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Posted 16 May 2010. PMN Crop News.

Alfalfa Weevil

Source: University of Illinois Press Release.

Urbana-Champaign, Illinois (April 29, 2010)--The recent warmer temperatures have resulted in alfalfa growth being ahead of normal and have speeded up alfalfa weevil development. Producers and agriculture dealers in northern Illinois 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 approximately 300 degree-days (base 48 degrees F), starting January 1, have accumulated. As of April 24 at Freeport and DeKalb, 276 and 265 degree-days have been accumulated, respectively. These degree-days are about 100 ahead of the 11-year average.

Degree-day accumulations for alfalfa weevil, and other insects can be found at

Feeding damage by the larvae begins as pinholes, and 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 three-eighths inch in length when mature.

University of Illinois recommends sampling for alfalfa weevil larvae by collecting 30 stems in a bucket at random locations by walking a U-shaped pattern through the field. Beat the stems against the sides of the bucket to dislodge the larvae.

A treatment guideline is when 25 to 50 percent of the leaf tips are being skeletonized and there are three or more larvae per stem. Before treating with an insecticide, consider when the first harvest will be made. It may be appropriate to harvest the alfalfa and not apply an insecticide.

Growers should always examine the stubble following the first harvest, because of the potential for larvae and adult beetle feeding on tender new growth.

Additional information on alfalfa weevil is available at and in the University of Illinois Pest Management and Crop Development Bulletin, Local Extension offices can assist individuals in obtaining this information.

Jim Morrison