Responses to Varroa by honey bees with different levels of Varroa Sensitive Hygiene.
John R. Harbo and Jeffrey W. Harris
Mite resistance that we had earlier called suppression of mite reproduction (SMR) is a form of hygienic behaviour that we have named Varroa Sensitive Hygiene (VSH). With VSH, adult worker bees (Apis mellifera) disrupt the reproduction of parasitic mites (Varroa destructor) by removing mite infested bee pupae from their cells. This study determines which brood cells are targeted by bees with VSH and which are not, and describes the relationship between brood removal and the sudden prevalence of sterile mites. We produced 26 colonies with different levels of VSH by backcrossing 14 queens from a high line, H (100% expression of VSH), and 12 queens from a low line, L (no VSH), to drones produced by an HL (high × low) queen. Because each of the 26 queens was mated to one drone, the resulting colonies were expected to represent the complete range of variability (0 to 100% of the alleles for VSH). To estimate brood removal, we measured mite populations in capped worker brood that was 0 - 3 days postcapping and again 7 days later when the cohort was aged 7 - 10 days postcapping. We correlated removal of mite-infested brood with the relative presence (at 7 - 10 days postcapping) of three classes of foundress mites: (1) viable: those with at least one daughter that could mature before emergence of the host bee; (2) nonviable: those with progeny but with no daughters that could reach maturity; and (3) no eggs: those with dead or nonovipositing foundress mites. As the rate of removal increased, both classes 1 and 2 showed significant declines, whereas class 3 was unchanged. Therefore, oviposition of the mite or something associated with mite oviposition provides the stimulus for bees with the VSH trait to remove mite infested pupae.