The odorous house ant, Tapinoma sessile, is one of the most common ants in the US, and people who’ve actually sniffed their fingers after crushing them typically liken the smell to rotten coconuts, cleaning solution, rancid butter, or blue cheese. As it turns out, at least two of the food items have chemicals in common with ant odor. The work is described in American Entomologist this week.
Ants communicate with smells, so using scent to identify them seems fairly practical. And while the human sense of smell is far less developed, most people – or 80% of 53 websites – agree that the smell of odorous house ant is reminiscent of rotten coconuts. One website was incredibly specific, calling it “sickening sweet… like a coconut piña colada that has been sitting out for a day.”
But ever since he crushed his first odorous house ant, North Carolina State’s Clint Penick has maintained that they smell like 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,” he explains to Wired. And during an informal survey at BugFest 2013, the majority of the 143 participants agreed with him. So Penick teamed up with Adrian Smith from the University of Illinois at Urbana–Champaign, who works on chemical communication in social insects.
To analyze the volatile compounds that characterize the smell of odorous house ants, the duo placed a special fiber used for micro-extractions into a container with live worker ants to absorb their odor. They did the same thing to a container brimming with blue cheese and a container of coconuts. Then, using a substance-identifying technology called gas chromatography–mass spectrometry, they analyzed the chemicals taken up by the fiber.
The major component of odorous house ant scent is called 6-methyl-5-hepten-2-one. And it belongs to the same class of chemicals found in blue cheese smell, called methyl ketones. No methyl ketones were found in fresh coconut or coconut oil. Then they decided to conduct the experiment again with a coconut that had been buried in the yard for at least three days. By then it was covered in blue-green Penicillium mold and had a rather pungent odor. Sure enough, that moldy smell contained those same methyl ketones.
Odorous house ants, they conclude, smell like blue cheese with a cautious nod to rotten coconut. “We say cautious, because it is not the ‘coconut’ in rotten coconut that smells like the odorous house ant, but the ‘rotten,’” they write. And why are these ants and Penicillium molds producing similar methyl ketones? “We think it may have beneficial antimicrobial properties,” Penick says in a news release, though this remains to be explored.
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