Odorous House Ants: The Ants We Gave Superpowers

Odorous house ants are near the top of the list of America’s Most Unwanted Insects.  Although scientists commonly refer to them as Tapinoma sessile, most people know them as ‘sugar ants’, or more colloquially, ‘piss ants’.  By any name, they are the plague of many an American kitchen from the east coast to Oregon, and, as such, most of the research on odorous house ants has been devoted to figuring out how to kill them.

There are actually several species of ants around 1/8” long that are lumped into the ‘sugar ant’ category of home invaders.  Odorous house ants can be distinguished from the rest of them by their uniform dark brown color and, as the name implies, by their distinct odor.  My colleagues Adrian Smith and Clint Penick have published an amusing study that claims the odor is akin to blue cheese, but I would counter that the smell is more similar to slightly rotten citrus.  Either way, once you have a wiff of these ants, you’ll never forget them.

Neither their home-plaguing habits nor their pungent chemistry is what inspired me to study odorous house ants for my graduate research.  I was more interested in understanding their ecology. Why?  Because something about living near humans gives these ants superpowers, and I wanted to figure out what.

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Odorous house ants with their young. Other groups of similarly-sized ants can be roughly identified by the “scratch-and-sniff test.” Argentine ants smell like musty socks, most formicine ants smell like vinegar (although one species of Lazius ants smell like citronella), and Forelius ants smell like flowery-perfume. Unfortunately, crushing the ants to smell them does not lend itself to collecting, er, intact samples. Original photo by: Janet Hurley, Bugwood.org.

When odorous house ants live out in the woods away from urban areas, they are the ant equivalent of the wimpy kid on the playground.  Their colonies are small, maybe a few hundred individuals.  Many times they have only one queen, but sometimes they have a handful of queens.  If some odorous house ant workers find a tasty dead bug and another species of ant comes along, the odorous house ants usually end up losing the fight for the food and running away.

However, when odorous house ants end up in urban areas, it’s like the wimpy kid suddenly drank Extra Strength Super Power Juice.  Odorous house ant colonies in cities grow to millions of workers in size with thousands of queens.  Such large colonies form by smaller colonies budding off of the founder colony over and over until there are huge networks of nestsites where the ants freely exchange queens and workers.  These so called “supercolonies” can function over an area of several city blocks.  Moreover, unlike in rural settings, these city-savvy ants tend to win fights over resources with other species of ants.

So what causes the change?

I thought it might be the way we modify the landscape around our homes, so I set off to do a survey of 24 urban and suburban yards looking for odorous house ants (and other urban ants).  I trapped ants in the yard, near the houses, and in garden beds away from the house.  I measured how thick the vegetation was, what the ground cover was, recorded the dominant plants, measured the amount of shade, and measured how deep any leaf litter was.  Because odorous house ants prefer to live under preexisting debris, I counted the number of potential nesting sites such as mulch, logs, rocks, and landscape timbers near where I trapped ants.

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This is what ant ecology looks like. I promise the hooka-like object I am using is really a device called an ‘aspirator’ for sucking up ant specimens. There’s a screen to make sure the ants stay in the attached vial and don’t actually get aspirated…usually.

Like many ecology research projects, I discovered much of what I spent long days meticulously measuring in the hot summer sun (while pregnant!), didn’t have any measurable relationship with odorous house ant numbers.  However, a few things did correlate with more odorous house ants being present: leaves, logs, and being close to the house.  What does this correlation mean?  Well, because this was just a survey and not an experiment, I can’t claim that any of these things cause higher number of ant, just that they might be related.  Maybe it means that odorous house ants do well in urban areas because they have more nest sites, or maybe our homes simply provide a convenient source of food or warmth.  More research involving actual experiments would need to be done to see if any of these potential causes are indeed the case.

Since my project, other researchers have continued to investigate the source of these ants success. Adam Sayler and his associates, did surveying work of ants in natural, semi-natural, and urban areas that suggests that odorous house ants may be helped by the fact that human disturbance is bad for other species for ants.  In short, odorous house ants can handle the bustle of the city, their competitors cannot.

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Odorous house ants tending scale insects. Photo by Susan Ellis, Bugwood.org.

Although not explicitly about ants, research coming out of my colleague, Steve Frank’s, lab has shown that tiny, tree sucking critters called scale insects, are more abundant in cities due to urban warming.  These insects secrete a sugary solution called “honeydew” that is used as a food source by many kinds of ants.  During the course of my research, I noticed huge trails of odorous house ants going up and down trees, presumably collecting honeydew.  Maybe the urban “hotspots” for scale insects, are helping fuel the massive colonies of odorous house ants.

Whatever the reason for their urban superpowers, it isn’t something that has happened just once.  Sean Menke and his associates found that many separate groups of odorous house ants have evolved the ability to make supercolonies in urban areas all over the US.  

So is it shelter, response to human disturbance, or heat-loving food sources that turns these ants from wimps to supervillains?  In all actuality, the secret to odorous house ant success is likely a combination of many of these factors, all of which are in some way related to the way we build our homes and maintain our yards. So the next time you grumble at the line of ants traipsing across your kitchen, keep in mind that these creatures may somehow be a pest of our own making.