Illinois growers busy with corn planting shouldn’t neglect scouting for alfalfa weevils, says Mike Gray, University of Illinois Extension entomologist.

Alfalfa growth is ahead of normal because of recent warm temperatures and that’s speeded up alfalfa weevil development. Producers and ag dealers across the state need to be on the lookout for this insect pest.

Alfalfa weevil activity can be predicted by monitoring degree-day accumulations. Young larvae typically begin to hatch when, starting Jan. 1, about 300 degree-days (base 48 degrees F) have accumulated. As of April 24 at Freeport and DeKalb, 276 and 265 degree-days had accumulated, respectively. These degree-days are about 100 days ahead of the 11-year average.

Check here for degree-day accumulations for alfalfa weevil, and other insects.

Illinois entomologists say larvae feeding damage begins as pinholes. As larvae mature, leaf skeletonization may occur. The larvae have a black head and are yellowish-green with a white strip along the middle of the back. They are about 3/8” in length when mature.

Alfalfa should be sampled by making a U-shaped pattern within a field, avoiding field edges, Gray says. Randomly collect 30 stems and place them in a bucket. Beat against the sides of the bucket to dislodge the larvae.

“If 25-50% of leaf tips have been skeletonized, and you find three or more larvae per stem, a management decision is required,” he says.

Growers may be able to harvest early rather than apply insecticide. Also, examine stubble following first harvest, because larvae and adult beetles may feed on tender new growth.

For more information, check out integrated pest management information on alfalfa weevil and the Pest Management and Crop Development Bulletin.