Start monitoring alfalfa weevils using degree days and scouting
Degree day accumulations for Kansas alfalfa weevils are well ahead of normal for the entire state this year (Table 1). A similar trend occurred in 2024. As air temperatures continue to increase, It is recommended that scouting for weevil activity should be occurring right now in all regions of the state.
Table 1. Alfalfa weevil degree days as of March 12, 2025. Kansas Mesonet, 2025: Kansas Mesonet Alfalfa Weevil Degree Days Accessed 12 March 2025, http://mesonet.k-state.edu/agriculture/degreedays/
| Accumulated Alfalfa Weevil Growing Degree Days | ||||
| Station | Actual | Normal | Departure | |
| Northwest | Cheyenne | 164 | 28 | 136 |
| Colby | 165 | 25 | 140 | |
| Southwest | Garden City | 236 | 59 | 178 |
| Meade | 257 | 61 | 196 | |
| North-central | Hays | 177 | 31 | 146 |
| Osborne | 168 | 15 | 153 | |
| South-central | Hutchinson | 190 | 35 | 158 |
| Harper | 216 | 61 | 155 | |
| Northeast | Manhattan | 135 | 27 | 108 |
| Corning | 110 | 13 | 97 | |
| Southeast | Cherokee | 197 | 55 | 142 |
| Woodson | 180 | 41 | 139 | |
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 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.
Table 2. Approximate degree days required for alfalfa weevil development. Excerpt from Whitworth et. al., Alfalfa Weevils, Kansas State University, October 2022 (MF2999).
| Degree Days or Thermal Units | Stage | Importance |
| 25-300 | Eggs hatch | In stems |
| 301-450 | 1st and 2nd instars | Leaf pinholing – start sampling |
| 450-600 | 2nd and 3rd instars | Defoliation |
| 600-750 | 3rd and 4th instars | Defoliation |
| 750+ | Pupa to adult | Adults – some feeding – oversummering |
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 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 (Table 3). When making management decisions, it is important to rotate modes of action as this is an effective way to prevent the development of resistance.
Table 3. Products registered in Kansas for control alfalfa weevil. Treatments listed are mainly used for treating alfalfa weevil larvae; products with an asterisk are also recommended for adult alfalfa weevil control. For more specific information relative to any insecticide, always refer to the actual label on the product.
| Trade Name | Chemical Name | Mode of Action Class |
| Fastac CS | Alpha-cypermethrin | 3A |
| Baythroid XL | Beta-cyfluthrin | 3A |
| Lorsban | Chlorpyrifos | 1B |
| Cobalt Advanced | Chlopyrifos + lambda-cyhalothrin | 1B+3A |
| Stallion | Chlopyrifos + zeta-cypermethrin | 1B+3A |
| Tombstone | Cyfluthrin | 3A |
| Proaxis | Gamma-cyhalothrin | 3A |
| Steward | Indoxacarb | 22A |
| Warrior II w/Zeon Tech | Lambda-cyhalothrin | 3A |
| Beseige | Lambda-cyhalothrin + chlorantraniliprole | 3A+28 |
| Lannate | Methomyl | 1A |
| Imidan 70-W* | Phosmet | 1B |
| Mustang MAXX | Zeta-cypermethrin | 3A |
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 2025 Alfalfa Insect Pest Management Guide that will be available online soon.
Anthony Zukoff, Extension Entomology Associate – Garden City
azukoff@ksu.edu