Behavioral mechanisms underlying parasite spread and host survival in the Varroa destructor / Apis mellifera parasite/host system

Abstract

This work has focused on understanding the remarkable survival of the honey bees (Apis mellifera) living without beekeeper intervention in Cornell University’s Arnot Forest, despite the bees’ infestation with the devastating parasitic mite Varroa destructor. The various mechanisms of intercolony transmission of the Varroa mite were investigated by examining the behavior of both mites and their honey bee hosts. Behavioral observations of mites and bees interacting on flowers demonstrated the plausibility of indirect mite transmission between colonies, using flowers as an intermediate substrate and staging ground for the infestation of new hosts. Experimentally placed boxes containing honey in combs were robbed by free-living bees in a forest setting, demonstrating the likelihood of mite transmission between colonies via honey robbing behavior. Detailed observations of worker and drone bees from light-colored colonies infested with a large number of Varroa mites and from nearby dark-colored colonies infested with few mites allowed us to correlate bee behavior with changes in mite populations. These correlations reveal that mites from dying colonies are carried into healthy colonies when the healthy colonies rob honey from the sick ones, and to a lesser extent when infested bees drift from the sick colonies into the nearby healthy ones. These investigations, taken together, show that the mechanisms of horizontal intercolony transmission have been underestimated in forests like the Arnot Forest. After demonstrating these multiple mechanisms of intercolony mite transmission, even when colonies are widely spaced as in forests, we reconsidered the earlier hypothesis that Varroa mites in the Arnot Forest had evolved avirulence. Instead, we investigated the behavioral mechanisms the Arnot Forest bees may be using to resist infestation by Varroa mites. We have demonstrated that the Arnot Forest bees possess multiple behavioral traits (brood hygienic behavior and mite grooming behavior) at high levels, which appear to confer partial resistance to Varroa mites. These traits are expressed at levels higher than is found in unselected populations, but lower than is found in populations that have been consistently selected by queen breeders for each trait, though whether this intermediate expression is adaptive or the result of incomplete evolution is still unclear.