Some areas of Iowa received rain recently, but according to the U.S. Drought Monitor, much of Iowa was still considered abnormally dry as of Aug.4. Two-spotted spider mites are erratic pests in some corn and soybean fields this summer, particularly in the driest areas of southern Iowa. “I’ve been getting a lot of questions about how to manage mites, including what insecticide products are active on the eggs,” says Erin Hodgson, Iowa State University Extension entomologist.

Regarding which products are available, Hodgson suggests you refer to an online list put together by Robert Wright and Julie Peterson from the University of Nebraska-Lincoln. They summarized pesticide groups labeled for two-spotted spider mites in corn and soybean fields.

2-spotted spider mites persist in corn, soybeans
Observations across several counties in southeast and south-central Iowa show that, despite some recent rain events and high humidity, two-spotted spider mites are persisting in both corn and soybean fields in the area. “These insect-like pests can be very difficult to identify,” notes Meaghan Anderson, ISU Extension field agronomist in southeast Iowa.

Anderson and Rebecca Vittetoe, ISU Extension agronomist in south-central Iowa, offer the following information to help you find and manage spider mites.

Mites colonize the underside of leaves in corn and soybean fields, where their feeding results in a stippled or speckled pattern on the upper leaf surface. As damage becomes more severe, areas of the field may be yellowed or bronze. Two-spotted spider mites usually move into the field via field edges, especially those near roadsides, drainage ditches or alfalfa fields. Also, spider mites usually work their way up the plant, so you need to check leaves lower in the canopy for any potential spider mites.

LOOK UNDER LEAVES: A close-up photo of spider mites, eggs and cast skins on the underside of a soybean leaf. 

Should you apply an insecticide treatment?
Once spider mites are spotted, continue scouting in a V or U pattern into the field, stopping in at least 20 places to check several plants for stippling and spider mite presence. Further into the field, leaf symptoms may not be as obvious but close investigation is necessary to determine if spider mites are present, if leaf stippling is beginning to appear, and where in the canopy the issue is located.

The soybean leaf shown below has moderate stippling to this leaf in the middle of the canopy. More severe damage will result in a bronzed appearance to leaves and eventual leaf drop. More extensive infestations will have webbing on leaves as well. These are signals to check for two-spotted spider mite presence on those leaves and inspect the field further to determine whether treatment is necessary.

LEAF SYMPTOMS: A soybean leaf from middle of the canopy with moderate stippling. It's always important to confirm the stippling is caused by spider mites. 

There are several tactics to try and identify spider mites themselves, as they are very difficult to see with the naked eye, says Anderson. One option is to bring a sheet of white paper, preferably laminated, and tap infested leaves over it. You should see small mites scatter on the paper. A second option is to use a hand lens or a cellphone macro lens to get a close-up view of the leaves.

Thorough scouting is the first step to determining if management is necessary. To learn more about the life cycle and management options for this pest, read Erin Hodgson’s related ISU Integrated Crop Management News article here.