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Carpenter Ant Control

- Carpenter ant nests in buildings should be treated to limit potential damage -

Summary: Carpenter ants that nest in structures can be controlled by creating an insecticide barrier around the building that ants must cross in order to forage for food and water outside. This barrier can be placed inside exterior walls and/or on the exterior foundation (perimeter). It is not necessary to locate the colony or colonies inside the structure itself.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

The standard treatment to control carpenter ants was to apply insecticide into exterior wall voids, completely around the structure, thereby creating a barrier that worker ants must cross to forage outside for food. Wall void treatments generally require drilling into these spaces and may be beyond the ability of most homeowners. However, wall void treatments may not be necessary if perimeter sprays and baits are used instead.

Perimeter sprays are used alone or in conjunction with wall void treatments for added protection. Typically, a perimeter treatment involves spraying an insecticide such as Termidor Insecticide around the base of the structure where the siding overlaps the foundation (see photo right).

The third, and newest tool, is carpenter ant bait. These special baits are designed for carpenter ants and may be used around the base of the structure or even indoors.

house foundation, "perimeter"

House foundation (grey), siding (white), and foundation vent (black). "Perimeter" sprays should be applied where the siding overlaps the foundation, under the lip of the siding and completely around the house. Photo by JD DeAngelis.


There is no need for periodic maintenance treatments, often recommended by pest control companies, nor is it necessary to locate individual carpenter ant nests. Treat only when ant activity is detected during regular inspections.

Treating carpenter ants

(1) Inspect exterior of structure at least once each year during warm months, morning or evening. Look for large ants moving into or out of the structure, often in more or less single file lines.

(2) Remove all vegetation in contact with structure, leave at least a 12" gap between plantings and structure. This ensures that you will be able to see ants that are moving into or away from the structure. Ants will also use vegetation to "bridge over" the perimeter treatment (see below). Be sure to also check for overhanging branches in contact with the structure, and remove as necessary.


(3) Treat the foundation perimeter (perimeter treatment), about where the house siding overlaps the foundation, with an insecticide registered for this application. Termidor Insecticide is currently labelled for this use. You can do this treatment yourself or have a pest control company do this step.

(4) Apply new Carpenter Ant Baits around the foundation.

Baits and Sprays are available here (DoMyOwnPestControl)


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Jack DeAngelis, PhD,  , email:  [email protected]

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