Early confirmation of twospotted spider mite

click image to zoomFrank Peairs, Colorado State UniversityPhoto 1. Twospotted spider mites have two dark spots on the top of the body, regardless of body color. Twospotted spider mite is an occasional pest of both corn and soybean in Iowa that is exacerbated by dry conditions. Most parts of Iowa are considered abnormally dry right now, according to the National Drought Mitigation Center in Lincoln, Neb. Soil moisture conditions this spring are similar to the last drought that hit Iowa in 1988; the same year Iowa experienced a statewide outbreak of spider mites.

Spider mites generally reach economically damaging levels in late July or early August when conditions are favorable for their growth. However, twospotted spider mite can start building populations in June during years with early-season temperatures greater than 85°F, humidity less than 90 percent, and low moisture levels. These are ideal conditions for the twospotted spider mite, and populations are capable of increasing very rapidly. Twospotted spider mites have been reported by several crop consultants in southwestern Iowa this week.

A hand magnifying lens is recommended to scout for the minute (< 1/60 inches long) twospotted spider mites. They can be mistaken for specks of dirt to the naked eye (Photo 1). Twospotted spider mite larvae have six legs and nymphs and adults have eight legs. Mites can be removed from collected leaves by shaking the leaves onto a white piece of paper and then looking for moving mites. Twospotted spider mites are typically a cream or green color when feeding on corn or soybean. They can also be an orange to red color when conditions are unfavorable for their growth.

click image to zoomPhoto by David CappaertPhoto 2. Plants heavily infested with spider mites are typically covered in webbing. Twospotted spider mites begin feeding on the bottom of the plant and move to the top as the plant’s health deteriorates. Although they lack wings, twospotted spider mites disperse with the wind to move from dying plants to areas with healthy plants. Therefore, it is important to scout healthy areas of an infested field that are downwind from damaged areas. Early symptoms of twospotted spider mite damage will appear as small yellow dots or stipples on the lower leaves of the plants. Prolonged feeding will cause the infested leaves to turn completely yellow, then brown, and eventually the leaf will die and fall from the plant. The webbing is visible on the edges and underside of leaves and is an indication of prolonged colony feeding (Photo 2).