Spider mites often show up when it’s hot and dry outside. Given our current weather (and the promise of more heat to come), it’s a good time to review our scouting and management recommendations. Spider mites feed on a wide variety of plants, and usually enter soybean fields from grassy edges – especially right after the edges have been mowed, which causes the mites to seek out a new food source. (If you have the option, consider holding off on mowing those roadsides and grass margins when it’s hot and dry). When scouting for spider mites, look for areas of the field with a yellow-brown or bronze coloration, especially near the edges. Spider mites feed on cellular fluid, which leads to a characteristic stipled or “sandblasted” appearance on the leaves. If you look closely at an infested leaf, you will likely see the tiny mites moving around, and if you examine the leaf with a hand lens you will see their small, globe-shaped eggs.
Application decisions for spider mites can be tricky. Use an economic threshold of approximately 20-25% of foliage discolored before pod set, and 10-15% of foliage discolored from pod set until pods are filled (mid-R6). Consider the extent of the damage and whether it is expanding, the growth stage of the crop (R4-R5 are especially critical), and the likelihood of continued hot, dry conditions. While spider mites are not insects, several insecticides will provide control, including the organophosphates chlorpyrifos and dimethoate, and the pyrethroid bifenthrin. Revisit the field 5 days after an application to determine if eggs or small mites are still present; these materials do not kill the eggs and have short periods of residual activity, so a follow-up application might be necessary. (If a follow-up application is necessary, switch the mode of action). Newer miticides, including products containing abamectin, are more effective and have a longer period of residual activity, but are usually more expensive.
Using the wrong insecticide when spider mites are in the field (whether the mites were your target or not) can be costly in more ways than one. Most pyrethroid insecticides will not provide control, and can actually make the problem worse by removing predators that normally keep mite populations in check. Remember, if an application is not going to make you any money, it’s best to keep it in the jug.
Contact: Dr. Nick Seiter, Field Crop Entomologist | firstname.lastname@example.org