Crop consultants in southwest Iowa reported finding two-spotted spider mites this past week. The tiny insect pest showed up in some fields as early as the first week of June this year. Scouting soybean fields and corn fields too, for signs of initial infestations is important to avoid yield loss. Foliar insecticide treatments should be made to fields before prolonged spider mite feeding causes leaf yellowing or leaf drop.
Iowa State University Extension entomologist Erin Hodgson, along with research entomologist Matt O'Neal and grad student Adam Vandenhorst, provide the following information and guidelines to help manage this insect pest.
Two-spotted spider mite is an occasional pest of both corn and soybeans 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.
Dry conditions this spring are similar to the last drought that hit Iowa, in 1988
Spider mites generally reach economically damaging levels in late July or early August when weather conditions are favorable for their growth. However, two-spotted spider mite can start building populations in June during years with early-season temperatures greater than 85°F and humidity is less than 90% and soil moisture levels are low. These are ideal conditions for the two-spotted spider mite, and populations are capable of increasing very rapidly. Two-spotted spider mites were reported by several crop consultants in southwest Iowa the first week of June this year.
It is recommended that you use a hand magnifying lens to scout for the tiny, minute (less than 1/60th of an inch long) two-spotted spider mites. They can be mistaken for specks of dirt to the naked eye (see photo). Two-spotted 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. Two-spotted spider mites are typically a cream or green color when feeding on corn or soybean plants. They can also be an orange to red color when conditions are unfavorable for their growth.
Two-spotted spider mites begin feeding on bottom of the plant and move to the top as the plant's health deteriorates. Although they lack wings, two-spotted spider mites disperse with wind to move from dying plants to areas with healthy plants. Thus, it is important to scout healthy areas of an infested field that are downwind from damaged areas.
Early symptoms of two-spotted 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, and then turn 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 (see photo).
There are not established economic thresholds for two-spotted spider mites in corn and soybeans, but scouting for initial infestations is very important to avoid yield loss. Two spotted spider mite is capable of reducing soybean yield by 40% to 60% when left untreated; drought-stressed plants could experience even more yield loss.
Organophosphate insecticides are the recommended insecticidal chemistry for control of two-spotted spider mites. Examples include dimethoate and chlorpyrifos. These products may not kill the eggs, thus a treated field should be scouted 7 to 10 days after application to determine if a second insecticide application is necessary. As always, refer to the label for the appropriate rates and re-entry intervals.
Pyrethroid insecticides should not be used to control two-spotted spider mites as pyrethroids are not as effective and can actually cause populations to flare up.
Under dry conditions, foliar treatments of insecticide are recommended when plants have substantial stippling or leaf-yellowing and spider mites are active (see photo). Because a naturally-occurring fungus can control spider mite populations, treatment of two-spotted spider mites may not be required when temperatures drop below 85°F and humidity levels are greater than 90% for an extended time. Mites that are infected by the fungus will appear brown and will not move on the piece of paper used for scouting.