What term describes the parasitic existence of cuckoo bumblebees who do not maintain a worker caste?

Cleptoparasite

The term cleptoparasite perfectly describes the existence of cuckoo bumblebees because it literally translates to 'theft parasite.' Unlike social bumblebees belonging to the genus Bombus, cuckoo females classified under the subgenus Psithyrus live a solitary life. They entirely forgo the demanding tasks associated with colony maintenance, such as establishing their own nests, building wax cells, collecting nectar, or gathering pollen. Their entire reproductive strategy hinges on exploiting the labor of another species, making the 'theft parasite' designation highly accurate for their lifestyle where they rely entirely on the work done by the host colony's established infrastructure.