How the Bees You Know are Killing the Bees You Don’t
USGS Bee Inventory and Monitoring Lab
(Inside Science) -- Sam Droege slides a drawer from a tall white cabinet, releasing an odor of mothballs. Row after row of small bodies stand skewered on pins, fragile limbs frozen, furry backs as bright as sunflowers. They are all examples of Bombus affinis, most collected from meadows where this bumblebee species no longer flies. The surrounding drawers hold nearly 2,000 more.
"Affinis was a dirtball species. It was super common. That's why there's drawers and drawers of them," said Droege, a biologist with the U.S. Geological Survey whose job sometimes involves identifying bees here at the Smithsonian National Museum of Natural History in Washington.
Earlier this month, the insects in the open drawer, more commonly called rusty patched bumblebees, were listed as endangered under the U.S. Endangered Species Act of 1973. The International Union for Conservation of Nature estimates that they have declined by more than 90 percent over the last decade. According to Rich Hatfield, a senior conservation biologist at the Xerces Society who helped assess the species for the IUCN, the leading suspect in their disappearance is disease spread by other bees -- commercial bumblebees raised by humans to pollinate crops.
Most people have heard that pollinators are in trouble, and with them agricultural products worth more than $200 billion annually. But public and policy concern largely revolves around western honey bees, a domesticated species whose population has actually risen worldwide over the past few decades, despite recent challenges faced by beekeepers. Their success stands in stark contrast to the more than 20,000 other distinct species of bees worldwide, many of which are thought to be declining or facing extinction.
Bees are the most important pollinators on earth, pollinating more plants than any other group of animals, said Hatfield. In appearance, they range from glimmering green gemstones the size of a gumball to brownish specks that could crawl through a cocktail straw. And while losing one bee species probably won't shatter ecosystems, losing enough of them will, said Droege. Diverse wild bee communities help buffer ecosystems, ensuring that something is left to pollinate plants even under the stress of climate change and other environmental challenges. They also help pollinate crops, sometimes more efficiently than the commercial bees farmers increasingly rely on.
Humans raise a small number of bee species commercially, and these managed bees are crucial for modern food production. But they don't always live in harmony with the natural world. Managed bees can spread diseases, compete with wild bees for food, invade new habitats and upset the balance of plant species.
Bees as livestock
Humans first domesticated honey bees, a group that includes several similar species, around 9,000 years ago. Honey bees are native to Asia, Europe and Africa, but humans have carried them around the globe, allowing them to establish feral populations. In most cases, the invasions happened so long ago that native bees have either adapted or gone extinct.
“We basically have no idea what the native bee fauna looked like prior to the introduction of honey bees,” said Dave Goulson, an entomologist at the University of Sussex in England. “It may have absolutely devastated the bee fauna of the Americas.”
Fewer than 50 years ago, people also began raising other types of bees for crop pollination, including several species of bumblebees. Bumblebees are capable of shaking pollen loose from plants such as eggplants and tomatoes -- crops honey bees can't pollinate. While honey bees still dominate the beekeeping world, farmers now use bumblebees routinely, most often in greenhouses.
From a typical bumblebee colony of a few hundred, only the young queens survive the winter, hibernating until they can start their own colonies in spring. Farmers usually buy new colonies from breeders each year. Honey bees, in contrast, live in colonies of up to 60,000 bees, and these colonies can persist indefinitely.
Recently, problems such as varroa mites, inhospitable farmland and colony collapse disorder have made it harder for beekeepers to maintain colonies and meet the rising demand for bees in agriculture. In the United States, beekeepers now lose about 40 percent of their colonies each year, four times the fraction they consider acceptable. They can replace lost colonies by splitting surviving colonies in two, but this technique can become unsustainable.
Nevertheless, the total number of managed honey bees worldwide has risen by 45 percent over the last half century. Honey bees are in no danger of extinction, said Hatfield -- and our reliance on them may come at a cost.
More on wild bees from Inside Science:
Slideshow: The Dazzling Diversity of Bees
Video: Are Bees Becoming Extinct?
How bee disease spreads
Managed bees may transmit new diseases to wild bees, or they may allow existing diseases to multiply and "spill back" into wild populations. Commercial beekeeping often involves maintaining bees at high densities, making it easy for diseases to pass from bee to bee. And companies routinely feed bumblebees with pollen gathered by honey bees, helping diseases to spread between species, said Peter Graystock, a conservation biologist at the University of California, Riverside. Some companies have started irradiating the pollen to kill pathogens, but others still feed it to bees as is.
In a 2013 study, Graystock and his colleagues tested imported bumblebee colonies that had supposedly been screened for disease, and found that more than three-fourths of them contained disease-causing microbes. When the researchers experimentally fed pollen and waste from the infected bee shipments to healthy bumble and honey bees, many of the new bees got sick.
However, flowers are probably the hotspots of disease transmission to wild bees, said Graystock. With colleagues, he recently demonstrated that two commercial bee species, buff-tailed bumblebees and western honey bees, can rapidly spread five kinds of pathogens between flowers. Some of the pathogens could only infect one of the two bee species, but they often hitched a ride on the other one.
“Imagine flowers as a dinner plate that you're eating from, but everybody else is also eating from that dinner plate as well,” said Graystock. “They could accumulate quite a lot of different microbes.”
Evidence from the field supports the idea that wild bees can catch diseases from managed bees. In Canada, Ireland and England, researchers have found elevated disease rates in wild bees living near greenhouses that use commercial bumblebees.
The lack of data on most wild bee species makes it hard to judge the threat facing their populations. Some ailments, such as the varroa mites that infest managed honey bees, appear to be harmless to other kinds of insects, including wild bumblebees. But other diseases, such as deformed wing virus and the dysentery-causing fungus Nosema ceranae, are known to harm multiple species that aren't closely related. Researchers usually have no idea whether a given wild pollinator can catch a particular disease, or how sick it would get if it did.
Still, circumstantial evidence suggests that the impacts on wild pollinators can be devastating. In North America, a species called Franklin’s bumblebee is thought to have recently gone extinct, and four other once-common species have suffered estimated declines of more than 70 percent.
Wiped out by plagues
While no one knows for sure what’s killing wild American bumblebees, the dominant hypothesis is one or more diseases introduced by commercial bumblebees imported from Europe, said Sheila Colla, a conservation biologist at York University in Toronto. The timing matches up with the rise of commercial bumblebees for pollinating greenhouse crops. At least two of the declining species have high rates of a fungal disease called Nosema bombi, and the strain they carry is genetically similar to the one found in bees from Europe, suggesting it may have been recently introduced. And many researchers think that the wild bees have vanished too quickly, and over too wide an area, for the culprit to be anything except disease. For example, the rusty patched bumblebees that just received endangered species protections have disappeared from around three-fourths of their historic range, according to a study by Colla and her colleagues.
"You can't really chalk it up to something like pesticide use, because that wouldn't explain why it disappeared in the Smoky Mountains," she said.
In the United States, it's now illegal to import any kind of bumblebee from overseas. But non-native bumblebees are still pouring into Argentina, even though two species have already turned invasive there. The second invasive species, known as the buff-tailed bumblebee, was first introduced to South America to pollinate crops. Feral buff-tailed bumblebees were first spotted in Argentina in 2006, and since then, they have spread across the region, wiping out native Patagonian bumblebees as they go.
Patagonian bumblebees are the largest bumblebees in the world, and the only ones native to the southern part of South America. They used to be a familiar sight in Chilean and Argentinian gardens, where their deep buzz and fluffy orange bulk made them hard to miss.
“They look like little bears,” said Marina Arbetman, a molecular ecologist at the National University of Río Negro and the National University of Comahue in Bariloche, Argentina.
Patagonian bumblebees have vanished so fast that they are almost certainly suffering from a disease epidemic, said Arbetman. She has an idea what that disease might be: Apicystis bombi, a parasite that attacks bees’ fat stores. The parasite was absent in the region until buff-tailed bumblebees invaded, but now it is found in native Patagonian bumblebees and feral buff-tailed bumblebees, as well as another bumblebee species that invaded earlier. Genetically, the parasites look similar to A. bombi parasites from Europe, suggesting that, like N. bombi in North America, they too rode in recently on commercial bees.
The loss of Patagonian bumblebees could have ecosystem-wide impacts, said Arbetman's adviser Carolina Morales, an ecologist at the National Scientific and Technical Research Council and the National University of Comahue. Patagonian bumblebees have longer tongues than the invasive species, enabling them to pollinate native plants with long, tube-shaped flowers. Invasive buff-tailed bumblebees can’t reach into long flowers normally, so they tear holes in them instead, stealing nectar without ever touching the pollen. One native plant appears to already be suffering reproductive costs, producing less fruit in landscapes taken over by the invasive bees, according to unpublished research by Morales and her colleagues.
Now, a few Patagonian bumblebees are hanging on across their former range, but sightings in most areas are rare. Arbetman spotted one last fall, but before then, she hadn't seen any in two years. "And you know I'm looking at every flower," she said.
Fighting over flowers
Competition for food is another source of conflict between managed and wild bees. Wildflowers are growing scarcer around the world, and wild bee populations are often limited by how much pollen and nectar they can find. Commercial beekeepers need places to keep their bees when they’re not on pollination jobs, and in the United States they sometimes use public land.
Resource competition between managed and wild bees is controversial, and in some cases, it appears not to be a problem. But in the majority of experimental studies where scientists have tested specifically for competition, they have found it, said Victoria Wojcik, research director for an international nonprofit called the Pollinator Partnership in San Francisco. Most such studies have focused on bumblebees, so there aren't enough data to make recommendations on preserving other types of bees. But if land managers have sensitive bumblebee habitat, they may want to think twice before allowing commercial honey bee hives on their land, said Wojcik. "There could be food-based competition, and the bumblebee loses in that situation," she said.
Bumblebees may be losing out to feral honey bees in a remote nature reserve in California, even though there are no managed hives around. The site had hardly any feral honey bees in the late 1990s, probably because of a mite infestation that swept through honey bee populations shortly beforehand. But since then, honey bees have recovered, and the native bumblebees have drastically declined. Diane Thomson, an ecologist and conservation biologist at the Claremont Colleges in California, monitored their populations over 15 years.
"I've been able to show, from looking at what plants they're using, that bumblebees appear to be getting squeezed off of certain floral resources that are important for them as the number of honey bees has gone up," she said.
These findings reinforce an earlier experiment Thomson conducted at the same site. In that study, she brought in honey and bumblebee colonies and placed them at various distances from each other. Bumblebees placed near honey bee hives had less reproductive success, producing fewer and smaller fertile bees capable of starting a new generation.
Now, Thomson's experimental hives are long gone, but native bumblebees are still being outcompeted. Their disappearance is especially striking because it happened in a protected area, far from pesticides, development and human disturbance.
“My site is really buffered from some of the potentially very harmful causes of bee declines in other places,” said Thomson. “And yet, we're still seeing a decline in these native bumblebee populations.”
What can be done to save the bees
People can reduce the risks to wild bees by not importing managed bees from other countries, by reducing the use of non-native species, and by thoroughly screening for diseases, policies that some countries already employ to varying degrees. Another simple strategy is to put screens on greenhouse vents before releasing bumblebees inside, a practice that Japan mandated after suffering its own bumblebee invasion, said UC Riverside's Graystock. But the most powerful way to help wild and managed bees coexist may be something beekeepers are already pushing for: creating more habitat where pollinators can find food.
Farmland used to have lots of rich pollinator habitat, with diverse plant species flowering at different times. Wild bees took care of most crop pollination, while honey bees were primarily used for honey, said Zac Browning, a fourth-generation commercial beekeeper based in Jamestown, North Dakota, who owns around 27,000 honey bee colonies. Browning said he has seen agriculture move toward giant, pesticide-soaked fields planted with a single type of crop -- a landscape hostile to wild and managed bees alike. Now, he said, farmland is "probably, in many ways, the most dangerous place to be, if you're a bee."
Since bees often can't survive in modern farmland, farmers now pay beekeepers to bring in bees just when their crop is flowering. The new system puts stress on both commercial and wild bees, forcing them to compete for scarce resources. This stress probably makes them more vulnerable to disease, said Graystock.
Modern crop pollination may also threaten food security. A growing body of research shows the crucial role wild insects still play in agriculture, ensuring stable production from year to year and boosting yields regardless of whether honey bees are present. And the more a crop relies on a single kind of bee, the more vulnerable it is to diseases and other crises that might strike that bee's population.
Project Apis, an agricultural nonprofit focused on honey bee health, is working to increase pollinator habitat on farmland by providing seeds for farmers to plant in unused patches of land. Its seed mixes include plants for native species as well as for honey bees, said Danielle Downey, the project's executive director.
"It would be great if there were not so much demand for every scrap of habitat that's left," she said.
Hatfield agrees. He works for the Xerces Society, a nonprofit based in Portland, Oregon that focuses on protecting wild insects and other invertebrates. "Our take-home message always comes back to 'if you want to do something, the right thing to do is to plant habitat,'" he said. "Habitat is going to support our native bees, and it's also going to support the honey bees."
Policy makers are also starting to realize that bee-friendly landscapes are important. The U.S. farm bill has offered incentives for landowners to restore pollinator habitat since 2008, and several states are starting to plant pollinator habitat along highways. And while the federal Pollinator Health Action Plan released last year has been criticized for focusing too much on honey bees, it does list habitat for all pollinators as a top objective.
Patagonian bumblebees and rusty patched bumblebees are flashy species that were once very common, so people noticed when they vanished. But they are the exception. For the vast majority of wild bee species, no one knows whether they are in danger, because no one has data on their populations. Often, we don’t even know they exist. According to one estimate, Earth holds another 20,000 bee species waiting to be discovered, on top of the 20,000 already described, said Sam Droege.
Droege has spent years trying to fill the knowledge gaps, amidst changing political landscapes that shuffled him between programs and agencies. He designed a nationwide monitoring program for U.S. bees, but the funding dried up before it could bear fruit. Now, he runs the USGS Bee Inventory and Monitoring Lab in Beltsville, Maryland, which catalogs local bees, creates identification guides, and helps land managers across the country conduct their own, small-scale surveys. But despite his efforts, there is still no comprehensive monitoring effort for U.S. bees.
"There's no program," he said. "There are no real statistics other than 'can't find it anymore.'"
Back at the museum, Droege picks up a tiny black bee by the pin and holds it in the circle of light at the base of his microscope. He turns the bee until light glances sideways off of minute pits and hairs, shadows revealing their arrangement. It is a female Pseudopanurgus, a poorly described genus, and Droege can’t tell whether this bee represents any named species. He has no idea whether she came from a thriving population, or one on the brink of extinction.
Editor's note (2/14/2017): This story has been updated to clarify the description of how buff-tailed bumblebees spread to Argentina.