Carpenter Ants In Homes

 Carpenter ant
 Carpenter ant worker

Description of carpenter ants

Carpenter ants are very abundant in Iowa and are common pests in homes and other buildings.  The most common carpenter ants are the familiar “large, black ants” that are one-half inch or more in length and shiny black.  Other kinds of carpenter ants, however, are as small as one-quarter inch and are reddish-brown or two-toned.  One consistent characteristic of all carpenter ants is the smoothly rounded outline of the thorax when viewed from the side.  See the ISU website for an identification guide to common ants of Iowa. 

Carpenter ant life cycle and habits

Ants have a complete life cycle of 4 stages, Egg, Larva, Pupa, and Adult.  Also, carpenter ants are social insects that live in large colonies with overlapping generations and castes that perform different tasks. Eggs are laid by a reproductive female (queen) and hatch into white, legless, grublike larvae. The larvae live for about three weeks before transforming to pupae and then to adults.  Workers are nonreproductive female adults and are the most numerous members of the colony.  Workers build the nest, forage for food, and care for the queen and larvae.  Mature colonies may have as many as 2,000 to 3,000 workers.

Carpenter ant female swarmer with wings
Winged female carpenter ant swarmer.

Carpenter ants are scavengers. Although they are most active between dusk and dawn, ants can be seen during the day. Inside a house, carpenter ants feed on all food scraps, especially syrups, honey, jelly, sugar, meat, grease, and fat. Outside, they feed on honeydew, plant exudates, live or dead insects, and animal carcasses. Food is carried back to the nest by workers where it is shared with larvae, nonforaging workers, and the queen.

Swarming.  Ants with wings, called swarmers, may appear at times in the home.  Swarmers are sexually developed males and females that leave a healthy, well-established colony on the slim chances of starting a new colony somewhere else.  Most swarmers will die of starvation, dehydration, or be eaten by birds, dragonflies, or other predators.  Though almost all will fail, just enough succeed to spread the species and ensure its survival.  All species of ants are capable of producing swarmers.  Carpenter ant swarmers are recognized by their large size (body length of one-half inch or more).

Damage caused by carpenter ants 

Carpenter ants do not eat wood but instead construct their nests in wood by chewing an interconnecting series of tunnels and cavities. Wood is removed as coarse sawdust that is discarded outside the nest. The sawdust may include other debris such as dead ants and parts of insects, and other food. Carpenter ant nests are common inside stumps, logs, hollow trees, dead limbs, landscaping timbers, porch columns, windowsills, door and skylight frames, building framing, roofs, deck, and fence posts.  Carpenter ants prefer naturally soft wood or wood that has been softened by moisture and decay (wood rot).  The decay caused by excess moisture in the wood is often more damaging to the building structure than carpenter ants.  Moisture and decay facilitate initial tunneling by the ants but are not required for nesting.  Nests may extend into dry, sound lumber and foam insulation boards. Carpenter ants may nest in existing cavities such as hollow doors or spaces around windows and doors.

Unlike the nests of termites and wood-boring beetles, ant galleries are free of soil, and powdery wood and are mostly free of sawdust. The walls of carpenter ant nests are usually smooth to the point of appearing to have been sandpapered.  Galleries do not follow the grain of the wood, unlike termite galleries.

Carpenter ants found indoors in the winter usually come from nests somewhere within the house.  Ants found indoors during spring or summer or on very warm days in winter could be invaders wandering in from outdoors, or they may be foragers from a nest in the wall or ceiling. While there is no easy way to determine the source, it does pay to check carefully before making any treatment.

Locating the source of carpenter ants is as important as it is difficult.  It is especially challenging if you see only a few ants at one time.  Our best suggestion is to observe ants to see if you can detect a pattern of movement.  In spring and summer, carpenter ants are more active at night, and observations after sunset, with a flashlight on the outside and inside of the house may indicate the source.  The presence of sawdust is an important clue in locating carpenter ant nests.

Management of carpenter ants

Carpenter ant control can be a do-it-yourself project or a job for a professional pest control operator. Shop around and compare prices and services when selecting a pest control service.

Under ideal circumstances, the best carpenter ant control comes by locating and treating the nest (indoors or out).  A small amount of insecticide sprayed or dusted directly into the nest will provide effective control. Effective treatment may require lifting molding strips or framing or drilling small holes through walls or siding to reach the nest.  Nests discovered during remodeling can be sprayed to halt ant dispersal, or the ants can be collected in a vacuum and discarded.  Drilling holes to inject insecticide into all wall voids may be of limited benefit, especially if the walls contain insulation.  Replace damaged or decayed wood and thoroughly seal the structure to eliminate future moisture problems.

If a nest is not found, the practical alternative indoors is to treat room edges, cracks, and crevices in the areas where foraging workers are abundant and hope to reduce the population through the gradual elimination of the foragers.  Outdoor perimeter treatments with residual insecticide spray or granule may also be useful when applied to cracks, gaps and potential entry points according to label directions.   Monthly spray treatments are not recommended for carpenter ant control.

Ready-to-use household insecticide products are readily available for indoor and outdoor use. Household sprays are usually identified as “ant and roach killer” or "home defense insecticide" and come in aerosol or pump dispensers.  Dust insecticides containing boric acid, diatomaceous earth, and silica aerogel also can be used.  Do not use lawn and garden insecticide products indoors.  OTC ant bait products that contain a sweet food may not eliminate carpenter ants.  However, specially formulated carpenter ant baits available to professional pest managers and have been very effective.  Place bait indoors where activity has been observed or is suspected.  Monitor bait and place more if it is consumed or remove it if there is no activity. 


Do you live in Iowa and have an insect you would like identified?

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them.  Please see our website for current forms, fees, and instructions on preserving and mailing insects.   

Contact information for each states diagnostic laboratory for U.S. residents.  If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.  

Last reviewed:
June 2021

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 12, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.