Alfalfa Weevil in Alfalfa
Crop Injury: Alfalfa weevil (Hypera postica) was introduced to the United States more than 100 years ago and has become an important and widespread pest of alfalfa. Larva and adult weevils defoliate alfalfa plants by consuming leaf tissue. Foliage appears skeletonized or shredded and plants may be stripped to the stems if feeding is severe. Feeding decreases alfalfa yield and nutritional value, reduces plant vigor, and can cause stand loss. In general, larvae cause the most plant injury and risk is most severe prior to the first cutting. Feeding by adults is typically minor, especially as they enter dormancy during the summer months.
Pest Description: Adult alfalfa weevils have a brown body with light brown to grayish scales. A darker brown stripe runs the length of the body. Legless larvae have a black head and a yellow or yellowish-green body. Later instars may appear darker green and have a whitish stripe running along the middle of the body.
Alfalfa weevil adult
Adam Sisson
Alfalfa weevil larvae
Iowa State University Integrated Pest Management
Life Cycle: Adult weevils overwinter near alfalfa crowns in plant residue in colder regions and as eggs in stems where it is warmer. The onset of egg laying varies according to latitude, ranging from late fall or early winter in the south to late winter or spring in the north. White to yellow eggs are deposited in stems. Newly hatched larvae briefly feed inside stems and then, after emerging from stems, migrate to plant terminals and eventually to nearby leaves. Net-like cocoons are formed on or near plants, protecting the insect during pupation. Emerging adults consume alfalfa tissue.
Alfalfa weevil life cycle in alfalfa.
Mimi Broeske, Nutrient and Pest Management Program, University of Madison-Wisconsin
Scouting: Alfalfa weevil can be confused with clover leaf weevil. Use a sweep net to determine the presence of alfalfa weevil. Accurate sampling of alfalfa weevil larvae can be made by hand collecting alfalfa stems, dislodging them into a bucket, and then determining the number of alfalfa weevils per stem.
Using a Sweep Net
Management: Insecticides can be used to manage alfalfa weevils; check with local extension resources for the most up-to-date economic thresholds. Sample larva using stem counts, paying special attention to very small larvae at terminals. Consider time until harvest and stem size when making treatment decisions, as well as risk to natural enemies that reduce alfalfa weevil populations. Early harvest and hay removal, winter grazing, and planting more tolerant alfalfa cultivars are additional management strategies.
Alfalfa Weevil Management in the Upper Midwest
Prepared and reviewed by the Alfalfa Pest Management Working Group.
Alfalfa weevil larva.
Alfalfa weevil feeding. Whitney Cranshaw, Colorado State University, Bugwood.org
Alfalfa weevil feeding. Whitney Cranshaw, Colorado State University, Bugwood.org
Alfalfa weevil larvae. Frank Peairs, Colorado State University, Bugwood.org
Alfalfa weevil injury in alfalfa field. Frank Peairs, Colorado State University, Bugwood.org.
Gallery images courtesy Adam Sisson, Iowa State University Integrated Pest Management, Frank Peairs, Colorado State University, Bugwood.org, and Whitney Cranshaw, Colorado State University, Bugwood.org.
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