Cuckoo Bees
In the world of the honey bee, Apis mellifera, we are used to concept of robber bees. The idea of a Cuckoo bee, Bombus vestalis, was news to me so here’s a quick summary.

Robbers are honey bee foragers who are after honey rather than pollen. Defending a hive against robbers and loss of honey can affect a honey bee colony but it is not a truly invasive problem.  
In contrast, the Cuckoo bee is a far more invasive pest.

Cuckoo bees are, in essence, free-loaders. They lack the pollen collecting leg hairs of a normal Bumblebee. They do not construct their own nests. They typically enter the nests of pollen-collecting Bumblebees and lay their eggs.

When the Cuckoo bee larva hatches, it consumes the host larva's pollen and, if the laying Cuckoo bee hasn’t already done so, it eats the host larva. In some cases, the invading Cuckoo bee may even remain in the host nest and lay many eggs, perhaps even killing the host queen. 
The Royal Society published research on the urban versus rural success rates amongst Bumblebees mentioned in a separate comment does not attempt to explain why the prevalence of Cuckoo bee is lower in urban areas. Surveys have recorded their presence in cities but it is possible that they are less abundant or simply that city bees are stronger and better able to resist their invasion. Air pollution in cities may be another factor, making the presence of pollen collecting colonies less easy to identify.

Jeremy Rosie
30 June 2018