For her master’s degree, Texas A&M University graduate student Valerie Renee Holmes studied red imported fire ants (Solenopsis invicta) and the viruses that afflict them. In the course of that work, she realized that a thorough review of immunity among eusocial insects in the order Hymenoptera (ants, bees, and wasps) hadn’t been done, and so she published such a review in December 2021 in the Annals of the Entomological Society of America. (Photo by Mikaela Egbert via iNaturalist, CC BY-NC)

By Melissa Mayer

Melissa Mayer

Nowadays, when most people hear the words “virus” and “epidemic,” the human realities of COVID-19 spring to mind. But insects that live in colonies also face viruses, and their lifestyle—which is the exact opposite of social distancing—is prime for epidemics and outbreaks.

In a research-review article published in December in the Annals of the Entomological Society of America, researchers from Texas A&M looked at what it means to be eusocial and respond to viral invaders. They report the ways insect communities use innate immune defenses and also act like a superorganism, deploying social immunity tactics to rise to the challenge.

A Risky Lifestyle

When graduate student Valerie Renee Holmes found herself stuck at home during the COVID-19 pandemic, she focused on her dissertation on fire ant (Solenopsis invicta) viruses. She soon realized that a thorough review of immunity among eusocial insects in the order Hymenoptera (ants, bees, and wasps) hadn’t been done.

Holmes’ background is public health and epidemiology, and she says colony life provides viruses unique opportunities to jump easily between hosts. Cozy living quarters mean hospitable temps and a ready supply of food, frequent contact between nestmates, and the presence of vulnerable larvae. Plus, insects in a colony share genetics, so a virus that knocks out one individual is likely bad news for the whole group.

“That’s basically insects in an airplane,” Holmes says. “We say in epidemiology that airplanes are an [epidemiologist’s] worst nightmare, and eusocial living is kind of the airplane of insects as far as immunity. So, they’re highly susceptible to epidemic disease outcomes.”

Fighting Viruses

Vertebrates have two types of immunity, including adaptive immunity, which recognizes pathogens from previous encounters and sends out targeted immune responses. Insects only have innate immunity, which simply recognizes that a pathogen is foreign and mounts a non-specific, whole-body response.

“When you don’t have access to individual, cellular-level immune response that’s very, very specific, then it’s basically like all the tissues in the insect saying, ‘Oh, ****, there’s somebody coming in here,'” Holmes says.

That non-specific immunity uses chemical pathways where one event (like recognizing a foreign invader) triggers a series of reactions. For insects, these include the Imd, Toll, and JAK/STAT pathways, which perform other important functions in the cell, like development or metabolism, but also switch on to combat a pathogen.

Holmes’ favorite is the RNAi pathway, a gene-silencing mechanism that also clears viruses. It relies on Argonaut proteins floating in the cell that recognize viral RNA as foreign and digest it so the virus can’t replicate.

While writing the review, Holmes says it struck her how interconnected the various pathways are. “Insects have gotten pretty darn efficient with using the pathways they have in order to regulate a number of different functions, both immune-wise and in development and just life in general,” she says. “It’s very difficult to really study individual pathways because they’re so extensively interlocked with one another.”

Eusocial hymenopterans also benefit from social immunity, acting like one big organism to help the colony survive. This may be why queens tend to be monogamous and mate in a single event, reducing the opportunity to encounter a pathogen.

But much of social immunity is tied up with workers. When foragers return to the nest, colony gatekeepers may refuse entrance to diseased foragers. Inside the nest, other workers may groom foragers to remove pathogens. The colony can even use a collective defense like social fever, clustering around a sick individual until their combined body heat kills the threat to the group.

Foragers may also bring back plant resins for the colony to consume to boost immunity. In some cases, even an individual’s own venom—which may be antimicrobial—can be slicked onto the exoskeleton for sanitary purposes.

The Big Picture

Figuring out how eusocial insects battle viral invaders helps unravel hymenopteran biology—but it also has tangible value. In the case of honey bees, which are economically important as livestock and pollinators, understanding immunity could help bolster the more than 30 percent of colonies that die off each year in the United States.

For Holmes, one of the most exciting applications is biological control. She says that, while it’s clear there’s no “magic bullet” for invasive fire ants, their viruses tend to be extremely host-specific, so a virus could wipe out a colony without the off-target effects of baits or insecticides.

The other thing that she says stood out to her is the opportunity for emerging entomologists to get in on this work. “What really surprised me is how much science is left to be done,” she says. “I was just really surprised at how many research questions immediately came to mind when I was reading these papers.”

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.