Structural Damage

Carpenter ants are a problem to humans because of their habit of nesting in houses (Fig. 1, 2). They do not eat wood, but they remove quantities of it to expand their nesting facilities. This can result in damage to buildings and, if the main structural beams are hollowed out, can result in an unsafe condition. Typical damage is shown in Fig. 3.

Most carpenter ant species establish their initial nest in decayed wood, but, once established, the ants expand their tunneling into sound wood and can do considerable damage to a structure. However, this damage occurs over 2 or more years, since the initial colony consists of a single queen. Workers are produced at a slow rate, so that a colony consisting of 200 to 300 workers is at least 2 to 4 years old.

Most problems in Washington caused by carpenter ants are due to Camponotus modoc and Camponotus vicinus. These species commonly nest in standing trees (living or dead), in stumps, or in logs on the forest floor. Since many houses are being built in forested areas, well established, vigorous colonies are readily available in the immediate vicinity to attack these dwellings. This is especially true when the homeowner insists that the home be built with a minimal removal of trees.

A number of workers from these large "parent" colonies will frequently move into a dwelling as a "satellite" colony. Communication and travel between colonies is maintained, and the satellite colony may contain larvae, pupae, and winged reproductives. Since these colonies are already established, damage to houses can occur in a shorter time and is not limited to decayed wood. Indeed, these ants may become established in houses still under construction. The size of a typical colony is probably 10 to 20,000 workers, and large colonies can have up to 100,000 workers. Not surprisingly, satellite colonies found in houses frequently contain up to several thousand workers.

The ants usually maintain a trail between the parent and satellite colonies. These trails follow natural contours and lines of least resistance and frequently cut across lawns. The trails are about 2 cm wide, and the ants keep them clear of vegetation and debris. Traffic on these trails may be noticeable during the day, but peak traffic occurs after sunset and continues throughout the night, sharply decreasing before sunrise.

The parent colony is often located in a tree, stump, or in stacked wood within 100 meters of the house. Wood and stumps buried in the yard when the house was constructed or stumps and decorative wood pieces used to enhance the beauty of a yard or driveway may also be the source of a parent colony.


Carpenter ants, genus Camponotus, belong to the sub-family Formicinae, which is characterized by a circular anal orifice surrounded by a fringe of hair. Carpenter ants are large, having queens about 16-18 m long. (Fig. 7A) and workers varying from 6-13 mm long (Fig. 7B. and C). When workers vary in size, they exhibit polymorphism (many sizes). The workers of some ants are monomorphic (one size).

For species identification of carpenter ants, you must collect the largest workers, called majors. Camponotus workers are easily recognized by the thoracic dorsum, which is evenly convex when viewed from the side (Fig. 8) Other ants that may be confused with Camponotus have a notch or depression on the thoracic dorsum (Fig. 9). Color is not a good means of identification, as Washington has several species of carpenter ants that vary in color from all black to red thorax with black gaster (the enlarged part of the abdomen) and head, to a light brown. However, the most common Camponotus infesting houses and other structures in Washington is Camponotus modoc. This species is black except for the legs, which are reddish.

Life History

All ants undergo complex metamorphosis, or change, and pass through the following stages: egg, larvae, pupa, adult. Under normal conditions, the egg to adult sequence takes about 60 days. Nests contain workers (sterile females), and single functional queen (usually), and may also contain winged queens and males (Fig. 7D), which are produced during the late summer and overwinter in the nest.

During the first warm days of spring (January-June, depending on locality) these reproductives emerge from the nest for their mating flights. After mating the males die. The inseminated queen selects a nest site, usually in a small cavity in a stump, log, under bark, or in the timbers of houses. The queen then breaks off her wings along lines of predetermined weakness, and within a few days lays her first eggs. These soon hatch into larvae, which are fed by the queen from reserves within her body. The queen does not leave the nest to forage for food during the entire time she feeds and raises this brood.

At the end of their developmental period, the larvae pupate and eventually emerge as workers. Since these first workers have been fed only on the reserves within the queen's body, they are very small and are called minors or minor workers (Fig. 7C). They usually number about 15 to 25. These workers then take over the functions of foraging for food, nest excavation, and brood rearing.

The queen's primary function from this point on is to lay eggs. The colony produces successive broods and, since the larvae are fed by foraging workers, the size of the workers increases; some may be very large and are called majors (Fig. 7B). The colony does not produce reproductives (winged males and queens) until it is from 6 to 10 years old and contains about 2,000 workers. Dorsal views of all adult forms are shown in Fig. 10.

While most carpenter ant colonies are probably initiated by a single queen, queens may also initiate colonies in close proximity of each other to create multiple queen colonies. These colonies are probably more successful and grow at a faster rate.

The natural food for these ants consists of insects and other arthropods and sweet exudates from aphids and other insects. They also are attracted to other sweet materials such as decaying fruits.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 7

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Fig. 8

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Fig. 9

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Fig. 10