Beeting Up the Competition: Western Honeybee Resistance to Varroa Destructor
Varroa mites are one of the biggest threats to the Western Honeybee. As of June 2022, the Varroa Destructor reached the last continent kept isolated from their wrath: Australia. To protect hives against these devastating and ever-present parasites, beekeepers yearly administer acaricides to diminish varroa populations (see our article on Oxalic Acid Treatment). Researchers have been trying to develop honeybees’ resistance to these parasites for years. Recent research is promising but also shows how much more work must be done before this parasite’s impact on the honeybee population is noticeably diminished.
Varroa mites are a parasitic mite that live on all life stages of the western honeybee. Being approximately 1.5mm, these mites are quite large compared to their hosts, who are 15mm in length. For comparison, that would be like a 5’6” (167cm) human having a 6” (15cm) parasite living on them.
These nasty mites feed off their hosts, creating malformations in honeybees who were fed on prior to adulthood. Bees infested with these parasites are known to be smaller in size and contain obvious body malformations. Because these mites create a hole in their hosts, an infected bee will be vulnerable to different infections. A hive overrun with these pests will not be able to sustain itself for very long before being afflicted by a virus or completely dying. Varroa mites enter the hive on an adult honeybee, but they only reproduce in brood cells (where honeybee eggs are laid and larvae develop into adults). They enter the brood cells by falling from infested adult bees. Once the brood cell is capped, the varroa mite will lay many eggs and feed on the larvae as it is developing.
A hive is considered “resistant” by possessing desirable genetic hygienic behaviors. These behaviors include grooming mites off adult bees, detecting and removing infected brood, more frequent swarming, smaller colony size, and smaller brood cell size. The removal of adults mites and breeding mites within the brood is advantageous to the honeybee for obvious reasons. More frequent swarming (though not preferred by most beekeepers), causes brood breaks which can throw a wrench in the mites’ life cycle. Because varroa breed in brood cells, having smaller brood cells can decrease the number of mites that can multiply in one cell.
Modes, Results, and Future of Research
Selective breeding of honeybees is required for a hive to achieve varroa resistant characteristics. Some techniques include artificial insemination of the queen bee in tandem with the use of only one drone. Having only one father to the colony decreases genetic variability allowing for easier selection of genetic characteristics. Another method of selective lineage was undertaken by crossing varroa resistant queens with a US commercial honeybee population. This method showed “promise in creating bees that have useful mite resistance and desirable beekeeping characteristics.”
In early 2014, a European team of honeybee researchers developed over one hundred small colonies with USDA approved Varroa Sensitive Hygiene (VSH). For a colony to be considered to have VSH characteristics, they must be able to detect and remove varroa-infested brood. This European team of beekeepers’ research supports the single-drone insemination technique for minimizing variation of genetic characteristics.
One review notes the daunting reality of the varroa infestation. Even with selective breeding in honeybees, this technique is far from reaching the world wide infestation of the varroa mite. If the technique of selective lineage is going to make a difference on a global scale, it will take years of informing beekeepers and decades of well-practiced VSH genetic selection.
References  Turnbull, T. (2022, June 29). Australia honey bees put in lockdown due to deadly Varroa parasite. BBC News. Retrieved July 27, 2022, from https://www.bbc.com/news/world-australia-61976446  Van Alphen, J.J.M., Fernhout, B.J., 2020. Natural selection, selective breeding, and the evolution of resistance of honeybees (Apis mellifera) against Varroa. Zoological Letters 6.. doi:10.1186/s40851-020-00158-4 Danka, R.G., Harris, J.W., Dodds, G.E., 2016. Selection of VSH-derived “Pol-line” honey bees and evaluation of their Varroa-resistance characteristics. Apidologie 47, 483–490.. doi:10.1007/s13592-015-0413-7  Great step forward in breeding Varroa Resistant Honeybees. Arista Bee Research. (n.d.). Retrieved July 27, 2022, from https://aristabeeresearch.org/press-release/  Guichard, M., Dietemann, V., Neuditschko, M., Dainat, B., 2020. Three Decades of Selecting Honey Bees that Survive Infestations by the Parasitic Mite <em>Varroa destructor</em>: Outcomes, Limitations and Strategy.. doi:10.20944/preprints202003.0044.v1