Drought Groupies

ROCKVILLE, Md. (DTN) -- Tracking twospotted spider mites is often as easy as checking the U.S. Drought Monitor. Like groupies, the tiny mites trail hot, dry conditions and add another stress to thirsty corn and soybean plants.

The pest is usually a late-summer phenomenon, but this year they are surfacing early in areas where drought conditions are mounting, such as Iowa, Michigan and Ohio, according to university Extension reports.

"They're a pest that can still have a high reproductive rate when temperatures are 90-plus degrees," Iowa State entomologist Erin Hodgson told DTN.

Widespread spider mite outbreaks are rare -- most recently, they dominated the Midwest during the 2012 drought, and older farmers might remember them from the 1988 drought, as well, Hodgson said.

Like soybean aphids, spider mites have a rapid life cycle, which allows populations to swell quickly and overtake plants.

Early symptoms are stippling and yellowing of lower canopy leaves, often at the edge of a field. Those leaf marks indicate permanent damage, Hodgson said. "They're fluid feeders, so they're smashing cells and sucking," she said. "When spider mites injure those cells, that area doesn't recover."

When feeding becomes heavy, you might notice webbing in the field, and the mites move up through the canopy of the plant. Leaves can die and fall off. At their most damaging, spider mites can kill entire plants.

The only natural solution to the mites is a change in the weather. Rain knocks them off plants, and cool, wet conditions allow certain pathogens and beneficial insects to prey upon them.

If the forecast ahead looks persistently dry and hot, growers should take a proactive approach to this pest, Hodgson said.

"You want to be on the leading edge rather than being reactionary because of this pest's rapid and overlapping life cycles, which makes it hard to take care of them," he said. "The goal is to try and avoid injury symptoms."

In severe cases, spider mites have caused 40% to 60% yield loss in untreated soybeans, Hodgson noted in a university newsletter. Plants under additional drought stress could see even higher yield losses.

Scouting is tricky, because the pests are so tiny. Most entomologists recommend taking a sheet of white paper and even a hand lens with you into the field. Hold the paper under the leaf as you shake it and look for tiny black specks to fall onto it -- those are the mites.

University of Minnesota entomologists Bruce Potter and Ken Ostlie have established a 0-to-5 rating system for spider mites that you can view here: http://bit.ly/…. Essentially, treatment becomes necessary around level 3, when you observe heavy stippling on lower leaves spreading into the middle canopy and some leaves are yellowing and dying.

Organophosphates are the preferred treatment method, but they do not kill the mites' eggs, so a second flush of pests often occurs and requires additional applications, Hodgson said.

Remember that pyrethroids are fuel to the spider mite fire, she added. "There's some evidence that the actual application of a pyrethroid changes their body chemistry so they produce more offspring," she warned. "It almost supercharges their reproductive potential, so they're producing more than before you sprayed."

So if you've treated your fields for another pest, such as soybean aphid, with pyrethroids, be prepared for a rise in spider mites if conditions are hot and dry.

Michigan State University entomologist Chris DiFonzo has noted that overtreatment for soybean aphids with pyrethroids and chlorpyrifos (an organophosphate) has decimated beneficial insect populations and led to some chlorpyrifos-resistant mite populations in Minnesota.

For more information on treating spider mites, see this Iowa State update from Hodgson: http://bit.ly/…. These guides from the Michigan State University provide helpful images for scouting and treatment decisions on soybeans (http://bit.ly/…) and corn (http://bit.ly/…).

You can read DiFonzo's full remarks on the connection between soybean aphids and spider mites here: http://bit.ly/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

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