Country ants make it big in the city

Urban life lets odorous house ants build empires with hundreds of queens

It’s a tale of bright lights, big colonies: Rural ants go wild in the city.

CITY ANT, COUNTRY ANT A colony of odorous house ants living in Indiana forests can fit into an acorn. But when the species goes to town, it goes mega-metropolitan. G. Buczkowski

The first systematic lifestyle survey of odorous house ants confirms how much a modest country dweller can change habits in the big city, according to urban entomologist Grzegorz Buczkowski of Purdue University in West Lafayette, Ind.

In the forests of Tippecanoe County, Ind., he found odorous house ants, Tapinoma sessile, in colonies with just one queen each. With no more than a hundred ants, each colony could live in a single acorn.  

Ants from city parks and other seminatural areas formed somewhat bigger colonies, he says. But in West Lafayette and other urban zones nearby, Buczkowski found that nests of odorous house ants connect via bustling ant trails to form supercolonies. Each of the 15 colonies he sampled typically held some 58,000 ants and 238 queens, he reports online February 26 in Biological Invasions. One supercolony across the street from Buczkowski’s office covered more than a city block and held 6 million workers and thousands of queens.

“In the forest, these odorous house ants have a pretty tough life,” Buczkowski says. Plenty of other species compete for food and shelter, and ants living in unheated acorns go dormant during the winter. But Buczkowski has documented that urban colonies stay active all year by retreating to warm refuges. “Even when it’s snowing outside, they can be happy inside reproducing,” he says.

The ant’s name comes from the odor they release when vexed, a smell that   Buczkowski describes as somewhat like a pi±a colada’s.

The ant doesn’t sting or bite or chew up houses. Yet Buczkowski says that pest control workers tell him they’re getting increasing numbers of calls from human city dwellers dismayed by heavy ant traffic in their houses.

“Here we have this native ant species that’s becoming a pest,” Buczkowski says. Unlike odorous house ants, most invasive ants live far from their native ranges. The infamous Argentine ant, for example, is no big deal in its South American home but forms supercolonies that are disrupting native ecosystems in the United States, Europe and elsewhere.

What horrifies home owners offers a great opportunity for biologists, according to ecologist Sean Menke of North Carolina State University in Raleigh. Because the whole business of going from country cousin to world-beater takes place in the same geographical region, researchers can narrow down the urban effect on lifestyle changes. 

Even though odorous house ants are one of the most widespread native ants in North America, ranging from coast to coast, and have been annoying home owners to some extent for decades, Menke says entomologists have started studying their basic biology and urbanization only in the last few years. “Most scientists became scientists to get out of the urban environment, he says.

Urbanization of odorous house ants occurred independently multiple times in different locations, Menke and his colleagues reported February 12 in PLoS ONE. In an analysis of 49 samples of odorous house ants from around the country, the researchers found large genetic variation. And rural ants proved the closest relatives to the urban dwellers in their own general region.

Menke points out that even in natural settings, odorous house ants have proved highly adaptable, surviving in palm oases in Baja as well as high in Colorado mountains. He says he has both seen and heard of odorous house ants in rural areas that have multiple queens, which could hint that the species has a smoldering invasive capacity.

“Many species are beginning to succeed and spread in urban environments,” Menke says. “We don’t know if it is because they are being forced there due to encroachment by people into their native habitats or because the species have altered their lifestyles.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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