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Start Scouting for Alfalfa Weevils (05/13/21)

Alfalfa weevils have one generation per year and overwinter as adults in shelterbelts.

Alfalfa weevils have one generation per year and overwinter as adults in shelterbelts. In the spring, the adult weevil emerges and will feed on the foliage to some degree. Adult alfalfa weevils are about ¼ inch long, with elbowed, clubbed antennae and a blunt snout. Adults are brown with short, somewhat thick golden hairs over the body and a distinctive brown stripe longitudinally along the center of the back. Females lay eggs in the stems of alfalfa.

Eggs hatch in one to two weeks. Larvae emerge from the stem and begin feeding on the growing tips of the alfalfa plants. Larvae pass through four growth stages (instars) before reaching maturity. Mature larvae are about ⅜ inch long and have a black head capsule and a wrinkled green body with a white stripe running lengthwise along the top. Younger larvae are similar in appearance but smaller.

Larvae continue to feed on tender leaf tissue as they grow. Mature larvae feed for two to three weeks before pupating. Mature larval feeding causes most of the defoliation and crop damage. A heavily infested alfalfa field will have a silvery appearance as the green material is eaten off the leaves by larvae. After larvae complete their development, they drop to the soil for pupation (non-feeding stage). The new generation of adult weevils emerge in 1-2 weeks from puparium and then overwinter in debris.

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The degree day model for alfalfa weevil (base temperature of 48 F) is a useful IPM tool. It predicts the timing of adult emergence, when different life stages will occur, and when we should scout for adult weevils or larval feeding injury in alfalfa fields. The degree day table (below) summarizes the Accumulated Degree Day (ADD) units and alfalfa weevil activities.

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In North Dakota, the ADD units range from a low of 172 ADD in the northeast area to a high >300 ADD in the southwest to south central areas (see map). Growers should start to scout their fields for adult alfalfa weevils after 300 ADD, and for larvae from 371 through 595 ADD. The heaviest feeding occurs as mature larvae appear (from 439 through 595 ADD), usually mid-June through mid-July.

To assess the insect DD model, go to the NDSU’s NDAWN website and Applications – Insect DD. Then, click on the Map tab and select 48 F for your base temperature and Degree Days (DD) for your map type. Then, click Get Map.

For more information, see the NDSU Extension publication on IPM of Alfalfa Weevil in North Dakota E1676

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Janet J. Knodel

Extension Entomologist

 

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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