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.

"Mites are found in every field, every year, and usually do nothing of consequence to producers," said Christian Krupke, Purdue extension entomologist. "However, stressed plants actually provide a more nutritious feast for spider mites than healthy plants do."  

Amino acids are more available to insects when they feed on stressed soybeans instead of healthy soybeans, he said. 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," Krupke said.  

Spider mite damage is typically most visible at first in the most stressed areas of the field; this often includes field edges. Soybean growers are likely to first notice foliar damage in the form of subtle 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, he said, 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.  

"To confirm the presence of mites, shake some discolored soybean leaves over a white piece of paper," Krupke said. "Watch for small, dark specks moving about on the paper. Also, look for very tiny, fine webbing on the undersides of the 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."  

According to Krupke, spider mites colonize fields in a patchy fashion that often begins at field borders. He suggested sampling at least five areas of a field to determine how far mites might have moved into the field.

If mites are positively identified in drought-stressed soybean fields, Krupke said, 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," he 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."