// The Varroa mite (Varroa destructor) is the most devastating parasite of Western Honey Bee populations. Originally from Asia, Varroa has spread to Europe, the Americas and beyond, infesting brood cells of honey bee colonies, reproducing and feeding on the larvae and pupae. The mite weakens its host, impairing its development and immune function. The Varroa mite is also a vector of debilitating diseases, including Deformed Wing Virus (DWV).1,2
Many beekeepers combine chemical treatments with other control measures in an integrated pest management approach to safeguard their colonies against Varroa. In the long-run, due to overuse of single products, Varroa has become resistant1 to some of the available chemical treatments and new product development and registration are challenging and both time- and cost-intensive. Treatment efficacy also depends on good beekeeping practices. Otherwise, mite control is often hampered by re-infestation when hive densities are high. As such, beekeepers are fighting a tough battle.
BartJan Fernhout, Program Director of the Arista Bee Research Foundation, knows this problem all too well. Years ago, he worked in the pharmaceutical industry but kept bees as a hobby. “I lost two extraordinary queens when I used a chemical to treat my colonies against Varroa. As a side effect, the queens stopped laying eggs. The worker bees did not like that and replaced them. I was devastated. I thought to myself, there’s got to be a better way.”
Seeking answers, BartJan discovered breeding work done at the United States Department of Agriculture in Baton Rouge.3 Encouraged by their results, building on their methods and supported by many highly-motivated bee breeders, BartJan started the Arista Foundation with the mission to create Western Honey Bee lines capable of ridding themselves of Varroa mites without external assistance. “This appears to be a behavior present to some degree in any honey bee strain,” explains BartJan. “What we’re doing is fashioning an environment that allows the behavior to emerge.”
Spanning 34 km at its widest point, Barbados enjoys a mild tropical climate with distinct rainy and dry seasons.
The behavior BartJan is talking about is called Varroa Sensitive Hygiene (VSH). VSH-positive worker bees detect and remove pupae that are infested with mites, effectively interrupting mite reproduction and holding their population at bay. Arista programs breed bee colonies and expose them to Varroa to find those capable of clearing the infestation from the worker brood; that is, they exhibit 100 percent VSH.
An important question about Varroa resistance is under which circumstances it emerges naturally in honey bee populations. We have seen this occur in the Asian Honey Bee (Apis cerana), but what would trigger this emergence in the Western Honey Bee (Apis mellifera)?
While vacationing on Barbados, BartJan heard about an extreme honey bee die-off in 2003, the year that Varroa arrived on the island. Nearly 80 - 90 percent of managed and feral honey bee colonies collapsed. Since then, however, the honey bee population has recovered. “Here were honey bees in the tropics where, without winters, Varroa mites can infest the bee brood year-round. The bee colonies nearly vanished and then came back!” BartJan wondered, “did they develop Varroa resistance on their own?” As an isolated island, Barbados offered a rare opportunity to study resistance development; a kind of Darwinian-evolution laboratory.
Collaborating with the Bayer Bee Care Center, BartJan and his team visited Barbados over four seasons in 2018 to study 10 - 12 colonies at three sites. The aim was to look for evidence of Varroa resistance and, if present, identify how the bees were handling mite infestations. “Being in a remote location, the project presented many unique challenges,” notes BartJan. “We overcame them all with support of local beekeepers and basically built an apiary from scratch. There, we spent long hours sampling the colonies and evaluating brood under our microscopes.”
BartJan opens cells of a brood comb to extract the pupae and count the number of reproductive and non-reproductive Varroa mites. VSH-positive honey bees detect and remove mainly the infected pupae on which the mites reproduce, effectively interrupting the mites’ development process.
Sampling the study honey bee colonies on Barbados. Unlike managed bee colonies in Europe and the USA, most hives in Barbados are established from feral colonies which tend to swarm and are more sensitive to disturbance.
The work was worth it. The honey bees in Barbados show evidence of strong Varroa resistance. In representative samples taken from the colonies, only a low percentage of the examined bees and brood cells had mites (graph 1), and a high proportion of those were non-reproductive. In a normal, non-resistant honey bee colony, up to 20 percent of the resident mites may be nonreproductive. As the number of reproductive mites is reduced, due to the bees selectively removing them or by suppression of mite reproduction by the brood itself, the percentage of non-reproductive mites in the colony automatically increases. So, higher percentages, as seen in the Barbados honey bee colonies, are connected with Varroa resistant behavior in the honey bees (graph 2). The colonies also appeared healthy, showing no signs of brood disease or Deformed Wing Virus (DWV). Nevertheless, BartJan points out that “while the data show that the bees are resistant, they are not conclusive about the resistance behaviors involved. We saw VSH during inspections and feel confident that the bees are using it, but exactly how they keep mite levels down must be tested directly in further experiments.”
The honey bees from Barbados will not be used in the Arista breeding programs, which focus on managed Western Honey Bee lines. Yet, this adds another count to a list of “very few genuinely Varroa resistant bee populations,” says BartJan. “To me,” he continues, “the results support my suggestion that VSH happens in nature. Extreme conditions like the dramatic knockdown of the honey bees on Barbados can cause the selection of a trait that renders them resistant to Varroa, and this trait can be maintained in the honey bee population assuming beekeepers – like those in Barbados – do not treat their colonies.”
A breeding project on Hawaii has produced colonies with 100 percent VSH and the Arista Foundation is working with breeders, researchers, industry and government to produce similar colonies in different honey bee stocks (e.g. Carnica, Buckfast). “Working together,” says BartJan, “we can balance VSH with other desirable traits from the start, to generate long-term healthy, Varroa resistant and productive bee stocks.”
1Ramsey, S.D., et al. 2019. Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph. PNAS 116: 1792.
2Rosenkranz, P., et al. 2010. Biology and control of Varroa destructor. J. Inv. Path.,103: S96.
3Harbo, H. and Harris, J. 1999. Heritability in honey bees (Hymenoptera: Apidae) of characteristics associated with resistance to Varroa jacobsoni (Mesostigmata: Varroidea). J. Econ. Entom. 92: 261.