MARSHALL - A mild winter can mean a greater insect population come spring, according to University of Minnesota extension entomologists, but a variable winter with no snow cover could cause high mortality among crop-eating bugs.
"This winter might be harder on insects," said Bruce Potter, entomologist with the U of M Extension office in Lamberton. "I wouldn't buy a bunch of Off and Cutters just yet."
Potter cautioned that the full effects will not appear until spring, but so far the lack of snow cover and the erratic freezing and thawing cycle could be hard on insect populations.
"Extremes are bad for them," Potter said. "Especially in the spring as insects break hibernation. With a long fall and warmer winter, insect eggs are vulnerable to fungi. Typically a Minnesota winter puts everything in a deep freeze. This winter we have a busted freezer."
But different species of insects react differently to cold weather, Potter said. Species such as the bean leaf beetle are not very cold tolerant, but cold temperatures have relatively little effect on the corn borer beetle.
The lack of snow cover the area has experienced lately may increase mortality among some insects.
"Snow provides a nice insulator for overwintering insects," said Ken Ostlie, an entomologist at the U of M campus in St. Paul. "When insects are in hibernation their metabolism is really low. Without snow cover it takes more energy for insets to handle the cold. And dramatically fluctuating temperatures are worse than cold for insects."
Temperatures fluctuating above and below freezing can cause insects to come out of hibernation before spring, depleting their biological reserves and causing high mortality when the temperatures drop again, according to Ostlie. However, some crop pests do not overwinter and migrate north from as far as the Gulf Coast, including black cutworms, army worms, corn ear worms, and lately corn rootworms.
"The northern corn rootworm dominates in Minnesota," Ostlie said, but lately the western corn rootworm has in moving in, and the western rootworm has been developing resistance to transgenic corn. If we continue to have cold weather with no snow cover if could be beneficial in knocking down the western corn rootworm."
Jeff Hahn, extension entomologist at the U of M, said with garden pests such as the Japanese beetle, the factor that keeps them from spreading is soil moisture rather than temperature.
"If you don't have them in the area, they're spreading out, you will someday," Hahn said. "Japanese beetles lay their eggs in the summer, and soil moisture is critical. When it's dry they don't do so well."
Hahn said Japanese beetles feed on grass roots in their immature stage and when mature feed on roses, grapes, and trees.