Not all bumblebees are happy-go-lucky pollinators. Some species are actually sneaky parasites. These duplicitous social insects sneak into other bumblebees’ nests and, through a variety of techniques, convince workers to raise their young, much like the cuckoo birds for which they are named do with their own eggs.
Cuckoo bumblebees haven’t traditionally received much attention from scientists, but a new paper published in the Finnish zoological journal Annales Zoologici Fennici suggests that it’s time we start looking at them more closely. Researchers from three universities in Finland examined the world’s cuckoo bumblebees and determined that they are in an even greater risk of extinction than the rest of the world’s bees.
The problem has to do with their nature as parasites. Cuckoo bumblebees have evolved so they simply cannot survive without their host species. Most cuckoo species depend on a single bumblebee species and have very limited ranges related to where the hosts live. As those host bumblebees disappear due to habitat loss or pesticides, so will the cuckoos. In fact, according to the paper, the cuckoos will disappear more quickly than their host species because the parasites exist at lower population levels than their hosts do and rely on them so heavily.
The researchers use an established term for this: co-extinction, a process in which one species’s decline or extinction causes cascading trophic effects and pushes other species into extinction.
The Finnish researchers concentrated on 10 European cuckoo bumblebee species, but we’re seeing the same declines in North America, says Rich Hatfield, a conservation biologist with the Xerces Society for Invertebrate Conservation and the IUCN Bumblebee Specialist Group, who was not involved with the paper. “We just finished analyzing the 47 North American bumblebee species,” he says. “Five of those are going to be listed as critically endangered.” Three of those critically endangered species—Bombus bohemicus, B. variabilis and B. suckleyi—are cuckoo bumblebees whose host species are also in dramatic decline.
B. bohemicus is perhaps the most interesting example among the cuckoos. In Europe it depends on a widespread host called the white-tailed bumblebee (B. lucorum). As a result of having a common host, the parasite is doing just fine in the European Union at this time. But B. Bohemicus also exists in the U.S., where until recently it was known as B. ashtoni (a taxonomic designation that has since been abandoned). Here it relies on two hosts, the critically endangered rusty-patched bumblebee (B. affinis) and the less-endangered but rapidly disappearing yellow-banded bumblebee (B. terricola). That’s enough to make B. bohemicus critically endangered in North America.
Should we care if these parasites go extinct? Well, like I said, they’re poorly studied so we don’t really know if what ecological role they provide. Hatfield says they do perform some small amount of pollination—about the same as butterflies—and it’s possible that like other predators they improve their prey’s health by removing the weakest individuals from the gene pool, but it’s going to take a lot more work to figure that out for sure.
The paper does say that if we hope to conserve cuckoo bumblebees long enough to understand them, the best bet is to conserve their host species. Hatfield says that’s easier than most people realize. “There’s a very easy thing that everyone can do, and that’s create habitat. You can plant flowers around your front porch and that’s habitat that makes a difference for bees.” (The Xerces Society has a guide to conserving bumblebees on their web site.)
Hatfield says he has numerous examples of people who have converted their lawns into bee habitat and seen a dramatic increase in the number of threatened insects. “They now have rare species observations in their front yard. I believe it’s a ‘build it and they will come’ scenario. It doesn’t really matter what the cause of the decline is. If we improve habitat and make habitat better, we can give our bees a better chance at recovery.” That’s the same for your happy-go-lucky pollinator bumblebees as well as the cuckoos that depend on them.
Photo: Bombus bohemicus, by S. Rae. Used under Creative Commons license