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Corn+Soybean Digest

Secondary Insects Bug Corn Growers

Wireworms, grubs have become more prevalent So-called secondary insects are chewing into profits in many cornfields. The major culprits are wireworms, white grubs and grape colaspis ("miniature" white grubs). They've become serious pests, warn university entomologists and crop consultants.

Entomologists say a combination of mild winters, less tillage, earlier planting and poor weed control are all possible contributing factors. There's also speculation that these insects, which traditionally have been found after pastures and forage crops, are adapting to corn-soybean rotations.

"During the past few years we've seen damage from white grubs, wireworms and grape colaspis," reports crop consultant Bill Craig, Carlinville, IL. "White grubs have been the biggest problem."

Craig says he has found many more annual white grubs than true white grubs, although entomologists say the latter are the greater menace. (True white grubs have a three-year life cycle; annual grubs have a one-year life.)

"However, we did have a 160-acre field of corn last year that had to be replanted due to true white grubs," he says.

When farmers tell Craig a field has stunted plants and there's no obvious cause, he digs up some of those plants. "Although there are various reasons for stunting, I often find white grubs eating on the roots," he says. "I've also found them eating root hairs on soybean plants, and have documented soybean yield losses due to grubs."

He has seen economic damage from wireworms in corn for the past 15 years, but has encountered grape colaspis more recently. "Grape colaspis damage last year was the worst I've ever seen."

In fields with a history of white grubs or wireworms, Craig recommends a broadcast, incorporated treatment of 2 quarts per acre of Lorsban. (There is no rescue treatment for either insect.) "That takes care of white grubs and wireworms for five years or more," he says.

Crop consultant Jerry Mulliken, Nickerson, NE, saw an increase in wireworm damage in 1998 and 1999. "They're a problem when soil temperatures are cool to moderate," he explains. "After soil temperatures warm up to about 70, they burrow deeper. Once they're in a field, they'll stay for years."

Prior to corn planting, Mulliken sets 10 corn/wheat bait traps per 40 acres to detect wireworms. If he finds an average of one or more wireworms per trap, he recommends a planter box insecticide treatment or a half rate of Force in the furrow.

University of Illinois entomologist Mike Gray says his office was flooded last summer with reports of grape colaspis injury to both corn and soybeans. It was the third consecutive year of extensive damage by that insect.

No spring soil insecticides are labeled for grape colaspis, says Gray. And University of Illinois insecticide research trials showed that currently available products, including the new seed treatments, are not effective against grape colaspis. (Many products in that trial greatly reduced white grub numbers but grubs were still found in every treatment.)

New seed treatments are available to combat several secondary insects in corn. Syngenta Seeds (formerly Novartis) offers ProShield, a seed coating with Force ST. The company says ProShield provides effective control of white grubs, wireworms and seed corn maggots.

Gustafson markets Gaucho as a seed-applied insecticide that, it says, protects corn against wireworms and seed corn maggots, equal to hopper-box products. It controls flea beetles through the first true leaf stage. A number of seed companies are offering Gaucho-treated seed in 2001, but usually only on limited hybrids.

Iowa State research in 2000 showed Gaucho equal to Kernel Guard Supreme and Aztec for protecting against wireworm.

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