A sawdust pile is spoil from digging inside the tree; the hollow left behind is the real question.
Carpenter ants do not eat wood: NC State Extension says they tunnel through it to build their nests and throw the debris out, coarse and fiber-like, like pencil shavings. That debris can be the pile at the foot of a tree, but ambrosia and Ips beetles leave sawdust too, so what a pile says depends on who made it.
General field guidance, not a diagnosis from a distance. A tree can hide problems from the fence line and hide strengths too.
What a carpenter ant pile is made of, and where it comes from
A carpenter ant pile is construction debris. NC State Extension's carpenter ant factsheet says that as the ants expand a nest, "they throw out piles of fiber-like wood debris and frass (fecal material) that is similar in appearance to pencil shavings," mixed with remains of insects the colony fed on. Damaged wood may have slit-like openings through which the ants push out sawdust and frass, and the tunnels usually run with the grain, the softer springwood dug first. Extension's list of outdoor nest sites includes tree holes, tree stumps and standing dead trees.
Ambrosia beetles, and beetles in infested pines, leave sawdust too
Not every pile comes from ants. Henderson County's Extension center says ambrosia beetles drill holes smaller than a pencil lead, gnaw the wood without eating it and push it out behind them, sometimes leaving a toothpick-like protrusion of sawdust at the hole. Buncombe County's center describes the granulate ambrosia beetle's "toothpick-like strands of boring dust emerging from the trunk" on trees such as dogwood, redbud, maple and crape myrtle.
Pines have their own version. In its southern pine beetle overview, posted by the NC Forest Service, the Southern Group of State Foresters says secondary insects arriving after southern pine beetle produce "pale sawdust at the base of the tree," a question for the NC Forest Service county ranger.
What the pile says about the wood, and why the hollow matters more than the ant
A pile shows insects at work in wood; whether that wood was sound is the open part. Extension's publication says debris "with a dark appearance is an indication of decaying wood," that carpenter ants prefer moist wood, and that some species tunnel into wood while others prefer to nest in existing cavities. Union County's Extension center says colonies invade wounds and scars, and that "Some researchers believe they attack entirely sound wood."
Union's center also says it has not been proven that carpenter ants can kill a tree by chewing alone, "but carpenter ant galleries in trees do leave branches and trunks more susceptible to wind damage." The USDA Forest Service's defect guide weighs the space, not the insect: "a cavity where the wood is missing can create a serious hazard." New Jersey's FACE investigators wrote of one tree that it had been damaged at its base by carpenter ants, making it unsafe to climb. Decay that hides behind sound wood gets its full treatment under dead and rotten trunks.
Worth watching, until another sign shows up beside the pile
Sawdust or ants below a tree get the fence-line check's third label, "Worth watching," the home page's general reading of the sign. Ants on a trunk are not always nesting in it; Extension says workers often climb trees for honeydew.
The reading changes when the pile has company. The USDA guide says multiple defects "that are touching or are close to one another should be carefully examined," and, where more than one defect sits on the main stem, says to assume the tree is extremely hazardous. A pile next to mushrooms at the base of a tree, under a crack at the fork or big dead limbs, or on a tree that has a lean that is new is a different conversation from a pile alone.
The insects are a pest question; the tree is a tree question
Carpenter ants and ambrosia beetles are pest questions first. NC State Extension's publication sends complex carpenter ant treatments to a pest control professional and general questions to the local N.C. Cooperative Extension county center, in Davidson County the Davidson County Center.
The tree belongs to tree work. Extension's own carpenter ant prevention list says to remove nearby trees that are dead or in poor health and to get advice from a tree specialist about the trees around the home. This crew's line on insects is among the questions answered under the Davidson County tree calendar; a tree that has to come out is tree removal; and the defect-and-target frame behind the check anchors the hazard tree field guide.
Where is the pile, and where is the hole above it?
Questions people actually ask.
Can carpenter ants get from a tree into the house?
Yes. NC State Extension's carpenter ant factsheet says carpenter ants may travel along tree limbs or shrubs that touch the siding and roof, gaining access to attic areas.
Why are carpenter ants on a tree with no sawdust under it?
Carpenter ants on a trunk may be foraging rather than nesting. NC State Extension says workers often travel up tree trunks and onto limbs in search of honeydew, and that their trails may extend up to 300 feet away from the nest.
Is sawdust at the base of a pine the same sign as a carpenter ant pile?
No, not necessarily. The Southern Group of State Foresters' pine beetle overview says ambrosia beetles, Ips beetles and other secondary insects produce pale sawdust at the base of pines already infested by southern pine beetle.
What does dark sawdust below a tree point to?
NC State Extension's carpenter ant factsheet says ant debris with a dark appearance is an indication of decaying wood, and that the color can be used as a clue in searching for the nest.