Just when you think there can’t be anything worth knowing about yet another maddening household pest, along comes the small, brown-black, pale-legged Tetramorium caespitum to remind us once again that nature is a complex entity in which no creature stands alone, even, or perhaps especially, when it has to adapt to a new environment.
Along with cluster flies, the Pavement ant was introduced to this country in 18th-century merchant vessels carrying soil ballast from Europe. Once in port, the soil was shoveled out of the holds and dumped on shore to make room for trade goods. From these dockside piles the tiny Pavement ant managed to spread out across New England and the Mid-Atlantic states into the Midwest and Florida, and is now found in parts of California and Washington and as far north as Canada, where it is known as the Sand ant.
Pavement ants form multiple-queen colonies containing thousands of workers in nests built underneath and along the sides of patios, driveways, curbs and sidewalks, cement slabs and foundations of homes. They will also nest under mulch, landscaping materials, and fallen logs or brush piles, always near a water source. The ant beds sometimes appear on the surface as small piles with a crater-like center. Most people are familiar with these ant mounds poking up like miniature volcanoes between sidewalk cracks or on bare ground. But as insignificant as they might seem, these mounds represent industrious underground excavation. Given time, Pavement ants can remove so much sand and soil from underneath roads, walkways, and shallow building foundations that these structures sink and settle, causing considerable damage.
With multiple queens each laying from five to forty eggs per day, a Pavement ant colony grows quickly. Workers live approximately five years and queens much longer. The colony can divide at any time (though usually in spring) by mass swarmings of reproductive queens and workers, a process that can create quite a spectacle. For example, the nature blogger Jim Conrad once observed an enormous 70-foot long ten-ant wide column of Pavement ants marching along a sidewalk toward a single hole in the grass; many of the ants were carrying pupae in their jaws. The Pavement ants greeting one another migration, Conrad writes, “went on until darkness fell and maybe into the night,” and continued the next afternoon, with equal numbers going in both directions, transporting pupae from one site to the other.
Such swarming activity can also lead to, or be a tactic of, massive ant wars that usually go on unseen by human eyes, with whole colonies locked in fierce battles to the death for territory and food. As ant scientist Edward O. Wilson puts it, “Ants are arguably the most aggressive and warlike of all animals… If ants had nuclear weapons, they would probably end the world in a week.” As it is, ants employ sophisticated and often lethal chemical weaponry, including noxious formic sprays and foul odors (such as the banana oil smell given off by the Pavement ant when threatened). One tropical species of ant has even evolved as a race of suicide bombers, capable of making themselves burst open to spray toxic secretions onto their adversaries.
Even so, and though the Pavement ant can bite and sting and on occasion make a nuisance of itself in vegetable gardens, in the outdoors it poses no great threat to daily human activity. Unlike some ant species, it is not a medical pest (though it is an intermediate host for a kind of poultry tapeworm). Inside homes, however, Pavement ants are singularly relentless. They are active foragers that will go searching in large numbers for their favorite foods, especially grease and sweets. Bread and cracker crumbs, uncovered fruit or fruit peels left in the sink, syrup blobs or jelly smears on countertops, the unwashed fry pan sitting on the stove –all are rich attractants for the Pavement ant. Pet food left in dishes on the floor is a particular favorite. But nearly any morsel of dropped, untended, or carelessly stored edible will draw them.
Once established, Pavement ants are not easy to deter. They come in along plumbing pipes, electrical wires, and through foundation cracks and spaces under siding and doorways. They are especially troublesome in houses built on concrete slabs. Seeking winter warmth, the ants will nest in wall voids and insulation, and under toilets and water heaters. Worker trails are set up along baseboards and carpet edging, in and along cupboards and counters in a seemingly infinite parade. As they travel they leave a chemical scent by wiping their abdomens on the ground as they walk.
In addition to these scent trails, Pavement ants use polarized light to navigate, and even dead reckoning if they must do so. Yet Pavement ants rarely travel more than 100 feet from the colony center, staying closer to home than many ant species. This is both a plus (the nest is not difficult to locate) and a minus (the nest is not far from the kitchen). Swarmers can also be attracted to light fixtures in drop ceilings, traveling there for the afforded warmth.
The Organic Outlook
Despite the nuisance factor perpetuated on human habitats, like other ants the Pavement ant in its excavations aerates the soil; in its foraging habit of harvesting dead insects and seeds for food, the colony performs a clean-up service as well as helping the spread of plant life. In its adopted North American environment, it serves us well in these capacities.
Moreover the Pavement ant functions as a protective host for caterpillars of the lycaenid butterfly. The caterpillars secrete a nutritious sugary substance that the ants consume, and so the colony tends them, allowing them to overwinter in the nest and protecting the caterpillars from predators such as parasitic wasps and other ant species. This is a not inconsiderable advantage for lycaenids in a time when butterfly numbers everywhere are under great stress from habitat loss and pesticide use. In fact, an experiment conducted in Colorado revealed that when groups of Silvery Blue butterfly larvae were isolated from their attending ants, the caterpillars survived at a rate of only 10 to 25 percent of those in the same fields that stayed with their ant hosts.
More oddly, Pavement ants serve as hosts for other, parasitic ant species, including the rare and rather bizarre European Teleutomyrmex schneideri or “final ant,” whose queens actually ride on the backs of Pavement ant queens and are fed by Pavement ant workers. This species has become so adept at subjugating their hosts to this arrangement that its own hunting and feeding organs have mostly disappeared along with its entire worker caste.
“[But] a price has been paid for this achievement,” as Edward O. Wilson points out. “The mark of the parasite is upon [T. schneideri]. . . There is no evidence that the adults can do more than mate, fly short distances, cling to their hosts, and beg. When separated from their hosts, they do not live for more than a few days.” Nonetheless, in evolutionary return, the presence of T. schneideri diminishes the host population, thus acting as a natural control on Pavement ant numbers. In other words:
Big fleas have little fleas
Upon their backs to bite ‘em;
And little fleas have lesser fleas
And so, ad infinitum.
Journey to the Ants, Bert Holldobler and Edward O. Wilson, 1994 Belknap Press of Harvard University Press, Cambridge.
Penn State Entomological Dept. Fact Sheet, “Pavement Ants,” by Steve Jacobs, Sr. Extension Associate.
Jim Conrad’s Naturalist Newsletter, September 25, 2005, http://www.backyardnature.net
University of Michigan Museum of Zoology Animal Diversity Web, Tetramorium caespitum, by Lynn Tarkington http://animaldiversity.ummz.umich.edu
West Virginia University Extension Service fact sheet, “Pavement Ants” by Peggy K. Powell, Ph.D
“Pavement Ant Elimination, Biology, Habits” www.pestproducts.com
“In Depth Science: Home Invasion,” by Paul Jay, CBc News, May 21, 2007, www.cbc.ca/news/background/science/ants