Early appearance of spider mites threatens soybean crop

Very dry conditions are encouraging two-spotted spider mites to lay claim to drought-stressed soybean plants throughout the much of the eastern Corn Belt.

When left untreated, spider mites can cause extensive and irreversible damage to soybean foliage, so growers need to keep an eye on their fields - especially if the weather remains dry.

"Two-spotted spider mites have the potential to cause more yield loss than any other insect," said Ron Hammond, Ohio State University Extension entomologist. "The damage caused by the mites is severe enough to kill the entire plant. Growers who have a bad infestation will not see any yield from the affected area. While it doesn't happen very often, in those areas where it does, the effects are devastating."

Spider mites thrive on stressed soybeans because amino acids are more available to insects when they feed on stressed plants than when they feed healthy ones. That means the mites can use these nutrients to synthesize proteins for use in reproduction.

"Under conditions where drought-stressed plants are abundant, mites thrive and quickly colonize large areas or fields where stress is more evident," said Christian Krupke, Purdue Extension entomologist. "Stressed plants actually provide a more nutritious feast for spider mites than healthy plants do."

He said spider mites are present in every soybean field every year, but normally "do nothing of consequence to producers."

While conditions in Ohio are not quite as dry as Indiana, spider mites still are appearing earlier than normal in Ohio soybean fields.

Typically Ohio growers aren't affected by two-spotted spider mites until late July or early August, because moisture levels in spring and early summer are usually high enough to keep the pests at bay, Hammond said. The dry, hot summer days typical of July and August are when conditions are conducive to spider mite infestations. But during that time, the spider mites are typically found only along the edges of a soybean crop and cause minimal damage.

"We're just in the middle of June and already dealing with this situation, so the concern is what will happen in the coming weeks, considering that the weather outlook calls for continued hot, dry conditions," Hammond said.

Hammond and Krupke encouraged soybean growers in both states to start scouting fields now.

Spider mite damage is normally first visible in the most stressed areas of the field, often including field edges. Soybean growers are likely to first notice foliar damage in the form of subtle yellow stippling of leaves, which can progress to bronzing.

If dry weather persists and mites are left unchecked and untreated, necrosis, or tissue death, can occur. Once foliage is bronze, the damage is done and cannot be reversed, even with treatment, Krupke said.

But before growers consider treatment, they need to be sure crop damage is a result of spider mite feeding and not one of the many other diseases, pathogens or nutrient deficiencies that can cause similar foliage appearances.

The way to confirm the presence of mites is to shake discolored soybean leaves over a white piece of paper and watch for small, dark specks moving about. Growers also can look for very tiny, fine webbing on the undersides of discolored leaves.

"Once spider mites have been positively identified in the damaged areas of the field, it is essential that portions of the entire field be scouted to determine the limits and range of infestation," Krupke said.

Spider mites colonize fields in a patchy fashion that often begins at field borders. Growers should sample at least five areas of a field to determine how far mites might have moved into the field.

If mites are positively identified in stressed soybean fields, pesticide application is typically warranted, and it sometimes takes more than one insecticide or miticide treatment.

"Surviving spider mites are able to repopulate a field much more quickly than their natural predators, which are usually also wiped out by these chemical applications," Krupke said.

Recommended pesticides include dimethoate and chlorpyrifos.

An even better "control" method, Krupke said, would be precipitation.

"Obviously the best plant stress reliever under dry conditions is rain," he said. "Significant rain doesn't control spider mites but helps the soybean plant become more vigorous and healthy. This, in turn makes the 'juices' of the plant less nutritious to the mites, and makes mites less likely to reproduce as quickly.

"We can't make it rain, but we can take steps to make sure that mite scouting and treatment is prioritized until conditions improve. Mites don't need to reach outbreak levels, but vigilance is important in early stages of infestation."

More than 50 percent of Ohio is abnormally dry, and nearly all of Indiana ranges from abnormally dry to the second stage of drought, according to the June 12 U.S. Drought Monitor.