Assessment of pyrethroid susceptibility, parasitism, and biotype variation in alfalfa weevil populations in Nebraska

Alfalfa (Medicago Sativa L.) is a key agricultural forage crop of the western United States, and the alfalfa weevil (Hypera Postica) is a major pest that poses a significant threat to alfalfa forage quality and productivity. Although various management strategies exist, including cultural practices (i.e., early harvest and grazing), biological control (parasitoids), and host plant resistance, alfalfa weevil remains a persistent pest heavily relied on insecticides to control. Resistance to pyrethroids has been recently reported in western states; however, whether alfalfa weevil populations in the central Midwest have developed resistance is unknown. The objective of this study is to assess alfalfa weevil larvae susceptibility to two pyrethroids: lambda-cyhalothrin (type II) and permethrin (type I) in Nebraska. Alfalfa weevil larvae were collected from alfalfa farms in 11 counties across Nebraska for vial bioassays to calculate median lethal concentrations (LC₅₀) at 24 hours post exposure.The LC₅₀ and mortality at a resistance diagnostic rate were compared among populations.Parasitism was found in some populations.Hence, parasitism rates were also estimated.Results indicated that all alfalfa weevil populations tested with permethrin remained highly susceptible. However, susceptibility to lambda-cyhalothrin varied among populations across Nebraska.Only one population is considered moderate resistance to lambda-cyhalothrin while the rest are susceptible.Parasitism was observed in 6 of the 11 collected populations with the highest parasitism rate being 3.5%.The detection of moderate resistance in one population highlights the need for continued insecticide resistance monitoring and the adoption of integrated pest management strategies in Nebraska.