High temperatures, humidity, and low rainfall experienced over much of Illinois the past weeks have created conditions favorable for outbreaks of twospotted spider mite, Tetranychus urticae. For example, feeding injury is apparent on euonymus.

Under moist conditions, when rainfall is sufficient, spider mites are generally not a problem because naturally occurring fungi keep the populations in check. However, under conditions of low rainfall, populations of natural fungi decline, allowing spider mite populations to increase rapidly.

Twospotted spider mites are considered warm-season mites because they are mainly active from late spring to early fall. Summer temperatures allow them to overwhelm populations of beneficial insects and mites that control them at moderate temperatures.

Twospotted spider mites feed on a wide range 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, including marigold, pansy, aquilegia, buddleia, clematis, daylily, delphinium, phlox, rudbeckia, salvia, shasta daisy, and verbena.

Twospotted spider mite adults are oval, 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 mite is viewed from above. The adults and nymphs can be found on all areas of plants but are often more numerous on older leaves. These mites produce fine silk, which is sometimes seen between leaves and between the petiole and stem. Rainfall usually washes this webbing away.

Twospotted spider mites mainly feed on leaf undersides, removing chlorophyll (green pigment) from individual plant cells with their styletlike mouthparts. They generally feed near the leaf midrib and veins where the highest levels of amino acids are present. The leaves appear stippled with small silvery gray to yellowish speckles. Heavily infested leaves appear bronzed, turn brown, and eventually fall off.

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

Management of twospotted spider mite involves maintaining plant health, sanitation, and/or the use of pest-control materials. Avoid any type of plant stress by proper watering and fertility as this minimizes potential problems with spider mites. For example, lack of moisture or overfertilizing plants, especially with nitrogen-based fertilizers, results in higher mite populations. Monitor for spider mites by knocking them off plant parts (branches) onto a sheet of white paper, where they can be seen more easily. Plant-feeding spider mites produce a green streak when crushed, whereas predatory mites produce a red streak. A hard spray of water (not a registered pesticide … yet) can dislodge spider mite eggs and live mites. Removing plant debris and weeds eliminates overwintering sites. Also, many weeds, especially broadleaves, are hosts for spider mites.

Pest-control materials recommended for managing spider mites outdoors include abamectin (Avid), bifenthrin (Talstar), dicofol (Kelthane), hexythiazox (Hexygon), insecticidal soap, and summer oil. Concentrate sprays on leaf undersides. Avid has trans-laminar properties, meaning the active ingredient penetrates the leaf surface and resides in leaf tissues--killing spider mites feeding on the underside of leaves. As a result, coverage of leaf undersides is less critical. Hexygon primarily kills the egg and nymphal stages, with no activity on adults. Avoid using organophosphate-based insecticides (for example, Orthene, Malathion, Dursban, and Diazinon) because they may stimulate female spider mite reproduction. Make spray applications before 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, potentially making continual use of these materials necessary once applications begin.