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Issue No. 11, Article 3/June 3, 2005

Twospotted Spider Mites in Soybeans Add Another Level of Concern

Across a lane from the field of soybeans in which soybean aphids were found in Ford County (refer to the article "The Soybean Aphid Season Has Begun"), we found an infestation of twospotted spider mites on VC stage (unfolded unifoliolate leaves) soybeans. The area infested was an estimated 5 acres, and the infestation appeared to be significant. Numerous mites and even more numerous mite eggs were found on the undersides of the unifoliolate leaves, the topsides of which showed classic symptoms of spider mite feeding injury--yellow stippling, with some signs of small, necrotic areas. The spider mites were just beginning to spin webs on the soybean plants.

The spider mites appeared to have moved from a destroyed area of alfalfa-bromegrass and also from some henbit (Lamium amplexicaule) that had been sprayed with Roundup. Webbing, mite eggs, and a few mites were still visible on the dying henbit.

(Just before submitting this article to our editors, I learned from Robert Bellm, Extension crop systems educator in Edwardsville, about a tentative report of spider mites infesting V1 stage soybeans in Jersey County.)

I looked through some 1988 issues of the Insect, Weed & Plant Disease Survey Bulletin, which evolved into the Bulletin. As many of you will remember (and not fondly), twospotted spider mites wreaked havoc with our soybean crop during the 1988 drought. Our first report of spider mite infestations in 1988 was published in issue no. 9 (June 3). The first reports of problems were from southern Illinois (Hamilton and Perry counties), where fields of red clover were severely damaged. The spider mites moved from the dying red clover plants into the edges of soybean fields. And that's how it all began in 1988, when an estimated 4.5 million acres of soybeans in Illinois were sprayed for spider mite control.

I certainly am not suggesting that the situation in 2005 will be the same as many of us experienced in 1988. However, this early sighting of twospotted spider mites indicates that in addition to monitoring for soybean aphids, crop scouts and growers should look for signs of spider mite infestations. The continuing lack of moisture and an onset of higher temperatures could contribute to a buildup of twospotted spider mite populations. According to the most recent Illinois Weather & Crop Report, "Conditions are particularly poor in central areas of Illinois where topsoil moisture is rated over 80 percent short to very short. The state average topsoil moisture was 17 percent very short, 51 percent short, 31 percent adequate, and 1 percent surplus."

Twospotted spider mite infestations are most often linked with extended hot, dry conditions. Although populations usually don't peak until late July and August, early establishment of spider mite populations enables a buildup to potentially threatening levels earlier in the season.

Injury to soybeans caused by twospotted spider mites usually is noticed along the edges of fields first, where the mites have moved from other hosts. Twospotted spider mites have an extremely broad host range, having been recorded on more than 150 species of plants. The following is excerpted from a chapter (by Thomas H. Klubertanz) in the Entomological Society of America's Handbook of Soybean Insect Pests: "Spider mites feed with long, styletlike mouthparts that are inserted into leaf cells. Contents of the individual, living cells are extracted by mites. . . . As mites feed on a soybean leaf, small yellow or white spots called stipples become visible. Stippling usually is evident on both sides of the leaf, especially for younger leaves. . . . Generally, spider mite feeding results in a reduction in photosynthetic capacity and chlorophyll content of leaves. As mites insert their mouthparts deeper into leaf tissues, the protective leaf epidermis becomes highly disrupted. Water loss through these wounds results in moderate to severe leaf water stress. . . . With increased mite injury, leaves become yellow, bronzed, brown, and eventually drop off the plant."

Injury to soybean leaf caused by twospotted spider mites (photo courtesy of Marlin Rice, Iowa State Unviersity).

When scouting for twospotted spider mites, you usually find them on the undersides of the leaves. You can tap soybean leaves over a piece of paper or cardboard to dislodge the mites for easier viewing. Twospotted spider mites are extremely small (adults are about 0.3 to 0.4 mm long), and adults have eight legs. They are greenish yellow to dull orange, with two large, irregular-shaped black spots on each side of the body, which actually are food balls seen through the mite's integument. Look also for webbing on the undersurface of leaves.

Twospotted spider mite (photo courtesy of Marlin Rice, Iowa State University).

We still have a less-than-satisfactory treatment guideline for twospotted spider mites in soybeans: treat when 20% to 25% discoloration is noted before pod set. In the field I visited in Ford County, 20% to 25% discoloration will occur very shortly if rainfall does not occur soon. The numbers of mites crawling on the leaves and, more significantly, the very large numbers of eggs I observed suggest that the population will increase in size very quickly. It is very likely that the infested portion of that field will require a miticide to control the spider mites and prevent further injury to the seedlings.

The two products suggested for control of twospotted spider mites in soybeans in Illinois are Dimethoate 4EC (1 pint per acre) and chlorpyrifos (Lorsban 4E or Nufos 4E at 1/2 to 1 pint per acre). The use of Lorsban 4E or Nufos 4E is restricted to certified applicators.

I offer a word of caution regarding application of insecticides for control of spider mites in soybeans: in general, the pyrethroids (i.e., Asana XL, Baythroid 2, Mustang Max, Proaxis, Warrior) labeled for use in soybeans have not been very effective. In fact, use of pyrethroids to control spider mites attacking other crops (e.g., corn, orchard crops) has resulted in mite "flare-ups," because the pyrethroids killed off mite predators. So, although soybean aphids can be controlled very effectively with these pyrethroids, it is advisable to use only chlorpyrifos or dimethoate for control of spider mites in soybeans.--Kevin Steffey

Kevin Steffey

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