Some areas of Montana are experiencing high populations of alfalfa weevil that have carried over to the second growth after first cutting. There have also been some reports of control failure after applying insecticide with pyrethoid active ingredients. This question of potential resistance is being researched this spring/summer and additional programing and information will be offered this fall and winter to address concerns related to emerging insecticide resistance.
If more than eight larvae per square foot are present under the windrows an insecticide application to the stubble is suggested (attached MontGuide). If earlier insecticide applications prior to first cutting that used pyrethroid active ingredients (Mode of Action, MoA group # 3A) were ineffective, consider using products in different MoA groups such as chlorpyrifos (MoA group 1B) and indoxacarb (MoA group 22A).
Additional information and a list of insecticide products can be found on the High Plains IPM Guide, https://wiki.bugwood.org/HPIPM:Alfalfa_Weevil.
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