From May onwards, Vestal Cuckoo Bumblebees start appearing in gardens, searching for established bumblebee nests. From an identification point of view, they are somewhat tricky to separate from Buff-tailed Bumblebees. That is not surprising as the Vestal Cuckoo bee’s patterning and colouration is mimicking the bee it parasitises, (the Buff-tailed), in order to sneak into its nest and take over. Once in the nest, she will destroy the host bee’s eggs and larvae, probably killing the Buff-tailed queen as well, though sparing the workers, who will now feed her own offspring, that she lays in the nest. Female Cuckoo Bumblebees have harder exoskeletons and larger mandibles in order survive attacks from the defending queen.
There are three main identification features that help separate the two species, visually. Here are some comparison images to help sort them out:
1 The rear legs
2 Wing colouration
3 Body stripes