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Monitor for Alfalfa Weevil (6/04/20)

Adult alfalfa weevils are about ¼ inch long, with elbowed, clubbed antennae and a blunt snout.

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. 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.

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The degree day model for alfalfa weevil (base temperature of 48 F) indicates that the Accumulated Degree Day (ADD) units range from 320 in northeast area to 503 in southwest area of North Dakota (see map). Growers should start to scout their fields for adult alfalfa weevils after 300 ADD, and for larvae from 371 through 595 ADD. Although adult weevils will feed on the foliage, larval feeding causes most of the defoliation. The heaviest feeding occurs as mature larvae appear (from 504 through 595 ADD), usually mid-June through mid-July. 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 and then overwinter in debris or alfalfa stubble.

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.

In alfalfa grown for hay, the most cost-effective method of managing weevils is to cut the hay early before extensive feeding injury (>30% defoliation in entire field).

Some years, populations of larvae continue to be high in the emerging regrowth and chemical control may be necessary. For regrowth, the economic threshold is eight or more larvae per square foot or two or more active larvae per crown.

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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