Carpenter Ant

Encyclopedia Article

 Carpenter ant
 Carpenter ant

In The Home

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.  https://hortnews.extension.iastate.edu/ants-iowa-common 

Damage caused by carpenter ants 

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, especially moist, decaying wood.  Carpenter ants will also excavate soft materials such as foam insulation boards.  Nests are made by chewing an interconnecting series of tunnels and cavities. Wood is removed as coarse sawdust 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 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 difficult if you see only a few ants 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 carpenter ant nests.

Sawdust from carpenter ants in a tree
Sawdust from carpenter ants in a tree

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).  Replace damaged or decayed wood and thoroughly seal the structure to eliminate future moisture problems.  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.  Outdoor perimeter treatments with residual insecticide may also be useful.

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.  Outdoor granular baits may be effective when applied according to label directions.

In Trees

Description of carpenter ants

Carpenter ant nests are very common inside trees, especially older trees that are hollow or have a significant amount of dead limbs and branches. The nests are usually in rotted, decayed wood, although some nests may extend into sound heartwood in the center of the tree.

Artist rendering of a carpenter ant
Artist rendering of a carpenter ant

Damage caused by carpenter ants

Carpenter ants in trees are not directly harmful to the tree. Control is not essential for the tree's health, as the ants are only taking advantage of an existing situation of soft, weak wood in which to establish their colony. Stress, mechanical injury, environmental conditions, disease or other insects are responsible for killing limbs or sections of the trees in which the ants are able to nest. Once injury has occurred, wood decay can set in if moisture is present; it is the wood decay that gives the carpenter ants the opportunity to colonize the tree. Carpenter ants use knots, cracks, holes and old insect tunnels to gain access to these areas.

Management of carpenter ants

Control of carpenter ants inside trees is difficult but can be done as a way to reduce invasion of the ants into adjacent structures. It is also possible for ant colonies located inside trees to form satellite colonies inside a nearby home wall. Available controls are not likely to permanently rid a tree of carpenter ants so retreatment every year or so may be necessary. Dust insecticides containing pyrethroids or carbaryl labeled for use on trees in the landscape are suggested for control. Apply the dust directly into the nest cavity. For more information please read this article.

Plugging or sealing tree cavities or treating tree wounds with wound dressings is not advised. Such treatments are unnecessary and will not eliminate nor prevent decay or carpenter ant activity. Also, cutting down otherwise viable trees that happen to be infested with carpenter ants is generally not necessary.

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.  

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