Get those fleas off my bees: Assessing the isoxazoline class as possible treatments for Varroa destructor control and safety towards honey bees (Apis mellifera)

Chemical resistance to pyrethroids (tau-fluvalinate), organophosphates (coumaphos) and formamidines (amitraz) in Varroa destructor populations has been observed worldwide. To find new active ingredients that could reduce V. destructor populations in honey bee (Apis mellifera) colonies, we examined the acute toxicity of insecticides from the isoxazoline class of chemicals. Isoxazoline insecticides are a commonly used to treat ticks and fleas on mammals through topical solutions. Here, we calculated the lethal dose that killed 50% of the population (LD50) and the selectivity ratios (honey bee LD50/V. destructor LD50) for afoxolaner, fluralaner, sarolaner, and lotilaner on V. destructor and honey bees by using direct topical application methods. These values were compared to amitraz, coumaphos, a solvent control and a positive control (dimethoate). Fluralaner (LD50 = 0.068 ng/V. destructor) was the most toxic isoxazoline insecticide and only 1.9× less toxic than amitraz (0.036 ng/V. destructor), but was still 26,200× more toxic than coumaphos (1788.692 ng/V. destructor). Sarolaner (selectivity ratio = 0.05) and afoxolaner (selectivity ratio = 0.12) were more toxic to honey bees than they were to V. destructor, while fluralaner was 126× more toxic to V. destructor than to honey bees. These results suggest that fluralaner could be a promising active ingredient due to its high toxicity to V. destructor and its relatively low toxicity to honey bees. Further research should be conducted to study the toxicity of isoxazoline insecticides, specifically fluralaner, on honey bee larvae as well as the sub-lethal impacts of these chemicals to adult honey bees.