Are dispersal mechanisms changing the host-parasite relationship and increasing the virulence of Varroa destructor [Acari:Varroidae] in managed honey bee [Hymenoptera: Apidae] colonies?

Authors: DeGrandi-Hoffman, Gloria; AHUMADA, FABIANA - Agscience Llc; Graham, Richard

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2017
Publication Date: 8/1/2017
Citation: Hoffman, G.D., Ahumada, F., Graham, R.H. 2017. Are dispersal mechanisms changing the host-parasite relationship and increasing the virulence of Varroa destructor [Acari:Varroidae] in managed honey bee [Hymenoptera: Apidae] colonies?. Environmental Entomology. 46(4):737-746. doi:10.1093/ee/nvx077.

Interpretive Summary: Varroa mites are the most serious pest of honey bees worldwide, and are difficult to control in managed colonies. We conducted a longitudinal study in commercial apiaries and found that that even with multiple miticide treatments in the summer and fall, mite numbers remained high and overwinter colony losses exceeded 55%. Furthermore, large heavily infested colonies treated in September perished shortly thereafter. Though Varroa can cause high colony mortality in managed apiaries, there is documentation of feral colonies that have coexisted with Varroa for years. Mite survival and dispersal strategies may differ between feral and managed colonies and affect colony survival. For example, in feral colonies, mites disperse when colonies swarm. In managed apiaries, mites disperse when collapsing colonies are robbed or when foragers with mites drift into other colonies. We use a honey bee colony-Varroa mite population model to simulate the effects of swarming on the survival of colonies infested with mites. The model predicts that at low initial mite populations, yearly swarming events keep Varroa at low levels and enable the colony to survive for more than five years. Without swarming, colonies with the same initial mite level collapse by year-3. Varroa dispersal on foragers requires that mites attach to foragers rather than nurse bees, and that foragers drift to other hives. We hypothesize that viruses transmitted by Varroa may be an underlying cause for drifting foragers with mites, because virus infections can affect the age of first foraging and cognitive function. Simulations using data collected from field studies indicate that low levels of drifting foragers with mites beginning in late summer create sharp increases in mite populations in the fall and heavily infested colonies in the spring. Finally, we suggest new research directions to investigate factors leading to the dispersal of mites on foragers. Mite management strategies that consider Varroa as a migratory pest also are discussed.

Technical Abstract: Varroa mites are the most serious pest of honey bees worldwide, and difficult to control in managed colonies. We show in a longitudinal study that even with multiple miticide treatments in the summer and fall, mite numbers remained high and colony losses exceeded 55%. Furthermore, large heavily infested colonies treated in September perished shortly thereafter. High colony mortality from Varroa in managed apiaries is a departure from the effects of Varroa in feral colonies where bees and mites can coexist. Differences in mite survival strategies and dispersal mechanisms may be contributing factors. In feral colonies, mites disperse when colonies swarm while in managed apiaries where swarming is controlled, mites disperse on foragers robbing or drifting from infested hives. We use a honey bee colony-Varroa mite population model to simulate the effects of swarming on the survival of colonies infested with mites. The model predicts that at low initial mite infestations, yearly swarming events curtail Varroa population growth and enable infested colonies to survive for more than five years. Without swarming, colonies collapse by year-3. Varroa dispersal on foragers requires that mites attach to foragers rather than nurse bees, and that foragers drift to other hives. We hypothesize that viruses transmitted by Varroa may be an underlying cause for drifting foragers with mites because of the effects that virus have on age of first foraging and cognitive function. Simulations using data collected from field studies indicate that low levels of drifting foragers with mites beginning in late summer create sharp increases in mite populations in the fall and heavily infested colonies in the spring. Finally, we suggest new research directions to investigate factors leading to the dispersal of mites on foragers. Mite management strategies that consider Varroa as a migratory pest also are discussed.