Greenhouse studies show corn rootworm has an appetite for foxtails, other weeds
GOLDEN VALLEY, Minn. – February 26, 2008 – More farmers are relying on Bt corn to stop rootworms before they cause yield-robbing damage. Rootworms are most vulnerable to Bt toxins during the first instar, when larvae are under 1/8 inch long. But what would happen if insects fed on something else during this stage? Say they chewed on weeds, for instance, and grew large enough to withstand Bt toxins?
A USDA-ARS research team at the University of Missouri tested just such a scenario in greenhouse studies. They found that western corn rootworms don’t like johnsongrass roots, but they will munch on foxtail roots.
Now the researchers are tracking isotopes to determine just how much of the larvae’s diet comes from corn roots versus weed roots. Results may be of particular interest to growers who plant Bt corn for rootworm control and employ total-post herbicide programs.
Von Kaster, entomologist with Syngenta Seeds, says field trials with its
Agrisure® RW corn rootworm trait have demonstrated excellent rootworm control and no apparent correlation between trait performance and early-season weed infestation. Nevertheless, Kaster says we should not underestimate corn rootworm’s ability to overcome control measures.
“We’ve already seen rootworms develop resistance to insecticides and crop rotation,” he says. “It’s an incredibly adaptable and damaging pest, so it’s in our best interest to know as much as we can about its biology and behavior to develop measures like planting of rootworm refuges to protect highly effective Bt technologies.”
In the USDA greenhouse studies, researchers measured adult beetle emergence from larvae feeding on Bt corn roots, weed roots and a combination of the two. Glyphosate herbicide was applied to the combination treatment to mimic a total-post weed control program. More beetles came from the combination than from either alone. In related studies, larval survivorship was relatively high in large crabgrass, giant foxtail, witchgrass, woolly cupgrass and green foxtail.
While these findings clearly demonstrate the potential for rootworm larvae to survive the first instar on grassy weeds, the effect on rootworm control in field trials has been minimal.
“At this point, it looks like the main reason for early-season weed control in corn is still to curb yield-robbing competition for nutrients and moisture,” says Kaster.
The AgriEdge® Corn program from Syngenta encourages early-season weed control in Bt corn fields through incentives to growers who plant hybrids with the Agrisure RW trait. Through its refuge reward, the AgriEdge program also makes it more profitable for growers to manage their refuge acreage with Force® 3G soil insecticide.
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