By Andy Michel and Kelley Tillmon, Ohio State University Extension Entomology, C.O.R.N. 2020-22
Hot, dry weather encourages certain pests in field crops, in particular spider mites in soybean and occasionally corn. Spider mites are a sporadic problem that most often occurs in August, but infestations in July are possible with sustained periods of hot, dry weather like some parts of Ohio are experiencing. Crop scouts in areas that have not received rain recently should be on the lookout for this problem; spider mites are easy to miss in early stages and can build quickly.
Look for light-colored stippling damage which is easier to spot than the mites themselves. In areas with heavy stippling you can confirm the presence of mites by tapping vegetation over a black piece of construction paper. (Many sources will say to use white paper; but insider tip: they are actually easier to see against a dark background.) The mites will look like specks of dust that move.
Stippling is common in the lower canopy even in non-outbreak situations. When the stippling extends up into the middle canopy and is common, treatment is recommended. We do not recommend edge treatments for this particular pest. Make the decision for the whole field. Most pyrethroid products with the exception of bifenthrin are not effective against spider mites and may even flare them. Lorsban and generics have been popular choices against mites but may be less available now. Check the field five days after application for resurgence because these products do not kill mite eggs.
There are specific miticide products that are particularly effective because they also kill mite eggs, eliminating the next generation. Two such products are abamectin (Agri-Mek SC), labeled for use on soybeans, and etoxazole (Zeal), labeled for use on corn and soybeans.
A resurgence of moisture will go a long way to reducing spider mite populations. Mites are particularly susceptible to fungal insect/mite killing pathogens which are favored by moist conditions (one of the reasons dry weather encourages mite outbreaks).