Carpenter Ants in the House

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.

Carpenter ants do not eat wood but instead construct their nests in wood such as hollow trees, stumps, logs, posts, landscaping timbers, and the lumber used in homes and other structures. Nests are made by chewing an interconnecting series of tunnels and cavities. Wood is removed as coarse sawdust-like material that is pushed from the nest. The sawdust may include other debris such as dead ants and parts of insects and other food.

Carpenter ants found indoors in the winter usually come from nests somewhere within the house. Ants found indoors during spring or summer 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 difficult if only a few ants are seen at one time. Our best suggestion is to spend time observing 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 give an indication of the source. Presence of sawdust is an important clue in locating ant nests.

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 most circumstances, the best carpenter ant control comes by locating and treating the nest (indoors or out). 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 gradual elimination of the foragers. Homeowners should use ready-to-use products such as ant and roach killer or boric acid dust.

Most ant baits (e.g., Terro ) are consistently not effective in eliminating carpenter ants. However, specially formulated carpenter ant baits that were recently introduced to the pest control industry have been very effective. I recommend consideration of professional treatment using these new baits when other techniques have been unsuccessful. Look for retail carpenter ant bait products such as Combat Bait Granules where home pesticides are sold. Read and follow label directions.

This article originally appeared in the May 10, 2002 issue, p. 60.


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 May 10, 2002. The information contained within may not be the most current and accurate depending on when it is accessed.