Researchers Determine the True Odor of the Odorous House Ant

The odorous house ant, Tapinoma sessile. Photo by April Nobile from www.AntWeb.org.


By Matt Shipman

If you live in the United States, you’ve probably seen an odorous house ant (Tapinoma sessile) — one of the most common ants in the country. And for more than 50 years they’ve been described as smelling like rotten coconut. But Clint Penick thinks they smell like blue cheese, and he’s proven it in an article in American Entomologist called “The true Odor of the Odorous House Ant.”

Matt Shipman

Penick is a postdoctoral researcher at North Carolina State University. Most of his work revolves around ants, and for years he’s been fascinated by the fact that you can identify some ant species by smell — such as T. sessile.

In grad school he heard that T. sessile smelled liked rotten coconut. But, not knowing what rotten coconut smelled like, he had no idea what that meant. And, since he went to grad school in the arid Southwest (where T. sessile are rare), he had no chance to find out.

But after grad school he moved to North Carolina, where T. sessile are common. And when he found some T. sessile in his own backyard, he moved in for a sniff. Blue cheese. Not coconutty at all.

So, being a scientist, Penick decided to do an experiment.

At the annual BugFest event held at the North Carolina Museum of Natural Sciences, Penick did a survey. He asked hundreds of people to smell T. sessile and fill out a survey on what they thought the ants smelled like.

The winner? Blue cheese.

And this is where the science really comes in.

To see if T. sessile really smelled like blue cheese, Penick got in touch with his friend Adrian Smith, a postdoc who studies chemical communication in social insects at the University of Illinois at Urbana-Champaign.

To analyze the characteristic smell of odorous house ants, the researchers placed an SPME fiber in a container with the ants to absorb their odor. They then did the same with a container full of blue cheese and a container full of coconut. The researchers then analyzed the chemicals caught by the SPME fiber using gas chromatography-mass spectrometry.

It turns out that the scents of blue cheese and T. sessile are both caused by the same class of chemicals, called methyl ketones. Coconut’s smell wasn’t related at all.

Then Penick tried one more thing: he buried the coconut in his yard.

When he dug the coconut up a week later, it was covered in a blue mold and smelled exactly like … blue cheese. When they tested it, Penick and Smith learned that the Penicillium mold on the coconut produces the same methyl ketones found in blue cheese.

“This was something Adrian and I did for fun — it’s not something we spent tax dollars on,” Penick said. “But while this started out as a joke, we got some interesting findings. For one thing, we’ve learned something new about one of the most common household pest species in the United States, and it’s something people can use to describe the species to homeowners, students, and entomologists. But we also learned that T. sessile and the Penicillium molds are producing these similar methyl ketones. Why? We think it may have beneficial antimicrobial properties, but that remains to be explored.”

Read more at:

The True Odor of the Odorous House Ant


Matt Shipman is a science writer and public information officer at North Carolina State University. He is the author of the Handbook for Science Public Information Officers (University of Chicago Press, 2015) and a contributing author to The Complete Guide to Science Blogging (Yale University Press, 2015). Follow him on Twitter at @ShipLives. This post also appears on NC State’s research blog, The Abstract.

Comments

  1. Karen Vail says:

    I always thought Argentine ants smelled more like blue cheese than OHA and am concerned that PMPs using smell as an identifying characteristic will now further confuse OHA and Argentine ants. A quick search through the literature revealed that Iridomyrmex sp. (a prior genera of the Argentine ant, Linepitheme humile) produces this same methyl ketone, 6-methyl-5-hepten-2-one, as does the ghost ant, Tapinoma melanocephalum. Could these researchers please simultaneously compare the odor of OHA and Argentine ants to determine if Argentine ants produce more of this methyl ketone or other methyl ketones so we can more accurately describe the odors of these two ants? I wouldn’t be surprised if the methyl ketone in the OHA odor was described as a delicate hint while that of the Argentine ant was full-bodied.

  2. Karen,
    6-methyl-5-hepten-2-one is the major compound present in the T. sessile headspace (it is ~100 times more abundant than other volatiles) as well as most other dolichoderine ants. But you are right to suggest that there are other compounds that make up their scent that are more subtle. Both species (Argentine ant and Tapinoma) also emit a class of compounds called “iridodials” that are named after the ant genus (Iridomyrmex). Most online sources report that Argentine ants are “musty” compared with T. sessile. I tried smelling both right after each other, and I can smell the difference. But smelling one ant alone, I think I would have a hard time. At least neither of them have a coconut smell, which is derived from lactones. I would agree with you that smell is tough identification character for humans to use, but at least in Raleigh we can narrow down whether someone has Argentine ants or Tapinoma rather than non-blue cheesy ants (fire ants, Monomorium, Brachymyrmex, etc).

  3. Peter Apps says:

    And to close the circle – some moulds that grow on cream cheese produce an odour of coconut – I worked on it many many years ago and the names of the compounds escape me, I do recall that they were ketones.

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