Carpenter ants, sometimes referred to as “termites of the northwest,” can be important structural pests which typically nest in moisture-damaged wood. In our region, swarms of flying reproductives (males and females) often leave the nest during January. After mating, the surviving queens will shed their wings and search for a new nest site.

Once a thriving colony is established, it has 10,000 to 50,000 or more individuals. The main nest is in dead wood, perhaps a tree, stump, or landscape timbers.

But that’s only part of the story.

The pupae and some workers are in a secondary nest, one that’s in a relatively warm, dry structure such as a house, garage, or shed. When a colony is about 6 to 10 years old, it produces its first winged swarmers (reproductive males and females). The females, at 16- to 18-mm (about 5/8-inch), are larger than the males, the latter a scant 1/2-inch long.

Key Facts…

• Secondary nests in structures begin in moisture-damaged wood.

• Buildings near a wooded area are more liable to attack than others.

• Carpenter ants don’t eat wood; instead, they excavate wood for housing.

• Carpenter ants feed on honeydew and captured insects.

• If winged ants emerge indoors from underneath the baseboard, the nest is probably in the wall; if via a ceiling light fixture, in the space above, in either the attic or ceiling void.

Locating the Main Nest

• Look for piles of fresh sawdust in the attic and crawl space.

• To determine where the ants are entering the structure, look for 2-way trails outdoors. The best time to look for trailing ants is from 10 PM to 2 AM during April through October. Inspect along the foundation and other architectural lines, in the crawl space, and where utilities (pipes and wiring) enter the structure.

• If a trail is found follow it to the main nest. In order to limit structural re-infestations, the main nest must be treated by a pest control company.

Prevention

• Create an airy clear zone by trimming, or removing, any plant material within 12 to 18 inches of a structure.

• Maintain a minimum gap of 8 inches between mulch and siding.

• Ensure roofing is intact.

• Inspect the perimeter of the structure periodically, to check for a 2-way trail which warns of an infestation.

Management Practices

• Correct any moisture problem(s) and replace damaged wood

• Hire a pest control company to treat the structure and, whenever possible, the main nest.

  NOTE: Over-the-counter ant baits available in the northwest are ineffective against carpenter ants.