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An Early Warning about Twospotted Spider Mites

May 25, 2001
The recent rainfall in many areas of Illinois probably has dissipated thoughts of droughts and the problems that frequently accompany hot, dry weather. However, areas in southern Illinois still are lacking moisture, and a return to hotter weather will re-invigorate concerns. That's why it's important to report about the first observation of twospotted spider mites in Illinois this year. On May 16, Dennis Epplin (crop systems educator, Mt. Vernon Extension Center) and Robert Bellm (crop systems educator, Edwardsville Extension Center) observed a red clover field in Hamilton County that had localized (5 to 20 square feet) but very heavy infestations of twospotted spider mites. The mites had killed the clover in the infested areas.

The primary reason I am alerting people about this situation is that similar problems began to show up early in 1988. Noel Troxclair, former IPM educator in Benton, reported to us early that year that he was observing areas of red clover fields that were being killed by spider mites. In fact, he sent us a lot of slides verifying the problems. As we all know now, those early infestations developed into the most devastating outbreak of twospotted spider mites that we have ever encountered. In 1983, another year noted for a widespread outbreak of spider mites, the infestations developed much later in the year (July and August) than they did in 1988.

Now, before anyone begins to freak, the situation in 2001, thus far, is different from the situation in 1988. As I indicated, concerns about drought have been alleviated for now by very timely and frequent rains. However, in areas where dry conditions still prevail, it's not too early to begin watching for buildups of twospotted spider mites along field margins. As populations of spider mites increase, the mites will move from their overwintering sites (usually undisturbed areas such as clover fields and field margins) into adjacent rows of soybeans. Watch for soybean plants that become stunted and yellow as a result of the mites' feeding activities, and look for webbing on the undersides of the leaves. Twospotted spider mites are extremely small (0.3 to 0.4 mm), green-yellow to dull orange, with two large, irregular-shaped, black spots on each side of the body. Nymphs have six legs, and adults have eight legs.

Infestation of twospotted spider mites at the edge of a soybean field.

Close-up of an adult twospotted spider mite.

So, stay alert out there, and let us know if you find infestations of twospotted spider mites developing anywhere under any conditions. Early knowledge of such infestations will help us prepare, if necessary.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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