Carpenter ants are the scourge of homeowners in the Pacific Northwest. They do far more structural damage to homes and other buildings than all other insects combined.

That said, they are part of our wood decay cycle and have been here far longer than humans have existed.

Reproductive (winged) ants now are emerging from existing colonies. If you find them emerging from your house, you have a problem. If you just see random large queens that have shed wings, it probably isn’t a problem, since they wander far and wide.

We have several carpenter ant species. Ants can be either all black (most common) or a designer combination of black and red. But don’t confuse it with the mound-building and harmless thatching ant, which is also black and red.

Carpenter ants have a smooth, concave thorax (directly behind the head), with a single “spike” (called a petiole) between the thorax and abdomen.

Carpenter ant size is variable. Ants grow with the food supply. If food was limited during their formative weeks, they may be quite small. If times were good, their size increases.

It is not uncommon to have a range of different-sized ants in the same colony. If you need them identified, bring them into the local county extension office.

Colonies can be small with less than a thousand individuals, up to colonies with over 50,000 ants. Main colonies are often located outside in stumps, rotten fence posts, or unused firewood piles.

These often contain the queen. Treating these colonies is crucial for control success. Satellite colonies can form inside the house. These may not have a queen. If they don’t, they gain new members from the main colony outdoors.

Carpenter ants don’t consume wood for food. They use wood as a place to live. Their destructive work is a consequence of gallery building.

They forage for food like any other self-respecting ant. Food can be protein sources, other insects, aphid honeydew (they protect the aphids from predators), seeds and much more.

It takes some detective work to find inside or outside colonies. Late-night observation on warm summer evenings can be helpful. (Tell your neighbors what you are doing before you follow the ants with a flashlight.)

Finding lots of larger ants persistently in one area of your house is an indication of a problem.

Here are steps a homeowner might take to reduce carpenter ant problems:

•    Treat wall voids with boric acid, Timbor, or desiccating dusts. This is especially useful during new construction.

•    Control moisture under the house and in the wall voids. While carpenter ants don’t require moisture like dampwood termites do, they prefer to start in moisture-damaged wood.

•    Prune and maintain vegetation at least 18 inches away from the walls of the house. Ants can cross insecticide treatments on plant limbs and wires. You can also reduce nearby aphid populations.

•    Treat outside nests if you find them.

Insecticide treatments are intended to interrupt their foraging paths. There are several compounds for ant treatment. Consider spraying a barrier 18 inches up the foundation and 12-18 inches out from the foundation in the spring and late summer.

There are also several insecticide bait products that are effective. Baits are designed to be carried back to the queen in sufficient quantities to kill her. That generally spells the end of that colony.

You may run into problems that resist exterior treatment. Often, wall-void treatments are required.

Wooden vaulted ceilings are also very difficult to treat and require expert help. It is then best to call professional pest control operators to get these situations under control.

Chip Bubl works with the Oregon State University Extension Service. He may be reached at 503-397-3462, or


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