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Western bean cutworm will be back this summer, experts say

Western bean cutworm will return to Midwestern cornfields this summer. Exactly where and in what numbers remains to be seen.

"It has established itself in Iowa; it will not go away," says Marlin Rice, Iowa State University Extension entomologist. "It is a permanent resident of our corn now."

While the northwest quarter of Iowa has been hardest hit by western bean cutworm since it became a pest of Iowa in 2000, Rice says Iowa State has documented the pest's presence in nearly all of the state's 99 counties.

And with moths captured in Michigan and Ohio in 2007, it appears western bean cutworm is continuing its march east through the Corn Belt. Last year, significant numbers of moths were trapped across northern Illinois and parts of southwestern Wisconsin, meaning extension entomologists in those states are paying close attention to the pest this year.

"Right now, I wouldn't call it an established pest in Illinois," says Kevin Steffey, University of Illinois extension entomologist. “But it will most likely be one.

"We have watched what has transpired in Iowa the last several years and we want to see if the trend is mirrored in Illinois. We should have expectations for ever-increasing numbers unless something drastic happens to suppress populations."

The theory of pest replacement -- where one insect grows in numbers and replaces a pest that has seen populations fall dramatically -- could be a reason for the growth of western bean cutworm. In this case, European corn borer populations have fallen dramatically with the introduction of in-plant traits.

"I've got a theory. I don't know how good it is yet, but I have to ask myself why western bean cutworm became a pest of Iowa in 2000," Rice says. "We know it was in the state for the previous 30 years, but it was rare.

"What has changed during the years prior to 2000 that would affect these insects, in my mind, is transgenic corn, which decreased European corn borer populations and allowed western bean cutworm, which is not affected by YieldGard Corn Borer, to expand. I have some data that supports that, but I'd hate to say that's the singular reason. We know the insect is affected by disease, and maybe diseases aren’t as severe here as in other states."

Steffey adds: "YieldGard Corn Borer has been planted on a lot of acres and is not effective against western bean cutworm. It's very effective on European corn borer and has some impact on corn earworm, so it may have cleared out a niche for western bean cutworm because its competitors -- European corn borer and corn earworm -- are no longer as prevalent."

In the last several years, the introduction of hybrids containing HERCULEX Insect Protection has provided corn producers with a Bt trait option that protects against western bean cutworm. Rice says they have seen HERCULEX provide 80% to 90% control of western bean cutworm.

"It will allow a little bit of ear-tip feeding, but nothing that causes economic damage," Rice says. "I've sometimes seen identical levels of economic damage between a conventional, non-Bt hybrid and a hybrid with the YieldGard trait."

While it's possible to control western bean cutworm with insecticides, it takes diligent scouting and timely insecticide sprays once threshold levels of western bean cutworm larvae are identified.

"It's not easy to control," Rice says. "You've got to have an estimate of the egg density in a field. The field has to be scouted; you need to know when eggs are starting to hatch, because it's just a matter of days before they crawl from the flag leaf down to the ear. Once they're in the ear, they're home free."

Rice adds that the fact eggs can hatch over a five-week period adds to the complexity of scouting and controlling western bean cutworm with insecticides.

"Scouting and application timing are the two weak links. It can be done, but it's not easy," he says.

When it comes to yield losses, the standard estimate is that one western bean cutworm larvae per plant can cause yield losses of 3.7 bu./A, but it is highly variable. Rice saw yield losses of up to 14 bushels per acre last year, and Nebraska Extension has seen yield losses ranging up to 50%.

"We will have a network of moth traps this summer, which should allow us to keep a handle on populations," Steffey says. "However, people need to crank up their vigilance as to what the moth flights are doing in their area, then get out and scout their fields."

Western bean cutworm will return to Midwestern cornfields this summer. Exactly where and in what numbers remains to be seen.

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