No. 21 Article 4/August 12, 2005

A Little More Information About Twospotted Spider Mites

Frustration continues about less-than-hoped-for efficacy of miticides applied to control twospotted spider mites in soybeans and about making control decisions. I don't think there's much more that we can add to the dialogue. However, you should know that the twospotted spider mite population in the soybean field (near Tolono) where our experiment is located has crashed across the board. The average numbers of spider mites per five leaflets in all eight treatments in the trial range from only 12 to 77, compared with numbers in excess of 800 mites per five leaflets in mid-July. The average number of mites per five leaflets in the untreated check plots was 20.5 on August 5. If this phenomenon repeats itself elsewhere (and there are reasons to believe it will), you may wish to consider whether applying a miticide this late in the summer is necessary. Obviously, if mite densities continue to increase from one sampling date to the next, a treatment decision may be easy, depending on the condition of the soybeans. However, if a field has been sprayed once or even twice before this summer, it may make more sense to wait for the crash than to spend more money as margins narrow. When all is said and done, we should be able to determine the impact of early, late, multiple, and no applications of miticides on soybean yield.--Kevin Steffey

Close this window