Bombus sylvestris, known as the forest cuckoo bumblebee or four-coloured cuckoo bee, is a species of cuckoo bumblebee, found in most of Europe and Russia. Its main hosts are Bombus pratorum, Bombus jonellus, and Bombus monticola. As a cuckoo bumblebee, Bombus sylvestris lays its eggs in another bumblebee's nest. This type of bee leaves their young to the workers of another nest for rearing, allowing cuckoo bumblebees to invest minimal energy and resources in their young while still keeping the survival of their young intact.