Honey bees can’t socially distance. Here’s what they do instead.

By Alison McAfee

July 1st, 2020

The air is warm with bodies and heavy with humidity. Nestmates huddle and rapidly antennate one another — chatting and sharing pheromones, information, and food. Living in a beehive is like living in a house with 40,000 siblings who have no concept of personal space. Honey bees can’t socially distance; rather, their very existence depends on social contact.

Colonies are a pathogen’s dream, and opportunities to spread from bee to bee and colony to colony abound. Accordingly, honey bees’ disease-prevention strategies are far more elaborate than simply staying away from each other when they’re sick. Millions of years of disease pressure has incrementally driven honey bees to gain surprisingly sophisticated disease-fighting strategies. They even have corollaries to hand-washing, medicine, vaccines, and quarantine.

Prevention first

The best strategies are those that halt an outbreak before it begins. For example, honey bees collect antimicrobial plant resins to coat the inside of the hive, with a special focus on the entrance. Much like the hand sanitizer at the door to the grocery store, this sticky substance, propolis, acts like a disinfectant for the tarsi (feet) of foragers, who are forced to walk over it as they return home from the risky, pathogen-sprinkled outside world. Indeed, the term “propolis” has Greek origins meaning before (pro) the city (polis), after early observations of honey bees building propolis curtains at their doorways.

But the power of propolis doesn’t stop there. Propolis is not only antibacterial, it’s also antifungal and antiviral, owing to the complex phytochemicals in tree resins. And when colonies catch chalkbrood — a fungal pathogen, Ascosphera apis, which mummifies larvae from the inside out — foragers will scout out more of the fungus-fighting resin from trees like cottonwood and poplar. It’s like going out to stock up on medicine when the kids get sick.

Dr. Renata Borba, who conducted her PhD research at the University of Minnesota and is currently the Alberta Beekeepers Tech Transfer Team leader, confirmed that the propolis envelope helps fight against American foulbrood, a bacterial disease, as well. Curiously, she found that the brood food in colonies with a propolis envelope had higher antimicrobial activity than in colonies without. It’s not clear if the antimicrobial compounds from propolis were making their way into the brood jelly, or if propolis stimulated nurses to secrete their own bactericidal molecules into the food. But either way, propolis does more than reduce the chances of bringing pathogens home — it also gives the vulnerable larvae a helpful boost to their immunity.

Dividing labor divides the risk

We don’t usually think of division of labor as being an immune defense strategy. But by having specific groups of bees do the jobs with higher risks of contracting novel diseases (foraging), while other bees stay home and look after the queen and the brood (nurses), the risk of transmitting diseases to new brood is mitigated. Nurse bees are typically so young that they haven’t yet ventured outside, where they can acquire new pathogens, so they are unlikely to transmit new diseases between brood cycles.

Only later in life, having fulfilled their nursing duties, do the nurses become foragers and pass on their care-giving duties to the next cohort. This division of labor helps to prevent foragers, who might have just been robbing out another colony or foraging on contaminated flowers, from having close contact with the queen and brood. It’s as much social distancing as the bees can manage in a dense colony — keeping those with the highest likelihood of transmitting pathogens away from those who are most vulnerable.

In fact, this kind of task-specialization is one of the original ingredients in social insects’ evolutionary recipe. Place normally-solitary, ground-nesting bees together and the dominant female will lay eggs while the strongest will dig, and the meanest will defend. They self-organize to do the tasks to which they are best suited. Scientists think that this kind of self-organization is one of the characteristics that enabled insect societies to evolve. For honey bees, this now-entrenched cooperative segregation between caring nurses and daring foragers also creates a social buffer of disease-naïve nurses around the most susceptible larval kin.

Vaccines for all

Honey bees even have not one, but two kinds of “vaccines,” though not the kind delivered with a syringe. In essence, workers can vaccinate the larvae they care for, and queens can vaccinate the eggs they lay. As far as we know, this does not offer the bees long-term immune protection, but it protects them when they’re most vulnerable. Nurse bees act like immune surveillance systems: If they are unlucky enough to become infected with a virus, they will produce and secrete antiviral molecules — called double-stranded ribonucleic acid, or dsRNA — in the brood jelly that they feed to their baby sisters. When the larvae consume the dsRNA, they gain this transmissible immunity and become resistant to the virus too, without having to actually become infected by it.

Those dsRNA molecules specifically ward off the virus that the original nurse was infected with, so this approach efficiently protects the larvae against exactly the types of viruses that are circulating in the hive. That’s because the dsRNA sequence is modeled off the virus’s own genome. When a larva receives this dsRNA, the dsRNA is incorporated into the larva’s own molecular machinery, which lets it seek out and destroy sequences that look similar to the dsRNA — the virus — much like a vaccine.

A different kind of vaccination pathway, called transgenerational immune priming, enables the queen to transfer fragments of bacterial and fungal pathogens to her eggs, which stimulates the immune system of the developing larvae. Some researchers are exploiting this form of immunity to engineer real, beekeeper-applied vaccines for queens, with the aim of protecting whole colonies from AFB and chalkbrood.

Don’t save the children

These are all sophisticated ways of preventing outbreaks, but honey bees also have a ruthless side. When an infection strikes, a specialized faction of workers will murder their own immature kin for the greater good of the colony. This kin-killing trait has a benign sounding name, hygienic behavior, and I found it morbidly fascinating as a young graduate student — so fascinating that I was willing to devote most of my doctoral dissertation to studying it.

I still remember the first time I saw a hygienic bee in action. I cracked the lid of a colony, gave it a quick puff of smoke, pulled out a frame covered in bees, and paused. Normally, the bees scuttle around, indignant at being disturbed. But one bee near the center of the frame kept working, apparently oblivious to my intrusion, doggedly determined to uncap a cell where a young larva sat, about to undergo metamorphosis into a pupa.

Something was wrong with that larva, and the hygienic worker had decided it must go. Maybe it had the initial stages of an infection, invisible to the human eye. Or maybe there was a parasite hiding in its brood cell, weakening the larva and transmitting pathogens. Whatever the reason, the hygienic worker was purposefully uncapping that cell, preparing for sacrifice.

Hygienic worker bees use their superior sense of smell to sniff out larvae and pupae that are stricken with disease. Once identified, the workers yank their targets from their cozy brood cells and drag them out of the hive, leaving the helpless, immature, sick bees out on the doorstep to slowly die.

It may seem harsh, but this brutal habit evolved because it improves the colony’s chances of combating disease. By selectively removing their sick siblings from the colony before the pathogens they harbor become contagious, this behavior reduces disease transmission and suppresses outbreaks before they can appreciably spread. It’s similar, in principal, to compulsory isolation or quarantine. But instead of caring for the sick and nursing them back to health, the worker bees abandon them, their corpses becoming food for other insects, rodents, and birds.

The scent of death

In my research, I wanted to figure out which odorants triggered the hygienic bees to execute this behavior in the first place. Through my experiments, I found that dying larvae and pupae consistently released an odorant called oleic acid. When I applied this odorant to otherwise healthy pupae, hygienic worker bees removed them from the colony.

This result was not surprising. Back in the 1950s, E.O. Wilson famously called oleic acid a “death pheromone” when he discovered that applying it to live ants caused their nestmates to carry them, still kicking, to the colony’s midden heap. We now know that oleic acid stimulates burial behavior in termites, and hygienic behavior in honey bees. That the same chemical compound causes synonymous behaviors across social insect clades suggests there is a common evolutionary root to this type of social immunity. And this root might even extend beyond the origin of social living.

That’s because solitary insects respond to oleic acid, too. If a cricket, cockroach, or caterpillar gets a whiff of it, they avoid the source. Researchers agree that this aversion is likely also an infection mitigation strategy: By avoiding the smell of death and distancing themselves from the dead, they also avoid the pathogens or predators that may have caused the demise. And solitary insects need only to look out for themselves.

But when insects began living in large groups, this avoidance strategy was no longer an asset. Instead, it was more advantageous to actively seek out and remove the source: the dying, infected brood. If they could do it fast enough, this prevented their nestmates from becoming sick too, to everyone’s collective benefit. And since every individual in an insect colony is highly genetically related and most are sterile, traits that benefit the group rather than the individual are favored by natural selection — a principle also known as inclusive fitness.

Sophisticated as these immune defense strategies may be, parasites like varroa and Nosema ceranae have still managed to thrive. With varroa as an ultra-efficient vector, viruses became a more serious threat. The bees’ vaccination strategy was overwhelmed. And there is always the threat of newly emerging parasites, pathogens, even predators (like the Asian giant hornet), on the horizon. Honey bees will adapt to these challenges — throughout history, they always have — and may even evolve new defenses to add to their elaborate repertoire. It just won’t occur on a beekeeper’s schedule.

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