Photo 1: Carpenter ant worker. Note the one segmented petiole and the evenly round thorax in profile.
Although carpenter ants can be found in homes anytime during the year, they seem particularly noticeable in the spring as the weather becomes warm. Many people think of carpenter ants as big, black ants. And it is true the most common species here is black and approaches ½ inch in length. However, you can't always go by size and color; there is another carpenter ant species that is red and black and about 3/16th inch long. A more sure method is to look for a one-segmented petiole between the thorax and the abdomen (ants either have a one or two segmented petiole. Also examine the shape of the thorax (the middle section of the body). In carpenter ants, the thorax is evenly round in profile while other Minnesota ants have unevenly shaped thoraxes.
Finding carpenter ants inside in the spring can mean that a nest is present there; the sooner they are present indoors with the onset of warm weather, they more likely a nest exists in your home. Also look for signs of coarse sawdust which is a sign of a nest. If you find a swarm of winged carpenter ants indoors, that is a sure sign of an indoor nest. Remember that not all winged ants you see are carpenter ants so be sure they are correctly identified. If you find just one or several carpenter ant queens (winged or wingless) in your home, they probably just wandered into your home accidentally and no nest is present.
There are two types of carpenter ant nests. Parent colonies nest in moisture damaged wood while satellites nests, offshoots of the main colony, can nest in sound wood as well as insulation and hollow doors. Carpenter ants have the potential to damage the wood in buildings as they excavate galleries and tunnels. Fortunately, this damage occurs relatively slowly and it usually takes years for it to become severe enough to be an issue.
If you believe that you have a carpenter ant nest in your home, it is very challenging to control it yourself. It is critical to deliver insecticide to the nest; just killing the foraging workers has no impact on the nest. Typically baits are used for ant problems. However the ant baits available to the general public are not sufficiently attractive or effective to successfully eliminate a carpenter ant nest.
The best control for carpenter ants is to contact a licensed pest management service to treat the nest. An inspection is very important to find the foraging trails and ideally the nest(s). There are several options for treating the nest. Many technicians use a non-repellant residual insecticide (e.g. Termidor), sprayed around the building's exterior. The carpenter ants pick up residue; take it back it to the nest where it gets spread through colony, ultimately eliminating it. Technicians may also set out baits to control carpenter ants. They have a variety of baits and the experience to choose the proper bait that will be most effective (it is not unusual for technicians to use more than one bait to be successful). Keep in mind that baits take time so it is important to be patient and allow carpenter ants to take back sufficient bait to control the nest. If the exact location of the nest is discovered, it can be treated directly, especially with a dust.
For additional information, including preventative steps, see the publication, Carpenter ants.