Carpenter Ants

-- Big black ants that can cause structural damage --

Summary: Carpenter ants (Camponotus spp.) are large, powerful ants that can destroy wood and other building materials that they nest in. They replace termites as the most important structural pests in many temperate (cool) climates such as northeastern and northwestern US.

Carpenter ants that nest in structures can be controlled by creating an insecticide barrier around the building that ants must cross in order to forage for food and water outside. This barrier can be placed inside exterior walls and/or on the exterior foundation (perimeter). It is not necessary to locate the colony or colonies inside the structure itself. Finally, special baits are available that can be deployed when ants are active.

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Jack DeAngelis, ext. entomologist (ret.) revised: 11/2018

Identification

Carpenter ants are large black, or red and black, ants that nest above ground in cavities such as tree stumps and in the walls, subfloors, or roofs of our houses. Nest building can cause extensive, and expensive, damage to structures.

Carpenter ants are generally larger than 3/8" and all black, or red and black. Winged ants, the so-called "reproductives" may be present in late spring but for most of the year carpenter ants, like other ants, are wingless.

carpenter ants

Life History

Carpenter ant colonies are started by a queen. Mated queens generally fly during late spring and locate a suitable cavity in which to begin a colony by laying eggs. She can use almost any protected cavity such as a wall void, under floor insulation or outside in a natural cavity like a tree stump. The cavity is not always associated with wood since, unlike termites, carpenter ants do not necessarily need wood for survival. As an example, rigid foam insulation is frequently nested in and damaged by these ants.

In the eastern US ant damage is often associated with moisture because colonies tend to start where wood has been moisture-damaged but this is not the case in the western US where colonies and damage can occur in very dry situations.

Once the first brood of female worker ants is produced the queen stays within the nest. The worker ants forage for food and water which they bring back to the nest. Carpenter ants are predators and scavengers and will consume a wide variety of foods however they do not eat wood. This behavior of moving between nest and foraging grounds, combined with their large size, makes them relatively easy to detect during routine inspections. You'll see ants "trailing", moving more or less in a line, between the structure and outdoors.

As the colony grows worker ants excavate and enlarge the original cavity. It is this colony expansion that causes damage to building materials. Colonies can continue to grow over many years and may split into satellite colonies when the original colony gets too large. This means that finding one nest may indicate that there are other satellite nests hidden elsewhere in the structure.

Control

There is no need for periodic maintenance treatments, often recommended by pest control companies, nor is it necessary to locate individual carpenter ant nests. Treat only when ant activity is detected during regular inspections.

Inspect exterior of structure at least once each year during warm months, morning or evening. Look for large ants moving into or out of the structure, often in more or less single file lines.

Remove all vegetation in contact with structure, leave at least a 12" gap between plantings and foundation. This ensures that you will be able to see ants that are moving into or away from building. Ants will also use vegetation to "bridge over" the perimeter treatment (see below). Be sure to check for overhanging branches in contact with the structure, and remove as necessary.

The standard treatment to control carpenter ants is generally a two (or three) step process. First, a barrier of insecticide is placed on the building's foundation so that foraging ants must cross it in order to get to their feeding grounds outside. By crossing the barrier worker ants contaminate themselves with insecticide which is then carried back to their nest. In addition to the foundation spray, borate powders such as Tim-bor can be placed inside exterior walls (wall voids). Termidor Insecticide can be used for the foundation barrier spray.

treatment_steps
Control
house foundation, perimeter
House foundation (gray), siding (white), and foundation vent (black). Foundation "perimeter" sprays should be applied where the siding overlaps the foundation, under the lip of the siding, and completely around the house. Photo by JD DeAngelis.

In the second (or third if wall voids are treated) step, special poison baits can be placed around the foundation where foraging workers will find and carry them back to the nest and destroy the colony. Carpenter ant baits are a new addition to the control arsenal and baits can be used in areas where conventional insecticides can not, or should not, be used.

Professional perimeter sprays, baits and wall void treatments generally are not available in home and garden stores but can be found at DoMyOwn, our affiliate.

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