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Carpenter ants

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Common name:  Carpenter ant

Scientific name:  Camponotus pennsylvanicus

Hosts:  Found in dead or rotting wood.

Wood that is soft and moist, or decaying is the carpenter ant’s preferred nesting site. The telltale sawdust expelled during the construction of these nesting galleries gives the insect its name. While often associated with wood that is dead or rotting, the carpenter ant is not the source of the deterioration, as is generally thought. In reality, the underlying cause of the wood’s decay is probably a more serious problem than the ants themselves.

The carpenter ant is a wood-nesting species, and will usually build the main or home nest outdoors in rotting stumps and trees. One or more satellite nests may also be built and these are often the source of ants found inside the home. Carpenter ants prefer the softer wood associated with standing dead trees and stumps. Overpopulation, however, will cause them to search for new nesting sites, with nests often being created in living trees that have some rotted wood present. They can gain access through cracks, wounds, knots, or any decayed or faulty place on the tree’s exterior.

Carpenter ants are also attracted to any decaying or damaged wood, including discarded construction materials, piles of firewood, or any site in a building where wood has rotted due to water damage. These sites include high-moisture areas such as bathrooms, laundry areas, and basements, as well as leaks in and around roofs, gutters, chimneys, and windows.

A nocturnal insect, the carpenter ant prefers to stay out of direct sunlight and is most often seen during its night time foraging excursions. The scouting ants will lay down a pheromone trail from the nest to the food source for the rest of the colony to follow. Carpenter ants feed mostly on dead and living insects and the honeydew emitted by aphids or other sap-sucking insects. They also feed on the juices of ripe fruits and grains of sugar or other sweets. This is usually what draws them inside the home. Carpenter ants may be in the home solely for the purpose of foraging. This activity does not necessarily mean that a nest is established in the house.

Carpenter ants are social insects, which means they live in colonies made up of a reproductive female or “queen”, sterile workers, eggs, larvae, and pupae. These insects live in the colony year round. The site of a colony can often be determined by the presence of discarded wood fragments or ‘sawdust’, deposited near the nest openings.

Reaching anywhere from 1/4 to 1/2 inch in length, the carpenter ant is one of the largest types of ants found in our area. Although the adult is usually black, some individuals can appear reddish or brown, with long yellow to grayish hairs on the abdomen. The worker ant is wingless and has strong mandibles (mouthparts) needed to excavate the nesting sites. The carpenter ant does not sting, but the larger workers can administer a sharp bite, which can become further irritated by the injection of the formic acid that they produce.

A queen starts a new nest by laying eggs which will hatch into a generation of workers. The workers care for the queen and the subsequent larvae that she produces. Most of the larvae of following generations become workers, but eventually a group of reproductive males and females are produced. These are often seen swarming in and around homes in the spring. The reproductive ants attempt to start new colonies, but luckily, many of them fail to do so.


Due to the carpenter ant’s preference for sweets, juices, and fats, proper food storage and sanitation is an important step in eliminating it as a household pest. Do not leave dirty dishes or food in exposed areas. Washing areas with a detergent solution where ants are seen foraging may reduce or eliminate the pheromone scent.

Conduct a careful examination of all structural areas for signs of water damage or wood rot. Replace soft wood and caulk all exterior openings. Do not place firewood directly on the ground and store it away from the house. If possible, cover the wood so rainwater cannot soak into it. Keep areas next to basement walls as dry as possible and provide proper air circulation or ventilation in areas subject to dampness.


Ant baits or ready-to-use insect sprays are available for nests indoors. Baits can take longer to work but are more effective in the long term. Foraging ants will take the bait back to the nest and share it with the colony. This eventually kills out the whole colony. Destruction of an indoor nesting site should be followed up with a thorough examination of the surrounding outdoor area for the home nest. If the home nest is left intact, these ants can re-populate the area.

If ants are found nesting in a living tree, there is likely to be wood rot in some part of the tree.  This can be more serious than the infestation of ants. Rather than trying to treat the ants, it is best to have a certified arborist evaluate the tree to identify any rotting wood. Companies with certified arborists on staff can be found through this website:

Contact the Plant Clinic (630-719-2424 or for current recommendations. Use pesticides safely and wisely; read and follow label directions. The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.