Changes in infestation, cell cap condition, and reproductive status of Varroa destructor (Mesostigmata: Varrroidae) in brood exposed to honey bees with Varroa sensitive hygiene

Authors: Harris, Jeffrey; Danka, Robert; Villa, Joseph

Submitted to: Annals of the Entomological Society of America
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2011
Publication Date: 5/7/2012
Citation: Harris, J.W., Danka, R.G., Villa, J.D. 2012. Changes in infestation, cell cap condition, and reproductive status of Varroa destructor (Mesostigmata: Varrroidae) in brood exposed to honey bees with Varroa sensitive hygiene. Annals of the Entomological Society of America. 105(3):512-518.

Interpretive Summary: Honey bees have been bred for a heightened ability to remove pupae infested by varroa mites (Varroa destructor) from capped brood. This behavior is varroa sensitive hygiene, and these varroa-resistant bees are known as VSH bees. The current experiment was conducted to look for evidence of biased removal of bee pupae infested by mites with offspring (fertile mites) as compared to mites that did not reproduce (infertile mites). Combs of mite-infested brood were introduced into colonies of VSH bees and commercial control stocks for a 1-week period. Afterwards, brood cells (> 200) from each comb were carefully examined for the presence of mites and their offspring. Additionally, each brood cell cap was carefully categorized as normal or recapped. Normal cell caps had never been manipulated by hygienic bees, while recapped cells had caps that had been punctured by bees and then re-sealed with wax. A comparison of frequencies of fertile and infertile mites from normally capped brood cells between VSH bees and commercial controls could indicate which types of mites were removed during the experiment. The combs exposed to commercial controls had ten times more pupae with fertile mites in normally capped brood as did the hygienic VSH bees. They also had three times more pupae with infertile mites in normally capped brood as did the VSH bees. Together, these two findings suggest that hygienic VSH honey bees selectively removed or uncapped and recapped pupae that are infested by mites with offspring. This preference for pupae infested by mites with offspring suggests that the stimulus triggering hygiene is enhanced by the presence of mite offspring. If this stimulus could be identified, it might be possible to measure responses of worker bees to a specific odor, and such a method might be used to breed for VSH behavior in honey bees.

Technical Abstract: Honey bees (Apis mellifera L.) bred for Varroa sensitive hygiene (VSH) selectively remove pupae infested with Varroa destructor Anderson & Trueman from capped brood that is inserted into the nest. After one week, remaining brood cells tend to have been uncapped and recapped, and remaining mites are mostly infertile. A primary goal of this experiment was to compare the reproductive status of mites that remained in recapped and normally capped cells after a 1-week exposure to VSH and control colonies. Differences in distribution of fertile mites in normally capped brood cells between VSH bees and control bees may suggest that the stimulus for hygiene is related to reproduction by mites. Identification of stimuli triggering VSH behavior could be used to develop new bioassays for selective breeding of this important resistance mechanism. Combs of capped brood that were exposed to control bees had ten times more pupae with fertile mites in normally capped brood as did VSH bees (6.7% and 0.7%, respectively). They also had three times more pupae with infertile mites in normally capped brood than did VSH bees (1.4% and 0.5%, respectively). Thus, VSH bees targeted fertile mites by a 3:1 ratio by either removing or uncapping and recapping their host pupae. Biased removal of mite-infested pupae with fertile mites suggested that stimuli triggering VSH behavior were enhanced by the presence of mite offspring within the brood cell. This bias for fertile mites is not seen during experiments of short 3-hour duration. The differing results are discussed relative to a behavioral threshold model for hygienic behavior in honey bees in which different experimental protocols may reflect activities of honey bees having different sensitivities to pupae infested by fertile mites. Additionally, mortality of mite offspring was significantly higher in recapped cells than in normally capped cells and contributed to decreased reproduction by the mites.