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Carpenter Ants

- Carpenter ants are potentially the most damaging structural insect pest in temperate climates -

Summary: Carpenter ants 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.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Carpenter ant identification

Most ants are small and nest underground, only rarely becoming anything more than a nuisance. Carpenter ants (see photo right), on the other hand, are large ants that nest above ground in cavities such as tree stumps and in the walls, subfloors or roofs of our houses where they can cause extensive and expensive damage.

winged & wingless carpenter ants
Winged (left) and wingless (right) carpenter ants

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

Carpenter ant colonies

Carpenter ant colonies are started by a queen (above, left). Mated queens generally fly during late spring and locate a suitable cavity in which to begin laying eggs. She can use almost any protected cavity such as inside a wall, under floor insulation or outside in 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 opportunistic predators and scavengers taking a wide variety of foods. This behavior of moving between nest and foraging grounds, combined with their large size, makes these ants relatively easy to detect during routine inspections. You'll see ants "trailing", moving more or less in line, between the structure and outdoors.

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

DIY carpenter ant control

The standard treatment to control carpenter ants is generally a two step process. First, a barrier of insecticide is placed around the building so that foraging ants must cross it in order to get to their feeding grounds on the outside. By crossing the barrier worker ants contaminate themselves with insecticide which is then inadvertently carried back to the nest. This insecticide barrier is sometimes placed inside the exterior walls (wall voids) of a structure and/or on the foundation wall below the siding (perimeter spray). New insecticides like Termidor Insecticide (see Using Termidor For Carpenter Ants) can be used for this perimeter barrier spray.

In the second 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. See Options For Treating Carpenter Ants for additional details about DIY control of carpenter ants.

Where to purchase perimeter sprays and baits: Professional perimeter sprays, baits and wall void treatments generally are not available in home and garden type stores but can be found here (DoMyOwnPestControl).

Related Articles

Carpenter Ants of the United States and Canada

Common Questions About Carpenter Ants

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Jack DeAngelis, PhD,  , email:  [email protected]

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