Conditions in southwest Minnesota took a turn for the cooler and wetter early this week, but area farmers dealing with pests like two-spotted spider mites may not be out of the woods yet.
Outbreaks of spider mites have been reported in parts of Minnesota this summer, said Bruce Potter, integrated pest management specialist with the University of Minnesota Extension. The extreme hot and dry conditions have been favorable for the mites to feed and multiply, he said. Areas hit hardest by drought have had the most mite problems. The mites cause damage to corn and soybean crops, and the signs of an infestation can be mistaken for drought symptoms.
Spider mites don't thrive in moist conditions or temperatures below 80 degrees, Potter said. It slows their rate of reproduction and encourages the growth of a fungus harmful to the mites. But the moisture and cool temperatures would need to be consistent before it would have an effect on an infestation.
"Just a rainstorm or two isn't going to do it," Potter said. Neither would a cool spell by itself. "Cool and dry isn't going to hurt them. It just slows them down."
Potter said spider mites were still found at Extension test stations after recent rains and cooler temperatures, although it appeared their spread into the fields had slowed. That kind of a slowdown could possibly give farmers more of a chance to treat infested fields.
"The nice thing about cooler weather is it gives you more time to react," Potter said.
The University of Minnesota Extension recently published an article written by Potter and Extension entomologist Ken Ostlie, which gives general guidelines for detecting and treating spider mite infestations. The Extension researched spider mite control during major outbreaks in 1988, 2007 and 2009.
Spider mites tend to live low down on plants, on the undersides of leaves. Ostlie and Potter said the mites cause discoloration and yellow spots on the leaves and also make webbing on the undersides of leaves. Spider mites are very small, Potter said, so the best way to check for living mites on a plant is to tap its leaves over a sheet of white paper and examine the paper with a magnifying glass.
Ostlie and Potter recommend farmers with a possible mite infestation check fields in a U-shaped pattern.
"If there are mites at the field edge, it's a good clue to look deeper into the field," Potter said. Potter and Ostlie recommended spraying for spider mites only if there is evidence of mites and damage throughout a field.