| Summary: | <i>Vairimorpha</i> (formerly <i>Nosema</i>) <i>apis</i> and <i>V. ceranae</i> are microsporidian pathogens that are of concern for managed honey bee colonies. Multiple treatments have been proposed to be effective in reducing the prevalence and intensity of <i>Vairimorpha</i> spp. infections. Here, we test the efficacy of these products in one lab-based experiment and three field experiments. In the lab experiment, we found no reductions in <i>Vairimorpha</i> spp. prevalence (proportion of individuals infected with <i>Vairimorpha</i> spp.) or intensity (number of <i>Vairimorpha</i> spp. spores per individual), but we did find a decrease in honey bee survival after treatment with Fumagilin-B, Honey-B-Healthy<sup>®</sup>, and Nozevit Plus. The first field experiment showed increased <i>Vairimorpha</i> spp. intensity in colonies treated with Fumagilin-B and HiveAlive<sup>®</sup> compared to a negative control (sucrose syrup alone). The second field experiment showed a weak reduction in <i>Vairimorpha</i> spp. intensity after 3 weeks post treatment with Fumagilin-B compared to Nozevit. However, <i>Vairimorpha</i> spp. intensity returned to levels comparable to those of other treatment groups after 5 weeks post treatment and remained similar to those of other groups for the duration of the experiment. The final field trial showed no positive or negative effects of treatment with Fumagilin-B or Nosevit on <i>Vairimorpha</i> spp. prevalence or intensity. These findings raise questions regarding the efficacy of the products currently being used by beekeepers to control <i>Vairimorpha</i> spp. We argue that the observed reduction of <i>Vairimorpha</i> spp. is more likely relevant to the phenology of spore prevalence and intensity in honey bee colonies than to chemical treatment.
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