Graphic detail | Social distan-sting

Just like modern humans, honeybees avoid each other amid plagues

They segregate behaviours in different parts of their hives to prevent parasites from spreading

HUMANS HAVE used social distancing to fight infectious disease for centuries. In 1582 a health officer in Alghero, Sardinia, which was suffering an outbreak of bubonic plague, required residents to carry six-foot-long canes in public, and venture no closer to each other than its length.

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Yet Homo sapiens is unlikely to have been the first species to keep apart for mutual protection. A recent study by Michelina Pusceddu of the University of Sassari, in Italy, and six colleagues found that honeybees in hives infected by parasites implement their own form of lockdowns. They achieve a degree of compliance that human public-health officials would envy.

Bees live in ordered communities that depend on social interactions. Their hives are split into sections. The periphery is mainly used to store food, whereas the core contains a nursery made up of “brood cells”, where larvae grow into adults. Individual bees also have well-defined roles. Queens lay eggs; drones fertilise queens; and young workers tend to look after the brood, before becoming foragers who leave the hive to search for food as they age.

Although the best-known threat to bees is the Asian giant hornet, which can swiftly decapitate all bees in a hive, vastly more bees succumb each year to the ominously named Varroa destructor. A parasitic mite, it attaches itself to bees’ bodies and helps itself to their stored energy resources.

To spread, Varroa depends on contact between the hive’s inner and outer parts. It enters a colony by hitchhiking on foragers, but needs to reach the nursery near the centre to reproduce. Once larvae in mite-infested cells emerge as young bees, they carry the newborn parasites with them throughout the hive, infecting other bees.

To determine how Varroa changes bees’ behaviour, the authors set up six colonies, half of which were infected with the mite. They studied two common activities: the “waggle dance”, which forager bees perform to communicate the site of a newly discovered food source, and “allogrooming”, when bees clean debris or parasites (including mites) from each other’s bodies.

The differences between the colonies were striking. In the Varroa-free group, waggle dances and allogrooming both occurred throughout the hive. By contrast, in infected colonies, most waggling occurred by the sides of the entrance, keeping potentially infected foragers far from brood cells. Conversely, allogrooming was concentrated in the centre of the hive, where parasite removal had the greatest impact.

Such changes probably come at a cost, just as lockdowns in human colonies (aka cities) do. Limiting where waggle dances are performed could reduce awareness of where to find food, and focusing allogrooming efforts in brood cells may worsen forager bees’ hygiene. Nonetheless, natural selection appears to have favoured this trade-off. It might behove sceptics of lockdowns by humans to take note.

Source: “Honey bees increase social distancing when facing the ectoparasite Varroa destructor”, by M. Pusceddu et al., Science Advances, 2021

This article appeared in the Graphic detail section of the print edition under the headline "Social distan-sting"

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