Figure 1. Black carpenter ant (Camponotus pennsylvanicus). Courtesy: Patrick Wagner
There have been numerous reports of carpenter ant activity across South Dakota this summer. The concern with carpenter ants is that they can become a structural pest if they nest in homes or other buildings. So far, almost all of the reported cases have been identified as the same species, Camponotus pennsylvanicus, commonly known as the black carpenter ant (Figure 1).
Carpenter ants can be distinguished from other ants based on a few characteristics. First off, most carpenter ants are quite large, ranging in size from 3/8 to 1/2 of an inch long. They also have dark-colored bodies that are either uniformly black or black and red. Carpenter ants have a uniquely shaped thorax (body segment directly behind the head) as well. Most ants have an indentation on the top of the thorax, whereas on carpenter ants, the top of the thorax is smooth and even (Figure 1). Carpenter ants also have only one node (or peak) on the pedicel (the thin segment connecting the thorax and abdomen). These features may be difficult to see without the assistance of a hand lens or microscope.
It is important to understand that, unlike termites, carpenter ants do not eat wood. However, they will burrow through and displace wood as they build their colonies. One of the main signs of carpenter ant activity is seeing piles of wood dust showing up near dead trees or wood structures (Figure 2). Carpenter ants prefer to nest in wood that has been softened by moisture. Such places include tree stumps, rotting logs, and damp wood in and around buildings. It is likely that the wet summer experienced throughout much of South Dakota this year has been favorable for carpenter ants, resulting in their increased activity.
The best management for carpenter ants is to prevent an infestation from occurring in the first place. Allow good air circulation in homes and other buildings to prevent moisture build up. If you have wood with water damage, replace it if possible. Make sure to remove rotting tree stumps, logs, and wood piles (i.e., firewood) from around buildings. Carpenter ants often migrate indoors from these areas, so it is best to keep them far away from the exterior walls. Keep in mind that seeing a few carpenter ants indoors does not mean that their nest is also indoors.
If you suspect a carpenter ant infestation, immediate treatment is recommended. One option is to use perimeter sprays which form an insecticide barrier that kills the ants on contact. This will prevent accidental invasions and also limit populations from making it indoors. However, these products are only effective for a limited amount of time and fail to eliminate the ants at the source. Setting out poison ant baits, either granular or liquid, is usually more effective. Ants not only consume the bait, but also take it back to their nest and feed it to the rest of the colony. Ant baits are relatively inexpensive and may be applied either directly or inside specially designed ant traps. There are a variety of other alternative methods such as using a mixture of boric acid and sugar as poison bait, or sprinkling diatomaceous earth near the nest to kill ants on contact. For severe infestations, you may consider hiring a professional pest control company to evaluate and treat the situation.
If you have questions or concerns regarding carpenter ants, please contact your nearest SDSU Extension entomologist for more information.
Reference: Hahn, J. 2018. Carpenter ants. University of Minnesota Extension.