Evaluation of the efficacy of the novel varroacide 1-allyloxy-4-propoxybenzene, and assessment of sublethal exposure on honey bee and Varroa destructor gene expression
Creators: Lu, Robert X
Publisher: name: University of Alberta Library
Year: 2024

<p>The ectoparasitic mite Varroa destructor (“varroa”) poses a major threat to sustainable beekeeping worldwide. Various chemical “varroacides” have been developed to combat infestations in commercial apiaries. While effective, these chemicals can be toxic to honey bees, humans, and leave unwanted residue deposition in beeswax and honey. Furthermore, varroacide-resistant mite populations are increasingly prevalent, necessitating the development of novel solutions. The dialkoxybenzene 1-allyloxy-4-propoxybenzene (codenamed “3c{3,6}”) is a promising new varroacide, capable of inducing paralysis and death in varroa in vitro. Prior field assays have demonstrated 3c{3,6} treatment to reduce varroa populations and improve overwintering survival in hives. My thesis continued to evaluate the viability of 3c{3,6} as a commercial varroacide.<br>
Across three experimental seasons, I expanded on existing 3c{3,6}field tests to evaluate efficacy and optimize treatment duration, dosage, and applicator design. In the fall  of 2021, I tested a 4 g/colony dosage over four weeks, using a wooden applicator was placed on the top bars of each colony’s broodnest. In the fall of 2022, the dosage was increased to 8 g/colony, applied over six weeks via a larger applicator consisting of three wooden strips suspended between broodnest frames. Cardboard was also tested as an alternative applicator material in this year. In spring 2023, a 3c{3,6} dosage of 10 g/colony was applied for 6 weeks using the same wooden applicator design as the fall of 2022. In all trials, colonies treated with 3c{3,6} were compared against a negative control containing no active compound, as well as a positive control treated with the commercially available varroacide Thymovar® in 2021 and 2022. In all years, 3c{3,6}-treated colonies exhibited significantly higher mortality of varroa  than untreated counterparts, with mortality rates and efficacy comparable to Thymovar® in 2021 and 2022. Nevertheless, 3c{3,6} treatment efficacy was lower in 2021 (~48%) versus 2022 and 2023 (&gt;90% in all cases), attributable to a combination of lower dosage, reduced applicator surface area, and a shorter treatment duration. Colonies treated with 3c{3,6} showed improved overwinter survival versus negative control colonies in 2022 and 2023, but not 2021. Cardboard applicators, though effective, were destroyed by worker bees within the hive.<br>
Secondly, I used an exposure bioassay coupled with transcriptomics to assess sublethal effects of 3c{3,6} vapour exposure on varroa and honey bees at an EC50 dosage (based on the concentration required to kill or paralyze 50% of a sample of varroa mites). I hypothesized greater perturbation in varroa metabolism than in bees, given that 3c{3,6} possesses higher toxicity towards the former. In a 2-by-2 design, cohorts of 10 nurse-age bees were either infested  with varroa mites, exposed to 3c{3,6}, had both mites and 3c{3,6}exposure, or neither. I found several hundred differentially-expressed genes between 3c{3,6}-exposed and control honey bees and mites, including putative genes pertaining to detoxification and odorant detection in the bees, and genes encoding calcium-dependent synaptic enzymes in the mites. However, no genes were significantly differentially regulated following a Benjamini-Hochberg correction. Additionally, no significantly differentially regulated ontology pathways were identified.<br>
Overall, my research further demonstrates 3c{3,6} as a promising novel varroacide. In addition, my trials helped optimize dosage and applicator design, though other materials may be tested in the future. However, the precise mode of action of 3c{3,6} on varroa remains unresolved, and further research is needed to investigate the impact of 3c{3,6} exposure on worker bee development and queen fertility.</p>