Evaluating the effects of 1-allyloxy-4-propoxybenzene on the parasitic mite Varroa destructor and synthesis of fluorescent probes to visualize the binding location(s) of the active compound in the mite

Varroa destructor is a serious threat to the eastern honey bees, Apis melliferra. The available varroa controls are either ineffective due to resistance of varroa mites or require correct temperatures for efficacy and are labour intensive. To search new effective varroa control agents, 15 compounds were assesed for the acaricidal activities like paralysis and death, against varroa mites in laboratory bioassays. The data from structure-activity assays revealed that allyloxy and propoxy groups at the para position of a benzene-based structure, are necessary for the acaricidal activities tested. Compound 1-allyloxy-4-propoxybenzene, known as 3c{3,6}, showed the highest acaricidal activites from a group of 15 compounds tested . Activity of 3c{3,6} was the same as that of thymol, a widely used varroa control agent in structure-activity relationship studies. 3c{3,6} prevented the mites from staying on the abdomen of bees, which is a major feeding site of the mites. The active compound 3c{3,6} initially paralyzed the mites and eventually a high number of the mites were observed on the surface of the glass dishes used for bioassays. The varroa acetylcholine esterase (VdAChE) was not inhibited by 3c{3,6} and hence, we conclude that VdAChE is not a target of 3c{3,6}. Fluorescent probes in which the structure of 3c{3,6} was modified with a linker, and the linker was attached either to a fluorescein or a rhodamine fluorophore, were synthesized to visualise the target binding location(s) in the mites. All probes showed variable acaricidal activies against varroa mites. The fluorescein-containing compound, 6-FAM probe-2, had similar acaricidal activities as 3c{3,6}. The confocal images highlighted fluorescent signals in the regions of the central nervous system (CNS) in the mites, suggest that there may be a binding target of 3c{3,6} in the CNS. Compound 3c{3,6} could be a potential varroa control agent that could be used in combination with the current varroa controls in an integrated pest management (IPM) system.