The Ants That Smell Like Blue Cheese—Or Is That Pine-Sol?

Researchers put these ants to a sniff test.
sniff test
Researchers put ants to a sniff test.Eleanor Spicer Rice

When an insect has the nick-name of "odorous house ant," you know it's smelly. This ant's scent has been described as blue cheese, rancid butter, or cleaning solution. Inquiring science minds wanted to know: What does it really smell like?

A pair of ant researchers used a public sniff test to try to identify the tantalizing fragrance of Tapinoma sessile, the odorous house ant:

C. Penick & A. Smith. 2015. The True Odor of the Odorous House Ant. American Entomologist 61(2): 85-87.

Why are entomological researchers huffing ants in the first place? Ants live in a world of chemical signals, and many are species-specific. With 1,000 species of ants in North America, smelling ants is a quick field test to identify a species without a microscope. Citronella ants have a lemony scent when crushed, for example. A carpenter ant releases formic acid, and smells a bit like vinegar.

The Odorous House Ant, Tapinoma sessile.

ADRIAN SMITH

An alternative common name for the odorous house ant is "coconut ants." That's how it's described in about 80 percent of published reports and online descriptions. But when Dr. Clint Penick crushed his first ant, expecting coconut to waft forth, what he actually smelled was blue cheese. "To me, it was like a personal grudge. I felt like I had been lied to and told there was this coconut ant. Then when I smelled it I was like "No, this just smells like blue cheese.... Let's get to the bottom of this."

Smell My Ant Finger

In 2013, Penick and crew held a sniff test at the 2013 North Carolina BugFest. Members of the public were asked to crush an odorous house ant between their fingers, and then choose between four choices: rotten coconut, rancid butter, blue cheese, and a write in of "other." One hundred and forty-three people willingly snorted an ant for science.

In sniff tests, 38% of people thought crushed odorous house ants smelled like Blue Cheese.

Penick & Smith, 2015.

About 38 percent of ant-sniffers thought they smelled like blue cheese, with rotten coconut next at about 25 percent. The rest of the responses fell into "other" and varied widely. Pine-Sol or cleaning spray was a leader in that category, although one little girl thought the ants smelled like her doctor.

High-Tech Ant Sniffing

Given the variability in human sniffers, the researchers next analyzed what chemicals contributed to these distinctive smells. They collected volatiles released by blue cheese headspace, and compared it to that from squished ants and coconuts. (Volatile cheese headspace describes a GC-MS technique, not overly excited Green Bay Packers fans.)

The first analysis found a group of methyl ketones in blue cheese and the odorous ants that were very similar. Eureka! But the coconut didn't seem to have any chemical resemblance to the ants.

Clint Penick explained: "We tried coconut oil and ... fresh coconut from the store. We didn't get any compounds of a similar structure where the other methyl ketones had been found. I thought 'Okay, that's it, I win, it's blue cheese, it's definitely not coconut.' But then I thought "Well, to be fair, it's really rotten coconut, everyone says rotten coconut."

But how does one acquire a rotten coconut? That's not something you can easily buy in a store or via a chemical supply retailer. Demonstrating the problem-solving skills common to scientists, Penick buried a fresh coconut in his backyard for about a week.

"I went down and I dug it up... It was clear the coconut was covered in a blue-green mold as soon as I dug it out of the ground. I held it up to my nose, I smelled it, and 'Yeah, so now it smells like blue cheese.'"

Just one problem: the two collaborators on this research were in different parts of the United States. Which is why Penick sent his collaborator Dr. Adrian Smith a rotten coconut via regular US Mail. "Normally when you send specimens, there's some need to get it there fast, you overnight it or something. But this was just rotten, so I was like 'All it's going to do is rot more, so I'll just put it in the regular mail and send it up there.'" Penick said.

And sure enough, once they re-ran the tests with rotten coconut, they found the same compounds that give blue cheese its distinctive smell. Methyl ketones are found in both rotten coconut and cheese. The blue-green mould Penick saw on the coconut might have contained Penicillium, which is what gives blue cheese it's distinctive color and flavor.

In the final sentence of their research paper, the authors lay it out for us: "Our results point to blue cheese with a cautious nod to rotten coconut... Or you could say that they smell like coconuts colonized by a Penicillium mold that metabolizes coconut oil to produce an odor similar to blue cheese, but by then, you could have said 6-methyl- 5-hepten-2-one three times fast."

I asked Penick if there were any ants that people should not smell. "You might not want to break into a fire ant mound and try to smell them. That won't go well."