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 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 implemented a novel colony-splitting approach to understand and quantify the impact of cuckoo parasitism for a common bumblebee species, Bombus terrestris.

The brood and workers from commercial B. terrestris colonies were divided into two boxes. One colony half was left with the original B. terrestris queen – forming the ‘queen-right’ colony – and a Bombus vestalis cuckoo bumblebee was introduced to the second half, becoming the ‘cuckoo-parasitized’ colony. This paired approach minimized differences between colony success from genetic variation or internal parasites and provided a controlled environment to ensure successful cuckoo invasion.

Brood parasitism resulted in a significant loss of host reproductive success, as compared to queen-right colonies. However, in cuckoo-parasitized colonies, we observed higher reproductive success for host B. terrestris than B. vestalis, the cuckoo bumblebee species. This appears to be driven primarily (but not entirely) by males developing from worker-laid eggs.

This work contributes to our understanding on the pervasiveness and persistence of brood parasitism in bumblebees. The reproductive ability of B. terrestris colonies may present an evolutionary ‘back-up plan’ for reproductive success in the face of brood parasitism.