Carpenter ants (Camponotus spp.) are the largest ants found in Montana and among the most structurally damaging insects in the region. Unlike termites, carpenter ants do not eat wood — instead, they excavate smooth, gallery-like tunnels inside wood to create their nests. The damage may look less severe than termite damage at first glance, but over time, carpenter ant colonies can compromise the structural integrity of beams, joists, window frames, and decks.
Montana's abundant forests and the prevalence of wood-frame construction make carpenter ants a common and serious concern. They strongly prefer wood that has been softened by moisture damage — so their presence often signals an underlying moisture or water intrusion issue that needs to be addressed alongside the ant treatment.
A mature carpenter ant colony contains 3,000–10,000 workers and is typically housed in an outdoor parent nest (often in a dead tree, stump, or log), with satellite colonies inside structures. Workers forage up to 300 feet from the nest, making them a common sight in kitchens and bathrooms as they search for food and water.
Size
¼ – ½ inch (one of Montana's largest ants)
Color
Black, or black with reddish midsection
Active Season
March–October; year-round in heated structures
Risk Level
Medium–High (structural damage)
Habitat
Moisture-damaged wood; nesting in walls, sills, beams
Colony Size
3,000–10,000 workers
SIGNS OF INFESTATION
Identifying a Carpenter Ant Problem
-
Large black ants indoors — Particularly in kitchens, bathrooms, or near windows — foraging workers exploring inside the structure.
-
Frass (sawdust-like debris) — Fine, fibrous wood shavings mixed with insect parts and soil deposited outside nest openings; unlike termite pellets, carpenter ant frass is coarse and fibrous.
-
Rustling sounds in walls — A faint crinkling or rustling sound inside walls at night, caused by ant activity in galleries.
-
Winged ants (swarmers) — Winged reproductive ants emerging from walls in spring indicate a mature colony inside the structure.
-
Moisture-damaged wood — Soft, spongy, or discolored wood around window frames, sills, or the base of exterior walls.
-
Trails of workers — Ant trails observed moving along the same path repeatedly, especially at night.
HEALTH RISKS
Carpenter ants do not transmit disease. They can bite if provoked — their bites are painful due to their size and the formic acid they inject — but bites are rarely medically significant. The primary concern with carpenter ants is property damage, not human health.
PROPERTY DAMAGE
Carpenter ant damage is slow but cumulative. A colony that has been active in a wall for 5–10 years can hollow out significant sections of structural lumber. Affected areas include window sills, door frames, deck support beams, roof rafters, and floor joists — particularly in areas with any history of moisture. The cost of the structural repairs often far exceeds the cost of professional pest control treatment — making early intervention critical.
