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Why Were Weevils Worse in 2002?

Monday, December 2, 2002
filed under: Insects

Why Were Weevils Worse in 2002?

Larry Charlet, entomologist at the USDA-ARS Northern Crop Science Lab in Fargo, N.D., points out that it is natural for insect populations to cycle, climbing and declining from year to year, depending on weather conditions and other variable factors, including management efforts to control the insect.

There are several possible reasons why weevils seem to be a bigger problem in dry growing seasons. For one, sunflower plants are more susceptible to stem damage and subsequent lodging from weevil activity when fields are stressed by drought.

“Insect pests have more impact on crop production during years of environmental stress,” says Sam Tutt, northern technical manager for FMC Corp., which makes crop protection products.

Dry conditions also tend to limit naturally-occurring diseases that can reduce insect populations. For example, several types of fungi favored by wet weather colonize grasshoppers, and can decimate their numbers. That’s why grasshoppers tend to be a bigger problem when it’s dry. This might also be the case with stem weevils; their naturally-occurring enemies may be less prevalent in dry weather.

Parasitic wasps are one natural enemy of stem weevils. These parasitic wasps lay eggs in living hosts—often other insects, including stem weevils—which hatch into larvae that feed on the host's tissues before emerging, eventually killing the host. As many as 11 species of wasps have been found to attack overwintering stem weevil larvae.

Cyclical populations of parasitic wasps may in turn contribute to cyclical populations of stem weevils.

Why was the stem weevil problem worse— at least this year—in the High Plains than in the Northern Plains, which has more sunflower acres? Charlet says that question is difficult to answer, and it may again be due to several factors, including a milder overwintering climate and more severe drought conditions.

“The overwintering effect on larval survival is an important consideration,” says Tutt. “Generally, mild winters have less impact on insect pest survival than more severe winters with extended periods of cold temperature without the insulating protection of snowfall.”

Charlet says a possible disparity in the species and number of parasitic wasps present in the northern sunflower growing area compared to the south may also be a factor. More research is needed to understand the different types of parasitic wasps, and how their population dynamics can be influenced to improve biological control of stem weevils in sunflower, he says.

Weather can also influence stem weevil propagation and control measures. Laboratory research indicates that females can produce up to five eggs per day for a total of 24 to 195 eggs, depending on temperature. The greatest number of eggs was deposited at 86º F.

Roger Stockton, Kansas State University extension crops and soils specialist, points out that pyrethroid insecticides such as Warrior, Asana XL, Baythroid, and Scout X-Tra are not as effective when applied during temperatures above 95 degrees. Such scorchers aren’t unusual on summer days in the High Plains.

Another factor, says Tutt, is that stem weevils lay their eggs over a considerable length of time— lasting several weeks to over a month—thus making proper timing of foliar insecticides more difficult.—Tracy Sayler

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