By STACY CAMPBELL
Cottonwood Extension District
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.
Degree day accumulations for Kansas alfalfa weevils are well ahead of normal for the entire state this year. A similar trend occurred in 2023. There were reports of treatable infestations in south-central Kansas during the last week of March, and in mid-April of 2023, fields were being treated, or close to being treated, in the central, southeast, and northeast regions of the state. It is recommended that scouting for weevil activity should be occurring now in all regions of the state, says Anthony Zukoff, K-State Extension Entomologist.
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, a couple of fields reported in central Kansas last year where lambda-cyhalothrin had reduced efficacy. 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 as this is an effective way to prevent the development of resistance.
One update to control options to be aware of for 2024 is the reversal of the EPA’s ruling to ban chlorpyrifos (Lorsban and others). On November 2, 2023, the Eighth Circuit issued a ruling vacating EPA’s final rule and sending the issue of chlorpyrifos tolerances back to EPA for further proceedings. 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.
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 upcoming K-State Research & Extension publication 2024 Alfalfa Insect Pest Management Guide that will be available online soon.
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.