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Corn+Soybean Digest

First-Rate Trouble

Corn rootworms may be the billion-dollar bug, but there are no shortages of other pests trying to make their mark.

For example, there are growing concerns about secondary insect pests, such as true white grubs, the adult Japanese beetle, wireworms and grape colaspis — all of which present challenges in trying to better understand their feeding habits and in finding effective ways to control them.

“It's often difficult to draw solid conclusions or provide precise advice on how best to control these secondary pests,” says Kevin Steffey, extension entomologist at the University of Illinois. “Past trials have been difficult and labor-intensive to conduct, and they haven't generated much data, especially in cases involving Japanese beetle grubs.”

Milder winters in recent years and early planting offer reasons why these secondary insects may be establishing a stronghold in some areas in the Midwest, Steffey says.

In many cases, secondary insects — like white grubs, grape colaspis and wireworms — are active during early corn planting when soils are cooler.

During later planting when soils are more likely to be warmer, the life cycle of these pests is usually near the end.

“We've offered scouting services since 1976 and have seen the numbers for secondary pests go up steadily each year,” says Dave Harms of Crop Pro-Tech, Inc., Bloomington, IL, which serves producers in Illinois, Indiana and Iowa. “Lately, I think producers are more aware and watching closely for them, too.”

Other factors contributing to secondary insect infestations include no-till planting into dying cover crops, fertilizing with animal or green manure and the presence of rotting weeds, says John Obermeyer, Purdue University extension entomologist.

Unfortunately, there aren't many natural enemies of subterranean pests. There are biological control products, such as Bacillus popilliae (milky spore disease), used in the turf industry to kill Japanese beetle grubs.

However, such controls aren't cost-effective in corn, and the material doesn't work well in this type of environment, according to Steffey.

White grubs attack corn and soybeans from April to mid- or late June before emerging as either Japanese beetles or May or June beetles, depending on the species. These adults then start feeding again.

Deciding whether to use a soil insecticide at planting depends on what type of white grub you're dealing with.

If “true” white grubs are present, treatment is highly recommended. If you have annual white grubs, treatment may not be necessary.

The reason? True white grubs have a three-year life cycle, and these grubs have the potential to feed all summer on developing root systems.

Annual white grubs complete one generation each year. By May or early June, they pupate and emerge as May or June beetles. So annual white grubs aren't around long enough during the growing season to cause extensive damage.

The adult Japanese beetle has been around for decades — mainly in the eastern half of the U.S. — but Steffey says the pest has been showing up in Iowa, Nebraska and even North Dakota.

Since the Japanese beetle can survive on several hundred different types of host plants, such as roses, fruit trees, ornamentals and even poison ivy, the pest may be hitching a ride and increasing its territory via the nursery industry, adds Steffey.

Japanese beetle adults feed on corn leaves, tassels, silks and pollen, and on leaves and flowers of soybeans. They can interfere with pollination in corn and defoliate soybean plants.

Adults emerge from the ground and begin feeding on plants in June. Individual beetles live about 30-45 days. Activity is concentrated over a four- to six-week period, beginning in July, after which beetles gradually die.

“The Japanese beetle is really a strange insect,” says Harms. “Its feeding habits seem to change daily. “It's definitely an insect that needs to be closely scouted and monitored.”

Wireworms have very long life cycles and can live in the soil as larvae for two to six years. Adults (click beetles) can live for 10-12 months, preferring to lay eggs in small grain stubble, sod or grass-infested fields.

Low or poorly drained areas within fields often support wireworms because of weed populations. In other situations, wireworms may be concentrated in high field areas.

A field history review (including the types of weeds) over two to four years and the previous conditions of infested areas may provide some insight into the potential for wireworm problems.

Wireworms attack either the seed or the base of the corn stem below ground level, damaging or killing the growing point. Infested fields usually have spotty stands with significant reductions in plant population in some areas.

Early detection of wireworms before planting is necessary for preventive measures since there are no effective rescue treatments after discovering the infestation. A baiting technique that aids wireworm detection before planting is often recommended.

“We set up wireworm traps in the spring only on fields suspected of problems or where we think they might occur,” says Harms. “If we find just two or more wireworms in each trap, we normally recommend a field treatment,” he adds.

Fields with greatest potential for wireworm damage include corn planted after small grains and grass sod. Growers who plant corn after soybeans double-cropped with wheat also may experience problems. So concern about potential wireworm damage does not justify the widespread use of soil insecticides on first-year corn planted after soybeans, Steffey says.

Seedling corn plants infested with grape colaspis larvae usually exhibit classic symptoms of injury-stunted, wilted plants with purple stems and “burned” leaf edges, says Steffey. It causes damage to the roots of corn and soybeans.

“Grape colaspis is a sporadic pest often found in corn planted after red clover or mammoth clover, and occasionally in corn planted after sweet clover, alfalfa or soybeans,” he says.

The grape colaspis has a life cycle of one generation per year, and overwinters during the early larval development period. The larvae become full grown during the summer (this occurs by around June 15 in central Illinois). The adults then emerge in July and are general feeders. Following mating, eggs are laid in the root zone of plants, especially in timothy and clover.

If scouting reveals problems with wireworms or seed corn maggots, Crop Pro-Tech, Inc. advises many of its clients to use seed treatments such as Agrox D-L Plus or 2-Way. “This treatment is not labeled for control of white grubs and grape colaspis, but we have seen some help in moderate infestations,” says Harms.

Harms says granular or liquid soil insecticides, such as Capture, Aztec, Force and Counter, work well as preventative measures.

“Rescue treatments with these insects are difficult, however, since we're dealing with ground-dwelling insects,” Harms says. “Getting good coverage isn't always easy below the soil surface; however, we've used Furadan successfully in some cases.”

There are also some newer neonicotinoid seed treatments available, such as Cruiser and Poncho — broad-spectrum insecticides, according to Jack Baldwin, Louisiana State University AgCenter entomologist.

Both will be available on pretreated seed only and offer control on a variety of corn insects, including white grubs, wireworms and grape colaspis. Poncho will also be available at only two rates (low and high), and the rates for Cruiser will range from 0.125 to 1.25 milligrams of active ingredient per seed.

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