The effects of raw propolis on Varroa-infested honey bee (Apis mellifera) workers

Self-medication plays a major role in the behavioral defense against pathogens and parasites that animals have developed during evolution. The conditions defining this adaptive behavior are: (1) contact with the substance in question must be deliberate; (2) the substance must be detrimental to one or more parasites; (3) the detrimental effect on parasites must lead to increased host fitness. Recent studies have shown that A. mellifera colonies are able to increase resin foraging rates when infested by V. destructor, whereas further investigations are needed for evidence of parasite and host fitness. In order to understand whether Varroa-infested colonies could benefit from increasing levels of resin, we carried out laboratory bioassays to investigate the effects of propolis on the fitness of infested workers. The longevity and energetic stress of adult bees kept in experimental cages and artificially infested with the mite were thus monitored over time. At the same time, in vitro experiments were performed to study the contact effects of crude propolis on Varroa mites. Our results clearly demonstrate the positive effects of raw propolis on the lifespan of Varroa-infested adult bees. A low narcoleptic effect (19-22%) of raw propolis on phoretic mites after 5 h was also observed. In terms of energetic stress, we found no differences between Varroa-free and Varroa-infested bees in terms of the daily sucrose solution demand. Our findings seem to confirm the hypothesis that resin collection and propolis use in the hive represent an example of self-medication behavior in social insects.