As scouting begins for the alfalfa weevil, be advised that the clover leaf weevil may be lurking in your alfalfa fields, as well. These two alfalfa pests commonly cause some confusion in identification at this time of year. Matt Montgomery, Extension unit educator in Springfield, found what he thinks are clover leaf weevil larvae in an alfalfa field in Sangamon County during the week of March 31. Although it's possible that alfalfa weevil larvae have hatched in Sangamon County (refer to the article "Development of Alfalfa Weevils Proceeds, Despite Colder Temperatures" in this issue of the Bulletin, it's probably a bit too early for alfalfa weevils in central Illinois.
Clover leaf weevil larvae. Note the tan head and pink border along the white stripe of the top specimen. (Photo courtesy of Matt Montgomery, Sangamon-Menard Extension.)
Alfalfa weevil larva. Note the dark brown head.
The alfalfa weevil and clover leaf weevil are similar in appearance but can be distinguished on closer inspection. Most important, the clover leaf weevil is a light green larva, with a white stripe down its back and a tan head. As the larvae get older, the white stripe develops a pink border. The alfalfa weevil is a smaller larva, also pale green, with a white stripe and a dark brown to black head. Clover leaf weevil larvae are larger (1/2 inch) than the alfalfa weevil larvae (3/8 inch). Alfalfa weevil larvae remain on the plant most of the time and prefer to feed at or near the tops of the plants. Clover leaf weevil larvae feed on the lower leaves of the plant, primarily at night. During the day, clover leaf weevil larvae can be found in the soil around the crowns of the plant or in debris. An adult clover leaf weevil is light brown with a wide, dark brown stripe on its back. The adult alfalfa weevil is about half the size of the clover leaf weevil adult and brown, with a dark, narrow stripe along the center of the wing covers.
Clover leaf weevil larvae on the soil in an alfalfa field.
Clover leaf weevils generally overwinter as larvae. Very early feeding on alfalfa leaves generally is caused by the clover leaf weevil. The amount of injury associated with the clover leaf weevil usually is insignificant in Illinois, compared with the amount of injury caused by the alfalfa weevil. Fungal organisms generally keep clover leaf weevil populations in check. It is important to differentiate between these two pests to correctly assess the problems at hand and to avoid spraying insecticides when they are not needed. As always, we're interested in what's going on in the fields. Keep us posted on any interesting findings.--Kelly Cook