Alfalfa weevil eggs are laid inside alfalfa stems in the fall and spring. Larvae feed on terminal and upper plant leaves early in the spring, resulting in damage occurring before the first cutting.
Alfalfa weevil degree days are a great way to estimate what might be going on in the field and serve as a useful tool to know when to time scouting. Alfalfa weevil eggs begin hatching after 300 degree days have accumulated.
Since we cannot determine if eggs present were laid the previous fall or the current spring, in Kansas, scouting should start after 180 degree days have accumulated starting from January 1.
How do I know how many degree days have accumulated?
For the most up-to-date alfalfa weevil degree day accumulations, visit the Kansas Mesonet Degree Days Calculator https://mesonet.k-state.edu/agriculture/degreedays/ then find the Mesonet weather station closest to you--click onto it, then click onto Alfalfa Weevil and click submit. That will show you the Alfalfa Weevil Growing Degrees accumulation.
The Hays Mesonet has 167 degree days accumulation at the time of this writing.
For a complete guide to alfalfa weevil management recommendations, please refer to the upcoming K-State Research & Extension publication 2025 Alfalfa Insect Pest Management Guide that will be available online soon. Just Google it and you should find it.
Be aware of insecticide resistance.
While warmer spring temperatures allow for faster alfalfa weevil development, be aware that dramatic temperature drops can slow down alfalfa growth, making the plants unable to keep up with feeding damage.
Treatment may be warranted in shorter fields between 3 and 7 inches tall when feeding is evident on the top inch of growth and 1 to 2 larvae are present. If a field is treated, it is important to verify that the expected amount of control was achieved.
In 2020, populations of alfalfa weevil resistant to lambda-cyhalothrin (Warrior II w/Zeon Tech) were verified in northwest and southwest Kansas and Oklahoma.
While this resistance has not appeared to become a widespread problem for Kansas producers, be aware and after application continue to scout fields. Oklahoma continues to have resistance statewide.
Numerous products are available for alfalfa weevil control in Kansas. When making management decisions, it is important to rotate modes of action, i.e. pyrethroid and organophosphate, this is one way to slow down the development of resistance.
For a comprehensive list and description of all currently recognized insecticide modes of action, consult https://irac-online.org.
One update to control options to be aware of was in 2024 a reversal of the EPA’s ruling to ban chlorpyrifos (Lorsban and others). Following that, the EPA issued a technical correction in which 11 special uses of chlorpyriphos were allowed. Among the 11 special uses pertinent to Kansas are alfalfa, cotton, soybean, and winter wheat.
If you have any questions contact your local county or district extension office.
Information provided by Anthony Zukoff, K-State Extension Entomologist.
Stacy Campbell is a Crop Production Extension agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact him by e-mail at [email protected] or by calling 785-628-9430.