The Chemical Deception of Symbiotic Beetles in Ant Colonies

Life may look like a paradise for beetles living in ant colonies. Plump, wriggling ant larvae and helpless eggs sit waiting to be devoured, while hundreds of thousands of ants stand at the ready to ward off predators and parasites.

There’s just one catch for these beetles, revealed in a new study published in the journal Cell. If one leaves the ant colony, it’ll be dead in less than a day.

From bacteria living inside insect guts to mycorrhizal fungi on plant roots, symbiotic organisms abound across the tree of life, says Joseph Parker, an evolutionary biologist at the California Institute of Technology and senior author of the study. All these symbiotes have something in common that can also be a liability—their evolutionary entwinement has progressed so far, they can no longer live without their hosts.

Parker and his colleagues wanted to know why. They tackled this question by investigating the odd relationship between a rove beetle known as Sceptobius lativentris and its host, the velvety tree ant.

One important challenge to entering an ant colony: Odor control.

When two velvety tree ants of the same colony meet, they tap antennae, sensing a specific odor profile that identifies the insects to one another. This is known as antennation, says Parker, and it’s possible because of chemical compounds called cuticular hydrocarbons, or CHCs.

All insects manufacture CHCs, which are waxy and serve a dual purpose as an anti-desiccant. Literally, CHCs stop insects from drying out. But a long, long time ago, ants also evolved the ability to sense each other’s CHC profiles and use them to recognize friend from foe. This means that every rove beetle that lives among ants must first get around the CHC security screening.

To get a better sense of how this works, the scientists looked at the CHC profiles of three species of rove beetles that practice varying levels of symbiosis with velvety tree ants. The beetles are distantly related, and each evolved its ant-loving lifestyle separately.

The rove beetle species Platyusa sonomae produces some of the same CHCs that ants do, but apparently not enough. “It’s never perfectly accepted inside the colony,” says Parker. “It lives on the periphery, and we think this is a kind of transitional step in evolution.”

Likewise, the rove beetle Liometoxenus newtonarum is another imperfect mimic. It appears to make up for the difference by producing other compounds that appease the ants and cause them not to attack. Similar to Platyusa, it does not live within the colony, but rather feasts on adult ants picked off outside.

And then there’s Sceptobius lativentris. Parker’s research revealed that the adult beetles turn off their ability to produce CHCs altogether. This likely allows Sceptobius to go stealth long enough to enter a velvety ant colony.

To complete its olfactory disguise, the Sceptobius beetle crawls onto an ant’s body to harvest its odors in a "grooming" maneuver. It “uses its mandibles to grip the ant antenna, and then it uses its feet to scrape against the ant’s body, and then it smears its feet all over itself,” Parker says. He and colleagues were astonished that the beetle’s resulting CHC profile was “a perfect mirror image” of that of the ants. “It’s scooping up the ant’s CHC profile and essentially basting itself with the ant’s chemical cues, so it then gets accepted as a nest mate.”

Once a Sceptobius beetle is properly perfumed, it becomes untouchable. Not only do the ants not attack—they will feed the beetles from their own mouths and even allow them to eat ant larvae and eggs without consequence.

There’s a catch to the beetles' deception.

If it ever became advantageous to evolve away from its relationship with velvety tree ants, the adult beetles would need to start producing their own CHCs again—which would expose the ruse and make them immediate enemies of their hosts. At the same time, the beetles cannot keep from drying out without the ants.

What’s more, Parker’s experiments revealed that those ant CHCs usually disappear within 20 hours or less and must be continuously replaced. That means the beetles are stuck.

“I have always been curious about why [the beetles] don’t persist when they’re separated from their ant hosts,” says Christina Kwapich, a behavioral ecologist at the University of Central Florida and author of The Guests of Ants: How Myrmecophiles Interact with Their Hosts. “I guess I always thought they weren’t being fed or weren’t eating enough, but it’s true, the amount of time it would take to dry out is much more rapid than the amount of time it would take to starve.”

While this may seem like an evolutionary dead end for Sceptobius beetles, another study published by Parker’s team in Current Biology in 2025 found that Sceptobius will attempt to approach and groom other species of ant when given a chance—at least in the lab. The ants attempt to tear the beetles limb from limb, of course, but it at least shows that Sceptobius will attempt to mimic other species, suggesting that under the right conditions, the species could potentially develop a different kind of symbiosis.

“They incorporated the chemical ecology angle, a molecular angle, and the behavioral angle,” says Kwapich, who specializes in myrmecophile crickets—those that live alongside ants. “They really hit this from every direction, and I think it’s a very nice, very strong story.”

Next up, Parker's lab hopes to learn now the beetles gradually evolved from defending themselves against ants to being attracted to living among them. Additionally, there's much to learn about how this symbiosis reached stability over time.

In particular, how is it that the parasitic beetles don't overrun their host's colonies? This delicate balance is "central to evolving a symbiosis that can last over deep evolutionary timescales," said Parker.