Adjuvants to improve efficacy of miticides in managed honey bee (Apis mellifera) colonies to control Varroa destructor

Beekeepers must manage Varroa destructor mites to maintain colony health. Large-scale beekeepers often use chemical treatments (miticides) to manage this pest. Miticide resistance drives a need for compounds with alternative modes of toxic action that can be used in rotation as part of a Varroa management plan. This research aimed to determine the efficacy of oxalic acid, clove oil, and fenpyroximate when delivered in glycerin soaked in strips and combined with a range of bee-safe adjuvants. Adjuvants are a group of compounds used in plant pesticide applications to increase the spreading and penetration of a pesticide. Laboratory cage trials tested a miticidal active ingredient (oxalic acid, clove oil, or fenpyroximate) and an adjuvant (Ecostep BC-12®, Ecostep SE-11®, Ecostep AE-13®, Ecostep CE-13®, or Silwet L-7500®) in glycerin-soaked strips. Field trials evaluated the best performing active ingredient-adjuvant combination from cage trials, oxalic acid combined with Ecostep BC-12® adjuvant in glycerin-soaked strips. Neither glycerin control, oxalic acid alone, or oxalic acid with adjuvant caused a significant change in Varroa per 100 bees in field trial year 1, when starting Varroa levels were high (average 11.8 Varroa per 100 bees across all treatment groups). In year 2, when starting Varroa levels were low (average 0.58 Varroa per 100 bees across all treatment groups), Varroa per 100 bees increased 2.6-fold for the glycerin control and 2.8-fold for oxalic acid alone, while a 29% reduction was observed in the oxalic acid with adjuvant treatment. Additionally, mite drop data indicated increased speed for the miticidal effect when an adjuvant is included with oxalic acid. This research informs formulation chemistries for oxalic acid and other miticides to help beekeepers maintain healthy hives.