Host-parasite adaptations and interactions between honey bees, Varroa mites and viruses

The ectoparasitic mite, Varroa destructor, has become the largest threat to apiculture and honey bee health world-wide. Since it was introduced to the new host species, the European honey bee (Apis mellifera), it has been responsible for the near complete eradication of wild and feral honey bee populations in Europe and North America. Currently, the apicultural industry depends heavily on chemical Varroa control treatments to keep managed colonies alive. Without such control the mite populations in the colony will grow exponentially and the honey bee colony will succumb to the development of overt virus infections that are vectored by the mite typically within three years. Two unique sub-populations of European honey bees (on Gotland, Sweden and in Avignon, France) have adapted to survive for extended periods (over ten years) without the use of mite control treatments. This has been achieved through a natural selection process with unmanaged mite infestation levels enforcing a strong selection pressure. This thesis reveals that the adaptation acquired by these honey bee populations mainly involve reducing the reproductive success of the parasite, that the different populations may have evolved different strategies to do so, and that this mite-resistant trait is genetically inherited. In addition, results of this thesis demonstrate that chemical mite control treatments used by beekeepers to inhibit the mite population growth within a colony can actually worsen bee health by temporarily increasing the bee's susceptibility to virus infection. The results of this thesis highlight the impact that apicultural practices otherwise have on host-parasite interactions and the development of disease in this system. Possible solutions to the threat of Varroa are discussed such as the potential to breed for mite-resistant honey bees, which may offer a sustainable long-term solution, and the need for better general beekeeping techniques that reduce the use of chemical treatments and inhibit the spread of disease