Effects of parasitization by Varroa destructor on survivorship and physiological traits of Apis mellifera in correlation with viral incidence and microbial challenge

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Abstract
Varroa mites (Varroa destructor) are serious ectoparasites of honey bees (Apis mellifera). This research addresses the impact of varroa mites on survivorship, viral incidence, and physiological traits of newly-emerged worker bees. RT-PCR confirmed our previous finding that varroa parasitization was linked to high levels of deformed wing virus (DWV). In non-treatment bees, varroa parasitization combined with increased viral levels altered survivorship curves from long-survival to shorter-survival types. After challenge with live Escherichia coli, the survivorship of mite-parasitized bees was significantly lower than mite-free bees. Deformed-wing, mite-parasitized bees died on average within 1 day, even without E. coli challenge. This was correlated with the absence of an important enzyme activity in insect immunity, phenol oxidase, lacking even in those bees challenged with immuno-elicitors. The lack of inducible phenol oxidase activity indicated that the bee immune system is not fully competent upon adult emergence. Varroa parasitism also significantly reduced body weight of the parasitized bees, but body weight was not significantly correlated with the survivorship of mite-parasitized bees. Our research indicates that the combination of mite parasitization, the interaction of DWV and microbes, and a developmental immune incompetency attribute to decreased worker survivorship and have a negative impact on colony fitness.

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