Wednesday, December 8th, 2021

Deadly virus turns honey bees into Trojan horses

Infected bees were less likely to be fed by other bees, a common behavior shown here.

Nathan Beach/University of Illinois, Urbana-Champaign; Tori Beach

Social distancing is nothing new to honey bees. When a colony is infected with the deadly Israeli acute paralysis virus (IAPV), bees are less likely to touch or feed their sick nest mates, according to a new study. But the virus appears to have an alarming counterattack: When sick bees try to enter a new colony, they do a better job of getting past the guards than uninfected bees. That has led the scientists to speculate that the virus has evolved a way to spread to new hives. 

The study opens an important window into a “coevolutionary arms race” between pathogens and social organisms, says Olav Rueppell, an evolutionary biologist at the University of North Carolina, Greensboro, who was not involved in the new research. The findings also highlight the dangers of putting commercial hives too close together.

Honey bees are particularly vulnerable to infectious diseases: They live in crowded conditions, and they touch each other all the time. Moreover, their immune systems are weak compared with other insects. They depend instead on hygienic behaviors, such as grooming or removing sick larvae.

Not much was known about how bees act when infected with viruses, says Adam Dolezal, an insect physiologist at the University of Illinois, Urbana-Champaign (UIUC), who studies IAPV. To find out, he teamed up with UIUC computer scientist Tim Gernat, who had developed an automated system to monitor bee behavior. The team glued labels onto the backs of about 900 bees in each of three colonies and tracked them with a camera. The camera took pictures every second, and a computer mapped the location and orientation of every labeled bee.

The researchers trained the computer to identify a behavior called trophallaxis, in which honey bees feed their fellow workers by regurgitating food from a pouch called a crop. Hungry bees approach other bees until somebody coughs something up.

To find out how they would react to the virus, Dolezal put 90 to 150 labeled bees into each colony after infecting them with IAPV. After 5 days of recording, the team found that healthy bees were avoiding contact with the infected bees. About half as much trophallaxis took place with sick bees compared with normal workers. But it wasn’t for lack of trying: The sick bees moved around the colony more than other bees, probably looking for someone that would feed them, Dolezal says. The findings show how bee behavior in a real colony can suppress an infection, says Christina Grozinger, a behavioral ecologist at Pennsylvania State University, University Park, who was not involved.

So how does the virus successfully spread? Dolezal wondered whether IAPV might have a way to sneak into other colonies, which are guarded by bees that keep out trespassers. Guards use their antennae to detect a collection of chemical signals on the outside of other bees, called cuticular hydrocarbons, that identify them as members of the colony or foreigners. 

When Dolezal and colleagues took IAPV-infected bees and placed them outside another colony, the guards let about 30% of them enter, compared with about 15% of healthy foreign bees that were allowed in. “It’s troubling that this level of virus movement is happening,” says Dolezal, whose team reports the results this week in the Proceedings of the National Academy of Sciences.

The researchers aren’t sure why IAPV is so successful at deceiving the guards, but it may be because it alters the abundance of various cuticular hydrocarbons, which differed between the sick and healthy groups. For example, they found that IAPV infection resulted in lower levels of octacosane, which has been associated with greater acceptance by other bees. The sick bees were also more submissive when challenged by guards and more likely to offer them food, and those behaviors may also help them spread the disease. 

IAPV isn’t the only problem these Trojan bees can bring. The new arrivals can also carry a parasite called the varroa mite, which has decimated bee populations worldwide. The mite feeds on bees’ fat reserves and can also carry IAPV and other deadly viruses. If IAPV helps these other pathogens spread, that could be a big problem for commercial beekeepers who pack colonies together for efficiency. “There’s very little you can do if you suspect a virus infection,” Dolezal says.

And when it’s easy for pathogens to spread to new hosts, Rueppell warns, they are much more likely to evolve to wreak new kinds of havoc.

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