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Plight of the Bumblebee
14 September 2012 6:25 pm
When Europeans arrived in the Americas, they carried measles, flu, and smallpox to the native peoples. Now history is repeating itself—in the world of bees. The introduction of a European bumblebee to South America—and the parasite that the bee carries—may have decimated populations of that continent's indigenous "giant bumblebee," scientists reported last week in Biological Invasions.
The queen bees of the spectacular native "giant bumblebee" of South America, Bombus dahlbomii, are the largest bumblebees in the world. B. dahlbomii once ranged across thousands of kilometers of Patagonia, the cool, southernmost tip of South America, where it was the only bumblebee species. But the species' burly size has not kept it safe: Populations of the native bee have declined sharply in recent years.
The spotlight of suspicion is now on a recent arrival, the European white-tailed bumblebee (B. terrestris), which was introduced into Chile in 1997 to pollinate agricultural crops. The European bee escaped from greenhouses and outdoor pollination sites into the wild; researchers observed it in Patagonia by 2006. At about the same time, the giant bumblebee began to disappear from this area.
The link between those two events, scientists suspect, may be a deadly single-celled parasite that hitchhiked to Patagonia along with the European invader. In the new study, scientists identified the parasite, Apicystis bombi, in three species of bumblebee—the native bumblebee; B. terrestris; and another European bumblebee, B. ruderatus—in Patagonia. The parasites wreak havoc on the bees, starting off as a gut infection and spreading to other parts of the body. They cause behavioral effects, increase worker bee death rates, and impede the founding of new colonies.
"There is evidence that this parasite was introduced with Bombus terrestris and spilled over to other species here in the region," says Marina Arbetman, lead study author and Ph.D. candidate at the National University of Río Negro in Argentina.
The parasite is certainly a relatively recent arrival: In the study, Arbetman and her team looked at preserved specimens of the native bee as well as B. ruderatus, also called the carder bumblebee. The carder bee first arrived in South America in the 1980s, but researchers could not detect the parasite's DNA in the preserved specimens.
The parasite is rare in bees in Europe, found in only 1% to 8% of white-tailed bees. However, it is surprisingly common in the European bumblebees living in Patagonia, the researchers found—almost half of the white-tailed bees in the region were infected, as well as the native giant bumblebees.
"We are not saying that the decline is only due to parasites," says co-author and bee biologist Carolina Morales of Argentina's national research council. For example, competition for food with the European bees could also be responsible for species decline. However, the speed of the native bee decline suggests that the parasite is a major factor, she says—and other native bumblebees north of Patagonia may be at risk from the parasite.
Indeed, "the giant bumblebee appears to have disappeared from 80% of its range," says bee biologist David Goulson of the University of Stirling in the United Kingdom. "I went out there earlier this year and hunted high and low for them, but couldn't find a single one." A clash over food or territory alone wouldn't be likely to decimate the giant bumblebees, he says. "The native bee has a long tongue and tends to feed on deep flowers that Bombus terrestris wouldn't feed on, so they really shouldn't be competing. The only sensible explanation that fits is that terrestris is carrying some kind of disease, and this one seems to be a pretty good candidate."
Goulson predicts the giant bumblebee species could be extinct within a few years—and adds that there could be wider ecological implications, as a lot of wild plants in the Andes will lose their main pollinator. "It is incredibly frustrating for people like me who are trying to conserve things that some idiot can do so much damage by bringing in an alien bee."
Still, it's not certain that European bees are primarily responsible for bringing this particular parasite to South America—or even that A. bombi is the only parasite involved. "There is correlational evidence to suggest that this parasite may have either been brought in or hugely increased in its abundance by the invasive bumblebee, but there is no causal evidence. It's not a smoking gun," says evolutionary biologist Mark Brown of Royal Holloway, University of London. "I don't think they can conclude that the parasite wasn't in native bumblebees in Patagonia prior to Bombus terrestris arriving, because the sample size was not large enough to do that." (That small sample size was due to the difficulty in locating surviving native bees, according to the researchers in Argentina.) Furthermore, Brown says, "if this parasite was introduced to native bees by commercial bees then it is highly likely that other parasites would have crossed over at the same time."
Morales, however, says that the study highlights a fearsome cautionary tale. Many companies export bumblebee species, sending them around the world to pollinate crops such as the tomato. The research, she says, paints a picture of what could happen if infected European species entered bumblebee-rich places such as China and Nepal—that is, assuming this parasite is just as detrimental to their bees.
*Correction 10:34 a.m., 17 September: The first invader species was B. ruderatus, not B. pascuorum.