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Corn+Soybean Digest

If Pests Infect your Corn or Soybean Field, Be Sure They Hit Economic Thresholds Before Spraying

A gut feeling to spray for bugs you expect to invade your corn or soybeans might become nauseating for your budget if the economic threshold numbers aren't there. And it could cripple your efforts if there's a jailbreak straight to your crops by seldom-seen harmful insects.

Managing your economic threshold (ET) the right way through proven formulas can mean the difference in profits in these tight times. The higher the crop price, the more economical it is to spray for bugs at lower population levels, and vice versa.

Phil Sloderbeck, Kansas State University Extension entomologist, says there are sufficient ET calculators to gauge the probable economic benefits of controlling most insects that regularly attack corn and beans. They are based on crop prices, chemical application prices and insect populations.

“You still may have to use your gut feeling on whether to spray bugs that haven't been consistent enough to provide good reliable data,” says Sloderbeck. “But for insects like corn borers or aphids, there are reliable computer models that can measure expected crop damage at various population levels.”

As an example, by knowing the number of European corn borer larvae, you can determine the ET by dividing the cost of control by the yield loss per borer, crop market value, yield, proportion of larvae killed and proportion of eggs laid divided by the number of eggs per mass (see table on p. 46).

A higher market price of $3.50 lowers the ET to 0.06. Last year's high $6.50 price lowered the ET to 0.03.

“If you have an equation value, you have a tool to plug in the numbers and know where the best economical range is for treatment,” says Sloderbeck. “You can make field-by-field, year-by-year readings. You need to at least run the formula on the farm once a year.”

VARIOUS UNIVERSITY entomology departments offer ET calculators for corn and beans. At Iowa State University (ISU), Extension Entomologist Erin Hodgson points out that using an ET chart to determine the need to treat against insect invasions should benefit growers.

Good sampling is a must before the use of ET equations or charts is effective, she says. For example, for bean leaf beetles in soybeans, use a sweep net to determine if densities are surpassing treatment thresholds.

In an ISU bean leaf beetle ET table (see below), the management cost of treatment/acre and price of soybeans are measured against the number of adult beetles to determine the ET.

For example, a bean price of $9/bu. and an application cost of $15/acre equates to a bean leaf beetle ET of 5.19 adult beetles/sweep. An $11 bean price and $15 treatment cost lowers the ET to 4.24/sweep.

A $7 bean price and $15 treatment cost raises the ET to 6.67 bugs. (For more on the chart, go to

“There are clearly defined ETs for various plant stages,” says Hodgson. “However, managing bean leaf beetles during the pod set and pod fill can be frustrating because adults may be feeding on pods for a couple of weeks before the population reaches the ET. In this situation, some loss in seed quality and quantity occurs before an insecticide application can be economically justified.”

Sloderbeck outlines reasons why treating fields without knowledge of current pest populations isn't recommended.

“One is that repeated insecticide applications can lead to insect resistance,” he says. “Another is that insecticides can have significant impact on non-target insects and beneficial organisms which can lead to secondary pest outbreaks.

“There are also potential health hazards to applicators and field workers. However, possibly the biggest reason is because — if the timing is wrong — the application may have little or no impact on the target pest.”

SLODERBECK ADDS THAT the economic threshold is based on getting 95% control. “If the timing is off, this could fall dramatically and in many cases there would be little or no economic benefit from making a treatment,” he says.

“Worse yet, if treatments are applied too early, the majority of the eggs could hatch after the insecticide residual is gone, and you could suffer severe corn borer injury even though one paid for an insecticide treatment.

“This can especially be true when insecticides are added to fungicide or herbicide applications without regard to proper timing for corn borer or other potential pests,” Sloderbeck notes.

That happened in 2007 and 2008 when corn prices reached historic levels. “We've had people who wanted to add an insecticide when making fertilizer or herbicide applications,” he says. “I have serious concerns with that. You still need a pest out there to apply an insecticide.

“So even when thresholds are much lower given the commodity prices, proper scouting and timing are necessary to collect the benefits from any insecticide applications,” Sloderbeck says.

Unneeded applications early on can also mean money isn't there if late-season emergency applications are merited. “Many late-season pests present a guaranteed yield loss,” he says. “I'd rather put my money there than just into random spraying.”

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