I have started seeing signs of Twospotted Spider Mites (TSSM) in some soybean fields around the area. The hot, dry weather is very favorable for large populations of these very small arthropods. The TSSM adults are around 1/60 of an inch and vary in color from white to light red. They usually appear along field edges first as they move into a field from other food sources. The signs are yellow to brown speckling on leaves starting at the bottom of the canopy and moving upward. It is common is see a discolored semi-circle from the edge of the field inward as the infestation beings.
If hot, dry conditions persist, populations can become very large and impact yield. TSSM can be viewed with a strong hand lens on the underside of the leaf or by uprooting a soybean plant and shaking it over a white piece of paper. The mites can be seen moving on the paper. Unlike most insects, TSSM are difficult to count so economic thresholds based on mite numbers have not been established. Instead, plant damage can be used to make the decision whether a spray application is needed. Once many plants along the field edge and/or within the field show speckling, a rescue application is needed. If the feeding is only occurring along the edges, a perimeter spray covering infested area and border area beyond noticeable feeding will likely take care of the population.
If there are a few mites found and a small amount of speckling, a rain may take care of the issue but will likely not reduce heavy infestations. Mite control products include dimethoate, chlorpyrifos and bifenthrin. For full details on the TSSM, view the fact sheet at: http://ohioline.osu.edu/factsheet/ENT-24.
Submitted by OSU Extension, Champaign County.