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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Big black ants in the home: Carpenter ants, Camponotus spp.


Carpenter ants excavate galleries for pupae and larvae in trees and stumps outdoors.


At this time of year ants make themselves known both indoors and out. On a recent visit to a friend’s home, we were somewhat dismayed by several large black ants roaming the floors of the living room. These were not the small odorous house ants we met in a previous episode of Bug of the Week, these were real bruisers of the ant world, robust, black giants.

The presence of large black ants in and around the home always raises the question “Are these carpenter ants, and should I worry?” Here is a little basic ant biology to help answer this question. As a group, ants are omnivores dining on meat in the form of other small arthropods, plant material including nectar and seeds, and in the case of leafcutter ants, fungi cultured on harvested leaves. Products of other insects like the carbohydrate rich honeydew produced by aphids and scale insects are also common fare. When a productive source of food is found, ants quickly establish a foraging trail to bring the goodies back to the colony where developing young and hungry queens await. Usually ants forage and gain sustenance for the colony outdoors, but sometimes their quest for food brings them indoors. A spill of syrup in the pantry or an unfinished meal in the pet’s bowl can initiate a busy parade of ants transporting food back to the colony.

Usually, the colony resides outside the home. However, one troublesome member of the ant clan, carpenter ants, often set up their home in your home. Carpenter ants have been around for millions of years, and in the natural world dead wood is home to the colony. Unlike termites that use wood as a source of nutrients, carpenter ants excavate wood and build elaborate galleries to house the colony. Moisture is important to the carpenter ant colony and decaying stumps, fallen wood, and hollow trees are excellent nest sites. Due to high ambient moisture level, these outdoor sites, called primary colonies, house egg-laying queens, juvenile ants, and workers. However, a plugged rain gutter, leaky pipe in a wall void, or water-soaked window sill or siding may provide a damp-wood situation that allows carpenter ants to invade a home. Workers of these satellite colonies create galleries for developing larvae and pupae. Satellite colonies can cause serious structural damage to homes and the longer the colony is in place the greater will be the damage.

Field ants are differentiated from carpenter ants by a distinct dent in the center of their thorax.

Carpenter ants are differentiated from field ants by the smooth contour of their thorax in profile.

Here are some clues to help diagnose a carpenter ant problem. First, the regular presence of large black ants indoors on floors and walls is an early warning sign. By large, I mean 3/8 to ½ inch. Another occasional home visitor in this size range is the field ant. These two large ants are easily distinguished by the shape of their thorax, the body segment just behind the head. In profile, the carpenter ant’s thorax will be smooth and even. In profile, the field ant will have a noticeable dent in the middle of its thorax. A hand lens or magnifying glass will help you observe this feature. Carpenter ants will leave other telltale signs indoors. As carpenter ants chew through wood, excess sawdust-like material called frass is cast away. Piles of frass on floors or windowsills often indicate a carpenter ant infestation. Carpenter ants will enter homes in spring in search of food and an occasional ant indoors does not necessarily signal the presence of an infestation. However, large numbers of wingless workers and the presence of several winged carpenter ants (these are reproductive queens and males) indoors in spring are highly indicative of an infestation.

Unpainted wet wood may allow carpenter ants to gain entry into a home.

What should you do if you have a carpenter ant infestation?  First and foremost, the carpenter ant colony must be located and destroyed, the damaged wood should be removed, and replaced, and any conditions leading to the founding of the colony, such as leaking pipes or other conditions creating wet wood, must be fixed to prevent recurring infestations.  Applications of insecticides to exposed colonies or the use of ant baits may be used indoors by homeowners to destroy colonies. All directions and precautions on the pesticide label should be read and strictly followed.  Treatments of concealed colonies in voids behind walls or in ceilings are for professional pest control applicators that have the knowledge, techniques, and tools to perform these services.

How can carpenter ants be foiled in the first place? Remove stumps, wood piles, and wooden objects near the home that might house parent colonies outdoors. Carpenter ants will live in the dead wood of standing trees. They do not kill trees. Unless the tree is a hazard because of structural defects it is not necessary to remove it simply because it is colonized by ants. Colonies in standing dead trees can often be eliminated with baits. Carpenter ants can gain entry to a home via branches that touch the home. Keep trees and shrubs near the home pruned to eliminate plant-to-house contact. With the onset of warm weather and abundant rainfall this spring, sucking insects and other sources of food for ants abound.  It should be a good year for ants.


Large carpenter ant workers are known as soldiers and their assignment is to defend the colony. Smaller workers care for brood inside the nest and forage for food.


The following web sites provide useful information in preparation of this episode. To learn more about carpenter ants please visit them.