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Why Western corn rootworm is so tough to control

Why Western corn rootworm is so tough to control
This native insect is so well adapted and genetically wired to overcome Bt that it will make you pay if you cut corners on refuges or rotations.

While Bt hybrids have worked well in controlling European corn borer, they have failed to consistently control Western corn rootworm.

Related: This iPad app helps you prep for corn rootworm risks

Tom Hoegemeyer, retired University of Nebraska agronomy professor and a corn breeder with Hoegemeyer Hybrids, lists a number of the reasons why:

•WCR is a native insect that is well adapted with a huge genetic pool of variability, compared to the less variable ECB that was transplanted to the U.S.

•Refuges of non-Bt corn haven't worked as effectively, partly because too few farmers plant refuges or the plant the required number of acres of refuges.

Western corn rootworm feed on an ear of corn. Photo: Purdue University

•Two to 20% of the insects that get a dose of Bt don't die

•There's a lot of continuous corn planted in the Western Cornbelt. This increases the WCR population.

•Mating between resistant and susceptible WCR isn't completely random because not enough susceptible beetles get into Bt corn fields.

•Resistance to Bt toxins is not a recessive genes. "It actually might be partially dominant," Hoegemeyer says. "We're seeing this at a scary rate."

•There isn't a significant "fitness cost" incurred by resistant insects, causing them to have a lower survival rate and fewer offspring than susceptible beetles. "Actually, they have a higher survival rate on Bt corn and do better on Bt corn than on non-Bt corn," Hoegemeyer says. Also, resistant WCR beetles are not more susceptible to natural enemies such as fungi or nematodes.

Crop rotation is the most effective tactic to use to control WCR beetles Hoegemeyer says.

"Rotate your crops and rotate your chemistries."

He also suggests planting Bt hybrids with a single, effective protein or a pyramid of multiple proteins. Scout corn and use a liquid or granular insecticide at planting time for larval control and post emergence treatments for larval and adult control when necessary. Follow protocol for non-Bt refuge plantings and other management tools.

"We will never control WCR with a single trait or insecticide," Hoegemeyer says. "Multiple tactics are the key to slowing down WCR."

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