Drought Raises Spider Mite Risk in Soybeans

By Scout Nelson

Drought conditions across South Dakota are creating additional challenges for soybean farmers. Dry and hot weather can weaken crops and increase the risk of pest problems. One pest that becomes more common during drought is the twospotted spider mite, which can cause major soybean yield losses when infestations are not managed.

Adam Varenhorst, Associate Professor & SDSU Extension Field Crop Entomologist, Patrick Wagner, and Philip Rozeboom highlighted the importance of scouting soybean fields for this pest during dry conditions. Twospotted spider mites have recently been found in eastern South Dakota soybean and corn fields.

Twospotted spider mites are very small and can be difficult to see without a hand magnifying lens. Farmers can also shake soybean leaves over a white sheet of paper and look for tiny moving specks. The mites are usually cream or green when feeding on crops, but they can become orange or red under unfavorable conditions.

The pests live mainly on the underside of leaves and produce thin webbing. Early damage appears as small yellow or white spots on lower leaves. As feeding continues, leaves turn yellow, then brown, and eventually fall. Infested areas may appear as yellow circles that spread across the field. Farmers should pay special attention to healthy areas downwind from heavily infested locations because mites can move with the wind.

Twospotted spider mites often reach high populations in late July and early August. If unmanaged, they can reduce soybean yields by 40% to 60%. There are no established economic thresholds, so regular scouting is important.

Organophosphate insecticides containing dimethoate or chlorpyrifos can help control nymphs and adult mites. Because eggs may survive treatment, fields should be checked again seven to 10 days after application. Some pyrethroid and neonicotinoid products can increase mite problems by reducing natural predators.

Management is most important when plants show heavy stippling or yellowing and mites are present. Cooler temperatures below 85°F and humidity above 90% for extended periods can slow mite growth and may support natural fungal control.

Photo Credit: pexels-alexasfotos