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October 23, 2002

Field survey: Corn borer worms its way into few Indiana crops

WEST LAFAYETTE, Ind. – A major corn-destroying pest was a minor nuisance in Indiana this year.

The European corn borer, a wormlike insect that tunnels through cornstalks and chews leaves, tassels and ears, inflicted nominal damage on most Hoosier corn crops. Poorer-than-average corn production contributed to the pest's unremarkable feeding activity, said John Obermeyer, a Purdue University Cooperative Extension Service entomologist.

Purdue entomologists recently completed a statewide fall field survey. The survey, conducted annually for 42 years, found corn borer populations about the same this year as they've been in recent years, Obermeyer said. Crop losses from corn borer were heaviest in counties where corn production was highest or where highly vulnerable corn varieties are grown, he said.

"According to our survey, corn borer levels are very much the same as they've been for the last three or four years," Obermeyer said. "When you compare it to 42 years of data, this year is average. Some areas had corn borer numbers, but most areas didn't have much at all. Our survey found that northwest Indiana counties seemed to be the high spot for the state. We also had a surprisingly high number in southwestern counties, where they grow white corn. White corn is very susceptible to European corn borer."

Pest counts indicated that fields in central portions of southwest Indiana averaged 2.28 corn borer worms attempting to overwinter per corn plant. Fields in northwest Indiana averaged 2.18 worms per plant. The state average was 1.15 worms per plant. Entomologists arrived at those numbers by counting corn borer larvae in 20 plants in five to six fields in each of the survey's nine geographical areas.

Entomologists also calculated estimated crop losses this season.

"We took the damage ratings we obtained from the fall survey and tried to apply some simple economics," Obermeyer said. "In other words, how much money did corn borers cost us? We found it ranged anywhere from an estimated zero dollars per acre loss to just over $34 per acre."

The per-acre estimated crop loss was highest among northwest Indiana counties, ranging from $4.94 per acre to $34.11 per acre. The smallest estimated loss range was in southeast-central Indiana, from 34 cents to $4.43 per acre.

"Every year we have a huge estimated loss range," Obermeyer said. "Of course, the zero dollars and $34 per acre estimates are few and far between, but we do have significant damage here and there with European corn borer."

Weather conditions last fall and this past spring likely played a role in keeping corn borer populations down statewide, Obermeyer said.

"The worst thing for European corn borer is not a very hard winter, as one would suspect, but rather, very mild fall and spring temperatures," he said. "The reason for that is the larvae are very susceptible to fungal diseases while they're hidden within crop residues or other overwintering sites."

Corn borers usually survive winter because the fluid within the larvae works as an antifreeze.

The pests emerge from their winter slumber, and in early to mid-May the larvae pupate inside their hosts. By late May the female corn borer moth begins searching for lush, green corn plants to lay eggs. A female moth can lay more than 400 eggs.

New corn borer larvae feed from June to mid-July. A second-generation corn borer feeding takes place from mid-July to mid-September. A third-generation corn borer infestation is possible from mid-August to October.

Obermeyer said it is impossible to predict next year's potential corn borer damage. However, farmers who suffered significant crop losses this year should carefully consider planting dates and seed varieties. Using corn borer-resistant seed will help, as well.

"If producers are planting early into a high-production field, many times those will be hit by that first-generation corn borer," he said. "Those I would consider high risk. That would be an excellent location to consider using Bt corn.

"On the other hand, we have fields that are planted late or are planted in late-maturing varieties. Those fields often will be selected by the second-generation corn borer for egg laying."

For more information on the fall corn borer survey and management considerations for 2003, read the Oct. 18 issue of the Purdue Pest & Crop Newsletter. The newsletter can be downloaded online.

Writer: Steve Leer, (765) 494-8415, sleer@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 site:
Purdue University Department of Entomology: http://www.entm.purdue.edu/

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


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