The lack of rain and the warm temperatures experienced throughout many regions of Illinois over the past month have created conditions conducive for outbreaks of two-spotted spider mite, Tetranychus urticae. Under moist conditions, spider mites are generally not a problem because natural fungi keep the populations in check. However, when rainfall is insufficient, the natural fungal populations decline, allowing spider-mite populations to increase.

There are two types of spider mites that attack trees and shrubs in Illinois: warm-season and cool-season mites. Cool-season mites, which include the spruce spider mite, Oligonychus ununguis, are active in spring and late fall. During the summer months, they remain in the egg stage. Warm-season mites, such as the two-spotted spider mite, are primarily active during late spring to early fall.

Two-spotted spider mites feed on a variety of trees and shrubs, including rose, azalea, maple, elm, redbud, ash, euonymus, black locust, and poplar. They are green to greenish yellow with two lateral dark spots that are visible when the mite is viewed from above. The two-spotted spider mite can be found on all areas of plants but is often more numerous on older leaves. It primarily feeds on leaf undersides within plant cells and removes chlorophyll (green pigment) with its stylet-like mouthparts. Spider mites generally feed near the midrib and veins. The leaves appear stippled with small silvery-gray to yellowish speckles. Heavily infested leaves turn brown and eventually fall off.

Warm and dry conditions favor rapid spider-mite development and increased feeding and reproduction. The life cycle from egg to adult can occur in 5 days at 75°F. Females, which don’t have to mate to reproduce, live 2 to 4 weeks and can lay 100 to 300 eggs. Two-spotted spider mites spend the winter in protected places, such as on weeds, in ground litter, or in debris. They do not overwinter on plants, which means that applications of dormant oil sprays are not effective.

Two-spotted spider-mite management involves maintaining plant health and/or the use of pest-control materials. Reducing plant stress through proper watering and fertility minimizes potential problems with spider mites. For example, lack of sufficient moisture or overfertilizing plants, especially with nitrogen-based fertilizers, will result in higher spider-mite populations. Monitor for spider mites by knocking them off branches onto white paper, where they can be seen more easily. Plant-feeding spider mites produce a green streak when smashed, whereas predatory mites produce a red streak when smashed. A hard spray of water can be used to dislodge spider-mite eggs and live spider mites.

Pest-control materials recommended for managing spider mites include abamectin (Avid), bifenthrin (Talstar), dicofol (Kelthane), hexythiazox (Hexygon), insecticidal soap, and summer oil. Be sure to concentrate sprays on leaf undersides. Make spray applications before spider-mite populations are high and aesthetic injury is visible. Note that many of these pest-control materials are harmful to beneficial insects and mites that naturally feed on spider mites, so that continual use of these materials may be necessary.