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5 tips for scouting corn

Agriculture.com Staff 05/14/2007 @ 8:42am

Remember scouting? You did it several years ago to manage European corn borer. Or assess black cutworm damage. Or count corn rootworm beetles to justify applying an insecticide when corn was again planted in that field.

Granted, many farmers and consultants still plunge into towering corn to pinpoint corn pests. Scouting is a key part of Integrated Pest Management (IPM), a multiple-step pest-management strategy.

However, transgenic technology has helped shift farmers away from IPM in corn and soybeans.

"The 'I' in IPM has been forgotten," says Mike Gray, University of Illinois (U of I) Extension entomologist. "Producers are increasingly not integrating their management tactics."

For example, transgenic Bt hybrids resistant to European corn borer (ECB) have dissuaded many from scouting for ECB and made treating based on economic thresholds passé.

"Yet, there are many areas in Illinois where they [ECB-resistant hybrids] aren't needed, due to low ECB densities," says Gray. Overuse of Bt hybrids where they are not needed will help hone selection pressure and ultimately trigger resistance by insects to these hybrids.

Meanwhile, scouting and using a mix of control tools will help preserve control methods like transgenic hybrids.

Here are five tips to remember as you get out in your fields.

Those who responded to an Agriculture Online® poll are doing a good job of hitting their fields. Of the 196 respondents, 73% scout their fields weekly, with 9% even going so far as checking their fields every day.

Still, increasing farm size and tight time budgets make it difficult to carve out scouting time. A good rule of thumb is to examine 10 plants in 10 areas as you walk the field in a zigzag pattern, says Suzanne Bissonnette,

U of I IPM Extension educator. Plan on regularly entering fields every seven to 10 days throughout the growing season.

"As humans, we have the tendency to see the worst thing and make decisions based on that," says Bissonnette. "That's part of the reason for zigzagging through the field. You can arrive at an average pest population, instead of taking the worst areas of a field."

Novice scouters can confuse insects, such as mixing up the plant-cutting black cutworm with the leaf-feeding dingy cutworm. Should black cutworms pass the 3% to 5% plant-cutting threshold in a field, insecticide applications pay.

Not so with the dingy cutworm. By often eating just leaves, it doesn't impact yields. Properly identifying the pest results in a justified insecticide application and protected yield, says Bissonnette.

Pests may stunt corn -- or not. It might be compaction.

"I'm seeing the impact of different types of tillage," says Michael McNeill, president of Ag Advisory, an Algona, Iowa, crop consulting firm. "I think growers should be aware of the impact that tilling soil at the improper time and moisture level has on a field. There is a great deal of yield reduction that results from soil compaction. Scouting can reveal if you're doing something you shouldn't and enable you to correct the problem next year."

You'd think crop rotation, soil-applied insecticides, and transgenic hybrids would be enough to make corn rootworm cry uncle. Still, there are times when adult rootworm beetles cover corn ears like flies on a rotting carcass.

"Corn rootworm populations tend to cycle, and we are in an upward cycle right now," says McNeill.

In many areas, rotating corn with soybeans continues to control corn rootworm. Ditto for soil-applied insecticides, particularly in moist years. And if you're looking for why farmers like transgenic hybrids, a 2006 U of I trial showed a transgenic hybrid outyielded a hybrid protected by a soil-applied insecticide by 100 bushels per acre.

In the spring, McNeill generally digs up two to three plants in five to six areas in a field to monitor larval root chewing. He even does this in fields planted to transgenic rootworm hybrids. "This gives you an idea of how much pressure is out there," he says.

That knowledge helps later in the summer, when adult corn rootworm beetles can threaten pollination via silk clipping. Thresholds can vary. The U of I's recommendation is to treat with an insecticide when five or more beetles infest each plant.

McNeill again scouts five to six areas in an 80-acre field for beetles. "You just have to stand really still in the field and watch for the beetles," he says. "I look at those already on the plant and those that are hiding in the silks."

The western corn rootworm variant originated in east-central Illinois and has extended outward from there. The variant lays eggs in soybean fields that will hatch when the field is rotated to corn the following year.

If you're in an area where variant populations exist, unbaited Pherocon AM yellow sticky traps can help predict next year's outbreaks.

Gray recommends evenly placing 12 staked traps in each soybean field, regardless of size, about the last week of July through the first three weeks of August. Replace traps each week. If levels consistently surpass five to six beetles per trap per day, it's likely that problems will surface in corn the next year. Some treatment, such as transgenic hybrids or soil-applied insecticide, is then recommended.

Remember scouting? You did it several years ago to manage European corn borer. Or assess black cutworm damage. Or count corn rootworm beetles to justify applying an insecticide when corn was again planted in that field.

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