While most bumble bees spend their days collecting pollen and provisioning their colonies, a small and fascinating group of these bees have evolved an entirely different way of life. These are the cuckoo bumble bees—social parasites that rely on the labour and resources of other bumble bees to reproduce.

Unlike their industrious relatives, cuckoo bumble bees invest nothing into nest-building or pollen gathering. Instead, cuckoo females seek out established colonies of other species (known as their hosts), infiltrate them, and assume control, taking over the host queen’s reproductive role. Once the female cuckoo has taken the colony over, the host workers rear her offspring as if they were the offspring of the foundress queen.

Although their behaviour may seem ruthless, cuckoo bumble bees are a natural part of functioning ecosystems and their presence is a signal of overall health in the local bumble bee community. Their survival is tightly linked to the abundance, stability, and timing of their host species, making them exceptionally sensitive indicators of ecosystem health. If habitats are degraded, floral resources decline, or host populations falter, cuckoo bumble bees are often the first to disappear. In this way, they act as an early warning system for broader pollinator declines.

North America is home to six species of cuckoo bumble bees, and several are of conservation concern. The Ashton cuckoo bumble bee (Bombus bohemicus) and Suckley cuckoo bumble bee (Bombus suckleyi) are now listed as Species At Risk across portions of their range. Because these species are entirely dependent on their hosts—many of which are also declining—their conservation cannot be separated from the protection of broader native pollinator communities and the habitats that support them. 

Photo: Bumble Bee Watch/Parker Lloyd

Despite their importance, many gaps remain in our understanding of cuckoo bumble bee biology. For several species, we still know very little about which host species they parasitize in different regions, where viable populations continue to persist, and how habitat type, landscape structure, or floral resources influence their presence. It is also unclear how climate change and land-use shifts are altering their survival and host interactions. Filling these knowledge gaps is essential for developing effective conservation strategies.

That’s where Bumble Bee Watch participants come in!

How Bumble Bee Watch Fills the Knowledge Gaps

Every photo submitted to BumbleBeeWatch.org adds a piece to the puzzle. By mapping community observations, researchers can visualize the distribution of these secretive bees and the relationships between cuckoos and their hosts.

For example, overlaying observations of cuckoos with those of their known hosts reveals areas where these interactions may be occurring, and where conservation efforts might be most needed. In the map below, I overlaid lemon cuckoo bumble bee (Bombus citrinus) observations with those of common eastern bumble bees (Bombus impatiens), their most-documented host species. 

Here you can see that the observations of lemon cuckoo bumble bees (B. citrinus) seem to be concentrated in areas of high common eastern bumble bee (B. impatiens) density, which makes sense - when a cuckoo’s host species is in high abundance, there is a higher likelihood of the cuckoos themselves being able to persist. 

However, this map doesn’t exactly explain the distribution of these cuckoo bees. Bumble Bee Watch observations show that lemon cuckoos (B. citrinus) are present on the island of Newfoundland, but common easterns (B. impatiens) are not. Thus, these bees must be relying on a different host species in this region.

Sure enough, when you overlay their next most-common host species, the half-black bumble bee (Bombus vagans), onto the map, you find a cluster of half-black (B. vagans) observations on the east shore of Newfoundland, right where the lemon cuckoo (B. citrinus) observations were made. Thus, if there was ever a push to try to conserve populations of lemon cuckoo bumble bees (B. citrinus) in Newfoundland, Bumble Bee Watch data gives baseline information for 1) where to start concentrating those conservation efforts (the east side of the island) and 2) what potential host species are being used by these bees, and thus require protection as well.