Alfalfa weevil activity began early this year for our Oklahoma neighbors and it appears that degree day accumulation for Kansas alfalfa weevils is well ahead of normal for most locations in the state. Only northwest Kansas is at or slightly behind normal. There were reports of treatable infestations in south-central Kansas during the last week of March and getting towards the middle of April, fields are being treated, or close to being treated, in the central, southeast, and northeast regions of the state.
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 (Table 2). Since we cannot determine if the 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.
Be aware of insecticide resistance
While warmer spring temperatures have allowed for faster alfalfa weevil development, at the same time in some areas dramatic temperature fluctuations have slowed down alfalfa growth making the plants unable to keep up with feeding damage. In shorter fields, between 3 and 7 inches tall, treatment may be warranted 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 were verified in northwest and southwest Kansas as well as Oklahoma. While this resistance has not appeared to become a widespread problem for Kansas producers, Oklahoma continues to have resistance statewide and it appears resistance to zeta-cypermethrin is developing there. There are numerous products available for alfalfa weevil control in Kansas. When making management decisions it is important to rotate modes of action as this is an effective way to prevent the development of resistance.
For the most up-to-date alfalfa weevil degree day accumulations, visit the Kansas Mesonet Alfalfa Weevil Degree Day Calculator (https://mesonet.k-state.edu/agriculture/degreedays/). For a complete guide to alfalfa weevil management recommendations please refer to the 2023 Alfalfa Insect Pest Management Guide.
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.