Colony success of the bumble bee, Bombus terrestris, in relation to infections by two protozoan parasites, Crithidia bombi and Nosema bombi

摘要

Crithidia bombi is transmitted both horizontally between and vertically within colonies of the bumble bee Bombus terrestris. By experimentally infecting or not infecting laboratory-raised colonies with a standard inoculum before their transfer to the field, this study attempted to evaluate the virulence of C. bombi under natural conditions. An unexpected finding was a substantial and seasonal increase in the infection of natural populations, so that all colonies quickly became infected in the field. Experimentally infected colonies showed positive signs of infection～ 9.5 days after exposure which was not significantly different from the 11.7 days in untreated colonies. No significant differences were found between the 2 experimental groups in terms of colony success, such as male and young queen production or time of emergence of sexuals. Overall, C. bombi showed low levels of virulence which fitted recent models for parasites with correlated horizontal and vertical transmission. The number of sexuals produced depended on the length of time over which reproduction could be sustained. Thus, early colonies and those with large first brood and large maximum size were at an advantage. Many colonies also became naturally infected with Nosema bombi.These infections were not associated with experimental treatment or colony size, but were correlated with an increased production of sexuals, particularly males.