Thursday, July 2, 2015

What's That Smell? The True Odor of the Odorous House Ant

When talking to folks about the odorous house ant, you’ve probably told them that the ant smells like rotten coconut when it’s crushed, after which they inevitably give you a “look” – one that tells you they think it’s disgusting that you readily squish ants! 

While the freshly crushed odorous house ant does produce a definite odor, describing it as “rotten coconut” never said a lot to me, namely because I have never smelled a rotten coconut! I have always thought the odorous house ant produced a clean smell, almost like glass cleaner, with more of a “punch.” 
In an effort to provide more accurate information on just how the odorous house ant smells, Clint Penick and Adrian Smith conducted a recent study investigating the volatile compounds released by the odorous house ant and the items most commonly associated with their scent. 

The vast majority of online sources identify the odor of the odorous house ant as “coconut-like” (Fig. 1A). This ran counter to the results of a smell test that Penick and Smith conducted as part of their study. In the smell test, participants most identified the smell of a freshly crushed odorous house ant as “blue cheese,” followed by “other” as a close second (Fig. 1B). The most common write-in candidate for “other” was cleaning spray (very similar to way I have always described the smell of odorous house ants!).


When Penick and Smith also used gas chromatography-mass spectrometry to determine the exact chemical composition of the odorous house ant scent. They found that the major component of the odorous house ant scent was a methyl ketone (Fig. 2) that has actually been identified in other ant species as well. As in turns out, the most prominent compounds associated with blue cheese are also methyl ketones. No methyl ketones were found in fresh coconut. Once coconut turned rotten, however, it released the same methyl ketones found in blue cheese. Interestingly, the microbes that turn coconut oil rancid – Penicillium mold – are the very same microbes used to make blue cheese!

So, what exactly IS that smell? According to Penick and Smith’s results, the odorous house ant odor points to blue cheese, with a cautious nod to rotten coconut. They emphasized cautious because it’s not the “coconut” in rotten coconut that smells like the odorous house ant, but the “rotten.” To sum up, odorous house ants do not smell like coconuts. They smell like blue cheese. Or you could say that they smell like coconuts that have been colonized by Penicillium mold that causes the coconut oil to produce an odor similar to blue cheese.

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