Should you worry about carpenter ants? Damage, signs and costs

The longer a colony stays in a structure, the more damage it can do, and severe damage can weaken structural wood, though it generally happens slowly and often takes years.² Extension services weigh the risk differently: Penn State says a nest's expansion into a building's wooden timbers will rarely cause structural damage, and Maryland says tunneling may cause serious structural damage in key timbers although nest sites rarely exceed a few square feet.^3-4

Maryland also says a colony inside a structure usually matters most as a sign of a past or present water problem, and that wood-rotting fungi around a persistent leak or seepage can threaten the structure as much as the ants do.⁴ Carpenter ants prefer wood with a moisture content of 15% or higher, which is why a carpenter ant problem often goes along with a moisture problem.¹ They tunnel through wood to build or expand their nests and don't eat it.¹

They don't sting, and handling one will at most get you a little nip from its jaws.⁴ If you do find a problem, Oregon State University Extension's advice is not to panic, because most damage develops over years and a short delay won't worsen the situation.⁵

Look at the ant from the side: a carpenter ant has a waist with one node and an evenly rounded thorax, and other ants that get mistaken for it have one or two nodes and an uneven thorax.² Minnesota's extension service describes workers as black or red and black, usually 3/8 to 1/2 inch long, with one species only 3/16 inch.² Ants of different sizes could all be carpenter ants from the same colony.¹ Field ants can easily be confused with carpenter ants because of similar size and coloration, but workers have an uneven thorax and the ants don't nest inside buildings.⁶

Penn State Extension says it's important to separate carpenter ants from termites because each needs different methods of control, and these are the differences between the black carpenter ant and the eastern subterranean termite:^2-3

Inside damaged wood, carpenter ant galleries look almost sandpapered, while subterranean termite galleries are filled with a mud-like fecal material.¹ If you can't settle it, Oregon State's ant publication says images can be submitted to your county Extension office for help with identification.⁷

Not by itself, since foraging ants inside your home don't necessarily mean they're nesting indoors.¹ The parent colony is often in a tree, log or waste wood within 100 yards of the house, and foraging trails may extend up to 300 feet from a nest.^{1, 3}

The time of year and the numbers you see change what a sighting means:²

• Carpenter ants seen indoors during winter mean there's an inside nest, although workers sometimes arrive in firewood and can't start nests or cause damage.²
• Finding the ants in late winter or early spring means they're coming from a nest in the building.²
• Large numbers of winged ants indoors are a sure sign of an indoor nest, but one or just a few winged queens don't mean one is present.²
• Consistent indoor sightings of 20 or more worker ants are another sign of a nest.²

Coarse sawdust that looks like pencil shavings indicates a nest is close by.⁴ NC State adds that dark debris points to decaying wood, that damaged wood may have slit-like openings, and that mature colonies can make rustling or gnawing sounds in the wood or void where they're nesting.¹

In homes, nests turn up in moist or decayed wood behind bathroom tiles, around tubs, sinks, showers and dishwashers, and under roofing or in attic beams.^{2, 6} Satellite nests can sit in drier spots such as insulation, hollow doors and wall voids, and finding one nest doesn't rule out others in the structure.²

Minnesota's extension service puts carpenter ant mating swarms from April through June, with occasional swarms indoors earlier during late winter on warm, sunny days.⁶ Workers search for food between sunset and midnight during spring and summer months.² Penn State Extension suggests using a flashlight to check the basement, attic, garage and building exterior from May through July between 10:00 p.m. and 2:00 a.m..³ NC State Extension says workers often show up foraging for water around sinks, dishwashers and showers beginning in early spring.¹ Oregon State University Extension says to treat nests in mid-April when they're active.⁵

Maryland's extension service says carpenter ants often enter homes as the weather warms in spring and cools in fall, and if that's the only time they're a nuisance, waiting a few weeks may resolve the problem without intervention.⁴

Penn State Extension says the ants are inactive during the winter but will be active if a nest is in a heated part of a building.³ A nest can exist in a house during winter without being noticed, and one in a spot without enough indoor heat or sunshine, such as a north-facing outside wall, stays dormant until spring.²

University of Minnesota Extension's approach has three parts: locate and destroy the nest, replace damaged or decayed wood and eliminate any moisture problems.² It says that in most cases you need a pest management professional, who has the experience, equipment and a wider array of products.² Oregon State says large colonies should be handled by professional exterminators.⁵ Penn State says both the parent and satellite colonies have to be destroyed or the ants will reappear in a few days or weeks.³

Treating a hidden nest can involve a professional applicator drilling small holes and applying insecticidal dust into the nest area, and Minnesota's page says not to do this yourself.² NC State likewise says injecting dusts or sprays into voids or nests is complex and may be too difficult for the average person to carry out safely, and it sends readers to a pest control professional.¹ You can help that professional by saying when, where and how many ants you've seen.²

Spraying the ants you can see won't eliminate a nest, because ants carry very little insecticide back and only a small percentage of a colony is out foraging at any given time.² Maryland's extension service warns that indoor spraying kills only a few ants, can compromise baits and needlessly exposes occupants to potentially toxic chemicals.⁴

Homeowners do have a bait option in containerized ant bait in small, sealed plastic trays, sold by pest control operators and retail stores.⁴ NC State says baits belong in locations pets and young children can't reach, and that spraying pesticides where bait will go makes the ants avoid it.¹ Timelines vary by page, from several days in Maryland's account to 7 to 10 days before activity declines significantly in NC State's and weeks or months in Minnesota's.^{1-2, 4} Minnesota also warns that carpenter ants have complex food preferences and baits may not attract them long enough to succeed.²

Any pesticide has to be used according to its label, which Minnesota's extension service calls the law.² If you suspect a child has been accidentally exposed to chemicals, Penn State Extension says to immediately call the Poison Control Center at 1-800-222-1222.⁸ When damage is severe, repairing or replacing the wood and then removing the nest may matter more than any treatment and may solve the problem in the process.¹

NC State Extension says prevention starts with inspection and doesn't start with a monthly or periodic pesticide application, and that the key point to remember is moisture.¹ Penn State's list includes keeping wooden parts of the house dry by repairing roofs, flashing, gutters and downspouts, replacing water-damaged or decaying wood and sealing holes where pipes and wires enter.³

Branch clearances differ by source, with NC State saying to keep limbs pruned 12 inches or more from the house and Oregon State saying to trim plants and tree branches at least 24 inches from it.^{1, 5} Firewood should stay off the ground and away from the house, and never on a porch or deck, against the side of your home or in a garage.^{1, 3} A crawlspace should stay dry and well ventilated, and a properly installed vapor barrier helps.¹

Oregon State recommends taking time each year to inspect your home, and says sawdust piles may indicate carpenter ants and can be brushed away so you can check for new activity later.⁵ It also says to document your findings each year to track changes over time.⁵

Published national ranges aren't quotes, and for ant treatment in general they run $110 to $300 a treatment.^9-10

Maryland's extension service says most pest control companies give free estimates, when they discuss a treatment plan, cost and warranties, and it suggests comparing recommended treatment programs and estimates among several companies.¹¹ Ask whether the price covers finding the nest, because nests are often hidden in wall voids, ceilings, subfloors, attics or hollow doors.² Ask about repairs too, since NC State says working out the extent of structural damage may require removing siding or portions of sheetrock.¹

Maryland's page also tells residents to make certain a company has an active certification license from the Maryland Department of Agriculture Pesticide Regulation Section.¹¹ You can read more in how to check a pest control company's license.

When you're ready, you can ask for quotes from pest control companies near you.