University of Illinois

No. 21/August 15, 1997

More News about Spider Mites

Recent rains have prompted a lot of questions regarding its effect on spider mite problems in soybeans. First, the rain, if adequate, should enable the soybean plants to tolerate the feeding of spider mites a little better. Second, the rain should raise the relative humidity, so the potential for an epizootic of the fungal disease we discussed in a previous issue of this Bulletin is enhanced. However, if the fungal organism is not present, the spread of the disease will not occur. In addition, it's a myth to believe that spider mite problems are solved completely after it rains. If densities of spider mites are increasing, rain may not help much.

Keep monitoring soybeans for spider mites and symptoms of their injury. They won't go away immediately, but several more timely and plentiful rainfalls likely will reduce the mites' impact on yields.

Also, as a potential explanation for some control problems with dimethoate, at least one individual told me that results with dimethoate improved after lowering the pH of the spray water. Although I do not know the specifics about dimethoate, I am aware that several pesticides are pH sensitive. I pass this information along in case any of you have had similar experiences with dimethoate and wish to share your advice with others.
Kevin Steffey, Extension Entomology, (217)333-6652