Scout Fields Routinely for Alfalfa Weevils

Photo from KY IPM website

While driving through the county to check out flood damage recently, I happen to come across some beautiful fields of alfalfa.  I anticipate these fields will make for some happy livestock later.  And in order to make sure that alfalfa is available to make for happy livestock, we need to be on the lookout for alfalfa weevils.  These little guys have a voracious appetite and can destroy an alfalfa field.

Alfalfa weevil larvae are the most important pest of the first cutting. They feed at the tip of the stem leaving many small rounded holes. Eventually, all of the leaves at the top of the plant may be destroyed. Heavily infested fields take on a bleached out appearance. In addition to reducing yield and quality of the first cutting, the second cutting may be stunted. Larval and adult feeding on the regrowth may set back recovery and development after the first harvest.

Adult alfalfa weevils are 1/8 to 1/4 inch long, grayish brown to black beetles with a broad dark stripe down the center of the back. The front of the head extends downward in a distinct snout. Groups of eggs are laid in live and dead alfalfa stems during the fall and spring. The newly-hatched legless, grub-like larvae are pale yellow. They soon become green with a white stripe down the middle of the back. After feeding for about 4 weeks, the full grown larvae spin a net-like spherical cocoon near the top of the plant and pupate. The adults emerge within 10 days. After a short feeding period, they leave the field and spend the hot summer months hidden and inactive. The adults fly back to alfalfa in the fall and remain there over the winter.

The best alfalfa weevil management decisions are based on stem sampling.  If this information is not available then control is recommended when 25% to 50% of the tips are being fed upon and 2 or more larvae can be found per stem.  Symptoms include small holes in the leaves and growing tips of alfalfa plants.  From a distance, damaged plants look as though they have had frost injury.

Insecticides that can be used for alfalfa weevil control include: Lorsban 4E, Chlorpyrifos 4E, Lannate SP, Baythroid XL, Bolton, Declare, Proaxis 0.5EC, Warrior II, Mustang MAX, Pounce 25WP and/or Ambush.  Also, Aza-Direct and PyGanic are approved for organic production.  Be sure to read the label carefully and follow the instructions when using any of these insecticides.   Harvest interval days vary for each of these.

Make sure that you scouting your fields routinely and be on alert for alfalfa weevils and other pest.  For more information on alfalfa weevil control, contact the McLean County Cooperative Extension Service at (270)273-3690.  Educational programs of the University of Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin.


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