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Issue No. 12, Article 3/June 10, 2005

More About Twospotted Spider Mites

Ben Reep, the independent crop scout who found twospotted spider mites infesting a soybean field in Ford County, has found spider mites in three other fields in Ford County. The spider mites were at the edges of fields near ditch banks or filter strips. The infestations in these fields were not as heavy as the infestation that was the focus of the article in the Bulletin last week. In addition, Robert Bellm, Extension crop systems educator in Edwardsville, confirmed a spider mite infestation in Jersey County. More recently, John Fulton, Extension unit leader in Logan County, reported finding spider mites infesting V1-stage soybeans in Logan County. Injury to the unifoliolate leaves was noticeable.

The continued hot, dry weather in many areas of Illinois is ideal for development of populations of twospotted spider mites, so vigilance for this economically threatening pest must increase. The U.S. Drought Monitor reveals that much of Illinois is "abnormally dry," and a large band from St. Louis to Chicago has moderate drought conditions. If you want to visualize how these conditions unfolded this spring, click on "6-week animation" to view the weekly changes from April 26 through May 31, 2005.

Following is a review of the key life history aspects of twospotted spider mites (author T. H. Klubertanz), from the Entomological Society of America's (ESA's) Handbook of Soybean Insect Pests:

  • "Twospotted spider mites in northern states overwinter as diapausing adult females in sheltered areas such as field margins."
  • "Spider mites disperse by crawling and by airborne movement. . . . Female mites on heavily infested plants congregate on the tips of upper leaves where wind may carry them aloft. This behavior can result in extremely rapid dispersal of mites throughout a field."
  • "Spider mite females deposit eggs on the undersides of leaves. Development includes one six-legged larval and two eight-legged nymphal stages, with periods of temporary inactivity during each molt."
  • "The twospotted spider mite has a tremendous reproductive potential. Generations are completed in 4-14 days with fastest developmental rates occurring above 91°F (33°C). Adult females produce as many as 300 offspring in the first month of egg-laying, resulting in exponential population growth."

The latter key aspect is of greatest concern at this point. If spider mite populations gain a foothold in some areas on early-stage soybeans, and if hot, dry weather conditions prevail, we could be in for it. These early signs of trouble do not ensure that an outbreak will develop, but we have to stay on top of it. However, as always, I urge patience when considering whether a miticide application is necessary. Quoting once again from the ESA's Handbook of Soybean Insect Pests: "The rapid recovery of mites after treatment significantly adds to the cost and difficulty of controlling this pest. Because spider mites have tremendous capacity to develop insecticide resistance, avoiding resistance is an important consideration." In 1988, some soybean fields were treated multiple times for control of twospotted spider mites. Multiple treatments may not be economical, and they increase the potential for development of insecticide resistance. So please make your management decisions carefully. This could be a long summer.--Kevin Steffey

Kevin Steffey

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