I found spider mites in part of one of my soybean fields. They haven’t done a lot of damage yet, and the soybeans are at R5 – R5.5. I’ve also heard that cooler temperatures, rain, and heavy dews slow them down. I will keep watching to see if they spread, but is there a point where it is too late to spray?
Spider mite populations are often higher when conditions have been hot and dry for a sustained period of time, but populations can change substantially after heavy rains. So if you are noticing what appears to be feeding from spider mites on soybean plants, and your area has had some recent rain, make sure to look for the mites and see what the current population is like. Spider mites are small (about 0.4 mm), so tapping the leaves over something that the mites can easily be seen on may help in determining their presence and levels.
The stages of soybeans that are most susceptible to spider mites are the R4 (full pod) through R5 (beginning seed – when seeds are filling) stages. Once the soybeans reach R6 (full seed or green bean stage) the feeding from spider mites will have less impact on yield.