Quick facts…
- Large black or brown carpenter ants can cause serious damage if permitted to remain in the structural wood of a building.
- Winged carpenter ants often are mistaken for termites, a more serious problem.
- Ants inside a home year-round indicate an infestation; seasonal presence is not as serious.
- Locating all nests is the most important and difficult step in controlling carpenter ants.
- Preventive measures will guard against carpenter ant infestations.
Introduction
Carpenter ants (Camponotus species) can be structural or nuisance pests and are particularly common in wooded areas and urban sites with ample tree canopy. They do not consume wood, but create extensive nest galleries in dead and down wood of both natural and human origin. Nests in structures are often satellites of larger outdoor colonies. Carpenter ants prefer to nest in decayed, often water-damaged wood. Occasionally, they move into the sound structural wood of a building. If they are permitted to remain in structures, serious damage can result. Although carpenter ants can cause significant damage, problems develop slowly and there is plenty of time to plan safe and effective ways to control the infestation.
Carpenter ants are large black or brown insects in wingless and winged forms (Figures 1 and 2). Winged carpenter ants often are mistaken for termites (Figure 3), a more serious problem.
Figure 1
Figure 2
Figure 3
Termites are easily distinguished from ants by straight rather than elbowed antennae, by forewings and hindwings of equal size, and, most notably, by the lack of the “wasp waist” just behind the legs that all ants have. Wingless carpenter ants are large, usually black, ants. They can be distinguished from other ants by a single segment forming the wasp waist at the front of the abdomen and by the smooth, arched profile of the back above the legs.
Large numbers of winged carpenter ants usually emerge from nests during spring or early summer. Winged ants disperse on mating flights and mated females subsequently search for new nest sites to establish a new colony. If originating inside a structure, winged ants may accumulate at windows and doors as they try to find new nesting sites. Winged forms emerging inside a home indicate a mature colony that has considerable damage potential.
Another sign of structural infestation is the year-round presence of wingless forms searching for food. They are attracted to all sorts of animal and vegetable matter.
If their presence indoors is seasonal, the nest is likely outdoors in a tree, stump, landscape timbers, or firewood, and the ants have entered the house in search of food. In such cases, there will be an obvious path which can be followed back to the outdoor nest. Carpenter ants also follow established paths between satellite nests and the main nest. Treat the paths and nests as indicated below. Remove infested wood and replace it if necessary.
Additional warning signs of carpenter ant activity are small piles of sawdust in isolated areas, such as crawl spaces or closets, and faint rustling noises in walls.
Control of Infestations
The most important and difficult step in controlling carpenter ants is locating all of the nests. Because many species of carpenter ants are nocturnal, conducting inspections at night may help to identify sites of infestation. Check areas of the home that have had excess moisture problems, because carpenter ants frequently get their start in moist, decayed wood. Moisture problems associated with leaky roofs, clogged gutters, badly fitted chimney flashing, etc., are an open invitation to carpenter ants. Closely examine wood in contact with soil, including decks and porch columns. Seal and caulk potential entry points. Check house areas with a history of ant activity. Tap suspect wood and listen for rustling noises or movement.
Hollow-sounding wood also may indicate a problem. A sharp knife will easily penetrate the surface of infested wood. Once located, carpenter ant nests are distinguished from other insect damage by the presence of wide, irregular galleries that are smooth-surfaced and free of debris. Do not end the search with the discovery of a single nest — check all potentially affected areas. Nests in houses may be satellites of larger outdoor nests that also need to be destroyed to avoid reinfestation. Extensively damaged wood may need to be replaced.
Nests and ant trails can be treated with insecticides; dusts, sprays, and baits of several active ingredients are available to control carpenter ants. Dust formulations are the most effective for carpenter ant control, as foraging workers become covered with the dust and carry it into the nest. Non-toxic dusts that kill insects by causing them to dry out also can be effective in certain situations. Boric acid powder (available in several commercial insecticide formulations) is another low toxicity product that can be used in areas inaccessible to children and pets.
Sprayable formulations are effective under some conditions, particularly as perimeter treatments around structures to reduce indoor ant foraging activity.
Poison baits can also control carpenter ants, though baits take longer to eliminate infestations than other formulations since they rely on ants exchanging food at the nest (trophyllaxis) to be effective. Offer a variety of baits to determine ant preference, and monitor the bait stations for bait acceptance and consumption. Ant preferences can change over the season; a bait that works well in spring may not work well in fall.
With any insecticide, read and follow all label instructions. These recommendations are provided only as a guide. It is always the pesticide applicator’s responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to changing product labels and registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author(s), Colorado State University, and Colorado State University Extension assume no liability resulting from the use of these recommendations.
Preventing Infestations
Several steps can prevent the occurrence or recurrence of carpenter ant infestations:
- Correct any moisture problems that exist around the home such as leaking roofs, gaps in siding, clogged gutters, or faulty plumbing. Ensure good air circulation in attics and crawl spaces.
- Trim shrubs and trees away from the house.
- Replace water-damaged or decayed wood. Wood should not be in contact with soil. This also is important in preventing termite damage.
- Remove stumps and dead trees.
- It is not necessary to remove healthy trees, but remove broken branches.
- Store firewood away from the house, preferably off the ground. Examine and discard heavily infested pieces. Bring inside only firewood that is about to be burned. (This also minimizes other insect problems associated with firewood).
- Seal possible entry points, including around doors, windows, and where utilities enter the building.
Reference: CSU Fact Sheet 5.554