Spider Mites Problems in Dry Areas of State By Wayne Bailey

Prior to rainfall from tropical storm Dennis, two-spotted spider mite populations were rapidly increasing in the drought areas of the state and had reached or exceed economic levels in some locations. The potential for this organism to continue as a pest of soybean remains high in those areas of the state where recent rainfall was limited or lacking.

Spider mites are small organisms most closely related to chiggers and spiders than to insects. The two-spotted spider mite, Tetranychus urticae, is often an economic pest of soybean and to a much lesser extent of corn in Missouri during periods of drought conditions. This pest gets its name from two dark spots on the sides of the abdomen which are visible through the mite’s translucent, greenish-yellow, white, orange, or red colored body. Spider mites feed on the underside of soybean leaves and are difficult to detect due to their small size of about 1/60 of an inch. Damage to soybean is thought to be cause by the mites piercing individual plant cells with their mouthparts and then feeding on cell contents. Spider mite injury to soybean initially appears as yellow stipples or spots on soybean plants growing along field margins (where mites often overwinter). As mite populations increase, damage moves across the field as mites infest additional soybean plants. In heavy infestations the yellow stipples are generally followed by the injured foliage turning yellow, then brown/bronze and finally dropping from the plant as leaves senesce or dry. Identification of this pest is best accomplished using at least a 10X magnification lens or by shaking infested leaves over a white paper and watching for the small yellow mites to crawl about the paper after being dislodged from the soybean plant.

Although good thresholds for this pest are not available, treatment of infested fields is recommended if drought conditions exist, stipples are present of soybean leaves, and live mites are present. Generally infestations of this pest move downwind, so it is necessary to scout the entire field to determine if mites are present in spots or throughout the entire field. If hot and dry conditions persist, the entire field may require treatment even if mite numbers are low in some areas of the field. Both soybean maturity and shattering of grain will be increased by the presence of economic levels of spider mites. Fields which received rainfall should continue to be scouted as damaging levels of spider mite may redevelop if drought conditions return.

Two insecticides are labeled for spider mite control in soybean. They include dimethoate, sold under several labels, and chlorpyrifos, marketed as either Lorsban 4E or Nufos 4E. The use rate for dimethoate varies depending on the formulation, whereas, Lorsan 4E and Nufos 4E are labeled for use at ½ to 1 pt per acre.

Wayne Bailey
Entomology
573-882-2838