With all of the dry weather occurring throughout much of Illinois, it is important to be aware of problems due to twospotted spider mite, Tetranychus urticae. Populations of this pest tend to multiply rapidly under hot and dry conditions—as we have experienced lately. When rainfall is low, populations of natural fungi decline, allowing twospotted spider mite populations to increase. However, when conditions are moist and rainfall is sufficient, naturally occurring fungi generally keep twospotted spider mites in check. Although we have recently received rainfall in much of the state, we have been seeing numerous cases of mite damage. These will increase if weather predictions of continued hot, dry weather are correct.|
Twospotted spider mite is considered a warm-season mite because it is active primarily from late spring through early fall. Summer temperatures allow twospotted spider mites to reproduce so rapidly that they overwhelm populations of beneficial insects and mites that can control them at moderate temperatures.
Twospotted spider mites feed on a wide diversity of trees and shrubs, including ash, azalea, black locust, elm, euonymus, maple, oak, poplar, redbud, and rose. They also feed on many herbaceous annuals and perennials, such as marigold, pansy, aquilegia (columbine), buddleia (butterfly bush), clematis, daylily, delphinium, phlox, rudbeckia, salvia, shasta daisy, and verbena.
Adult twospotted spider mites are oval and about 1/16-inch long. They vary in color from greenish yellow to reddish orange, with two lateral dark spots that are visible when the adult is viewed from above. Adults and nymphs can be found on all plant parts but are more numerous on older leaves. Twospotted spider mites produce a fine silk, which is sometimes observed between leaves and between the petiole and stem. This webbing protects populations of twospotted spider mites from their natural predators. A heavy rainfall usually washes this webbing away.
Twospotted spider mites feed on leaf undersides, removing chlorophyll (green pigment) from individual plant cells with their stylet-like mouthparts. They typically feed near the leaf midrib and veins, where the highest concentrations of amino acids are present. Leaves that have been fed on appear stippled with small silvery gray to yellowish speckles. Plant leaves heavily infested with twospotted spider mites will appear bronzed, turn brown, and eventually fall off.
Warm, dry conditions favor rapid development, increased feeding, and enhanced reproduction of twospotted spider mites. The life cycle from egg to adult can occur in 5 days when temperatures reach 80ºF. Twospotted spider mite females, which don’t have to mate to reproduce (this doesn’t sound like much fun), can live for 4 weeks, laying up to 300 eggs. Twospotted spider mites spend the winter in protected places, such as weeds, or in ground debris. Unlike spruce spider mite (Oligonychus ununguis), twospotted spider mite does not overwinter on plants, so applications of dormant oil sprays are not effective.
Twospotted spider mite management involves maintaining plant health, practicing proper sanitation, and using appropriate pest control materials (miticides). Avoid plant stress through proper watering, fertility, and mulching, which will minimize potential problems. For example, inadequate moisture or overfertilizing plants, particularly with nitrogen-based fertilizers, will result in higher than “normal” outbreaks of twospotted spider mites.
Monitor for twospotted spider mites by knocking them off plant parts such as leaves and branches onto a white sheet of paper, where they can be observed more easily. This is particularly important after periods of rainfall, because damaged plants may no longer have high spider mite populations. Plant-feeding spider mites such as twospotted spider mite and spruce spider mite produce a green streak when crushed, whereas predatory mites produce a red streak.
A very effective and cost-efficient way of dealing with twospotted spider mites is to apply a hard spray of water, which dislodges all the life stages, including eggs, from plants. (Remember, water is not registered as a pesticide—yet). Removing plant debris and weeds eliminates overwintering sites. In addition, many weeds, especially broadleaves, are hosts for twospotted spider mites.
Pest control materials recommended for controlling twospotted spider mites outdoors include abamectin (Avid), bifenazate (Floramite), bifenthrin (Talstar), etoxazole (TetraSan), hexythiazox (Hexygon), insecticidal soap, and horticultural (summer) oil. Both Avid and TetraSan have translaminar properties, meaning that the active ingredient penetrates the leaf surface and resides in leaf tissues, thus killing any twospotted spider mites feeding on leaf undersides. Coverage of the undersides of leaves is less critical with these materials. All of the other products listed have contact activity only. Hexygon primarily kills the egg and nymphal stages, with no adult activity. Be sure to make applications before twospotted spider mite populations are “high” and aesthetic injury becomes noticeable. Many pest-control materials recommended for twospotted spider mites are harmful to beneficial insects and mites that naturally feed on them, potentially leading to continual use of these materials once applications are initiated.