Carpenter Ants

Carpenter ants are one of the most common household pests in the Midwest. Besides being a nuisance indoors, these ants damage wood by hollowing it out for nesting space. While carpenter ants do not eat wood, large colonies can be destructive; however, are more of a nuisance than a threat to structural integrity.

Carpenter ants are the largest pest ants in the United State. The black carpenter ant (Camponotus pennsylvanicus) is common in the Midwest. The typical adult, known as a “worker” ant, is black, wingless and varies from ¼- to ½-inch in length. The size and color of carpenter ants vary considerably between species and even between ants from the same colony, so these features cannot be relied upon for identification. Instead, look first for the carpenter ant’s smoothly rounded thorax (viewed from the side) and single node (the small triangular connection between the abdomen and thorax; some ants have two nodes).

Carpenter ants are fast moving and stop only to feed or share food with other ants. They are most active at night. Workers emerge from the nest about 15 minutes after sundown. Like other ants, they follow chemical trails in search of food -- sometimes hundreds of feet from the nest – and often create permanent, well-beaten trails like cow paths through the grass. A colony may use the same path from year to year.

A mature carpenter ant colony may contain as many as l0,000 individuals. Typically, only 10 percent to 15 percent of the workers are outside the nest searching for food including insects and a variety of human foods such as meats and sweets. Other workers engage in nest construction and repair, colony defense, and feeding and caring for the larvae, pupae and queen. The workers’ variation in size enables them to specialize for different tasks.

Each year, carpenter ants become active in the spring (March-April) and remain so through early fall (September-October). A mature carpenter ant colony usually releases reproductive individuals in springtime. The reproductives have wings and, like winged termites, are commonly known as “swarmers.” The swarmers’ purpose is to mate and, in the case of females, to fly to a new location, lay eggs and establish a new colony. In winter, most carpenter ant colonies become dormant, although indoor nests may show some continued activity.

Carpenter ant nests consist of smooth, clean tunnels and excavations in wood that run with or against the grain. In contrast, subterranean termite tunnels are lined with a mud-like material and always run in the same direction as the wood’s grain.

A carpenter ant colony is often composed of a series of nests. The main nest, or parent nest, is usually located outdoors, often in woodpiles, logs, stumps, or trees – sometimes several feet above the ground. The nest contains the queen, some workers, larvae and pupae. It may be joined by sub-nests, or satellite nests, containing workers, and older larvae and pupae. The colony’s reproduction takes place in the parent nest where the queen lays eggs. Larvae hatch from the eggs, are cared for and later may be transported to satellite nests. There, the larvae will undergo pupation and complete their metamorphosis to become adult workers.

It is the satellite nest that is most often encountered in structures. A satellite nest is often established in an area where wood has become moist. Common sites include wood around leaking chimney flashing, attics, skylights, bathtubs, windowsills, doorframes, porch supports, columns, soffits, wood siding and shingles, and flat roofs. Carpenter ants also will nest in fiberglass and foam insulation.

Destruction of the parent nest is perhaps the best way to control carpenter ants. If the parent nest can be located, it can then be treated with dust or liquid residual insecticides. This eliminates the queen, preventing further reproduction without which the colony cannot long endure. Finding the parent nest is thus the critical step in ridding a structure of carpenter ants. Eliminating indoor satellite nests may afford a degree of control, but the ants from the parent nest outside the structure may reinfest later.

Locating parent nests outdoors can require some sleuthing. Remember, carpenter ants are mostly nocturnal, so you may need to inspect after dark with a flashlight. Try to locate where the ants are trailing, and follow any ant that is carrying a bit of food back to the nest. You also can set out sugar water, honey or freshly killed insects along the ants’ path, and track them back. Remember, the nest may be up to 100 yards away.

Inspecting indoors means looking for satellite nests. Possible indoor nest sites are not limited to those listed above. Again, watch ants that are trailing through the house. If they disappear into a hole in the wall, remember the nest could still be several feet away. Also, look for piles of sawdust. The sawdust accumulates where the ants are tunneling. Refuse, in the form of bits of non-digestible food such as insect legs and wings, is also periodically kicked out of the nest.

Sound also can be used to detect the presence of carpenter ants. By clicking their mandibles, disturbed ants in an active colony produce a rustling sound, like crinkling cellophane,. Tapping on suspected wood members sometimes excites the ants enough for this sound to be heard.

As indicated, locating the nest(s) is the key to carpenter ant control. Applying pesticides around an infested structure’s foundation, or into the open space in an attic, usually provides little or no control and unnecessarily exposes non-targeted organisms (including people) to pesticides. For reliable control, nests must be treated directly. This may require drilling into voids in walls, doors, window frames, etc., to allow dust or liquid insecticide injection into the nest. Bait formulations also are available for carpenter ant control, though they may be a slower and less effective method compared to finding and treating nests. For more information about using baits for ant control, see RESOURCES.

Methods of sanitation and exclusion can be employed to help prevent carpenter ant home invasions or to supplement control of existing infestations. Since carpenter ants require wood and moisture, effort should be made to reduce the availability of these. Stumps, logs and woodpiles should be eliminated or moved as far away from structures as is practical. Openings into the structure -- that is, cracks in the foundation, gaps around doors and windows, and spaces where utility lines penetrate the structure -- should be sealed. Also, correct moisture problems that occur from grades sloping toward the foundation, clogged gutters, leaky plumbing and faulty seals around chimneys, skylights, doors and windows. Overhanging tree branches should be cut back to prevent them from contacting the structure and providing an easy access route for ants.

NOTE: When pesticides are used, it is the applicator’s legal responsibility to read and follow directions on the product label. Not following label directions, even if they conflict with information provided herein, is a violation of federal law.

This document was published with the assistance of the University of Illinois Extension. 

For more information, contact the Illinois Department of Public Health, Division of Environmental Health, 525 W. Jefferson St., Springfield, IL 62761, 217-782-5830, TTY (hearing-impaired use only) 800-547-0466.

• Ohio State University: Ohioline Yard and Garden
• University of Kentucky: Entomology Facts