Scouting is best done in times of prolonged hot, dry (low humidity) weather. In soybeans the most important time is during the reproductive stages of R1-R5 and where a synthetic pyrethroid insecticide has been used.

Shake plants over a piece of white paper and look for tiny white moving specks. A hand lens is very useful in determining if the specks are actually mites.

In corn, scouting is much more difficult and less is known about making a control decision. The mites are no harder to find, simply follow the instructions for soybeans, but using corn leaves. One simply wishes to determine if the infestation is only on the outer edges or is across the field.

Large and widespread, mite infestations usually happen when hot, dry and low humidity conditions have occurred early in the season (like this year!). In these circumstances, scouting should start well before R1 (soybeans) and in younger corn.

Cooler temperatures and high humidity allow a natural fungus to control spider mites. Rainfall will help the plant tolerate the infestation, but will not reduce the mite population.

Application of synthetic pyrethroid insecticides and fungicides may worsen the infestation, because these insecticides don’t work well on mites and the fungicide may hamper the naturally occurring fungus that kills the mites.

If direct control is required, consider using an organophosphate insecticide like chlorpyrifos or dimethoate. If you must use a pyrethroid consider bifenthrin. An application is warranted when most plants are infested with spider mites and leaf speckling and discoloration are apparent.

Reference: Field Crop Insects. Iowa State Univ. CES. CSI 0014. Jan. 2012