Two-Spotted Spider Mite
Description
Adult: Females are 0.4 mm long and oval; males are slightly smaller with a tapered hind end. They are orange during the winter and early spring, turning yellow-green with two dark spots once feeding begins. Adults have red eyes, and as they mature, their bodies turn dark brown, and the two dark spots become less distinct.
Egg: Eggs are round, 0.5 mm in diameter, and translucent to opaque.
Immature stages: Initially round, translucent to pale green, with three pairs of legs when first hatched, then oval, dark green with black spots on back with four pairs of legs as it matures. Stages include larvae, protonymph, and deutonymph.
Life History
The mites overwinter in non-crop and weedy areas, such as grassy banks along irrigation ditches and roadsides, on weeds, in fallowing fields, and in pastures. Spider mite populations increase in mid to late summer, spread onto entire corn plants. Mites can complete their development (one generation) in as quickly as 1 week; in cooler weather, it may take a month. Eggs hatch within 3 to 19 days, depending on temperature. Webbing produced by spider mites helps fasten eggs to leaf surfaces and provides protective cover, making the eggs difficult to see. Unfertilized eggs develop into males and fertilized eggs develop into females.
Damage
Spider mites feed by piercing leaf cell walls with their mouthparts, sucking out the cell’s contents, and causing characteristic stippling damage (small spots). Heavily infested leaves are yellow or brown and may also appear burnt on the upper surface. Severe damage from mite feeding causes leaves to dry and fall off, the stalk to break, and kernels to shrink. Infestations start on the undersides of lower leaves and gradually move into the upper part of the plant. This pattern occurs especially along the field borders or near grassy areas within fields.
Management
Cultural
- Ensure adequate irrigation. Mites are more likely to develop economically damaging populations in fields that are moisture-stressed during the drier and hotter summer months. Frequent overhead irrigation or heavy rain suppress mites.
- Use scouting to detect mite infestations. Check the undersides of leaves for minute webbing on discolored leaves. Check plants that are on the field edges, especially in fields that are close to dusty roads, ditches, and grassy areas. Shake discolored leaves over a white piece of paper and look for dark specks that move. Use a hand lens or magnifying glass to see the tiny mites.
- Control weeds. Keep fields, field margins, and irrigation ditches clean of weeds. Spider mites use weeds as alternate food sources.
- Avoid creating heavy dust. Spider mite populations may increase rapidly in areas where dust deposits are heavy on foliage.
Chemical
Miticides are typically necessary when 15% to 20% of the leaf area is covered with mite colonies, leaf damage is noted, and hot, dry conditions are predicted.
Biological
Many fields don’t require chemical treatment because mite populations are held in check by natural enemies. The most important natural enemies of spider mites are a predatory mite, Amblyseius fallacis, minute pirate bug, Orius insidiosus, and Stethorus, a small black lady beetle known as the “spider mite destroyer.” Other predatory mites, thrips, and lacewing larvae prey on spider mites and offer some natural control.