sealPurdue News

January 22, 2003

Purdue expert urges farmers to monitor rotation-resistant beetle

WEST LAFAYETTE, Ind. – A rotation-resistant corn rootworm beetle has set up camp in the northwestern and west-central portions of Indiana, and even though it's been around for awhile, many farmers don't monitor for the beetle and may not know whether or not it's a concern.

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"A producer's best bet is to monitor for the beetle using traps," said John Obermeyer, a Purdue University Cooperative Extension Service entomology specialist. "That tool will let them know whether this insect is a problem and help them make the best decision for their farming operation."

A set of criteria developed by Obermeyer and other Extension entomologists suggests producers in the northern portion of the state should use an insecticide if:

• numbers of western corn rootworm beetles on traps in soybean fields averaged five or more per trap per day during any trapping week.

• rootworm larvae caused damage in corn and there was no monitoring for western corn rootworm beetles.

• the average number of larvae in soil samples is two or more per plant by hand sorting or eight or more per plant by washing.

Farmers in southern Indiana should consider using an insecticide if:

• a field is being planted to corn following a soybean crop that had a high population of volunteer corn with rootworm beetles present.

• the average number of larvae in soil samples is more than two per plant by hand sorting or eight or more by washing.

Obermeyer said farmers who didn't monitor for the beetle should ask neighboring farmers if they've had problems and check in their own fields for rootworm damage. If there was rootworm damage, farmers should consider a soil insecticide, especially if they're located in the high-risk area, he said.

"A soil insecticide is not needed for rootworm larval control where no or very few rootworm beetles were observed in last season's soybeans," Obermeyer said.

The fight against rootworms didn't always involve chemicals. For years, the typical corn-to-soybean rotation served as the main means of controlling the pest. In the early 1990s a strain of rotation-resistant insects started laying their eggs in east-central Illinois soybean fields, anticipating a crop of first-year corn and wreaking havoc with farmers' rootworm control methods. Shortly after the savvy beetle made its way into Indiana. The variant is found in Illinois, Indiana, Michigan and Ohio. Iowa and Wisconsin are looking into possible cases.

"Until this western corn rootworm variant, producers were able to rotate corn and soybeans and had little reason to use soil insecticides, which cost about $15 to $17 per acre," Obermeyer said. "Now producers, especially those above Interstate 70, must consider using this 'insurance' to prevent economic losses to their corn."

Rootworm beetle numbers in 2002 were highest in the northwestern and west-central counties, a mixed bag in other northern counties and lowest in the southern half of the state, Obermeyer said.

Extension entomologists determined 2002 beetle numbers by using sticky traps in soybean fields around the state. The traps are rectangular pieces of yellow paper scented with pheromones. Corn rootworm beetles are attracted to the pheromones and become trapped on the paper, which allows scientists and farmers to estimate the number of beetles in a field.

An Extension publication about monitoring for western corn rootworm beetles in soybeans is available on the Web.

Local Purdue Extension educators can help in locating traps and setting them up. They also can suggest soil insecticides to combat corn rootworm.

Educators in several counties around the state are involved in a corn rootworm study. Information about the study can be found on the web.

"Farmers are in a Catch 22 on the decision to remain with a corn-to-soybean rotation or move to a lengthier rotation that includes more soybeans," said Bob Nielsen, Extension corn specialist.

He said the traditional corn-to-soybean rotation no longer suppresses the variant western corn rootworm, but the longer rotation increases the risks of developing serious soybean pests such as soybean cyst nematode, white mold and sudden death syndrome.

"There is no clear solution," Nielsen said.

Writer: Kay Hostetler, (765) 494-6682,

Sources: John Obermeyer, (765) 494-4563,

Bob Nielsen, (765) 494-4802,

Ag Communications: (765) 494-2722; Beth Forbes,;

Related Web sites:
Extension publication detailing how to monitor for western corn rootworm beetles
Center for Integrated Pest Management corn rootworm study home page

Western corn rootworm risk in Indiana is divided into three categories: high, moderate and low. This map shows those areas for 2003 based on 2002 beetle numbers. (Purdue entomology Extension graphic)

A publication-quality graphic is available at

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