Rootworms Resistant to Bt Gain Ground
Western corn rootworms (CRM) resistant to Bt appear to be gaining ground. Initially, resistance concerns were limited to Cry3Bb1; however Aaron Gassman, Extension entomologist, Iowa State University, has since found resistance to mCry3A and reports problems with Cry34/35Ab1. Early reports out of central and east central Illinois suggest the problem may have taken a turn for the worse in that state, while pressure was down in Minnesota hotspots.
We haven’t confirmed resistance to Cry3Bb1 yet in 2013, but with resistance previously confirmed in northwestern Illinois, it looks suspicious,” said Mike Gray, Extension entomologist, University of Illinois. “There we were looking at long-term continuous corn using the same trait without rotation. In Kankakee and Livingston counties, problems with root feeding and lodging were found in first-year corn with Bt traits.”
Doug Flageole and his son David raise corn and soybeans in a 75:25 ratio near the epicenter of the first-year damage. He said traits continue to work; however, some need the aid of a soil-applied insecticide.
“I can drive down the road and see my neighbor’s corn flat on the ground,” he related. “If they didn’t have the right ‘event’ going on, their corn fields are a disaster. I’ve been scouting my fields. I’ve seen no root feeding, and my corn is standing well.”
Flageole credited planting the bulk of his corn acres to Pioneer hybrids with the Herculex Cry34/35Ab1 trait. Acres planted to DeKalb hybrids with the Cry3Bb1 CRW trait, as well as the 20 percent refuge acres in fields planted to Herculex trait hybrids, received a soil application of Force this year. It is a practice Flageole described as “preparation for resistance,” something he will extend to Herculex acres if necessary.
“I keep a close watch on the resistance test trials Pioneer is running,” said Flageole. “I’m concerned problems could develop with Herculex as well, but so far it is showing no resistance, so I have no plans to change.”
PROBLEMS ARE SPREADING
Loss of crop rotation as a viable option is a concern. However, the news that Western CRW resistant to Bt may have developed in another area comes as no surprise to Bruce Tabashnik, population geneticist and head, Department of Entomology, University of Arizona. He advised that the dose rates of Bt traits toxic to corn rootworm make resistance inevitable.
“The immediate problem is Cry3Bb1, but the other toxins have a similar profile in terms of response of the Western corn rootworm,” said Tabashnik. “Unlike the Bt trait for European corn borer, they don’t provide a high dose that kills more than 99.99 percent of the target pest. They kill about 94 percent to 98 percent. Survivors are more likely to be resistant and to mate with other resistant survivors. You’re eliminating nearly all of the susceptibles from the population, but probably not the hybrid off spring (heterozygotes) produced by matings between resistant and susceptible beetles.”
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