Jeffrey Hahn, University of Minnesota Asst. Extension Entomologist
Although it is cold outside, that does not stop some ants from being active in buildings. A common indoor winter ant is pavement ants. A pavement ant is 1/8 inch long and is reddish brown (it can actually range from light brown to dark brown). If you examine a pavement ant closely, you will see a two-segmented petiole, the 'waist' connecting the abdomen with the thorax. With magnification, you can also see a series of fine grooves etched into their heads as well as a pair of small spines on the back of their thorax.
Pavement ants typically nest in the soil, usually under objects, such as stones, bricks, sidewalks, and driveways. When they are found during winter, they are nesting in the soil under the concrete slab. When the nest is kept warm from the building's heat, the ants stay active, move through cracks in the concrete and actively forage for food and water. Ironically, many people that see pavement ants during winter do not see them in the summer when the ants are more likely to forage outdoors. Pavement ants prefer to feed on greasy food, including meats, dry pet food, and peanut butter.
If you are having a problem with pavement ants, first see if you can determine where they are coming from. If you find them moving through a crack, e.g. in an expansion joint, you can try to seal it to help keep pavement ants out of your home's interior.
If this isn't possible, the best long-term control is baiting them. Select a bait that is effective for grease feeding ants and place it where you are commonly finding the ants. Don't be surprised if there is an increase in the number of workers that are active around the bait. They will recruit additional foragers to take advantage of the newly discovered food source.
You might be tempted to spray the ants with a household insecticide but the number of foraging workers represents just a small percentage of the total number of ants in the nest. You can't destroy a nest through attrition by killing the workers you see. You can get some relief from their presence but it will only be temporary and the ants will return. Insecticides will also interfere with the ability of the workers to take bait back to the nest. The less bait that is brought into the nest, the less effective it will be in eliminating it. You can also consider hiring a professional pest control service to treat your ants.
If you ignore pavement ants, they will probably start foraging outdoors as warmer weather arrives, and 'disappear' from inside a building.
Keep in mind that not all ants seen during winter are pavement ants. You may also see carpenter ants, Pharaoh ants, yellow ants, and thief ants. Their habits differ as well as their treatment. If you have any doubt as to what kind of ant problem you have, get them identified them by an expert.