Agriculture online
06-22-07
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."