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Alfalfa Weevil: Degree-Day Accumulation Update

May 4, 2001
Bob Scott, Illinois State Water Survey, has provided us with another set of degree-day accumulation maps that are of great help as we continue to offer management tips for alfalfa weevils this spring. From January 1 through April 29, 300 degree-days (base 48°F) had accumulated as far north as a west-to-east line from Galesburg to Watseka, respectively (Figure 3). After 300 degree-days have accumulated from January 1, first-instar weevils can be observed in folded terminal leaves. These yellowish larvae with black heads create small pinholes in leaf tissue; however, this injury is not of economic importance. As larvae mature and reach the third-instar stage of development, they can begin to skeletonize leaves, potentially resulting in economic losses. Please refer to issue no. 3, April 13, 2001, of the Bulletin for details concerning economic thresholds and suggested insecticides.

Growers in southern Illinois have by now dealt with weevils for several weeks. Dave Fischer, Extension dairy educator, Edwardsville Extension Center, suggests that the optimum date for harvesting alfalfa in the southern one-third of the state is at hand. Although harvesting will destroy many weevil larvae, producers are encouraged to monitor the stubble very closely. Control of weevils may be warranted after a cutting, when larvae and adults are feeding on more than 50% of the crowns and regrowth is prevented for 3 to 6 days.

Projected degree-day accumulations (Figure 4) indicate that by May 13, alfalfa growers, even in the most northern counties of Illinois, should begin to see evidence of alfalfa weevil feeding. Please let us know how these projections match your observations in the field. We look forward to your reports.--Mike Gray

Author: Mike Gray

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

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