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Alfalfa Weevils Are Active in Southern Illinois

April 12, 2002
Several people from southern Illinois reported alfalfa weevil activity during the past week. For the most part, the larvae being found are quite small, and the injury is limited to pinholes in the terminal leaves. However, in some fields in which environmental conditions have accelerated weevil development, the larvae have reached the third instar and the damage is quite noticeable.

Vince Ochs, with Fruit Belt Service Company in Vienna (Johnson County), found "a lot" of small alfalfa weevil larvae (about 1/8 inch) in a field on April 5. Alan Mosler, with Twin County Service Company in Marion (Williamson County), has found alfalfa weevil larvae in about 60% of the tips he examined, with one to two larvae per tip. Omar Koester, Extension unit assistant, crop systems, in Randolph County, checked several fields in southern Monroe County and western Randolph County on April 8 and discovered varying sizes of larvae and levels of infestation. In most of the fields the larvae were small. However, in a few fields Omar found four to eight first through third instars per tip, with 50% or more skeletonization. The alfalfa was only 2 to 3 inches tall. Omar indicated that these infestations were among "the most intense insect pressure [he has] seen in 40 years." Matt Montgomery, Sangamon/Menard Extension unit educator in crop systems, may have found some early-instar alfalfa weevils in a field in Menard County. However, we need to verify the report.

The time to scout for alfalfa weevils is now. Be sure to look for alfalfa weevil larvae and the symptoms of their feeding injury throughout the field, not just along the edges. We recommend walking in a U-shaped pattern through the field, stopping at least 30 times to check for alfalfa weevils and to measure the height of the alfalfa. The best way to count the larvae is to snap a stem off at ground level and place it top down into a white bucket. After collecting 30 stems, you can beat the stems, a few at a time, against the sides of the bucket to dislodge the larvae. You also can select a sample of 10 of the stems to measure the height.

An insecticide for control of alfalfa weevils may be warranted when you find 25% to 50% are being skeletonized and three or more larvae per stem. However, a more dynamic set of guidelines for alfalfa weevil control are presented in Table 4. These guidelines incorporate alfalfa height, value of alfalfa hay, and numbers of alfalfa weevil larvae per stem for making decisions.

Insecticides suggested for control of alfalfa weevils are listed in Table 5. Please follow all label directions and precautions.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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