Garbarczyk, JessicaEuropean honey bees (Apis mellifera) are essential pollinators of crops and flowering plants globally. Despite their importance, honey bees are threatened by unexplained colony collapse. The combined stress of many different parasites and pathogens is often an attributing factor in failing colonies. One such parasite is Lotmaria passim, a trypanosomatid gut parasite that infects honey bees worldwide. In vitro, L. passim growth is limited by both high temperatures and the acidification of the environment by Firm-5 Lactobacilli. To assess if these interactions are upheld within the honey bee host, we developed an in vivo model using microbiome-depleted honey bees inoculated with L. passim in the presence or absence of Firm-5 Lactobacilli. Post-inoculation, the bees were reared for 5 days across a range of biologically relevant temperatures. Following completion of the trial, each bee gut was dissected, hindgut pH values were measured, and DNA was extracted from the recovered guts. Next, L. passim and Firm-5 Lactobacilli were quantified via qPCR. Preliminary results suggest that although symbiont-mediated parasite inhibition was observed, it is unlikely this interaction was pH-mediated. In the presence of Firm-5 Lactobacilli, L. passim levels were significantly lower than in the absence of Firm-5 Lactobacilli, and lower L. passim levels were significantly correlated with rising temperature. These results provide ecological and mechanistic insights for an understudied host-parasite system and can inform potential treatment strategies targeting a parasite of these essential pollinators.enBiology Honors ProgramJay D. EvansUSDA-ARSDisease EcologyHoney BeesOther