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Corn+Soybean Digest

How To Judge Defoliation Damage

There are a slug of bugs - bean-leaf beetles, green cloverworms, grasshoppers and more - that like to feast on soybean leaves. And if they do a lot of their chewing when pods are forming, they can put a hefty hit on yields.

But if there is sufficient leaf canopy, soybeans can withstand considerable defoliation without suffering serious yield damage.

Today, university entomologists have developed new, more accurate guidelines for evaluating foliar damage to soybeans by insects. The revised standards are based on research that indicates soybeans can tolerate more defoliation than once was believed.

The good news is that the research should mean less spending for insect treatment.

Entomologist Leon Higley at the University of Nebraska, and colleagues at universities across the Soybean Belt, have been doing extensive research on soybean defoliation.

"We now have a better understanding of how insects cause yield loss to soybeans," Higley reports.

"We've found that, if soybean plants have a canopy large enough to intercept more than 90% of the available light, they can tolerate 50-70% defoliation without yield loss. However, if the canopy is small, low levels of defoliation (20-25%) may be enough to reduce yields."

As a result of the research, entomologists are drawing new guidelines for determining economic thresholds for spraying. In essence, it will take more insects than in the past to trigger a treatment.

"We'll have guidelines both for the seedling stage - where feeding damage also can be serious - and the reproductive stages of growth," Higley notes.

Those guidelines, when determining insect thresholds, consider both the value of soybeans and the cost of treatment. That is, as soybean prices drop and/or application costs rise, it takes more insects to justify a treatment.

Entomologists also are developing a system where farmers or crop scouts can take simple measurements to estimate canopy size.

"It still will require insect counts," Higley explains, "but the canopy charts will help determine whether a canopy is large or small. Since a large canopy means soybeans can tolerate more insects without yield loss, treatment guidelines will be adjusted to the canopy size."

Some of those canopy charts could be available this summer, explains Higley. Check the University of Nebraska's entomology Web site (www.ianr. dept.htm) for the latest developments.

"Farmers often have a sense of canopy size," Higley points out. "Therefore, even if they don't have the formal guidelines this summer for assessing canopy size, they may want to do some estimations on their own. If they feel that a canopy is large, they can raise the treatment threshold accordingly."

Also, a commercial light meter, called the LAI-2000, can measure canopy size, notes Higley. It can be used in conjunction with insect economic thresholds. However, at a cost of about $1,500, it's more practical for crop consultants who cover thousands of acres than for individual farmers. (For information, call Li-Cor, Inc., 402-467-3576.)

Canopy size is determined more by planting date and growing conditions than by row width or population, Higley says.

"If soybeans are planted on time at the recommended seeding rate, and receive plenty of moisture, they usually will form a large canopy regardless of row width," he says. "But late-planted soybeans or beans that are under drought stress won't develop a large canopy, even if in narrow rows."

Initially, there will be different treatment guidelines for different insects and row widths. However, Higley explains, there eventually will be programs that consider a mix of insects and put them on a common basis.

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