Viruses plague not only us humans – the same applies to honeybees. Researchers have now investigated how a bee virus can be passed on from one hive to another. It turned out that within a beehive, the insects seem to recognize and avoid infected animals – they do social distancing, so to speak. But at the entrance to a strange floor, infected workers are even given preferential treatment because the virus manipulates their scent. The guardian bees do not chase them away, but even exchange food with them. This in turn favors the transmission of the virus to the new colony.
Honeybees have a hard time: beekeepers are observing that more and more bee colonies have perished in recent years. In addition to pesticides and insufficient feed from agricultural monocultures, researchers attribute this primarily to the infestation with the parasitic varroa mite. These mites, imported from Asia decades ago, suck the blood of bee larvae, pupae and adults and weaken their defense. In addition, the parasites often transmit pathogens, including the deadly wing deformation virus, but also other viruses. Studies indicate that both the mites and the pathogens they transmit are spread between different types of bees and between different hives of honeybees. However, exactly how this is done and what role it may play in changing the behavior of infected and sick bees has so far been unknown.
Social distancing in the beehive
Amy Geffre from Iowa State University in Ames and her colleagues have now investigated this in more detail. To do this, they used an automated system with which they could continuously track and evaluate the movements and behavior of more than 900 honeybees individually marked with a QR code in three beehives. In the first experiment, the researchers infected some of these bees with a non-disease-causing but immune-stimulating DNA fragment or with the Israeli Acute Paralysis Virus (IAPV). This virus, which is transmitted by the Varroa mite, is suspected to be crucially involved in colony death in the United States. They then observed whether the behavior of the bees on the hive changed.
It turned out that the reaction of the healthy honeybees in the hive changed both in relation to the bees infected with the virus and also to the only immunostimulated animals. Although the affected bees were still scanned intensively with the antennas, the so-called trophy ax – mutual feeding with liquid food – was largely absent. “Honeybees use the trophy practice to share food, but also hormones and other signaling molecules,” explains co-author Gene Robinson from the University of Illinois at Urbana-Champaign. “This exchange takes place by touching the mouth parts and antennas in pairs, and each bee does this with hundreds of partners a day.” But as the experiment showed, the bees clearly recognize when a fellow species is fighting a pathogen or their immune system is activated – and is avoided him. “We think that this reduction in trophy practice is an adaptive social mechanism to reduce pathogen transmission through physical contact in the beehive,” the researchers say. Since this avoidance reaction occurred both against the bees infected with IAPV and only against the immunostimulated bees, this reaction is apparently non-specific, as the scientists explain.
(Video: University of Illinois)
Access to the other floor easier
In a further experiment, the researchers investigated whether an infection with the bee virus also changes the behavior of honeybees from a foreign hive towards the infected congeners. “In modern beekeeping, bees are often kept at higher densities than naturally – the hives are often less than a meter apart,” explains Geffre and her colleagues. Some beekeepers temporarily place hundreds of colonies in an orchard or field to pollinate crops. At the same time, however, it is known from previous observations that weakened or sick honey bees often get lost on the way back to their hive and then land in front of other beehives. In theory, this could favor the transmission of pathogens and varroa mites. How the guardian bees behave towards such strangers is therefore crucial. To test this, the researchers either placed healthy, immunostimulated or IAPV-infected workers in front of the opening of a foreign floor.
Their observations revealed that the healthy and only immunostimulated workers were touched by the guardian bees on the strange hive and recognized as foreign. The guards threatened her and did not let her go upstairs. The situation was different, however, among the infected workers: “We found that the bees infected with IAPV experienced significantly less aggression,” the researchers report. “At the same time, these bees evoked more grooming and trophy practice from the guards.” Apparently the guardian bees did not recognize these workers as strangers and treated them more like a stick companion. “Somehow, the infected bees manage to bypass the guards’ immune response on the alien stick,” said co-author Adam Dolezal of the University of Illinois. However, this increases the likelihood that the IAPV virus will be transmitted to the foreign floor.
Manipulation of the fragrance
In contrast to the changed reaction of honey bees within a bee colony, the reaction of other conspecifics also seems to be specific for honey bees infected with IAPV. “Something about them has to be different,” said Dolezal. In order to find out how the honeybees recognized the infected conspecifics, the researchers analyzed the hydrocarbons present on the animal’s shell – scents, which the bees use to identify the members of the hive and the condition of their conspecifics. And indeed, there were striking differences in eight of these compounds. “The infection resulted in a molecule profile that was different from that of the controls and the immunostimulated bees,” reports Geffre and her team. “The virus apparently changes the smell of the bees.” This could explain why the guards at the entrance to the hive reacted less aggressively to the infected workers than to healthy strangers.
At the same time, these results demonstrate how a disease-causing virus can manipulate its host to optimize its spread. “For a virus, it is far more beneficial to be transferred to a new group of hosts, such as from one city to another,” explains Dolezal. “And how do you do that? In the case of IAPV, there is a greater chance that an infected bee from floor A will have access to floor B. ”
Source: Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2002268117