Warmer Weather Increases Feeding Potential for Alfalfa Weevil in Ohio

Cocoon Of Alfalfa Weevil. Photo: Thinkstock.

WOOSTER, Ohio – The return of hotter weather to the region also makes cozy conditions for alfalfa weevil larvae to grow and start eating away at alfalfa crops, with the potential to cause significant damage.

The pest, which causes major alfalfa damage in its larval stages, should only be treated if it grows to populations large enough to cause economic loss to growers, said an entomologist with Ohio State University’s College of Food, Agricultural, and Environmental Sciences.

The high temperatures experienced across Ohio recently have caused alfalfa weevil larvae to develop rapidly, said Andy Michel, an Ohio State University Extension pest expert.

As a result, growers need to start scouting now for the pest, said Michel, who also has an appointment with the Ohio Agricultural Research and Development Center. Growers who find large populations of the weevil in their alfalfa crops may want to consider an early cutting, he said.

OSU Extension and OARDC are the statewide outreach and research arms, respectively, of the college.

“This is a big issue for alfalfa growers,” Michel said. “With the heat that we’ve seen, alfalfa weevils are growing and feeding rapidly, and growers can expect to see some significant feeding in alfalfa crops.

“Once alfalfa crops get to be 16 inches or higher, growers may consider doing an early cutting instead of spraying. You don’t want to spray alfalfa if you don’t need to. “

While some growers think that they’ve grown alfalfa varieties that are resistant to alfalfa weevil, Michel said that is not possible. Some types of alfalfa are resistant to potato leafhoppers, another significant pest, but that does not mean they are also resistant to weevils.

“There aren’t any alfalfa varieties that are resistant to weevil,” he said, noting that, “All alfalfa crops should be checked for weevils.”

The concern for growers is that adult alfalfa weevils can lay large quantities of eggs in the plant stems. The hatched larvae then start feeding within the folded leaves at the growing tips. Plants that experience heavy feeding can develop a frosted look and can result in yield reductions because of stunted plants.

Growers can scout with the bucket sampling method, in which a series of 10-stem samples are randomly collected from various locations in a field. Each stem should be carefully picked off at the base and placed top-down in a bucket and vigorously shaken, so growers can count the number of larvae collected, Michel said.

The shaking will dislodge the late third and fourth instar larvae, which cause most of the foliar injury. Growers who find one or more larvae per stem on alfalfa that is 12 inches or less in height can use a rescue treatment, he said.

“When alfalfa is between 12 and 16 inches in height, the action threshold should be increased to two to four larvae per stem depending on the vigor of alfalfa growth,” Michel said. “When alfalfa is 16 inches in height and there are more than four larvae per stem, early harvest is recommended.”

Information on insecticides that can be used for alfalfa weevil can be found at go.osu.edu/alfalfaweevil

Tracy Turner
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Andy Michel