Plant Health Initiative

Mites - Agronomic impact

Damage begins in the lower canopy and progresses upwards. Photo credit: Phillip Glogoza and Ian MacRae, University of Minnesota Extension Click on image to view a larger version

Leaf stippling. Photo credit: Phillip Glogoza and Ian MacRae, University of Minnesota Extension Click on image to view a larger version

Spider mites pierce the leaf cells with their mouth parts and suck
plant fluids from the leaves. This injury produces a white or yellow spots or “stipling” that is heaviest on the underside of the leaves. The leaves lose photosynthetic surface as feeding continues. Water loss from damaged leaf surface is uncontrolled. If the infestation is unchecked by parasites, predators, or miticides, spider mites may kill the entire plant.

The economic threshold is not well defined for spider mites. Keeping in mind that a leaf loss of 20 to 30% during pod fill can reduce yields, it is necessary to back that off to a lower value to avoid actual losses. University of Illinois entomologist Dr. Mike Gray has recommended a treatment timing when 10 to 15% of the leaves are discolored during pod fill.

Source
Twospotted Spider Mite Infestations in Soybeans Intensify as Drought Conditions Persist. Illinois Pest Management and Crop Bulletin newsletter.