- Pest Type: Insect
- Crops Affected: Soybeans, Potatoes, Corn
- Scientific Name: Tetranychus urticae
- Pest Order: Acari
Eggs are tiny, spherical, and straw-colored or pearly white; and are laid on the underside of the leaves. Nymphs are six-legged, colorless immature mites, and resemble the adults but are only the size of the eggs. The adult female two-spotted mites range from 0.01 to 0.02 inch (0.25 to 0.5 mm) long, have eight legs, and are yellow to dark green. Both the adults and immature stages have two dark spots on either side of the dorsum. Males are smaller with more pointed abdomens.
During periods of extended hot, dry weather, spider mites leave preferred grasses and other weeds that are dying around fields and invade the crop. Thus, injury from two-spotted spider mites begins along the edges of fields adjacent to weedy hosts or near weedy patches within the field. Spider mites feed by inserting piercing mouthparts into leaf surfaces (usually the underside of leaves) and suck sap from plants, giving the upper leaf surface a speckled or mottled appearance. Leaves of mite infested plants may turn yellow and dry up, lose vigor, and die.
Spider mites develop differently than insects, going through five stages in their life cycle - egg, six-legged larva, eight-legged protonymph, deutonymph, and adult. This complete cycle from egg to adult may require as few as 7 days in mid-summer, but the normal interval is approximately 19 days. Females will lay 50-100 eggs during their lifetimes. Eggs normally hatch after about 5-10 days. Spider mites overwinter as adults deep in the crowns of grasses and other weeds. In the spring, as temperatures warm, adults begin feeding on common hosts such as clovers, chickweed, and various grasses; then they begin migrating to tender, young crop plants.
Control measures are often justified when 10-20% of the leaf area is discolored due to spider mite colonies, and hot, dry conditions are expected to continue. For observation of spider mites, a 10X or 15X magnifying glass is needed. Spot treatments along the edges of the soybean field may suffice, since these mites normally migrate in from fencerows, etc. Proper spray coverage is important for control of these spider mites. If control measures may be necessary, contact local advisers.