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May 31, 2002

Late start in the field may lower corn rootworm problems

WEST LAFAYETTE, Ind. – Even though late planting may have a negative impact on fall yields, the delayed start could keep insects from bugging farmers as much during the growing season, said a Purdue University entomologist.

John Obermeyer, Purdue Extension integrated pest management supervisor, said corn rootworms may be less of a problem this year. Corn rootworm eggs overwinter and hatch from mid-May in the southern part of Indiana until the beginning of June in the north.

"Corn rootworms that are hatching don't have any corn roots to feed on and are obviously going to starve to death," Obermeyer said. "We saw a similar situation in 1997 and 1998. We had huge rootworm beetle populations in 1997 but a tremendous amount of starvation and drowning of larvae in 1998 because of delayed planting and very wet soils."

The corn rootworm eggs are durable and can sustain flooding conditions for weeks, but once the larvae hatch they are very vulnerable to drowning, Obermeyer said.

Since farmers are getting a later start planting corn, the amount of granular insecticide applied can be reduced, which will cut down on expenses and fill time on planter boxes, Obermeyer said. However, producers who experiment with reduced rates are responsible for the product's performance, which may mean other insects won't be effectively controlled.

"Farmers putting corn in the ground between now and June 10 are safe to reduce their granular insecticide by a quarter," Obermeyer said. "Applying three-quarters of the labeled rate will produce a result of greater efficacy. We feel that it is safe to stop using rootworm insecticide after June 10 because most of the threat from the hatched eggs is over."

Obermeyer said producers applying reduced insecticide rates should:

  • Set up and accurately calibrate insecticide applicators on planters to deliver the insecticide at the targeted rate and proper location.

  • Read and follow label instructions and not reduce the labeled rate by more than 25 percent.

    Recommendations for using reduced rates apply only to granular insecticides. Liquid soil insecticide recommendations are not available because the performance of these products at full labeled rates is inconsistent, Obermeyer said. He recommends not lowering the rate of these insecticides.

    Writer: Jennifer Doup, (765) 494-6682, doupj@purdue.edu

    Source: John Obermeyer, (765) 494-4563, obe@purdue.edu

    Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; http://www.agriculture.purdue.edu/AgComm/public/agnews/

    Related Web sites:
    Pest and Crop Newsletter (May 24, 2002)
    Purdue Extension Delayed Planting

    Purdue News Service: (765) 494-2096; purduenews@purdue.edu


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