Cold may aid farmers by killing pests

February 20, 1994|By Frank D. Roylance | Frank D. Roylance,Sun Staff Writer

Farmers and gardeners who lost crops and plants in last month's deep freeze can take some comfort in knowing that the cold has probably killed insect pests, too.

The freeze does not discriminate, however. The victims may include some beneficial insects, such as lady beetles, which eat enormous quantities of destructive aphids.

"Lady beetles over-winter in groups under leaf litter and bark," said Dr. Galen P. Dively, extension pest management specialist at the University of Maryland College Park. "I think they probably will be hurt pretty badly this winter."

But aphids are cold-hardy. They can even remain active and reproduce during mild winters. They are harmful to ornamental plants, vegetables, wheat, barley and other field crops.

After extremely cold weather in January 1977 and December 1990, Dr. Dively said, spring brought abnormally large populations of aphids, probably the result of lady beetle mortality. Based on that, he said, "I'd predict we're going to see an overall increase in aphid problems." Vulnerable insects are affected when the cold persists long enough to drive the frost line deep into the soil where they spend the winter as eggs, larvae or adults.

"I was talking to a grower a couple of weeks ago and he indicated the freeze line may have gone down 15 to 18 inches. That's unusual," Dr. Dively said.

Normally in Maryland only the top few inches of soil freeze, and only for a short time. "But when it goes this deep it's going to kill some over-wintering insects," he said.

Most affected are insects that winter in the upper soil. These include the corn ear worm, which is a major threat to sweet corn, and flea beetles, which damage cabbage, sweet corn seedlings and "just about every vegetable crop, even tobacco," Dr. Dively said.

Cyst nematodes, which are a serious problem for soybean growers on the lower Eastern Shore, probably have survived the cold in their tough capsules. But nematodes that don't form the protective cysts, like lesion and root knot nematodes, have probably been reduced, he said.

"Just because you have freezing temperatures doesn't mean you're freezing insects," Dr. Dively said. The Colorado potato beetle, for example, is cold-hardy.

Other insects, such as white grubs that damage turf grass, burrow deeper to escape the frost.

Cyrus Lesser, chief of the mosquito control section at the state Department of Agriculture, believes this winter's conditions have produced at least the potential "for an abnormally high spring for mosquito problems."

But, as with other insects, predicting more mosquitoes at this point is difficult. If spring is warmer than normal, for example, the standing water that typically are home for mosquito larvae will evaporate, and the mosquito population will be reduced, experts said.

The Associated Press contributed to this article.

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