Plant Health Initiative

Aphid treatments may flare spider mites

Yellow "stippling" or speckling on green leaves are a diagnostic sign of spider mite infestation. Photo credit: Phillip Glogoza and Ian MacRae, University of Minnesota Extension Click on image to view a larger version

How you manage aphids may increase the number of two-spotted spider mites that attack your soybeans.

According to Dr. David Ragsdale, entomologist at the University of Minnesota, pyrethroid insecticidesr commonly used to control soybean aphids do not control spider mites and the organophosphates that control spider mites do not control eggs, which often makes two applications necessary. The use of both insecticides might reduce the number of insect predators that naturally control spider mites, which can make things even worse.

Aphids attack your soybean plants from the top down and spider mites attack from the ground up. Dry, hot weather and a reduction in spider mite predators create ideal conditions for an explosion in spider mite populations. An increase in the average temperature from 68 degrees F to 78 degrees F can multiply the number of spider mites by tenfold. Add another 10 degrees F and the number jumps to one hundred times.

Ragsdale advises using regular scouting and a 250-aphid-per-plant threshold for spraying to conserve the natural enemies that prey on aphids and spider mites. He says aphids in fields planted with an aphid-resistant variety tend to reproduce slower and have smaller offspring. A higher threshold of perhaps 500 aphids per plant may work in those fields, but quickly adds that researchers across the region are still working to come up with the appropriate threshold for aphids on resistant plants.

Reprinted with permission. Originally printed in the Minnesota Soybean News & Views, Spring 2010, Volume VIII, Issue 2.