Photo by Eugene E. Nelson, Bugwood.org

Hot, dry weather and two-spotted spider mites are typically found in tandem. Many farmers are still trying to forget the drought of 1988 and the severe outbreak of mites that occurred across much of the Corn Belt. Mite infestations have been reported in soybean fields that have missed the widely scattered rain showers.

Generally, soybean plants along field margins are the first to show the characteristic bronzing and mottling of leaves. "By tapping the leaves over a sheet of white paper, you can observe mites moving about the surface of the paper," Gray says. "Often, webbing is present on the lower surface of leaves."

A 1988 study found that as spider mite injury to soybean leaves intensified, photosynthetic efficiency decreased, stomatal resistance increased, transpiration rate decreased and the total chlorophyll content of leaves decreased. Symptoms included pale green leaves and some yellow mottling.

If mite injury to plants is evident along field margins, and mites are found on plants with relative ease throughout a field, consider a rescue treatment, especially if hot and dry weather is expected to continue. The most common insecticides used as rescue treatments include chlorpyrifos (Lorsban 4E and generics) and dimethoate. In 1988 many fields were treated multiple times due to the continuing drought and the residual activity of the products lasting approximately one week. Many of the fields in which only border rows were treated ultimately required full-field sprays.

"At this point, it's difficult to predict where this summer is headed with respect to this pest," Gray says. "Let's hope we begin to see more widespread and abundant precipitation across the state. If this occurs, two-spotted spider mite infestations will abate."