Spider mites are the most destructive of the economic pests found on soybean plants in our hot interior valleys. Destructive species are the two-spotted mite, Tetranychus urticae K., and the Pacific mite, T. pacificus McG. High population densities of several hundred to over a thousand per leaflet may build up. The light to moderate white-stippled type of injury intensifies so that leaves turn yellowish, then brown, and drop prematurely. This causes a reduction of pod set and yield of seed. The studies have indicated that mite numbers should be kept at or below 4 to 6 per unit count to obtain any sort of satisfactory seed yield. After pod set commences, it appears necessary to have some type of mite control in or on the soybean plants or serious damage and seed loss will occur. Superior mite control has been obtained by chiseling a granular pesticide, such as Thimet, into the soil at planting time and again as a side-dressing in the seedling stage. Kelthane has afforded the best control of the foliage sprays tested. Trithion was fair but multiple sprays were needed. Pesticides discussed in this article have not been registered for use on soybeans and are not recommended for use at this time.

John Campbell, Nurseryman, Department of Entomology, U.C., Davis assisted in these studies. Seeds were obtained and field plantings carried out with the assistance of the Agronomy Department, U.C., Davis and the West Side Field Station, U.C., Five Points. These investigations were financed by funds allocated from the Oil Seed Crop Research Trust.