The relative effects of season and imported bumblebee colonies on the distribution of ten common pathogens in wild Swedish bumblebees

Abstract: Due to recent declines regarding pollinators, The Swedish Civil Contingencies Agency launched a project investigating possible biological threats towards wild bumblebees. This led to an extensive collection of bumblebees from areas in southern Sweden during the summer of 2018. The purpose of this project was to investigate the prevalence of ten bumblebee pathogens and see if there were any difference in prevalence in areas with and areas without commercial bumblebees. This study also aimed to investigate if the prevalence of the pathogens tended to fluctuate during the season. Of the collected bumblebees (Bombus spp) the ones from the late summer season were analyzed in this project, and data from the already analyzed samples from the early summer season was used. To screen the bumblebees for the pathogens a laboratory process was followed with dissections – homogenization – DNA and RNA extractions – conversion of RNA to cDNA and lastly real-time quantitative PCR. The results showed significant differences between areas with and without commercial bumblebees for some pathogens, but the results were only consistent between early and late season for Apicystis bombi and Black queen cell virus. Apicystis bombi had a higher prevalence (2,0 and 3,0 for late and early season) in areas with commercial bumblebees. Black queen cell virus had a higher prevalence (1,3 and 1,6 for late and early season) in areas without commercial bumblebees. For the seasonal fluctuation a significant difference between early and late season could be seen for the pathogens – Crithidia bombi, Apicystis bombi, Locustacarus buchneri and Sphaerularia bombi.