No. 4 Article 5/April 15, 2005

Scouting for Alfalfa Weevils

Degree-day accumulations have topped 200 through the southern half of the state and are expected to exceed 300 and 400 throughout the entire state within the next two weeks (IPM Degree-Day Calculator). Those who have not been out to their alfalfa fields should make that first scouting trip soon. As degree-day accumulations near 400, larval tip feeding may become severe as larvae approach third- and fourth-instar development (Table 1).

Reports of alfalfa weevil larval feeding are starting to slowly trickle in. Matt Montgomery, crop systems educator, Sangamon-Menard counties, spotted some slight alfalfa weevil feeding in a field this week. Early larval feeding can be identified by the small pinholes on the terminal leaves of the alfalfa plant. Larvae will begin feeding on the inside of the terminal leaves and move to foliage lower on the plant. As larvae continue to grow, they begin feeding between the veins of the leaf, skeletonizing the plant.

When 25% to 50% of the leaf tips have been skeletonized and there are three or more larvae per stem, a management decision should be made. One option is to harvest the crop as early as possible. Cutting the hay removes food and shelter from the larvae and exposes them to harmful rays from the sun. Remember to scout alfalfa regrowth for signs of alfalfa weevil damage! Larvae and adults have the potential to prevent or slow regrowth by feeding on new shoots. Control may be warranted after a cutting when feeding is occurring 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 2005 Illinois Agricultural Pest Management Handbook and listed in Table 3.

Alfalfa weevil larvae in curled alfalfa leaflet (provided by Matt Montgomery, Sangamon-Menard Extension).

Alfalfa weevil larval feeding on alfalfa plants.

--Kelly Cook

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