Courtesy of Magdalena Sorger

What makes ants such successful pests? There are many reasons but two of the biggest are their immense variation and, surprisingly, people. To better understand pest ants, we need to examine both of these factors individually and together.

In truth, less than 50 of the more than 12,000 described ant species regularly act out as pests. That is to be expected with such abundant diversity of behavior and anatomy. Pest ant species tend to share similar traits often described as “tramp characteristics.” These characteristics include broad diets, a wide range of preferred nesting habitats, colony expansion through budding, extensive tolerance of many climates and reduced aggression toward ants of the same species. Not all pest ants share all these characteristics, but it is common for ants that display many of these “tramp” traits to become increasingly difficult pests to control. It makes sense when you think about it from a pest management perspective:

Figure 1. A large plot of Purdue campus with the suspected interconnected nest locations of a single odorous house ant colony likely containing hundreds of queens and hundreds of thousands of workers. Purple circles represent overwintering sites for the colony while red circles and lines indicate satellite nests and interconnecting trails present in the summer.
  • Pests that eat a variety of food will be able to thrive in lots of places and potentially avoid insecticidal baits placed by pest management professionals.
  • Pests that nest in a variety of material will require more time spent identifying where to focus any treatment.
  • Pest colonies that grow in number and safely split in two are more likely to survive, instead of taking a risk by sending newly mated individuals to potentially dangerous places.
  • Pests that can thrive in diverse climates will not be as impacted by seasonal changes and may have expanded ranges or habitats where they are found.
  • Pests that do not compete as aggressively among their own kind facilitate additional pest infestations of the same species that can fill in any vacated nests.

“Tramp” behaviors enable pest ants to thrive in the presence of human development, encouraging the accidental spread of these pests on a global scale into new environments as potential invasive species. Some invasive species can infiltrate both natural and urban habitats, commandeering resources and outcompeting native ants. These ants are called “drivers of change” because they drive down ant diversity and destabilize ecosystem balance. This can become problematic because a reduction of ant diversity often can lead to a surviving single ant species experiencing a perpetual positive feedback loop and growing into a giant pest problem (consider the massive Argentine ant supercolony, Linepithema humile, covering more than 500 miles of California).

Figure 2. A black walnut is a common nesting site for natural odorous house ants, likely with only one queen and less than 100 workers.

On a local scale, every ant colony you see in the urban habitat is, in a sense, invasive because these ants have spread from their natural roots into an alien environment generated by humans. However, these ant species merely benefit from native habitat destruction and simply survive to exploit the remaining resources with limited competition. These ants are called “passengers of change” because they indirectly receive a benefit caused by some other factor (typically urban development), diminishing ant diversity prior to their arrival. Ants that are “passengers” can be more easily controlled by limiting natural habitat disruption and, more importantly, focusing control efforts in the developed areas where the ants can be found. Therefore, proper pest ant control can greatly benefit from identifying if the pest ant acts as either a “driver” or “passenger” of change in the target area of management.

CONTROL TECHNIQUES.At Purdue, we discovered odorous house ants (Tapinoma sessile) adjust their behavior to act as both “drivers” and “passengers” of change throughout different phases of urban invasion (see Figure 1). Odorous house ants found in their native natural habitats, like North American forests, live in small colonies with just one queen in one nest (see Figure 2). We found these small colonies appear subordinate, living in natural spaces that house a total of 28 ant species. However, only 19 species were found in urban environments representing an approximate 32 percent reduction caused by urban development.

Figure 3. The relative decline of ant species diversity from natural habitat into urban habitat is 32%. The relative decline of ant species diversity in urban habitat when odorous house ants are present is 63%.
Courtesy of Charley Eiseman on BugTracks.com

Upon reinvading their newly urbanized habitat, odorous house ants likely benefit as “passengers of change” because of the reduced ant competition. Urban sites with odorous house ants contained only seven total ant species, resulting in a 63 percent reduction of possible surviving urban ant species (see Figure 3). These results support the hypothesis that, over time, urban odorous house ants capitalize on their “tramp” characteristics to exploit a large variety of nesting sites and food resources, reproduce quickly through budding, monopolize large territories, and thus become “drivers of change” by dominating resources and driving out other urban ant species. The initial benefit from urbanization combined with an observed reduction of urban ant diversity in the presence of odorous house ants leads us to conclude Tapinoma sessile fulfill both roles as “back-seat drivers of change.” However, we still don’t understand what triggers odorous house ants’ change of behaviors from single queen and nest colonies into multiple queen and nest supercolonies.

The remarkable transition of odorous house ants from a small, natural forest colony expanding into a large infestation that covers more than 1 acre of urban territory reveals several pest management implications. While a pest control technician may eliminate a problematic odorous house ant colony, permanent control of all odorous house ant pests is unlikely because the native ant population can re-infest any North American urban environment. Although baiting is highly effective against odorous house ants, once a colony has been eliminated through baiting, they are opportunistic nesters and a new colony may soon discover the vacated nest cavity left behind and move in. This underscores the importance of sealing up cracks and crevices whenever possible.

Educate your clients to set expectations appropriately (even with perfect pest control, odorous house ants may become an intermittent problem requiring regular management effort). Finally, if you find a property with low ant diversity, that property may be susceptible to a flourishing odorous house ant infestation.

The authors are all at Purdue University. Dr. Adam Salyer is an urban entomologist, Dr. Gary Bennett is a professor of urban pest management and Dr. Grzesiek Buczkowski a research associate professor.