Varroa mites exhibit strong host fidelity despite spatial desegregation of mite and host species in Vietnam

Parasitic mites of the genus Varroa pose a serious threat to global apiculture. Host switches from native to introduced Apis species have occurred in multiple Varroa species, in some cases with devastating results. Additional host shifts and hybridisation among mite species and mite lineages on different hosts represent significant risks for apiculture and natural ecosystems. Quantifying this risk is thus an essential step toward management. The risk of hybridization among mite lineages on different hosts is likely to be at its highest in eastern Asia, where multiple host and mite lineages occur sympatrically. The available evidence, however, is somewhat contradictory, suggesting gene flow is occurring in some localities, but limited or absent in others. In this study, we elucidate the relationships between Varroa mites collected from A. mellifera and A. cerana in Vietnam, a region of high overlap of mite species and hosts, using a combination of genome-wide SNPs and mitochondrial sequence data. Our results indicate an absence of any new host shifts and a lack of gene flow across host species and between mite species, despite considerable overlap in mite and host species ranges. This confirms the findings of several earlier studies in this area but contrasts with evidence of hybridization in other regions, suggesting regional differences in the potential for gene flow between host/mite combinations. Nonetheless, we suggest this area remains one of high risk, because of (i) considerable overlap of mite lineages which elsewhere have been shown capable of interbreeding, (ii) evidence we find of human-mediated long-range dispersal events, which increase the chance of interaction among different mite lineages, and (iii) the persistent threat of secondary factors such as disease transmission, which may be facilitated by both of the factors above.