Issue No. 5, Article 5/April 23, 2004
Degree-Day Accumulations and Economic Thresholds for Alfalfa Weevil
Though we've had few reports of alfalfa weevil feeding this year, degree-day accumulations for much of the state are nearing 300 degree-days. Light tip feeding may be occurring in much of the state. As degree-day accumulations approach 400, larval tip feeding may become more severe as development of alfalfa weevils approaches third and fourth instars (Table 1). For a map of degree-day accumulations and projections in Illinois, visit our degree-day calculator (http://www.ipm.uiuc.edu/degreedays).
Continue to scout for alfalfa weevil larvae to determine whether economically damaging levels are present. Economic thresholds can be determined by the percentage of tip feeding or number of alfalfa weevils per stem. When 25% to 50% of the tips are being skeletonized and there are three or more larvae per stem, treat immediately. Do not apply insecticides when alfalfa is in bloom; instead, cut and remove the hay. Control 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. Table 2 shows some economic thresholds based on numbers of alfalfa weevil larvae per stem at different alfalfa heights and values of alfalfa hay. These thresholds are published in Pest Management of Alfalfa Insects in the Upper Midwest, (1999, Leopold Center for Sustainable Agriculture, Iowa State University, Ames).
Suggested insecticides for alfalfa weevil control are summarized from the 2004 Illinois Pest Management Handbook and listed in Table 3.--Kelly Cook