[Cover Story] Hot On The Trail!

The inspection is the single most important component of a carpenter ant control program. Finding the nest(s) and foraging trails is essential to success.

April 7, 2003

Ants belong to the family Formicidae, a diverse and cosmopolitan group of social insects in the order Hymenoptera. About 9,000 to 10,000 living species have been described (Bolton 1994), but the actual number may exceed 20,000 (Hölldobler and Wilson 1990). In the U.S., about 35 species have adapted to and become pests in urban and agricultural environments (Thompson 1990).

Indeed, in the United States ants are considered by pest management professionals to be the top economic pest (Jenkins 2001), and by homeowners to be a more serious pest than cockroaches (Whitmore et al. 1992). In Texas, pest control companies consider the red imported fire ant to be the No. 1 structural pest, and in the Pacific Northwest, the majority of homeowner calls to professionals are for carpenter ants (Hedges 1997). The following article, excerpted from the upcoming Ninth Edition of the Handbook of Pest Control, will focus on the identification and control of carpenter ants.

IDENTIFYING CHARACTERISTICS. Carpenter ants have a one-segmented petiole in the form of a vertical scale, and a terminal acidopore with a circular orifice fringed with hairs. The workers are polymorphic and characterized by their evenly convex thoracic dorsum.

DISTRIBUTION. Various species of carpenter ants are distributed throughout the United States ranging from sea level to more than 9,000 feet. Brief profiles of the most common species, by region, are featured below.

Midwest And Eastern States. The black carpenter ant, C. pennsylvanicus (DeGeer), is the most common carpenter ant pest in this region. It is dull black with pale yellow or light pubescence. Workers are 6 to 13 mm long. Other eastern species include C. ferrugineus (Fabricius), C. caryae (Fitch), and C. herculeanus (Linnaeus).

Northern States. The New York carpenter ant, C. novaboracensis (Fitch), is found throughout the northern United States and southern Canada. Along with C. herculeanus, it is common in Minnesota and is also found in North Dakota (Wheeler and Wheeler 1963). C. pennsylvanicus is prominent from New England west to Minnesota and the Dakotas.

Southeastern States. The Florida carpenter ant, C. floridanus (Buckley), ranges from Florida north to North Carolina and west to Mississippi. This ant ranges in size from 5 to 10 mm long. It has a red head, yellow or yellowish-red thorax and petiole, and black gaster. C. tortuganus Emery resembles C. floridanus and is also a common structural pest in central and south Florida (Klotz et al. 1995). C. decipiens Emery is found along the Gulf Coast states and north to North Dakota. Its head, thorax, and petiole are yellowish-red or red, and its gaster black. Workers are 4 to 8 mm long. C. planatus Roger is a small carpenter ant that is found occasionally in buildings in southern Florida.

Western States. The most common species on the West Coast is C. modoc Wheeler. In the northwestern United States. it is the principal structural pest. It is referred to as the western black carpenter ant, and resembles C. pennsylvanicus. Workers are 6 to 13 mm long. In Washington, C. vicinus Mayr is next in importance as a structural pest (Hansen and Akre 1985). It is variable in color, with the most common variant having a red thorax, and a black head, gaster and legs. The Hawaiian carpenter ant, C. variegatus (Fr. Smith), was introduced into Hawaii from Southeast Asia and is a serious nuisance pest on all the islands. Its head and thorax are yellow, and gaster yellowish-brown. It has been collected on the coast of Washington (Hansen 1995a). C. acutirostris Wheeler exists only in the mountain canyons of Texas, New Mexico and Arizona. Nuisance species include C. laevigatus (Smith), C. hyatti Emery, and C. clarithorax Emery.

BIOLOGY AND HABITS. Carpenter ants enter buildings to nest or forage. They are called "carpenters" because they excavate their nests in wood, creating smooth tunnels and galleries. They generally excavate in wood that is decayed or damaged by other insects. Fowler (1986), however, stated that carpenter ants are more often found in wood that is structurally sound.

Carpenter ant colonies are established after the mating flights of winged male and female reproductives (Hansen and Akre 1985). The nuptial flights usually begin during the first warm days of spring. After mating, the males die. In most species, the colonies are monogynous and begin from a single queen. She often starts the nest in a small cavity in a dead or live tree where she lays her first eggs. In two to three weeks, the eggs hatch into larvae that are fed by the queen. At the end of larval development, they pupate and later emerge as minor workers, numbering 10 to 25 individuals. The minors begin foraging, excavating, and rearing brood for the colony. Mature carpenter ant colonies have many sizes of workers.

In two years, a population of workers ranging in size from small minors to large majors will be present. In three to five years, colonies of C. pennsylvanicus start producing alates (Pricer 1908). Populations of ant colonies can reach tremendous numbers. For example, more than 50,000 workers have been found in colonies of C. modoc. This is relatively small in comparison with C. vicinus, whose colonies may number more than 100,000 workers (Akre et al. 1994a). Part of the explanation for this vast difference in numbers is the presence of multiple queens in C. vicinus colonies where as many as 41 functional queens have been collected in a single colony (Akre et al. 1994a). Most species of carpenter ants are monogynous, and as a consequence, the colonies are smaller and require years to reach maturity.

Mature, or parent colonies, establish satellite colonies nearby whenever a need exists for more territory, resources, or a drier, warmer nesting site for development of their larvae and pupae. The queen, workers, and small larvae are always present in the parent colony whereas the satellite colonies contain workers, larger larvae, and pupae (Hansen and Akre 1990). Except during winter diapause, workers travel between various satellites of the colony that are connected by well-defined trails. The distance between parent and satellite nests varies, but has been measured as far as 750 feet in C. modoc (Hansen 1995b).


• Hollow porch columns
• Above porches and dormers
• Above/below bay or box windows
• Soffits, especially those with no
• Hollow curtain or shower rods
• Hollow core doors
• Upholstered furniture
• Stored cardboard boxes
• Wall voids below and above
• Wall voids above doors
• Wall voids around fireplaces
• Ceiling voids around skylights
• Hollow ceiling beams
• Under bathtubs
• Under showers
• Under hot tubs
• Under crawlspace and attic
• In false ceilings beneath

Parent colonies containing the queen, workers, winged reproductives, and larvae overwinter in a metabolic state termed diapause. In temperate regions, diapause is a period of dormancy during which the ants are in a state of "suspended animation." The encasing wood of the colony’s residence provides them with insulation from cold temperatures. In addition, larvae, workers, and reproductives have glycerol, a compound that acts as antifreeze (Cannon and Fell 1992).

In temperate regions, colonies break diapause from January to June (depending upon the latitude, elevation, and habitat), and the queen begins her first egg-laying of the season, lasting for seven to 10 days (Hansen and Akre 1985, 1990). The voracious appetites of the developing larvae trigger increased foraging activity. The most intense foraging of the season is due to the increasing food requirements of the rapidly developing larvae. A second peak of activity occurs in June when the queen again lays eggs for a period of seven to 10 days. The foraging activity in the second peak is shorter and less intense. The colony enters into diapause in September or October along with the late summer brood, which overwinter as larvae and complete development in February. Colonies are perennial and may exist for more than 20 years.

Since carpenter ants are primarily nocturnal, they rely heavily on physical cues and chemical trails for orientation to and from the nest. Well-maintained physical trails and trunk lines of carpenter ants serve as roadways through vegetation and debris. In extreme northern latitudes, carpenter ant trails will often travel underground following natural hollows, such as those left by decaying tree roots in the soil. These tunnels are usually 1½ to 3 centimeters in diameter and may be as deep as 1 meter below the surface (Hansen and Akre 1985). The distance traveled to obtain food varies. For example, the surface trails of a C. modoc colony in California were 200 m long (David and Wood 1980), and the total lengths of tunnels for a C. herculeanus colony in Ontario was 185 m (Sanders 1970).

C. pennsylvanicus has a distinct cycle of food preferences (Tripp et al. 2000). During the spring and early summer, when brood production is high, the ants have a strong preference for proteins which are fed to the developing larvae. For example, freshly diced mealworms are mobbed by workers from May through July but are less attractive when offered in August or September. Conversely, carpenter ants recruit slowly to simple sugar or honey baits in the spring, but any carbohydrate source is rapidly depleted from July through the end of colony activity at the time of approaching winter. Workers need carbohydrates for energy throughout the year, but the mass provisioning in the fall, before the onset of diapause, may contribute to overwintering survival.

In structural infestations of carpenter ants, the parent colony is generally located outside in a tree, stump, stack of firewood, or landscape logging. In a tree, nests are frequently located in hollows or dead limbs. Satellite colonies may be found in similar sites in one or more neighboring trees and in adjacent structures. Such colonies may be found in a variety of places, including attic rafters, roof overhangs, bay windows, fascia boards, floor joists, box headers, wall voids, hollow curtain and shower rods, hollow doors or columns, behind dishwashers, under or behind insulation in attics and crawlspaces, bath traps, under cabinets, and in ceiling voids next to skylights and chimneys. A parent colony found inside is typically associated with a water leak or other constant moisture source.

A house built on the outskirts of town in a woodland habitat is a prime candidate for carpenter ant infestation. The numerous trees, landscape timbers, wooden porches and fences, and bay or box windows are all potential "hot spots." Leaky pipes or roofs can cause moisture damage to soffits, clogged gutters, and decay fungi or moss on the roof, all of which attract carpenter ants. Homes with flat roofs, dormers or hollow porch columns are potential sites for ant infestation. Often, moisture damage, especially in a void space, is an open invitation for a carpenter ant infestation. Homes with multiple roof lines, if not sealed properly and adequately ventilated, often lead to moisture damage in the attic. A chimney with improperly fitted flashing often leads to moisture accumulation in the soffits, attic, or walls. Other factors which contribute to infestation include holes and cracks where utility lines enter the house, earth-to-wood contacts, tree branches in contact with the building and inadequate ventilation.

Carpenter ant control is notoriously costly and often frustrating. Indeed, professionals consider carpenter ants to be the most frequently encountered and problematic of all the ant pests (Granovsky 1990).

CONTROL TECHNIQUES. The inspection is the single most important component of a successful control program for carpenter ants. Finding the nest(s) and foraging trails is essential. Carpenter ant nests, however, are notoriously difficult to find. Night inspections are helpful in this regard, because most species of carpenter ants are nocturnal — large numbers of ants emerging after sundown and an equally dramatic number of ants disappearing into the nest at sunrise.

Sites to inspect include stacked lumber or woodpiles; wood in contact with soil; wood or other material that has been damaged by water; sites with improper drainage; vegetation in contact with a building; fences attached to a building; timbers used in landscaping; and trees or stumps. Feeding foraging ants a little dab of honey or sliced-up insects, and then following them on their homeward journey can help the professional locate nests (Akre et al. 1994b).

All evidence of carpenter ants should be documented with a written record in the form of an inspection diagram that indicates locations of ant activity and nests both indoors and outdoors. Documentation provides the professional with a record of the infestation for future reference. It is also an important tool for customer education and for protection against possible litigation.

The professional should thoroughly review the inspection diagram with the client, emphasizing the contributing factors to the infestation. The client should also be educated as to the biology and detection of carpenter ants and the signs of infestation, so that he/she has an early warning of their presence. This allows for corrective measures before the ants can cause significant damage.

Most customers contact a pest control service for carpenter ants not because of the concern for structural damage but because foraging workers have become a nuisance (Klotz et al. 1994). The emergence of swarmers is the second most common complaint.

The client needs to know what measures to take in the form of repairs and physical alterations to eliminate probable sources of infestation (Hansen 1984a). Trimming back vegetation providing possible access for ants onto a building and sealing or caulking potential entry points are two nonchemical techniques helpful in preventing infestation.

Carpenter ant control is easiest to achieve when the parent and satellite colonies can be located (Hansen 2001). Because the parent colony requires high humidity for the development of eggs and young larvae, it is most often located outdoors in dead wood (e.g., stumps, tree holes, landscape timbers). Dry, warm attics and wall voids are typical sites for satellite nests (Hansen 1984b). Figure 2 on page 45 lists a number of sites where carpenter ant satellite colonies might be found.

Residual treatments or baits are directed at nests and trails, which have been located during the inspection. Three insecticide formulations are commonly used in carpenter ant control (Hansen 2002):

Perimeter Treatments. Non-repellent insecticides are effective as perimeter treatments when applied to the exterior foundation, around door and window frames, and under the lower edge of siding (Hansen 1989). Trails should also be treated when they can be located. In locations prone to infestation (e.g., wooded lots), a perimeter treatment may be applied biannually, or at least in the spring, when carpenter ants become active. These same perimeter treatments can be used to treat around tree trunks harboring carpenter ants, as well as any nests, which may be found inside a tree. These materials are effective at low concentrations and are readily transferred to other ants, thereby reducing the total population. Perimeter treatments with non-repellent insecticides are especially effective when satellite nests are indoors and the parent nest is outdoors. The interchange of ants between the nests spreads the active ingredient to the entire colony. Such treatments, however, usually do not result in total control of an infestation. Location and treatment of the parent colony should be the goal, with perimeter treatment assisting in reducing ant activity inside the building.

Dusts. For nests found in structural voids or inside wood timbers, a dust product can be applied in very light, thin layers. Dusts become repellent when applied too heavily. Compressed air or cordless, electric dusters are helpful in treating wall voids because of their very light output and excellent coverage (Akre et al. 1995). Dusts can be injected into wall voids either through existing openings around plumbing and electrical lines or by drilling access holes. (See Figure 3 above.)

Baits. Carpenter ants forage on the ground and in trees and other vegetation for insects and honeydew. Therefore, baits should be applied outside, along foraging trails. A variety of baits should be offered to the ants to determine their preference. Baiting is most effective during the foraging season and requires monitoring to determine bait acceptance, amount consumed, and reduction in the ant population (Hansen 2000).

Baiting is often slower in achieving results than residual treatments but may be preferred by pest control clients and is important to reach those colonies that may be out of reach for treatment, such as high in trees or on a neighboring property.

In an initial treatment for carpenter ants, the professional may drill and remove switch plates to apply dust into wall voids and also may apply a perimeter treatment outside (Akre et al. 1995). If baits are used in combination with residual treatments, the baits should be applied first, permitting ants several days to feed on them so the active ingredient can be distributed through the colony.

The author is an associate research and extension entomologist at the University of California, Riverside. He can be reached at jklotz@pctonline.com.

AKRE, R.D., L.D. HANSEN, and E.A. MYHRE — 1994a. Colony Size and polygyny in carpenter ants (Hymenoptera: Formicidae). J. Kan. Entomol. Soc. 67:1-9. 1994b. Do you know where your parents are? PCT, May. Pp. 44, 46, 55, 58, 60, 64. 1995. Home wreckers! PCT, January. pp. 54-60,77.
BOLTON, B. — 1994. Identification Guide to the Ant Genera of the World. Harvard University Press, Cambridge, MA., 222 p.
CANNON, C.A. and R.D. FELL — 1992. Cold hardiness of the overwintering black carpenter ant. Physiol. Entomol. 17:121-126.
DAVID, C.T., and D.L. Wood — 1980. Orientation to trails by a carpenter ant, Camponouts modoc (Hymenoptera: Formicidae), in a giant sequoia forest. Can. Ent. 112:993-1000.
FOWLER, H.G. — 1985. Alloethism in the carpenter ant, Camponotus pennsylvanicus (Hymenoptera: Formicidae). Entomol. Gener. 11:69-76. 1986. Biology, economics and control of carpenter ants, pp. 272-289, In: S.B. Vinson (ed.), Economic Impact and Control of Social Insects. Praeger Publishers, New York. 422 pp.
GRANOVSKY, T.A. — 1990. Chapter 12: Ants. In: Mallis Handbook of Pest Control, 7th Ed., Cleveland: Franzak and Foster. pp. 414-479.
HANSEN, L.D. — 1984a. Carpenter ant infestations in Washington. Proc., Washington State Ent. Soc. 46:715-716. 1984b. A pco’s guide to carpenter ant control. Pest Control Technology, April. Pp. 56-58. 1989. Control approach for carpenter ants. Ent. Newsletter for county agents. Coop. Ext. Washington State University. January (1):10. 1995a. Distribution and categorization of Camponotus spp. north of Mexico as nuisance or structurally damaging pests. In: Proceedings of the Fifth International Pest Ant Symposia and the 1995 Annual Imported Fire Ant Conference, S. Bradleigh Vinson and Bastian M. Drees, eds. 1995b. Notes from seminar for Oregon Pest Control Association. 3/25/95. 2000. Successful bait development is more than a matter of taste. Pest Control, May. pp. 52,54,58. 2002. Carpenter ant update. Pest Control, April. pp. 56,58,60,62,80.
HANSEN, L.D. and AKRE, R.D. — 1985. Biology of carpenter ants in Washington State (Hymenoptera: For-micidae: Camponotus). Melanderia 43:1-62. 1990. Biology of carpenter ants, pp. 274-280. In: Vander Meer, R.K., K. Jaffe, and A. Cedeno [eds.], Applied Myrmecology: A World Perspective. Westview Press, Boulder, Colo. 741 p.
HEDGES, S.A. — 1997. Chapter 12: Ants, pp. 503-589. In: Handbook of Pest Control. 8th Ed. Mallis Handbook and Technical Training Company.
HÖLLDOBLER, B. and E.O. WILSON. — 1990. The Ants. Harvard University Press. Cambridge, Mass. 732 pp
JENKINS, M. — 2001. Battling a common enemy. Pest Control 69(9):S10-S12,S16,S27.
KLOTZ, J., G. GOVEIA, L. DAVIS and B. REID — 1994. Surgical strikes. PCT, May. pp. 32-33,36,38,42.
KLOTZ, J.H., J.R. MANGOLD, K.M. VAIL, L.R. DAVIS JR., and R.S. PATTERSON — 1995. A survey of the urban ant pests (Hymenoptera: Formicidae) of peninsular Florida. Florida Entomol. 78 (1):109-118.
PRICER, J.L. — 1908. The life history of the carpenter ant. Biol. Bull. 14: 177-218.
SANDERS, C.J. — 1970. Distribution of carpenter ant colonies in the spruce-fir forests of northwestern Ontario. Ecology 51:865-873.
THOMPSON, C.R. —1990. Ants that have pest status in the United States. In: Vander Meer, R.K., Jaffe, K. and Cedeno, A., eds. Applied Myrmecology: A World Perspective. Boulder, Colo.: Westview Press, pp. 51-67.
TRIPP, J.M., D.R. SUITER, G.W. BENNETT, J.H. KLOTZ, and B.L. REID — 2000. Evaluation and control measures for black carpenter ant (Hymenoptera: Formicidae). J. Econ. Entomol. 93:1493-1497.
WHEELER, G.C. and J. N. WHEELER — 1963. The Ants of North Dakota. The University of North Dakota Press, Grand Forks, N.D. 326 pp.
WHITMORE, R.W., J.E. KELLY, and P.L. READING — 1992. National home and garden pesticide use survey, final report, volume 1: executive summary, results, and recommendations. U.S. Environmental Protection Agency.