The pavement ant is an invasive species thought to have come over from Europe during the colonial era, and despite its slow spread in North America, it is now one of the most abundant species on the continent. They can be found in large numbers on sidewalks during the spring and summer, engaged in their infamous territorial battles. This incessant fighting between different colonies of pavement ants is thought to play a role in hindering its spread throughout the continent.
The pavement ant in North America was until recently known as Tetramorium caespitum (Linnaeus), but now has a new scientific name, Tetramorium immigrans (Santschi). Pavement ants are usually easily identifiable. They are dark brown to black ants and are 1/16 to 1/8 inch in length. They have a two-segmented pedicel (waistline area between thorax and abdomen), as do other ants from the Myrmicinae subfamily, such as thief ants (Solenopsis molesta), little black ants (Monomorium minimum) and acrobat ants (Crematogaster spp.). What distinguishes pavement ants from other Myrmicinae ants is the presence of a raised carinate ridge that surrounds the antennal socket. The distinguishing feature of North American (and Japanese) pavement ants is the presence of longitudinal, parallel ridges on the head that are not intersected by shorter, latitudinal ridges. (See illustrations.)
In the 1980s, the Japanese pavement ant, Tetramorium tsushimae (Emery) was introduced to St. Louis, Mo., from Japan. It is so similar in appearance to T. immigrans that correct identification even under high magnification requires the measurement and statistical analyses of several body parts on a sample of workers. Alex Wild describes an easy trick to distinguish the two species in a blogpost, stating that the younger workers of T. tsushimae have a lighter colored thorax than T. immigrans, resulting in much greater color variation among the workers in the Japanese species. In addition to Missouri, it can now be found in Illinois, New York and Tennessee. Studies on the management of this particular species has not been extensive, but it is assumed that their biology is similar enough for similar techniques to work.
BATTLE ROYALE. Researchers at Purdue University have been conducting studies of pavement ants and investigating how management strategies might be tailored to exploit their aggressive territoriality and dominance over other ant species in urban areas.
Among the ant species, pavement ants dominate the urban landscape. A survey of ant activity was conducted in 2016 over the summer in West Lafayette, Ind., at two residential areas. It showed that pavement ants constituted 91 percent and 69 percent, respectively, of visible ant activity at those locations. This affirms that pavement ants have a major presence when it comes to residential areas, as they are well adapted to nesting in spaces between soil, cement and paving stones.
The researchers have been mapping the pavement ant territories at those two sites, and this is revealing that many colonies can exist in a small area. Because colony foraging territories are distinct, treatment efforts with insecticide bait need to be more extensive. This is because insecticide bait accessed by workers of a certain colony will largely only spread the toxicant to members of their own colony. A blanket approach to baiting is recommended so that all colonies will have their own access to the bait.
Pavement ant activity at the two sites has been found to correlate with temperature. As expected, numbers of ants were shown to increase with the rise in temperature, as well as decrease with a decline in temperature. Treatment outdoors should thus be focused during the warmer months of the year, and possibly in the spring or early summer before the populations get a chance to grow very large. Treatment during the cooler months can then be done indoors if the ants are found there.
When territorial battles do occur, the pavement ants have been seen in very large numbers around the bait, but seem to spend a lot of their time fighting instead of feeding on it. Therefore, baiting should be focused at nest entrances or away from where territorial battles take place wherever they are observed.
After an initial blanket treatment, the researchers also found that subsequent target baiting of individual colonies also was effective. There were colonies that did not get access to the baits due to being excluded by colonies that own the territories where the initial baits were placed. Bait can then be placed where those ants and their nest entrances are visible.
TREATMENT TIPS. What are pest management professionals to do when encountering pavement ants? Here’s what we suggest:
- Treatment outdoors should be focused during the warmer months of the year (e.g., spring or early summer). Treatment during the cooler months can then be done indoors.
- Blanket baiting with insecticide bait should be employed initially and should be placed at all locations where nest entrances or pavement ant trails occur and the surrounding areas. Where territorial battles occur, baits should be focused closer to where the ant trails of the different colonies are coming from. If there are still surviving colonies, they can be targeted for subsequent treatment where trails and nest entrances are visible.
Darren Chin is an entomology research technician and Gary Bennett is a professor in urban entomology, both at Purdue University.