OSU Expert: Now’s the Time to Scout for Alfalfa Weevil in Ohio

WOOSTER, Ohio – Now’s the time for growers in southern and central Ohio to start scouting for alfalfa weevil. Northern Ohio field crop growers should prepare to start scouting for the pest by next week, said an entomologist with Ohio State University’s College of Food, Agricultural, and Environmental Sciences.

The pest, known to cause significant alfalfa damage in both its adult and larvae stages, typically starts showing up in southern Ohio first, slowly progressing its way to northern Ohio fields, said Ron Hammond, an Ohio State University Extension entomologist.

This winter’s relatively warmer days have contributed to the pests’ damage potential, said Hammond, who also has an appointment with the Ohio Agricultural Research and Development Center.

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

“Now is the time that growers need to pay attention to their fields to ensure that alfalfa weevil isn’t rearing its ugly head,” he said. “This winter hasn’t been exceptionally cold since early December, which contributes to the potential for alfalfa weevil damage.

“We do have some areas that have outbreaks, so growers in southern Ohio should have already started checking their fields this past week, while central Ohio growers should begin scouting now, with northern Ohio growers OK to start scouting next week.”

The major concern for this pest 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, Hammond said.

“If the feeding gets heavy enough, it will result in the crop developing a frosted look and can cause yield reductions because of stunted plants,” he said.

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, counting the number of larvae collected, Hammond 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.

“Application of an insecticide to prevent excessive defoliation of alfalfa by weevil is justified when one or more late instar larvae are found feeding per stem and the stand cannot be harvested early,” Hammond said. “But where 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.

“Once alfalfa is 16 inches or more in height, it may be better to cut early and start a second growing cycle rather than spraying.”

More information on alfalfa weevil can be found in an OSU Extension fact sheet at http://go.osu.edu/weevil.