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Growers have several treatment options

Prolonged drought always increases the potential for spider mite problems in soybeans, and this year is no exception.

Spider mite infestations have reached treatable levels in scattered droughty areas throughout Minnesota, says Ken Ostlie, University of Minnesota Extension entomologist. Ostlie says one factor complicating spider mite management decisions is soybean aphid, and vice versa.

"Even though we've had significant rain in southern Minnesota in early August, don't assume rain has eliminated aphid or mite problems," Ostlie said. "Heavy rains of two to six inches remove a lot of stress on the plant, but you need to make sure that ongoing aphid or spider mite problems don't continue to rob yields. Scout now for spider mites, especially if you're contemplating a spray for soybean aphids."

When it comes to treating spider mites, the producer has a handful of viable options, according to The Ohio State University entomologist David Shetlar. First and foremost, regardless of what treatment used, is early diagnosis.

"Early detection of spider mites, before damage is noticed, is important. The tiny spider mites can be detected by taking a piece of white paper or cardboard and striking some plant foliage on it," Shetlar says. "The mites can be seen walking slowly on the paper. If 10 or more mites per sample are common, controls may be needed."

If in smaller fields, mites can be controlled through "syringing, Shetlar says. This simple control method uses a forceful jet of water to knock the invading mites from leaves. Another natural form of control is through introducing predatory insects like lady beetles or lacewings.

"Be sure to check listings to determine which species is appropriate," Shetlar says. "Some species are host-specific and each predator works better under different weather conditions. If predators are used, do not apply pesticides that will kill them."

The most common control measure used for spider mites in soybeans is miticides. It's important to check for miticide specifically on an insecticide label.

"Pesticides claiming 'for mite suppression' are usually weak miticides and will not perform well," Shetlar says. In general, a dimethoate or chlorypyrifos product is recommended for spider mite control.

Ostlie recommends checking fields every four to five days, and if an infestation warrants spraying, don't hold up a spray waiting for rain, he advises.

"If rain is unlikely to occur before the spray dries, go ahead with the spray," Ostlie said. "Reducing spider mite pressure will allow the crop to take full advantage of the moisture."

It may be small consolation for soybean producers, but spider mites haven't reached levels seen during the severe 1988 drought, according to Ostlie. That's due to a wetter spring and milder temperatures, compared to 1988.

Prolonged drought always increases the potential for spider mite problems in soybeans, and this year is no exception.

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