[Pest Spotlight] Bright Lights, Big City

One of the most common household ant species might have been built for living in some of the smallest spaces in a forest, but the ants have found ways to take advantage of the comforts of city living.

Grzegorz Buczkowski, a Purdue University research assistant professor of entomology, found that odorous house ant colonies become larger and more complex as they move from forest to city and act somewhat like an invasive species. The ants live about 50 to a colony with one queen in forest settings, but explode into supercolonies with more than 6 million workers and 50,000 queens in urban areas.

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Odorous House Ants
at a Glance

Latin Name: Tapinoma sessile

Size: ¹⁄8-inch

Color: Brown to black

Number of Nodes: One

Shape of Thorax: Uneven

Antennal Club: None

Tip of Abdomen: No circle of hairs

Geographic Range: Throughout the United States

Key Characteristic: Workers give off rotten coconut odor when crushed

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"This is a native species that’s doing this," said Buczkowski, whose results were recently published in the early online version of the journal Biological Invasions. "Native ants are not supposed to become invasive. We don’t know of any other native ants that are out-competing other species of native ants like these."

Odorous house ants live in hollow acorn shells in the forest. They’re called odorous because they have a coconut- or rum-like smell when crushed. They’re considered one of the most common household ants in the United States.

In semi-natural areas that are a cross of forest and urban areas, such as a park, Buczkowski said he observed colonies of about 500 workers with a single queen. He said it’s possible that as the ants get closer to urban areas they have easier access to food, shelter and other resources.

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A Highly Adaptable Ant

Some urban entomology research does not provide practical control solutions, but simply explains more about the evolutionary history or reasons behind the behavior of certain pests.  Such is the case with the article featured here. Grzegorz Buczkowski, a Purdue University research assistant professor of entomology, found that odorous house ant (Tapinoma sessile) colonies become larger and more complex as they move from forest to city and act somewhat like an invasive species.

Buczkowski theorizes that in forest settings, where odorous house ant nests can be small enough to fit inside a hollow acorn, the ants face competitors that force them into literally smaller niches.  In urban habitats, however, the odorous house ant has prospered with little interference from its normal competitor ants.

Marion Smith, a graduate student in entomology in the 1920s first proposed expanding the then common name "odorous ant" to the "odorous house ant" in a paper published in the Annals of the Entomological Society of America in 1928 (Ann. ESA 21:307-330). He noted that during the mid 1920s this ant had become the most important pest ant in homes in Illinois, but that the phenomenon appeared to be of recent origin, hence it deserved being called a house ant.

While many or most of our indoor ant pests are exotic species, the odorous house ant is an exception. Its native-American heritage shows how adaptable insects are, and how structures can function in some ways as an "exotic land" enabling even native insects to escape their competitors and natural enemies. This insight may not make you a better PMP, but hopefully will give you a better appreciation of this common urban ant. – Mike Merchant

The author is an entomology specialist for Texas AgriLife Extension. The preceding article appeared on Merchant’s blog, "Insects in the City," which can be found at http://insectsinthecity.blogspot.com.

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"In the forest, they have to compete for food and nesting sites," Buczkowski said. "In the cities, they don’t have that competition. People give them a place to nest, a place to eat."

Buczkowski observed the ants in three different settings on and around the Purdue campus. He said it might be expected that if the odorous house ants were able to multiply into complex colonies, other ants would do the same.

But Buczkowski found no evidence that other ants had adapted to new environments and evolved into larger groups as the odorous house ants have. He said it’s possible that odorous house ants are better adapted to city environments than other ant species or that they had somehow outcompeted or dominated other species.

"This raises a lot of questions we’d like to answer," he said.

Buczkowski said understanding why the supercolonies form could lead to better control of the pests in homes, as well as ensuring that they don’t out-compete beneficial species.

Future studies on odorous house ants will include studying the ant’s genetics and trying to understand the effects of urbanization on the pest which is becoming a growing problem in the pest control industry.

(Source: Purdue University News Service)