No. 3 Article 2/April 23, 2010

Don't Forget to Examine Alfalfa Stands for Weevil Activity

In the haste to plant corn, producers with stands of alfalfa should not neglect to scout for alfalfa weevils. Alfalfa weevil activity can be predicted each spring by tracking degree-day accumulations (base 48°F) since January 1. Larvae typically begin to hatch when 300 degree-days have accumulated.

On April 20 I sampled a stand of alfalfa just south of the University of Illinois campus and found no weevil activity. Based on heat unit accumulations through April 19 for Champaign (317 degree-days), this makes sense. However, in southern Illinois, 424 degree-days (base 48°F) had accumulated near Dixon Springs as of April 19. Some light leaf feeding by first and second-instar larvae is likely underway in many fields across southern Illinois. In order to check on degree-day accumulations for a variety of insect pests, including alfalfa weevils, visit the degree-day calculator. We thank Bob Scott of the Illinois State Water Survey for his cooperation in maintaining this important resource.

Alfalfa should be sampled by making a U-shaped pattern within a field, taking care to avoid field edges. Randomly collect 30 stems and place them in a bucket. Following the collection of all stems, dislodge the larvae from the stems within the bucket and determine the number of weevil larvae per stem. If 25% to 50% of leaf tips have been skeletonized and you find three or more larvae per stem, a management decision is required. Early harvest of the first hay crop is a sound management option. If this cultural management approach is selected, be sure to monitor the regrowth carefully for any signs of feeding activity. An insecticide rescue treatment may be required following an early harvest if both larvae and adults are causing injury to more than 50% of the crowns and regrowth is not occurring for 3 to 6 days. For a more complete description of scouting procedures and life cycle information on this insect pest, visit this web site.--Mike Gray

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