsanto Company and Dow AgroSciences. The companies report that their SmartStax hybrids, which include the Cry3Bb1 and Cry34/35Ab1 proteins, performed well this season despite heavy rootworm pressure. The high populations were a result of the mild 2011 winter, early planting and favorable soil conditions during larval hatch.
Doug Heatwole, customer agronomist for Mycogen Seeds, says some of his customers found lodged corn plants in fields but only in their refuge acres. Under heavy rootworm pressure, the refuge plants had a rating of 2.5 on the Iowa State University Node-Injury Scale of 0 to 3. At a rating of 0.25, yield loss can occur; at 1.0, lodging is more likely. A rating of 2.5 means 2½ nodes were consumed and significant yield loss is likely, especially under hot and dry conditions.
However, Heatwole says, the average rating for the SmartStax plants was 0.015, indicating minimal feeding scars to the roots and excellent protection.
Gray agrees that SmartStax hybrids held up well against rootworm across the Corn Belt.
"I’ve also told producers that [their use] may be shortsighted," he contends. "You may get through a couple more seasons, but you’re putting more pressure on the single effective Cry34/35Ab1 protein, and now at a reduced refuge percentage. The long-term durability of this protein may be compromised."
Entomologists Fred Gould of North Carolina State University and Bruce Tabashnik at the University of Arizona, want significant reform in the use of refuge acres. To protect Bt efficacy, they would like the Environmental Protection Agency to increase refuge requirements to 50% for
hybrids with one Bt toxin and 20% for hybrids with two toxins.
Root of the problem. Gray points out that rootworm resistance is the result of more than a lack of adequate refuge compliance. "It’s a more complicated problem than just that," he notes.
He explains that some combination of the following four factors contribute most often to rootworm resistance: continuous corn; the low-dose control efficacy of Bt rootworm traits; a lack of rotating those traits; and inadequate refuge compliance.
Farmers who address these key factors effectively and implement best management practices, he adds, stand the best chance of keeping rootworm at bay in their corn fields.