One release of wasps can halve corn borer damage

November 13, 2000

ITHACA, N.Y. -- In a war against the European corn borer, a major pest of sweet corn, Cornell University scientists have found that an army of tiny wasps, released just once and early in the season, can reduce damage to ears of corn by half.

For sweet corn producers in New York and several other Northeastern and Midwestern states this is "potentially big news," says Michael Hoffmann, Cornell associate professor of entomology and director of the university's New York State Integrated Pest Management program. In field tests in which wasps were released, Hoffmann says, 6 percent of ears on average were damaged, compared with 12 percent in fields in which wasps were not released.

Details of the test are described in "Biological Control of European Corn Borer with Inoculative Releases of Trichogramma ostriniae ," authored by Hoffmann; Mark Wright, Cornell research associate in entomology; Thomas Kuhar, Cornell postdoctoral researcher in entomology; and Sylvie Chenus, entomology technician. The article will be submitted to the Journal of Biological Control .

The European corn borer attacks field and sweet corn, costing American farmers about $1 billion annually in damage and control expenses. The pest first arrived in the United States in the 1920s.

Cornell scientists have found that by releasing an army of tiny, beneficial T. ostriniae wasps early enough in the sweet corn growing season, the borers can be greatly suppressed without additional wasp releases or insecticide applications. However, the grower can spray insecticide again if necessary, and a portion of the wasps will survive and continue to help control the borers.

Typically, these wasps are released throughout the summer growing season in large numbers. But the new research shows that this is unnecessary. The scientists found that only one early release was needed -- when the sweet corn is knee-high. And growers do not need many wasps -- about 30,000 per acre. The total cost, including packaging and placement in the field, is less than a single application of insecticide. The Cornell scientists conducted their field research in Tompkins, Tioga, Cayuga and Broome counties in New York.

As late-stage larvae, borers overwinter in corn stalks. By late spring, the larvae pupate and become adults -- corn borer moths -- and begin laying eggs. Within a few days the eggs become larvae and begin attacking the corn. The corn borer can tunnel through the stalk and destroy the plant's vascular system. When the wasps are introduced to the cornfield, the tiny females insert their eggs into corn borer eggs, effectively killing off the borer embryo. In time, two or more wasps emerge from each borer egg. As another cycle of wasps emerges, the female wasps seek out yet more borer eggs and repeat the process.

"The developing borer is killed and does not have a chance to damage the corn," says Hoffmann. "The wasp is strictly an egg parasitoid and it searches for more corn borer eggs. We found 50 percent less damage to sweet corn fields where the wasps were released compared to the control fields."

In upstate New York, the beneficial wasps do not overwinter and growers have to inoculate their fields every season. "We were hoping they would overwinter and become permanent residents in corn fields," says Hoffmann. "We wanted them to become a permanent member of a complex of natural enemies that suppress the corn borer. Maybe further south they will overwinter and become established."
Research into this problem has been conducted since 1992. Prior to 1998, funding came from the New York State Integrated Pest Management program. Since 1998 the Pest Management Alternative program of the U.S. Department of Agriculture (Cooperative State Research, Education and Extension Service) has provided a three-year, $155,000 grant.

Cornell University

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