Twospotted spider mites have been noted in fields across much of Iowa already this summer, as much of Iowa is in abnormal or extreme drought (D0-D3, US Drought Monitor). Scouting for spider mites in field crops is encouraged with prolonged drought. Twospotted spider mites can increase whenever temperatures are greater than 85°F, humidity is less than 90 percent, and moisture levels are low. These are ideal conditions for the twospotted spider mite and populations can increase very rapidly.
Spider mites can typically be found feeding on the bottom side of crop leaves, and they begin feeding at the bottom of the plant. Initial feeding looks like yellow or white spots (called stippling; Photo 1) on the leaves. Prolonged feeding will cause infested leaves to turn completely yellow, then brown, and eventually the leaf will die and fall from the plant. This is often mistaken for “firing” of the lower leaves during drought stress in corn. Webbing often is visible on the edges and underside of leaves and is an indication of prolonged colony feeding (Photo 2). Scout fields by looking for small mites that look like specks of dirt on the undersides of leaves, beginning at the bottom of the plant and working up.
Exact treatment thresholds for spider mites in corn and soybean do not exist. Instead, consider how long the field has been infested, mite density including eggs, mite location on the plant, moisture conditions and plant appearance. A general guideline for soybean is to treat between R1-R5 (i.e., beginning bloom through beginning seed set) when most plants have mites, and heavy stippling and leaf discoloration is apparent on lower leaves. Foliar insecticides are recommended in corn from R1-R4 (i.e., silking through dough stage) when most plants have mites at or around the ear leaf and 15-20 percent leaf discoloration. Several insecticides and miticides are labeled for spider mites in corn and soybean in Iowa. Refer to the spider mite encyclopedia article for more detailed scouting and management information.
In soybean, insecticidal control decisions should take into account whether soybean aphid is also present in the field. In many areas of the Midwest, soybean aphid has documented field-evolved resistance to bifenthrin, which is one of the only insecticide active ingredients available for twospotted spider mite. See the soybean aphid encyclopedia article for more information.