Jan 18, 2008

Earworm Becoming a Legitimate Threat for Sweet Corn Growers

The image of corn earworms floating on the winds is a bit tough to handle, but, of course, it’s not the worms themselves that float. It’s the moths that float in, and then they lay the eggs that give rise to the ugly larvae that gross out sweet corn customers.

Adding to the ugly side, these worms have gotten harder to control with pyrethroid insecticides in recent years.

University of Illinois entomologist Richard Weinzierl has been monitoring this pest for several years. In 2006, he and other entomologists in six northern sweet corn-producing states began to cooperate in a North Central Region IPM Project. The project includes participants from Illinois, Indiana, Michigan, Minnesota, Ohio, Wisconsin and Ontario. It’s led by Bill Hutchison of the University of Minnesota, along with Weinzierl and Rick Foster of Purdue University.

Working together, they assess earworms to find out how susceptible they are to pyrethroid insecticides, work to improve methods of monitoring and predicting when migrations will occur and evaluate insecticides to control them. This is the third year in which they have been making recommendations to growers about what to do to achieve control.

Weinzierl described the program during a sweet corn session at the Great Lakes Fruit, Vegetable and Farm Market EXPO in Grand Rapids, Mich., in December.

Earworms, like potato leafhoppers, don’t live year round in most areas of the Midwest that they infest during the summer. Earworms won’t overwinter north of southern Illinois (40 degrees north latitude). Instead, the moths ride on columns of rising warm air from their place of origin in south Texas and western Louisiana.

This “insect pump” moves the moths hundreds of miles north and drops them out in bands along the frontal boundaries where hot air meets cooler air and rainstorms result. Rains bring water, and also insects looking for host plants to lay eggs on. Earworm moths lay eggs on the new silks of corn.

Major migrations occur in August, but in some years – two or three years out of 10 – they can begin in June.

All this is, of course, not new. But since the late 1990s, the arriving moths have been harder to kill with pyrethroid insecticides.

“The level of control has decreased,” Weinzierl said. “It used to be 90 percent and more. Now it is 70 to 75 percent or even less: 50 percent.”

The onset of resistance begins at the source.

“Earworms that infest our late season sweet corn, tomatoes, peppers and snap beans come from southern source regions,” Weinzierl said. “Primary host crops in those source regions are cotton, sorghum, soybeans and corn. These crops receive numerous applications of pyrethroid insecticides. Pyrethroid resistance that evolves in southern crops may present a problem in the northern destinations of migrating earworms.”

Resistance does not build up from use of pyrethroids in the north, since the insects there are not destined to remain in the breeding population. They do not survive over winter.

While the potential exists for massive corn earworm control failures, so far only a few in-field failures have occurred in sweet corn fields in the north, Weinzierl said.

While the effectiveness of pyrethroids is declining, they are still the recommended method of control.

Here is the program recommended by Weinzierl and his cohorts for sweet corn growers in the north this coming summer:

Buy a wire Hartstack pheromone trap and lures that attract earworms. Use the trap to monitor earworm moth flights.

Continue to use pyrethroid insecticides as needed based on pheromone trap catches. Pyrethroids are still the best insecticides for corn earworm control. These include Capture, Warrior, Baythroid and Mustang-Max. Expect that Warrior, Proaxis, Capture, Discipline, Baythroid, Mustang/Fury and Pounce/Ambush will all be affected by any resistance mechanism.

If traps are catching moths, getting a first application on at row tassel or by first silk may improve control compared to starting sprays within two days of first silk, especially where adult control over a large acreage is accomplished. Small plots and sequential plantings may be more severely affected than large fields if moths continue to fly into these plantings from surrounding areas.

Application intervals of two to three days are especially important right after silking has begun.

Tank mixing (not rotating) with Lannate, Larvin, Sevin or SpinTor/Entrust may improve control.

Alternative insecticides include SpinTor/Entrust and Radiant (spinetoram). These are more expensive.

Bt sweet corn greatly reduces earworm numbers but does not give complete control (and does not prevent damage from western bean cutworm).

Read Extension newsletters for updates. Weinzierl’s updates are published in the University of Illinois Fruit and Vegetable News, online at http://www.ipm.uiuc.edu/ifvn. The Web site at Penn State, which will have up-to-date flight info, is www.pestwatch.psu.edu/sweetcorn/tool/tool.html.

Report any apparent control failures to the entomologists involved in the project or to your local Extension office.