Less attention has been paid to host–parasite systems where both host and parasite are closely related, sharing similar life history traits. This is known as social parasitism or brood parasitism and is found in both birds and social insects. Host bumblebees and their social parasites, cuckoo bumblebees share the same, but timeshifted annual life cycle, and corresponding environmental conditions.
Host bumblebee colonies are comprised of drones (males), workers and a single bumblebee queen who initiates nest foundation in early spring. During colony development, workers are produced exclusively and only at the end of the season are new sexuals (drones and queens) produced for the forthcoming season.
Cuckoo bumblebee females invade host nests in spring, killing the host queen and leaving host workers to take care of the cuckoo female’s brood. In contrast to the host, cuckoo females produce only male and queen offspring, lacking a worker caste. This kind of parasitism is assigned to queen-intolerant inquilines. Within this asymmetric, inter-specific arms race, cuckoo bumblebees may be specialist or generalist, being a mono- or multiple-host social parasite, depending on the host species range.