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Science Insider

The Virus Spillover Situation

- February 1, 2023 - Alison McAfee - (excerpt)

Evidence for viruses spreading from honey bees to native bees is patchy, but we should work toward minimizing risk

Nina Sokolov woke up in her tent, pitched in the Tahoe National Forest, ready to start another day of sampling. From the campsite, her research sites in the subalpine meadows of the Sierra Mountains are just a short drive away, and by 9:00 a.m. both she and the bees are getting to work with the day’s tasks. “Luckily, the bees are very reasonable with their activity hours,” Sokolov jokes.

Hiking through the lively meadows, Sokolov swoops her insect net over flowers, sampling bee specimens for a pinned collection. She carefully snips the colorful blossoms and preserves them for a diversity collection, too. Finally, she collects a subset of the bees to check for something a little less beautiful: viruses.

Sokolov, a disease ecologist and Ph.D. student at the University of California, Berkeley, is not just enjoying the scenery. She is using these high-elevation sites in the Sierra Mountains to study if new-coming honey bees are spreading viruses to native bees, and vice versa.

Many of the viruses that afflict honey bees, like deformed wing virus and black queen cell virus, can also infect native bees, raising alarm bells that these viruses could be spilling over from honey bees into more vulnerable bee populations. But they could be spilling back, too; the direction of transfer is just difficult to deduce.

Experiments in laboratories and greenhouses have shown that both honey bees and bumble bees can transmit microbes between each other and move them from flower to flower.1,2 But that doesn’t necessarily mean that it happens appreciably in the wild. In a more realistic setting, with more space and more foraging options, it is not clear how often or in what direction this transfer actually occurs.

The subalpine meadows give Sokolov a rare opportunity to study disease transmission in the broader landscape, where apiaries are sparse and their locations are known. Sokolov has been working with one beekeeper who brings colonies up to the meadows seasonally, using the sites as isolated mating yards, before heading back down to the valley for winter.

“Beekeeping isn’t quite as popular up there due to the bears and the harsh winters,” Sokolov says. It is not hard to find meadows that remain honey bee-free, creating a semi-natural experiment where Sokolov can examine viral infections in wild bee communities at sites with and without honey bee tenants, as well as over time after the mating yards pop up.

The data are still coming in, but Sokolov has a hunch that a number of viruses will be present in native bees — even ones which haven’t lived near honey bees. “I know that these viruses are circulating and they can infect many types of bees. It makes sense that a subset of them would be circulating in wild bees originally.”

Exactly how often viral spillover occurs remains debatable, and these data will be a valuable part of the framework. “Usually, we think about spillover in the other direction, from ‘livestock’ honey bees to native bumble bees,” Sokolov explains. “But I think the other direction can be possible too.”

And, while deformed wing virus likely originated in honey bees, for dozens of other viruses, the original source is not even known. Sokolov thinks she will see deformed wing virus spilling over from honey bees into wild bees, but for other viruses, she isn’t sure. “I have no idea which way it could go, and that’s very exciting.”

How do insect viruses spread?

The idea that viruses can spread between different species of bees is not new, but how they get there is still unclear. Leslie Bailey, a researcher at Rothamsted Experimental Station, England, showed that acute bee paralysis virus — which, at the time, was thought to originate from honey bees — persisted in some wild bumble bees as early as 1964.3 But, as Sokolov points out, host origins are difficult to decipher, and exactly how viruses move from bee to bee is tough to pin down, too.

Only recently have researchers actually begun to untangle transmission networks between species. That web is complicated, and the anchoring threads are ambiguous. Speaking of webs, spiders might become “incidental” hosts of viruses by consuming infected bees;4 however, it is not clear if those spiders are testing positive because they are truly infected, or because they have infected bee tissue in their stomachs. The same goes for wasps, scavenging cockroaches, and insects living in close proximity to honey bee colonies, like some ants or small hive beetles.

But the group of species receiving the most attention, aside from honey bees, are the native bees, especially bumble bees, mason bees, and leafcutter bees in America. For these bees, there is more concrete evidence that the same viruses found in honey bees can actively replicate within them.

Native bees and honey bees also forage on many of the same flowers, which means they have more opportunities than most other insects to bump into each other or pick up pathogen particles left behind. Samantha Alger, who is now an assistant professor at the University of Vermont, and colleagues coined the “dirty doorknob” hypothesis [see Scott McArt’s ABJ article from November 2021], referring to how sharing flowers could facilitate the spread of pathogens.

In research published in PLoS One,5 Alger and colleagues found that bumble bees caught near honey bee apiaries had higher levels of deformed wing virus and black queen cell virus than bumble bees caught far from apiaries. Flowers near apiaries were more likely to harbor virions, too. Sharing germs at flowers was, therefore, a tantalizing explanation that connected these dots.

However, Alger’s subsequent work shows that, while honey bees can leave virus particles behind on flowers, which can be picked up by bumble bees (and vice versa), those bumble bees appear to fail to actually develop pathological infections.6 Alger herself writes that transmission via flowers may actually be a “rare occurrence” in nature, and the most common acquisition route appears to remain a mystery.

This highlights one more big unknown in the “pathogen spillover” paradigm: We know that many viruses are present in native bees, but we still don’t have a grasp on whether those bees are really sick. Shifting our focus toward disease, or actual pathology, rather than just the abundances of microbes, will be an important next step toward understanding how infections are actually impacting native bees. 

Controlling varroa to minimize risk to wild bees

Even if it isn’t yet clear how the viruses are transmitted, or who is transmitting to whom, the sheer biomass of honey bees means that they have a huge potential impact as pathogen reservoirs and vectors. Furthermore, the number and density of honey bees in ….