Eristalis flower flies can be mechanical vectors of the common trypanosome bee parasite, Crithidia bombi

<p><b>Flowers can be transmission platforms for parasites that impact bee health, yet bees share floral resources with other pollinator taxa, such as flies, that may be hosts or non-host vectors (i.e., mechanical vectors) of parasites. Here, we assessed whether the fecal-orally transmitted gut parasite of bees, <i>Crithidia bombi</i>, can infect <i>Eristalis tenax</i> flower flies. We also investigated the potential for two confirmed solitary bee hosts of <i>C. bombi, Osmia lignaria</i> and <i>Megachile rotundata</i>, as well as two flower fly species, <i>Eristalis arbustorum</i> and <i>E. tenax</i>, to transmit the parasite at flowers. We found that <i>C. bombi</i> did not replicate (i.e., cause an active infection) in <i>E. tenax</i> flies. However, 93% of inoculated flies defecated live <i>C. bombi</i> in their first fecal event, and all contaminated fecal events contained <i>C. bombi</i> at concentrations sufficient to infect bumble bees. Flies and bees defecated inside the corolla (flower) more frequently than other plant locations, and flies defecated at volumes comparable to or greater than bees. Our results demonstrate that <i>Eristalis</i> flower flies are not hosts of <i>C. bombi</i>, but they may be mechanical vectors of this parasite at flowers. Thus, flower flies may amplify or dilute <i>C. bombi</i> in bee communities, though current theoretical work suggests that unless present in large populations, the effects of mechanical vectors will be smaller than hosts.</b.</p>