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Twospotted spider mites are very small greenish, yellowish to orange arachnids with two dark spots on their abdomen. Adults can barely be seen with the naked eye, and they have 8 legs (not 6 as in insects). Spider mites progress through three life cycle stages (egg, nymph, and adult), and complete multiple generations during the growing season. Egg to adult development takes 5 to 19 days, with the lesser amount of time at hotter temperatures.
Drought is a trigger for spider mite outbreaks in soybean fields. Hot and dry conditions reduce natural fungi that infect mites, and increase their reproductive rate. Soybean plant tissue also becomes a more attractive food source for mites under drought stress conditions, and movement to soybean plants from other vegetation increases.
Figure 1. Spider mites on the edge of a soybean field.
Spider mite infestations typically are first noticed near field edges or where soybean plants are stressed (Figure 1). They establish colonies on the undersides of soybean leaves. The colonies produce a webbing on the leaf surface that earns them the name “spider�? mites. They can be carried on a balloon of their webbing by wind and dispersed over a wide area.
Figure 2. Spider mites on soybean plants.
Soybean spider mite damage is often noticed before the mites. They injure soybean leaves by piercing cells and sucking out the contents (Figure 2). This produces white or yellow spots or “stipling�? most noticeable on the underside of the leaves. Feeding damage begins in the lower canopy and progresses upwards. As mite colonies grow and feeding intensifies, plants take on a yellowed then bronzed appearance. The leaves can eventually die and plants could defoliate with continued mite pressure.
Soybean plants injured by mites can mature early, have increased shattering, and produce smaller and wrinkled seed. Yield can be significantly reduced with mite injury occurring during late vegetative and early reproductive growth stages.
Recognizing the speckling or stipling effect on the lower leaves when foliage is still green is important for early detection of mite feeding. Soybean fields should be scouted weekly for evidence of developing spider mite populations. Fields should be checked more often if drought conditions persist since damaging infestations can develop quickly.
When scouting for mites, look at soybean plants at the field edge first, especially adjacent to drainage ditches, alfalfa, or corn. Examine leaves from the bottom of the plant upwards. Look at the undersides of leaves and note any webbing, speckling or stipling. Spider mites can be spotted by shaking plants over a white sheet of paper and look for moving specks on the paper. A hand lens is useful to observe the relative abundance of mites in egg, nymph, and adult stages. Examine how far the plant mites and symptoms have progressed. Then walk a “U�? pattern checking plants along the way moving at least 100 feet into the field.
Figure 3-5. Progression of spider mite injury on soybeans from leaves showing speckling or stipling (Bottom) to yellowing of leaves (Middle) to the bronzing and dying of leaves (Top). Photos courtesy Ronald B. Hammond
Rescue treatments should be considered when spider mite feeding is commonly observed extending into the middle canopy. Treatments should be made before mites cause leaf bronzing and leaf drop. There are few insecticides that have good mite activity.
Spider mite infestations can be treated with chlorpyrifos and dimethoate. These products will not control eggs and have short residuals, so mites can begin rebuilding their population in a few days. Therefore, more than one application may be necessary to maintain control under the right conditions. It is also important to have adequate water volume and spray pressure since mites are on the undersides of leaves.
Dual infestation of soybean aphids and spider mites can complicate insecticide decisions. Pyrethroids used to control aphids have little to no control of spider mites. They can reduce the beneficial insects that keep spider mite numbers low causing populations to increase to levels greater than in untreated fields. Chlorpyrifos insecticides can be used when both mites and aphids warrant treatment.
Hammond, R., Michel, A., and Eisley, J. 2009. Twospotted Spider Mite on Soybean. The Ohio State University Extension Fact Sheet FC-ENT-0024-09. http://ohioline.osu.edu; Ostlie, K. and Potter, B. 2009. Managing Two-Spotted Spider Mites on Soybeans in Minnesota. University of Minnesota Extension http://www.soybeans.umn.edu. web sources verified 6/13/16.