Cuckoo Bumble Bees: What We Can Learn From Their Cheating Ways (If They Don’t Go Extinct First)

By Meredith Swett Walker

Raising kids is a lot of work; it takes a lot of energy to feed and defend helpless offspring. So, brood parasites trick someone else into doing it for them.

This reproductive strategy is well known in birds, such as cuckoos or the brown-headed cowbird, but brood parasitism is much rarer in insects. Some of the best examples are the cuckoo bumble bees in the subgenus Psithyrus.

Cuckoo bumble bee queens employ sophisticated con artistry to infiltrate the nests of their hosts, but they don’t simply dump their eggs in another bumble bee nest, like a cuckoo bird. Because their host bumble bees are eusocial, cuckoo bumble bees have to have to trick the entire colony, not just mom and dad. These bees are also considered “social parasites” because they exploit the whole colony, tricking the host workers into rearing cuckoo brood.

Learning how cuckoo bumble bees cheat the eusocial system can tell scientists a lot about how insect sociality evolves and how hosts and parasites coevolve antagonistically with each other. But, despite their fascinating lifestyle, the ecology of half of the 28 species of cuckoo bumble bees is totally unknown. That lack of knowledge is due in part to their rarity and also, perhaps, to some anti-parasite bias.

In research published this month in the Annals of the Entomological Society of America, Patrick Lhomme, Ph.D., postdoctoral researcher at the International Center for Agricultural Research in the Dry Areas in Morocco, and Heather Hines, Ph.D., assistant professor of biology and entomology at Penn State University, review what we know about the ecology and evolution of cuckoo bumble bees and why our opportunities to study the fascinating bees may be running out.

Nest usurpation—when one queen invades the nest of another, kills the original queen and adopts her brood—is common among bumble bees and usually occurs within a species. This is called “facultative social parasitism,” and it’s a strategy deployed by bumble bee queens only under certain ecological conditions. But cuckoo bumble bees are “obligate brood parasites”—in other words, they cannot reproduce without their hosts. They cannot produce their own workers, they lack pollen baskets on their legs and so cannot collect pollen to feed their own offspring, and they cannot produce enough wax to build their own nest.

Instead, cuckoo bumble bees must find a host colony of another bumble bee species, and it has to be just the right size. Too large, and there will be too many workers defending the nest and the cuckoo will be killed. Too small and there will be too few workers to raise the cuckoo’s offspring. So, cuckoo bumblebees must be selective. They also have to be tough fighters to defend themselves from attacking workers as they infiltrate the nest and kill the host queen. Thus, cuckoo bumble bees are heavily armored with larger and stronger mandibles, a hardened abdomen, and a thicker, more powerful sting.

After it infiltrates a nest, the invading cuckoo must defuse the battle and integrate into the host colony. Some cuckoo bumble bees do this by mimicking the chemical cues used by their host species. Other cuckoos produce few recognition chemicals of their own and then take on the “scent” of the colony via contact with nest materials and workers.

Finally, once hatched, cuckoo larvae must trick the host workers into feeding them. How this works is largely unknown. Previous research by Lhomme suggests that colonies taken over by cuckoo bumble bee queens may lose their ability to recognize outsiders in general and so be more accepting of cuckoo larvae when they hatch.

Despite their fascinating adaptations, cuckoo bumble bees are understudied. This is partly due to their rarity in space and time. Because they produce no workers of their own, they are not out and about foraging for long. Cuckoo queens are only above ground and active for a short period while searching for host nests and mating with the short-lived male cuckoos. So, it can be difficult for researchers to find and observe cuckoo bumblebees. And there are just not very many of them: Sampling suggests that, even accounting for the lack of workers, their population sizes may be small.

But another factor that may contribute to our lack of study of these bees is our general attitude towards parasites. In the past, they may have been viewed as enemies of the “real bumble bees” says Lhomme. In addition, they are not effective pollinators and were therefore not seen as “useful” to humans.

Biologists may now be fascinated by brood parasites, but the public is often less enamored. “The first question people often ask me when I tell them about my research is ‘Why do you study bumble bees that are so mean!?’,” says Lhomme. “It’s said as a joke of course, but I think it’s characteristic of what people really think. I believe the public has less sympathy for parasitic species in general.” And this attitude may hurt conservation efforts.

Many bumble bee species are declining alarmingly due to climate change, loss of habitat, and introduced diseases. When populations of their host species decline, cuckoo bumble bees decline as well. They are particularly vulnerable to extinction because of their relative rarity as a brood parasite. But, people tend to forget that parasitism, in its various forms, is widespread in the living world. Around half of the species on Earth are parasites of some sort. Parasites “are not only important in shaping the populations or the behavior of free-living species, they are an integral part of the ecosystem and the complex food web,” says Lhomme. “They are thus as important to protect as free-living species.”

Lhomme and Hines hope their review will stimulate further research into cuckoo bumble bee biology and promote conservation efforts. Perhaps with better understanding, more people will see the beauty in these brood parasites. Lhomme refers to a quote from esteemed scientist and author Stephan Jay Gould: “We cannot win this battle to save species and environments without forging an emotional bond between ourselves and nature as well—for we will not fight to save what we do not love.”

Meredith Swett Walker is a former avian endocrinologist and wannabe entomologist. She now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. Find a sampling of her work at www.magpiescicomm.com.