By Ben Beckman and Samantha Daniel
Alfalfa Weevil Resistance to Pyrethroids
Alfalfa weevil is a serious pest of alfalfa and can cause significant damage, usually in April and May before the first cutting. Reports of reduced efficacy of pyrethroids in Nebraska as well as confirmed resistance to pyrethroids in several western states led to a project in 2025 to determine if Nebraska weevil populations are developing resistance.
During the 2025 season, alfalfa weevil larvae were collected from 12 counties across the state, including the Panhandle, southwest, south-central and eastern Nebraska. The larvae were shipped live to the PREEC Entomology Lab in Scottsbluff, where they were exposed to the label rate of two types of pyrethroid insecticides: lambda-cyhalothrin and permethrin.
The study found that five of the 12 weevil populations had 90% or less mortality when exposed to lambda-cyhalothrin. This indicates that some Nebraska weevil populations may be losing susceptibility to lambda-cyhalothrin. In contrast, all 12 weevil populations had a 100% mortality rate when exposed to permethrin.
What does this mean for alfalfa weevil management?
Currently, there are only two insecticide modes of action available for chemical control of this insect: pyrethroids and indoxacarb (Steward). Because of this, resistance management and the use of alternative control methods is critical. Make sure to scout and only spray when thresholds are reached, use the highest labeled rates, rotate control measures to delay resistance development.
Source : unl.edu