Brood parasitism reduces but does not prevent Bombus terrestris reproductive success.

Cuckoo bumblebees are obligate brood parasites that must invade a colony of their host bumblebee species in order to reproduce. As the host queen is typically killed in the colony invasion process, cuckoo bumblebees are thought to severely reduce the reproductive success of the host bumblebee species. Here, we introduce a novel colony-splitting method to investigate and quantify the reproductive impact of cuckoo parasites, applying it to common bumblebee species, Bombus terrestris and its brood parasite, Bombus vestalis. The brood and workers from commercial B. terrestris colonies were divided into two boxes. One colony half was left with the original Bombus terrestris queen-forming the "queen-right" colony-and a B. vestalis cuckoo bumblebee was introduced to the second half, becoming the "cuckoo-parasitized" colony. This paired approach minimizes differences between colony success from genetic variation or other parasites and allows for comparisons between host and cuckoo reproductive success in a controlled environment. In our experiment, cuckoo parasitism resulted in a significant loss of host reproductive success as compared to queen-right colonies. We also record a significant influence of cuckoo lifespan on the number of new cuckoos and hosts produced in parasitized colony halves. In cuckoo-parasitized colonies, we observed higher reproductive success for host B. terrestris than B. vestalis, the cuckoo bumblebee species. We suggest further refinements of this method may be needed before drawing conclusions on the comparative reproductive success of host and cuckoo bumblebees in natural environments.