Scattered thunderstorms have brought moisture relief to some parched areas of Indiana. Now the question concerning spider mites is whether the rain will "control" them. Before attempting to answer the question, let's review the factors that have combined to create a spider mite problem in soybean fields.
Extended hot and dry conditions will:
1) encourage the movement of spider mites from drying field margins and roadside weeds onto soybean
2) favor rapid (sometimes explosive) reproduction of spider mites
3) cause spider mites to increase their feeding rates, while also allowing greater nutrient uptake from stressed plants. This creates a feedback loop that makes plants more stressed, leading to more mites, etc.
4) dramatically reduce fungal pathogens that normally keep spider mites in check
In contrast, a significant rainfall (1 inch or more) followed by high humidity will:
1) hydrate plants, removing stress and reducing the concentration of nutrients available to mites in the tissues
2) encourage the growth, development, and spread of beneficial fungal pathogens
3) physically kill spider mites by dislodging them from the plant (this is relatively minor)
The bottom line here is that rainfall does not make the spider mites go away, it merely helps plants recover and slows down the reproductive rate of mite populations. Mites are always in each and every field at some level. Unless fungal pathogens wipe them out (given continual high humidity), they will lurk and may possibly return to an outbreak situation should the weather return to hot and dry. Spider mite damage is irreversible. That is, leaves that are severely bronzed or brown will not "green back up" and contribute to the yield of the field.
Two pesticides are recommended to control spider mites. These include dimethoate and chlorpyrifos. Dimethoate is the most efficacious of these compounds for mite control. If soybean aphid is also present in the field, then chlorpyrifos would be a good choice to suppress both pests. Neither of these products will control spider mite eggs, however, and each will provide about 7 to 10 days at most of residual activity. Proper placement of these pesticides is the key to successful control. Nozzle pressures of 40 psi with fine to medium droplet size and 30-40 gallons of water/acre for ground application helps distribute the pesticide throughout the foliage.