Spider Mites in Soybeans in Some Dry Areas of the State

June 23, 2000
For those of us in areas of the state that have been inundated with rain, talk about twospotted spider mites in soybeans seems silly. However, in the dry areas in western Illinois, spider mites have begun to make their presence known. Denny Copp with Lincolnland FS in Auburn (Sangamon County) reported that economic infestations of twospotted spider mites have been noted in edges of soybean fields in his area. Dimethoate has been applied to some fields to stop the spider mites from advancing throughout the fields.

As all of you realize, infestations of twospotted spider mites are associated with prolonged drought. Spider mites feed by inserting their long, needlelike mouthparts into leaf cells and extracting the liquid contents. Spider mites ingest a more nutritional diet when water is in short supply, resulting in greater fecundity of females (more eggs per female). In addition, hot, dry weather speeds up their development, so generations overlap and populations build quickly.

Close-up of twospotted spider mite adult.

Stages of growth of twospotted spider mites include egg, larva, protonymph, deutonymph, and adult (Figure 3). Adults are extremely small (0.002 inch) and have eight legs. They are green-yellow with two large, irregularly shaped black spots on each side of the body. Twospotted spider mites can complete their life cycle in 10 to 20 days, depending on weather conditions.

Twospotted spider mite injury to soybean plants at the field edge.

Twospotted spider mites clumped at the tip of a soybean leaflet.

Twospotted spider mites overwinter as females in noncrop areas such as field margins. Spider mites disperse by crawling and by airborne movement. Their crawling from the weeds in field margins to soybean plants usually results in initial infestations along field edges. Initial symptoms of injury include yellow stippling of the leaves and stunting of the plants. As injury becomes more severe, leaves turn completely yellow and may turn brown, die, and drop from the plants. If densities of the mites continue to increase, the mites group up at the tips of leaflets and spin out strands of webbing that catch in the wind, which carries the mites to other areas of the field.

Management of twospotted spider mites in soybeans depends greatly on vigilance. At the first sign of injury caused by spider mites, you should examine the injured area to look for the mites. A quick way to check for mites is to hold a piece of paper under soybean leaves and tap the leaves gently to dislodge the mites. If mites are present, you will be able to see them moving across the paper. A good magnifying glass will enable you to see some of their morphological characteristics. Another telltale clue of a spider mite infestation is the presence of webbing on the undersides of the leaves.

If you find spider mites only in field edges, spot treatments to prevent additional damage and to halt their movement may be justified. However, you should make certain that spider mites are not present throughout the field. Even healthy-appearing plants may be supporting a few spider mites, and a few spider mites can become a lot of spider mites rather quickly. Dimeth-oate (check various labels for formulations and rates of application) and Lorsban 4E at 1/2 to 1 pt per acre both have provided satisfactory control of spider mites in the past.--Kevin Steffey

Author: Kevin Steffey