Social distancing protects honey bees from parasites

Bees show highly complex social behavior. (Image: Dr Michelina Pusceddu, University of Sassari)

The more contact individuals have with one another, the easier it is for pathogens such as viruses, bacteria and parasites to spread. The strategy of reducing social contacts in order to limit the spread is not only found in humans during the Covid-19 pandemic, but also in the animal kingdom. A new study now shows how honey bees operate social distancing in their hives when their colony is threatened by the Varroa destructor mite. Accordingly, depending on the social and spatial situation, the bees find a compromise between distancing and caring for affected conspecifics.

The Varroa destructor mite represents one of the greatest threats to bee colonies. If it is introduced into the beehive by infected workers, it can multiply in the brood chambers and spread via newly hatched bees. The mite damages the bees in several ways: On the one hand, it sucks the body fluid of the insects and weakens the infected individuals in this way. On the other hand, it is an important vector of viral diseases, which in many cases lead to the mass death of entire bee colonies.

Spatial separation between old and young

A team led by Michelina Pusceddu from the University of Sassari in Italy has now observed in the wild and under laboratory conditions how honey bees react to varroa mite infestation. “Our study shows that honey bee colonies respond to invasion by an ectoparasitic mite with significant changes in behavioral traits associated with social immunity, both at the colony and individual levels,” the researchers write. “These results strongly suggest that honeybees limit the spread of parasites within the colony through social distancing.”

The states of honeybees are already fundamentally organized in such a way that parasites have a hard time getting to the brood. The older bees, who, as workers, have a lot of contact with the outside world and can thus represent entry gates for parasites, mainly stay in the outer areas of the beehive. The queen, brood and young bees, on the other hand, are inside the hive. “This spatial separation within the colony leads to a lower frequency of interactions between the two compartments than within each compartment and makes it possible to protect the most valuable individuals, i.e. queen, young bees and brood, from the outside world and thus from the introduction of diseases “Explain the researchers.

Increased distance in the event of mite infestation

Pusceddu and her colleagues suggested that bees exacerbate this division when their colony is actually threatened by a parasite such as the varroa mite. To test this thesis, the researchers first examined beehives with and without mite infestation in nature. In order to be able to observe the behavior of the bees inside the hive, they installed small cameras in the beehive. And indeed: if some of the bees in a colony were infested with the Varroa mite, the bees retreated from the inner compartment even further inside, while the workers remained outside.

For example, they limited their food dances, with which they point out other bees to food sources, to the entrance area of ​​the hive instead of crawling into it as usual. They also reduced contact with other bees. “The observed increase in the social distance between the two groups of bees within the same parasitic colony represents a new and somewhat surprising aspect of honeybees’ development in combating pathogens and parasites,” says Pusceddu. “Their ability to adjust their social structure and reduce contact between individuals in response to a disease threat enables them to maximize the benefits of social interactions and minimize the risk of infectious disease when necessary.”

More mutual personal hygiene

Inside, on the other hand, the bees in mite-infested colonies spent more time cleaning each other. This behavior also serves to combat the parasites: Although the mites are protected to a certain extent from cleaning efforts by the bees, honey bees can succeed in removing the pests from the bodies of their fellow bees. Especially inside the hive, where newly hatched bees could be infested by the next generation of mites, mutual hygiene helps to limit the spread of the pest.

In addition, Pusceddu and her colleagues observed groups of young bees in the laboratory, with some individuals in one group being infected with Varroa destructor and not in the other. As expected, these bees also showed increased mutual cleaning behavior, which is suitable for reducing mite infestation. However, they also increased behaviors that could promote the transmission of the parasite, including feeding each other and making contact via the antennae. “These results contradict our assumption,” write the researchers. “The infected bees were cared for more than the unaffected ones.” Although this could reduce the parasite load for the individual bee, it could also increase the risk of spreading. On the other hand, the intensive social contact between the bees is probably important to maintain the social network.

Weighing up between distance and care

“Our combined results suggest that social distancing occurs on a large scale (at the colony level) but not on a smaller scale (within a bee cohort) where caring behaviors appear to be predominant,” the researchers conclude. “This suggests that the trade-off between protection and risk of transmission due to social interactions could vary depending on the spatial scale and the cohort considered.”

The researchers also see their results in the context of current infection control measures: “Social distancing as a behavioral response to a disease is certainly costly for all social animals, as humanity is also experiencing during the current COVID-19 pandemic, but the widespread use of this strategy in the Nature suggests that the benefits might outweigh the costs, ”they write.

Source: Michelina Pusceddu (University of Sassari, Italy) et al., Science Advances, doi: 10.1126 / sciadv.abj1398

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