Honey Bees Guarding Hives Are Twice As Likely to Let in a Trespasser If It Is Infected With Deadly Virus

Honey bees are twice as likely to let in a trespassing insect from another hive if that insect is infected with a deadly virus, according to new research.

Israeli acute paralysis virus (IAPV) is a deadly pathogen that has been linked to colony losses. According to new research published in the Proceedings of the National Academy of Sciences, IAPV adjusts bees' behavior and physiology in a way that is expedient to the virus and helps it to spread between colonies.

Specifically, it is able to manipulate behavior in such a way that infected bees are more likely than non-infected bees to breach another hive.

"Somehow, the infected bees are able to circumvent the guards of foreign colonies, which they shouldn't be able to do," said Adam Dolezal, a professor of entomology at the University of Illinois at Urbana-Champaign who led the new research.

Dolezal and colleagues tagged individual bees from three colonies with a QR code that enabled them to track their movements and monitor their interactions for five days continuously. The researchers noticed changes in bees' behavior that appears to prevent IAPV from spreading within a colony yet facilitates its transmission between colonies.

Specifically, authors looked at a behavior called trophallaxis. Trophallaxis describes mouth-to-mouth transferral of liquid food from one individual to another. In addition to food, bees can pass on hormones and other molecules that can alter their behavior.

"They do it in pairs by touching their mouthparts and antennae, and each bee does this with hundreds of partners a day," said Robinson.

"Trophallaxis is essential to the spread of information and nutrition throughout the hive, but unfortunately, a behavior performed with such close social contact also allows viral infections to be transmitted through a hive."

The results suggest that IAPV-infected bees will engage in less trophallaxis with other members of the colony so as to limit the spread of infection—an adaptive behavior called a social immune response.

On the other hand, the results suggest IAPV-infected bees from outside the colony will engage in more trophallaxis, which the authors suspect serves as a way to increase their odds of being accepted into a new hive.

According to the paper, more than 30 percent of IAPV-infected bees were allowed into a colony—twice as much as the control group.

"We know that honey bees from one hive can mistakenly join another colony," Dr. Eyal Maori, a virologist and Chief Science Officer at Tropic Biosciences, told Newsweek. "However, such a drift is treated as a potential invasion, and is usually prevented by 'guard bees.'"

"This is an interesting study that demonstrates an arms-race between a honey bee host and its viral pathogen at the behavioural level," said Maori, who was not involved in the research.

The researchers analyzed the chemical make-up of the bee's hydrocarbons and found that the IAPV-infected honeybees were emitting a different odor. The authors suspect that these changes in odor could be associated with reduced aggression that helps them gain entry to a new colony.

"It seems that the virus is changing how the bees smell, and perhaps the infected bees also are behaving in a way that is meant to appease the guards by engaging more in trophallaxus," Dolezal said.

The researchers explained these responses are specifically linked to IAPV, implying the pathogen is manipulating its host's behavior.

"An interesting point that requires further research is whether IAPV also indirectly facilitates the transmission of other pathogens and pests, such as Varroa mite, to new host colonies," said Maori.

He said it is quite likely that other viruses have also evolved means to enhance their transmission by manipulating the host behaviour and physiology.

According to the paper, colonies in a natural setting are spread out at a relatively low density. There may be around two per square mile or one every square kilometer. However, intensive farming and modern-day apiaries can keep competing colonies less than a meter apart.

This research highlights how intensive farming practices may enable pathogens like IAPV to spread at a much faster rate.

"If you're a virus, it's much more valuable to get transmitted to a new family group, like traveling from one city to a new city," said Dolezal.

"And so how do you get there? You increase the chances that the sick bees leaving colony A are more likely to get into colony B."

The article has been updated with comments from Dr. Eyal Maori.

Correction 04/30/2020: A previous version of this article incorrectly stated Dr. Eyal Maori is currently based at Cambridge University.