No. 17/July 18, 1997
Spider Mites Cause a Few Problems in Northern Illinois
In areas of Illinois that have received little rainfall since May, twospotted spider mites are beginning to cause injury to soybeans. I have received reports from people in northern counties (LaSalle, Lee, Ogle) that spider mite injury is apparent in field margins and that some soybean fields have been treated with miticides. Obviously, this situation will get worse if the weather remains hot and dry. However, there's no reason yet to assume that the situation will parallel what happened in 1988. During that infamous drought, spider mite populations began building up in June, and the outbreak was widespread and intense in July. What has occurred thus far in 1997 is more similar to the circumstances in 1983, when spider mites were problematic in late July and August.
Let's review some fundamental information about twospotted spider mites and the knowledge we gained in 1988. Twospotted spider mites are not insects; rather, they are related more closely to ticks and spiders. An adult female is oval and has eight legs; an adult male is narrower and has a somewhat pointed abdomen. The mites are usually pale yellow to brown. The food contents accumulate in two spots on either side of the mite, and you can see these spots through their cuticle (skin); hence the name of the mite. After mating, females lay light-colored, spherical eggs, usually on the undersides of leaves. The eggs hatch into six-legged larvae, which grow into adults in a matter of days. Spider mites complete their life cycle from egg to adult in less than a week (4 to 5 days) when temperatures are high. Thus, you usually can find all stages of the mites on soybean leaves. In addition, as the mites continue to reproduce, overlapping generations occur and densities increase rather rapidly.
As experience and research have shown us, outbreaks of twospotted spider mites in soybeans occur only during hot, dry weather. These spider mites overwinter in Illinois in undisturbed areas, including grassy and weedy areas adjacent to soybean fields. However, during most years when rainfall is plentiful and temperatures are not very high, spider mites usually remain in these noncrop areas, reproducing and feeding contentedly. When the weather becomes hot and dry, spider mites begin showing up in the edges of soybean fields. If these weather conditions persist, densities of spider mites increase rapidly and the problem spreads into the fields.
Although the information in the research literature is not entirely consistent, it is clear that drought conditions favor outbreaks of twospotted spider mites. There are numerous explanations for this: (1) Drought provides a more favorable thermal environment for the mites' growth and development; (2) drought-stressed plants are more attractive to and acceptable for the mites; (3) drought-stressed plants are physiologically more suitable for the mites; (4) drought conditions do not favor natural enemies and pathogens of spider mites; and (5) drought might induce genetic changes in the mites.
Some research has shown that high temperatures significantly increase spider mites' reproductive rates and decrease generation times. Another researcher found that high temperatures increase the mites' metabolic rate by increasing demand for dietary fluids. However, other research has suggested that outbreaks of twospotted spider mites during drought years should not be attributed solely to drought-induced physiological parameters within soybean plants. Research at Iowa State University published in the early 1990s showed that the spread of a fungal disease occurs only during sustained cool and humid weather. We can speculate that this fungus suppresses spider mite populations in Illinois when conditions are not hot and dry. The bottom line is that no single factor causes an outbreak of spider mites when drought conditions prevail. The relationship among the mites, soybean plants, and weather is rather complicated.
Twospotted spider mite adults are quite small (about 1/60 inch, or less than 1 mm), often compared to a period at the end of a sentence. Consequently, a good magnifying lens comes in handy as you begin to scout for these pests. As mentioned, infestations usually begin at field edges, so that's where symptoms of injury first appear. The mites use their piercingsucking mouthparts to remove fluids from plant cells; and the cells collapse, resulting in stippling of the leaves. A symptom of feeding injury caused by spider mites are leaves that turn yellow, bronze, and then brown as they die. Lower leaves typically are injured first. Also, webbing usually is evident on the undersides of infested leaves. As the infestation increases, dead leaves begin to fall from the plants, and the injury in the field spreads. Although symptoms of injury usually appear first at the field edges, yellow spots in the interior of the field also may occur. The mites sometimes move by "ballooning," a process by which they extrude a strand of webbing that catches the wind and carries them elsewhere. Consequently, when you scout for spider mites, you need to look throughout the field, even on apparently healthy plants, to assess the extent of the infestation.
To examine plants for spider mites, hold a sheet of paper or cardboard under the leaves and tap the leaves to dislodge the mites. Yellow or brown specks that move across the paper or cardboard probably are spider mites. Remember, sample both injured and healthy plants to determine if the infestation is spreading. Also, be certain that the yellowed soybeans are caused by spider mites. Other factors can result in soybeans that turn yellow.
If the infestation of twospotted spider mites is isolated within a soybean field, a "spot" treatment with either Dimethoate 400 or Lorsban 4E is warranted. However, I re-emphasize that scouting the entire field is necessary. Apparently healthy plants may be infested, in which case a spot treatment does not prevent additional injury to the soybeans.
A word of caution about Dimethoate 400. I am aware of at least one instance in which two sprays of the miticide did not control spider mites adequately. The applicator then applied Lorsban 4E, which provided effective control. This individual contacted the supplier of Dimethoate and was told that some resistance of twospotted spider mites to dimethoate (the active ingredient) is a possibility. However, this does not mean that we have resistance to dimethoate in the twospotted spider mite population in Illinois. Our use pattern with this product on spider mites in soybeans is infrequent, so it seems unusual that resistance to dimethoate could occur so quickly in spider mites. Nevertheless, you need to be aware of this information when you make a decision to apply a miticide. Also, when you evaluate the performance of a miticide, be certain you evaluate as accurately as possible. If performance seems poor, review your application records. Because the mites are usually on the undersides of the soybean leaves, adequate pressure and sufficient gallons per acre are required to obtain effective results. Be sure to let us know if you have any concerns about the performance of either Dimethoate or Lorsban.
Kevin Steffey and Mike Gray, Extension Entomology, (217)333-6652