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Twospotted Spider Mite

August 1, 2001
The high temperatures, humidity, and low rainfall generally experienced throughout many regions of Illinois over the past weeks has created conditions conducive for outbreaks of twospotted spider mite, Tetranychus urticae. 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, the natural fungi populations decline, allowing spider mite populations to increase.

Twospotted spider mites are considered warm-season mites because they are primarily active during late spring through early fall. Summer temperatures allow spider mites to overwhelm the populations of beneficial insects and mites that are able to 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, poplar, redbud, and rose. They also feed on many herbaceous annuals and perennials. Twospotted spider mites are green to greenish yellow, with two lateral dark spots that are visible when the mite is viewed from above. The twospotted spider mite can be found on all areas of plants but is often more numerous on older leaves. These mites spin fine silk, which is sometimes seen between leaves and between petiole and stem. However, rainfall easily washes this webbing away.

Twospotted spider mites primarily 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. 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 degrees Fahrenheit. Females, which donít 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, which means that 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. Avoiding 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, results in higher spider mite populations. Monitor for spider mites by knocking them off plant parts (that is, 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 when crushed. A hard spray of water can be used to dislodge spider-mite eggs and live spider mites. Removing plant debris and weeds eliminates overwintering sites for spider mites. In addition, many weeds, especially broadleaves, serve as a host 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. Be sure to concentrate sprays on leaf undersides. Avid has translaminar properties, which means the active ingredient penetrates the leaf surface and resides in leaf tissues--killing spider mites that are feeding on the underside of leaves. As a result, coverage of leaf undersides is less critical. Hexygon primarily kills the egg and nymphal stages of twospotted spider mites, with no activity on adults. Avoid the use of organophosphate-based insecticides (that is, Orthene, Malathion, Dursban, and Diazinon) because these materials tend to stimulate female spider mite reproduction. Make spray applications before spider mite populations are high and aesthetic injury is visible. It is important to note that many of these pest-control materials are also harmful to beneficial insects and mites that naturally feed on spider mites, making continual use of these materials necessary once applications are started.

Author: Raymond A. Cloyd


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