Identifying Resistance to Lambda Cyhalothrin Insecticide on Alfalfa Weevil (Coleoptera: Curculionidae) in Oklahoma.

Alfalfa, Medicago sativa L. (Fabacae), is the third most valuable crop in the United States following maize, Zea mays L., and soybean, Glycine max (L.) Merr., and provides revenue of more than $104,000,000 annually in Oklahoma. The alfalfa weevil, Hypera postica Gyllenhal (Coleoptera: Curculionidae), is the primary pest causing significant defoliation, yield loss, and less stand. While economical and effective, pyrethroids, especially those containing lambda-cyhalothrin, are recently less effective. The objective of the study was to evaluate resistance to insecticide and potential cross-resistance to pyrethroids, specifically lambda-cyhalothrin, to control alfalfa weevil in Oklahoma. Alfalfa weevils at four locations (southwest, southcentral, north-central, and northwest) were assayed for resistance to lambdacyhalothrin, two other labeled pyrethroids, one combination, and one non-pyrethroid. Commercial formulations were prepared in acetone, using recommended field rates listed on the label by manufacturer. To corroborate bioassays in a laboratory, a field experiment was evaluated at the south-central site. Resistance of alfalfa weevil to lambda-cyhalothrin was evident in bioassays causing an average 21.6% efficacy, while all other products resulted in 100%. The experiment in the field supported resistance to lambda-cyhalothrin and resulted in 53.3% efficacy at the greatest labeled field rate. Results indicated that continued use of lambda-cyhalothrin could render the most used and economical pyrethroid ineffective and promote crossresistance to other pyrethroids. Without rotation of insecticides development of resistance eliminates management options and reduces profitability for producers. [ABSTRACT FROM AUTHOR]