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Title: Responses to Varroa by honey bees with different levels of Varroa Sensitive Hygiene

Author
item HARBO, JOHN - USDA,ARS, RETIRED
item Harris, Jeffrey

Submitted to: Journal of Apicultural Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2008
Publication Date: 7/1/2009
Citation: Harbo, J.R., Harris, J.W. 2009. Responses to Varroa by honey bees with different levels of Varroa Sensitive Hygiene. Journal of Apicultural Research 48(3):156-161

Interpretive Summary: We have selectively bred honey bees (Apis mellifera L.) for resistance to the varroa mite (Varroa destructor). Originally, we called these SMR bees because they had a trait that suppressed mite reproduction. However, we recently found that the SMR trait is a form of hygienic behavior that we have named varroa sensitive hygiene (VSH). With VSH, adult worker bees patrol the brood nest and remove mite-infested pupae from capped brood cells. Removal of the host pupa effectively stops the reproduction by the invading mite, and populations of varroa mites eventually decline in colonies with high levels of VSH genes in the worker bee population. The primary objective of this experiment was to provide information on the type of mite-infested pupae that are removed by VSH bees. In particular, our previous research had suggested that VSH bees preferentially removed pupae that were infested by mites that had produced offspring. We tested this hypothesis by producing two sets of colonies: one group had a high level of VSH genes, and the other group had a low level of VSH genes. These two groups were produced by backcrossing 16 queens from a 100% VSH line (high group) and 19 queens from a 0% VSH line (low group) to drones that were produced from a hybrid high × low queen. Each queen was single drone inseminated, and we expected a range of colony phenotypes to be produced within each group. After colonies were well established, a comb of mite-infested worker brood with pupae aged 0-3 days postcapping was placed into each colony for 1 week. The initial infestation rate (mites per sampled pupa) was determined for each comb of brood before the experiment, and a final infestation rate was determined at the end. We also determined the reproductive status for all mites found at the end. We correlated the ratio of the final infestation to initial infestation of capped brood (infestation ratio) to the ratio of ovipositing to nonovipositing mites (mite oviposition ratio). Results showed that as the intensity of removal by VSH bees increased fewer worker pupae were found with ovipositing mites. Pupae with nonovipositing mites remained at similar levels between high and low VSH colonies. Therefore, we concluded that oviposition of the mite or something associated with mite oviposition provides the stimulus for bees to remove varroa-infested pupae.

Technical Abstract: The mite-resistance trait called suppression of mite reproduction (SMR) is a form of hygienic behavior that we have named varroa sensitive hygiene (VSH). With VSH, adult worker bees (Apis mellifera) disrupt the population growth 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 frequency of ovipositing mites. We produced 35 colonies with variable levels of VSH by backcrossing 16 queens from a high line, H (100% expression of VSH), and 19 queens from low line, L (no VSH), to drones produced by an HL (high × low) queen. Because each of the 35 queens was mated to one drone, the resulting colonies were expected to represent the complete range of variability (0 to 100% of the genes for VSH). To estimate brood removal, we measured mite populations in capped worker brood that was 0 - 3 d postcapping and again 7 days later when the cohort was aged 7 - 10 d postcapping. We correlated the ratio of the final infestation to initial infestation of brood (infestation ratio) to the ratio of ovipositing to nonovipositing mites (mite oviposition ratio). Ovipositing mites were mites that lay at least one egg after invading a cell. The results showed that as the intensity of removal by VSH bees increased fewer worker pupae were found with ovipositing mites; pupae with nonovipositing mites remained at similar levels between high and low VSH colonies. Therefore, oviposition of the mite or something associated with mite oviposition provides the stimulus for bees to remove varroa-infested pupae.