Cold winter weather will probably not slow western bean cutworm

Early instars of the western bean cutworm (left) are lacking the identifying black rectangles that develop with older larvae (right). Photo by J. Obermeyer at Purdue.

Corn farmers who might have hoped that a new insect threat would be slowed by this winter’s frigid temperatures could be disappointed, says a Purdue University Extension entomologist.

The western bean cutworm is likely to emerge from winter in numbers capable of exacting a toll on the corn crop this summer, said Christian Krupke.

“A question I’ve gotten a lot from farmers is, with the colder-than-average winter will we have a lot of mortality of the overwintering larvae?” Krupke said. “The answer is probably not. That’s not because of the temperature of the air; it’s more because we’ve had so much snow and relatively few days without snow.”

Snow cover insulates crop fields and “keeps the temperature in the soil higher than it would be if the soil were bare, which actually helps the larvae survive,” he said.

Fortunately, timely scouting of fields, insecticide treatments and some biotech (Bt) corn varieties have proved successful in controlling the bug.

Western bean cutworm (WBC) caterpillars feed on pollen and, if not controlled, the corn ear itself. That process begins after the female moths lay eggs on corn leaves about a week before corn reaches the pollination stage.

The insect was first detected in Indiana in 2006 after migrating from western Corn Belt states. Crop damage reached a peak this past year, with the most severe cases occurring in northwestern Indiana counties. WBC is most common in continuous corn and corn grown in sandy soils and no-till cropping systems.

According to Ohio State University entomologists, western bean cutworm increased its presence in Ohio again in 2010. Compared to 566 moths caught in 2009, 2,695 adult moths were caught. Most of the moths were caught in northwest Ohio, with more than 400 moths collected in both Defiance and Fulton counties alone.

While economic damage has been minimal from western bean cutworm in Ohio, the rising population is a significant convcern.

“One caterpillar per ear in a field can cause up to 4-5 bushels per acre yield loss,” Krupke said. “With commodity prices being what they are, producers are extremely reluctant to risk even lower levels of damage.

“This pest has come under fairly heavy scrutiny because it’s new, some of the Bt hybrids don’t work and infestations are so heavy in some parts of the state that a lot of growers have learned about it on the fly.”

Krupke urged corn growers to plan now to carefully inspect their fields during moth flight and be prepared to apply a pyrethroid-based insecticide if conditions warrant. WBC caterpillars look similar to corn earworms but have two distinctive dark rectangles separated by a cream-colored line behind their heads.

“Growers should scout for this pest the last two weeks of June and first two weeks of July. That’s when the moths are flying, mating and laying eggs,” he said. “Right now they’re overwintering in the soil.

“Scouting is usually very effective if done at the appropriate time. We recommend an economic threshold level of 5%. So if you scout 20 plants and you see an egg on just one of them, you’re over the threshold and you’re going to want to treat with an insecticide.”

WBC caterpillars are highly vulnerable to insecticides. In one field Krupke visited in late June 2010, 50% of the corn plants were infested with the pest. The farmer ground-applied a pyrethroid insecticide.

“When we went back to that field in September, we could not find a single kernel on a single ear that was damaged,” Krupke said. “The control level was 100%, and you won’t hear that too often in pest management of insects.”

For more information, visit the Purdue Extension corn insects scouting page at and click on the western bean cutworm.

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