Cuckoo Combo: Re-Classification Makes Bombus flavidus World’s Most Widespread Bumble Bee

By Melissa Mayer

Most people picture bumble bees as fuzzy insects whose social colonies epitomize the idea of working together for a common purpose, but one group of bees has a less friendly approach: cuckoo bumble bees (subgenus Psithyrus) steal into other bumble bees’ nests, take out the queen, and force the workers to rear their brood instead.

While these clever usurpers are easy to distinguish from other bumble bees, it’s hard to tease them apart at the species level. A new article published last week in Insect Systematics and Diversity describes a multi-evidence approach researchers used to combine two previously described cuckoo bumble bee species—Bombus flavidus and Bombus fernaldae—into a single species. The change, which re-classifies B. fernaldae as B. flavidus, makes B. flavidus the most broadly distributed bumble bee in the world. The team also named a subspecies mostly found around the Appalachian Mountains—Bombus flavidus appalachiensis—which may bolster efforts to better understand and conserve cuckoo bumble bees.

Clever Cuckoos

Cuckoo bumble bees have reduced wax glands and lack a worker caste altogether, so their only option is to wait until other bumble bees start their nests and then take them over. To do that, they’ve evolved some sneaky adaptations. Some are odorless; they pick up the scent of the nest they invade or mimic the odor of the target bumble bee. Some cuckoo bumble bees stick with brute strength, relying on stronger jaws and tougher abdomens to take over a nest by force.

When lead author Patrick Lhomme, Ph.D., joined the Hines Lab at Pennsylvania State University to investigate how cuckoo bumble bees control a host nest’s workers, his advisor Heather Hines, Ph.D., associate professor of biology and entomology at Penn State, saw an opportunity to address a question she’d been pondering since grad school.

Back then, Hines sequenced some Psithyrus bumble bees, including Old World Bombus flavidus (found from Scandinavia to Russia) and New World Bombus fernaldae (found across much of Canada and the United States). “The Old World version … ended up having almost identical sequences to the ones in North America, which suggested an unusual trend,” Hines says. “In bumble bees, almost every species is either Old World or New World, and they don’t usually cross that area. They speciate when they do that. So, this was an unusual pattern, and it suggested they could actually be just one global species.”

A Multi-Pronged Approach

To explore that question, Lhomme, Hines, and colleagues at Penn State, the University of Mons in Belgium, and Stockholm University in Sweden examined genetic information as well as body color and wing morphology. They also looked at the chemicals secreted from the bumble bees’ heads to help them find mates.

The team found some differences in mitochondrial DNA, which is passed directly from mother to offspring and can help trace how populations are related to one another. These differences define western and eastern lineages for these cuckoo bumble bees and come together in northern Michigan, the only place with both lineages so far. They didn’t find any differences that indicate speciation in the nuclear DNA, which is the genetic information that shuffles with sexual reproduction.

Body color and wing morphology couldn’t distinguish the cuckoo bumble bees from each other, either—although the named subspecies tended to have smaller wings, possibly because those bee specimens were smaller overall. When it came to the head glands, there were some differences in relative amounts of pheromone compounds, but they weren’t distinct enough to separate out species.

Together, the data suggests that these cuckoo bumble bees are two lineages that make up a single species: Bombus flavidus.

So Many Bees

This is a big deal when it comes to bumble bee distribution. “The biggest, broadest conclusion that we have is that a socially parasitic bee actually has a broader distribution than any social bumble bee,” says Hines. That’s interesting because parasites are expected to have a narrower range than their hosts, but these cuckoo bumble bees are probably generalists that can snatch the nests of a wide range of bumble bees. “I think, in some ways, this parasitic lifestyle—being a generalist parasite—enabled them to spread into this broader distribution than other bumble bee species,” she says.

Clearly defining this species and naming the Appalachian subspecies may help scientists better understand the cuckoo bumble bees’ biology and their biogeographic history. That’s important because Psithyrus bumble bees have been declining, and the Appalachian subspecies may be a population of specific concern. “We know that in some parts of the world this particular species is rare, and in some parts of the world it’s abundant,” says Hines. “Recognizing those boundaries will be really important for setting conservation priorities.”

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.