The more people come into contact with each other, the easier it is for pathogens such as viruses, bacteria and parasites to spread. A strategy to reduce social contact to limit spread occurs not only in humans during the Covid-19 pandemic, but also in the animal kingdom. New research shows how honeybees practice social distancing in their hives when their colony is threatened by Varroa destructor mites. Therefore, depending on the social and spatial situation, bees find a compromise between distancing and caring for the affected individuals.
The Varroa destructor mite is one of the greatest threats to bee colonies. If it is transferred to the hive by infected worker bees, it can reproduce in the breeding chambers and spread through newly hatched bees. The mite damages bees in several ways: on the one hand, it sucks the body fluids of insects and thus weakens infected individuals. On the other hand, it is an important carrier of viral diseases, which in many cases lead to the mass death of entire bee colonies.
Spatial separation between the old and the young
A team led by Michelina Pusceddu from the University of Sassari in Italy observed how honeybees responded to infestation by Varroa mites in the wild and under laboratory conditions. “Our study shows that honeybee colonies respond to ectoparasitic mite infestation with significant changes in behavioral traits associated with social resistance, both at the family and individual level,” the researchers write. “These results strongly suggest that honeybees reduce the spread of parasites in the colony through social distance.”
Honeybee colonies are already essentially organized in such a way that the parasites have difficulty penetrating the brood. Older bees, which as worker bees have many contacts with the outside world and can therefore act as entry points for parasites, mainly live in the outer areas of the hive. There is a queen, a brood and young bees in the hive. “This spatial separation within the colony leads to a lower frequency of interactions between the two compartments than in each compartment, and allows the most valuable individuals, ie mothers, young bees and brood to be protected from the outside world and thus from disease introduction,” explained the researchers.
Increased distance in case of mite infestation
Pusceddu and her colleagues hypothesized that bees increase this division when their colony is actually threatened by a parasite such as the Varroa mite. To test this thesis, scientists first studied hives with and without mite infestation in nature. In order to be able to observe the behavior of the bees in the hive, they installed small cameras in the hive. Indeed, if some of the bees in the colony were infected with Varroa, the bees would withdraw from the inner compartment even further inwards, while the worker bees remained outside.
For example, they have limited their food dances, which they use to alert other bees to food sources, to the hive entrance zone, rather than crawling around as usual. They also reduced contact with other bees. “The observed increase in social distance between two groups of bees in the same colony infected with parasites is a new and somewhat surprising aspect of the evolution of honeybees in the fight against pathogens and parasites,” says Pusceddu. “Their ability to adapt their social structure and reduce contact between individuals in response to the threat of disease allows them to maximize the benefits of social interactions and minimize the risk of infectious diseases when needed.”
More mutual grooming
On the other hand, the bees in the mite infected colonies spent more time cleaning each other. This behavior also serves to combat parasites: although the mites are protected to some extent from the efforts of the bees, honey bees can effectively remove pests from the bodies of other bees. Especially in a hive where newly hatched bees may be attacked by the next generation of mites, mutual personal care helps to limit the spread of the pest.
In addition, Pusceddu and her colleagues observed groups of young bees in the laboratory, with some Varroa destructor infected in one group and not in the other. As expected, these bees also showed an enhanced mutual cleaning behavior which helps to reduce mite infestation. However, they also enhanced behaviors that could encourage transmission of the parasite, including feeding each other and antennae making contact. “These results contradict our assumptions,” write the researchers. “The infected bees were looked after more than the uninfected ones.” This can reduce the parasite load for a single bee, but also increase the risk of spreading. On the other hand, intense social contact between bees is probably important to maintaining a social network.
Balancing distance and care
‘Our combined results suggest that social distance occurs on a large scale (at the colony level) but not on a smaller scale (within the bee cohort) where caring behavior predominates,’ the researchers conclude. “This suggests that the trade-off between protection and the risk of transmission due to social interactions may vary depending on the spatial scale and cohort under consideration.”
The researchers also see their findings in the context of current infection control measures: “Social distancing as a behavioral response to disease is certainly costly for all social animals as mankind is experiencing during the current COVID-19 pandemic, but widespread use of this Strategy in Nature suggests the benefits may exceed the cost, ”they write.
Source: Michelina Pusceddu (University of Sassari, Italy) et al., Science Advances, doi: 10.1126 / sciadv.abj1398