Spider mites respond to dry conditions, stressed beans
The most severe damage occurs when the infestation starts in the early stages of plant growth and builds throughout the season (extended drought). Before applying controls carefully consider that, depending when damage is noted, multiple insecticide/miticide applications may be necessary. This is because surviving spider mites are able to repopulate a field much more quickly than their natural predators, which are usually also wiped out by these chemical applications. If leaf discoloration is apparent in multiple areas of the field, spider mites are positively identified as the culprit, and hot, dry conditions are expected to persist, it is recommended that a control be considered.
If a control is warranted, two pesticides are recommended for use. These include dimethoate (multiple generic formulations are available) and chlorpyrifos (Lorsban 4E and generics). Dimethoate is the most efficacious of these compounds for mite control. Neither of these products will control spider mite eggs, however, and each will provide a maximum of 7 days of residual activity. Proper placement of these pesticides is the key to successful control results. Nozzle pressures of 40 psi with fine to medium droplet size and 30-40 gallons of water per acre for ground application helps distribute the pesticide throughout the foliage. There are several products on the market with a combination of chlorpyrifos and synthetic pyrethroids. At this time we cannot recommend them, as field data for their efficacy for twospotted spider mites is limited. In general, do NOT use synthetic pyrethroid products when spider mites are present in fields, as they kill the natural predators and many perform poorly against mites.