Variable Cuckoo Bumble Bee
As a brood parasite, the variable cuckoo bumble bee does not raise its own young; instead, it “tricks” its host species, the American bumble bee, into tending to its eggs and raising its larvae to adulthood. Its name is a reference to cuckoo birds, many of which lay their eggs in the nests of other bird species.
About This Species
The variable cuckoo bumble bee is one of the rarest bumble bee species in the country. It hasn’t been seen in the United States in over 20 years, and its decline has been documented since the 1920s. The variable cuckoo bee is a brood parasite, with the American bumble bee (Bombus pensylvanicus, another bumble bee species under federal consideration for U.S. Endangered Species Act listing) as its primary host.
This species is so specialized in brood parasitism that it has completely lost its ability to collect and store pollen or secrete wax to make and tend a nest. The conservation of this species is heavily reliant on the success of its host species.
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Physical Characteristics
A key characteristic of the variable cuckoo bumble bee is the absence of pollen baskets on its hind legs. Because it tricks another species into rearing its young, it does not collect pollen. These bees have a fully black face, a black circle in the center of the thorax (the middle section of the bee), and a fully black abdomen (the third section).
Range
The variable cuckoo bumble bee has historically been predominantly an eastern species. Colorado is at the western edge of its historic range.
Habitat
In the west, variable cuckoo bumble bees mainly live in open grasslands. Habitats well-suited for its host, the American bumble bee, are key habitats.
Diet
Like all bumble bees, the variable cuckoo bumble bee is a generalist forager, visiting a diversity of flowers, depending on what is available on the landscape. As a brood parasite, larvae are fed pollen collected by their host species, the American bumble bee.
Reproduction
Cuckoo bees, as obligate brood parasites, are not able to rear their own offspring, but depend on their host. Females emerge from hibernation later in the spring, after their host species has begun establishing nests. To usurp an American bumble bee nest, a female cuckoo bumble bee kills the queen and lays her eggs in the host nest; she tricks the nest workers into rearing her young with chemical cues that manipulate their behavior Once the offspring have emerged as adults, they will leave the nest to mate and forage before finding a location to hibernate for the winter.
Threats to Species
As an obligate brood parasite, the variable cuckoo bumble bee faces its own threats, but is also indirectly affected by threats to its host species, the American bumble bee. Commercially managed bees (both bumble bees and European honey bees) pose a significant threat to native bumble bees. Managed bees can drive disease spillover, when a disease jumps from one population to another. Ongoing research demonstrates that honey bees can outcompete native bumble bees for resources and disrupt their natural foraging behavior. Pesticides also pose a threat to the western bumble bee, with both lethal and nonlethal consequences; bees can be exposed both directly through contact and indirectly through contaminated pollen, nectar and soil. Habitat loss is another major threat to the variable cuckoo bumble bee, as they require healthy populations of flowers throughout the growing season and access to suitable host nest sites. Finally, climate change poses a significant threat to the variable cuckoo bumble bee in many ways, affecting the bees directly and indirectly through the ways their habitats will change..
Additional Information
One of the best ways to support this species is to ensure its host species is readily available on the landscape. There are a number of ways to do this: 1) plant native flowers incorporating species that bloom across the entire growing season, 2) avoid the use of pesticides (both herbicides and insecticides), and 3) participate in community science projects like iNaturalist or the Bumble Bee Atlas to help expand our understanding of species distribution and abundance.
If you would like to help CPW understand and protect our state’s bumble bees, you can participate in the Mountain States Bumble Bee Atlas. CPW is partnering with the Xerces Society for Invertebrate Conservation to get the Mountain States Atlas off the ground. Learn more here about how to get involved. If you are not able to participate in the Bumble Bee Atlas formally, you can submit photos of bumble bees you spot as one-off observations to the Bumble Bee Watch.