A Moisture Problem Wearing an Insect Costume: Carpenter Ants, Satellite Colonies and the Fifteen Per Cent Threshold

1. Introduction: the insect is the symptom

A homeowner finds large black ants in the kitchen and calls a pest control company. The company treats the ants. Some months later the ants return.

This sequence repeats because the treatment addressed the wrong object. In most carpenter ant cases the ants are indicating a moisture condition in the building, and the moisture condition is what a competent response has to find.

2. They do not eat wood

The single most consequential misconception about this insect.

Carpenter ants are so named because these social insects excavate galleries in wood to create nest sites, using their strong mandibles to remove quantities of wood as they expand the nest. Unlike termites, carpenter ants do not consume wood.¹

The clearest demonstration is that they can live in creosote-treated railroad ties, since they do not actually eat the wood.³ A wood preservative that works by making timber inedible does not deter an insect that was never eating it.

2.1 What they actually eat

Carpenter ants have a variable diet including insects dead and alive, honeydew from aphids and other sweet substances, meats and fats.⁴ They feed mainly on honeydew produced by homopterous insects such as aphids, mealybugs and scales that feed on plant sap.⁷

This matters operationally. The ants crossing a kitchen counter are foraging for food, and the food is not the building. The nest and the food source are separate locations and a trail connects them.

2.2 They are not only pests

Worth stating for accuracy. Carpenter ants are an important component of many forest ecosystems and prey upon forest defoliators including budworms and tent caterpillars.⁴

They also do not kill trees, and unless a tree is hazardous for structural reasons it is not necessary to remove it simply because ants are present.⁵

3. Reading the galleries

The excavation has a distinctive appearance that identifies the species and distinguishes it from the more serious wood-destroying insect.

Carpenter ant galleries are smooth, with a sanded appearance, and are very different from termite-damaged areas, which have mud packed into the hollowed-out spaces.³⁸

The ants excavate along the softer spring growth and leave the harder summer grain, which serves as walls separating the tunnels, cutting openings in those walls to allow access between them.⁶ Galleries are kept clean and the tunnels look smooth as if sanded.⁶

3.1 The windows

Runways and galleries are excavated with openings or windows cut at intervals to expel sawdust.⁸

That expelled material is the most useful field sign available. Coarse wood shavings, often mixed with insect fragments, accumulating below a void indicate an active nest above it. Termites produce nothing comparable because they consume what they remove.

3.2 The diagnostic value

Smooth clean galleries with expelled shavings indicate carpenter ants. Mud-packed galleries with no frass indicate termites,³ which, as discussed elsewhere in this journal, is a rarer but more serious finding in Manitoba than most people assume.

4. The moisture threshold

The association with water is specific enough to be measurable.

The parent nest is established in damp, decayed wood, and is associated with wood that has a moisture content greater than 15 per cent, a situation in structures often caused by rain, leaks or condensation.¹

A study cited in the applied literature reports the same threshold, noting that Camponotus pennsylvanicus prefers wood with moisture content above 15 per cent, often found in areas such as leaky roofs, poorly sealed windows or water-damaged crawlspaces.²

4.1 Why a number changes the inspection

A moisture meter is a relatively simple and affordable tool for locating moisture in hidden areas, and pest management professionals are advised to use one.³

This converts a subjective search into a measured one. Rather than looking for ants and guessing where they came from, an inspector can survey wall and framing moisture and identify the locations that meet the condition the species requires.

4.2 The qualification

The association is strong but not absolute. Carpenter ants prefer to nest in moist wood, but wood previously saturated may be soft enough to hollow out as well, and they occasionally nest in dry, sound wood.³ Smaller species may live in pre-existing voids such as curtain rods, hollow-core doors, between wall studs and around windows.⁴

So a dry reading does not exclude them. It makes them less likely and points the search elsewhere.

5. Parent and satellite

The colony structure is the part that most directly determines treatment outcome.

Carpenter ants typically construct two types of nest: the parent and one or more satellite nests. The parent nest houses the queen, eggs and small larvae, and the satellite nests contain larger larvae and pupae.¹

5.1 The critical asymmetry

Satellite colonies consist of workers, larvae and pupae, but never eggs. The carpenter ant queen always stays in the original parent colony. Since the main colony usually needs a steady supply of moisture it is often outdoors, while satellite colonies are most common indoors.³

Parent colonies require a moist primary nest often outdoors, while satellite colonies may exploit drier indoor areas if connected to a moisture source.²

5.2 Colony scale

A mature parent colony contains an egg-laying queen, brood and around 2,000 workers, and average colony size for most carpenter ant species runs from 3,000 to 6,000 workers.⁴ A black carpenter ant colony is usually three and a half to four years old before it can produce swarmers, at which point mature colonies typically contain roughly 2,000 to 4,000 individuals and produce swarmers annually.¹

The maturation interval is worth noting. Swarmers indoors indicate a colony that has been established for several years, not a recent arrival.

6. Why this defeats the obvious treatment

Assemble §5 and the failure mode is clear.

The nest inside the building is, in most cases, a satellite. It contains no eggs and no queen.³ Eliminating it removes workers, larvae and pupae and leaves reproduction entirely intact in a parent colony that may be in a stump, a fence post or a tree across the yard.

6.1 The perimeter spray version

A repellent perimeter application may reduce indoor sightings without affecting the parent colony, producing exactly the pattern described in §1: apparent success, later return.

It also carries the dispersal risk described elsewhere in this journal for repellent chemistry, since a colony under chemical stress may relocate satellites rather than die.

6.2 What baiting offers

Colonies in standing dead trees can often be eliminated with baits.⁵ Bait works here for the same reason set out in the horizontal transfer article published in this journal: foragers carry it back to a nest the applicator cannot reach.

The constraint is that carpenter ants feed heavily on honeydew and other sweet substances,⁷ so competing food sources, including an aphid population on a nearby tree, reduce bait uptake.

7. The foraging range

One figure substantially widens where an inspection has to look.

Carpenter ants can travel more than 100 yards to search for food and establish satellite colonies.⁹

Structures surrounded by large trees are consequently often targets of carpenter ant infestation.⁹

7.1 What that means on an urban lot

A radius of over a hundred yards from a Winnipeg house takes in neighbouring properties, boulevard trees, back lanes and often a park. The parent colony may not be on the affected property at all.

This is the carpenter ant version of the building-scale argument made throughout this journal. The relevant unit is not the house.

7.2 The access route

Carpenter ants can gain entry to a home via branches that touch it, so trees and shrubs near the home should be pruned to eliminate plant-to-house contact.⁵

A branch resting on a roof is a bridge that bypasses the entire ground-level perimeter, which is where most exclusion and treatment effort is directed.

8. Where they nest in a building

The locations follow the moisture.

Parts of a house such as around and under windows, roof eaves, decks and porches are more likely to be infested because these areas are most vulnerable to moisture.³

Satellite nests are established in areas including roof trim, siding, rafters, joists, sheathing, decks, porches, steps, sills, subflooring, doors and window frames, and also inside hollow areas such as hollow doors or small voids produced during construction. Most often they are in areas where the wood is moist or has been damaged by moisture.⁶

In structures, timbers damaged by water leaks attract these ants, and they select damp, soft, decaying areas for nesting.⁷

8.1 The progression into sound wood

One detail matters for damage assessment. Ants can move from decaying portions of the wood into sound lumber in the process of enlarging the nest.⁶

So while establishment requires wet wood, the excavation does not stay confined to it. A nest that began in a rotted sill can extend into framing that was never wet.

9. How much damage, honestly

This section exists because our own industry is not consistently honest about it.

Carpenter ants cause damage to structures through their nesting behaviour. Yet carpenter ant damage rarely compromises the structural integrity of a building.⁹

A different extension source frames it more seriously, describing the greatest concern as satellite nests in structural wood which can cause serious damage in locations including rafters, joists and sills.⁶ And carpenter ants nest in wood and cause serious damage when infestations occur in structures.⁸

9.1 Reconciling these

We read the difference as one of timescale and degree rather than contradiction. Damage accumulates slowly, can become serious in a long-established nest in a load-bearing member, and rarely reaches the point of compromising a building's integrity.

The honest statement for a homeowner is that this is a problem to address and not an emergency, and that a company presenting it as a structural emergency is overselling.

9.2 The damage outside buildings

Carpenter ants do cause damage to utility poles and urban shade trees, and in New Jersey 75 per cent of urban shade trees sampled in 1981 to 1982 were infested. Excavation not only weakens trees but leads to infestation by other pests and pathogens.⁹

That prevalence figure is worth holding alongside §7. If most urban shade trees in a sampled city carried carpenter ants, then parent colonies within foraging range of a house are the normal condition rather than the exception.

10. Winter activity and the heated building

A seasonal point that connects to the thermal argument made elsewhere in this journal.

Carpenter ants are typically inactive during winter. However, the ants can continue to be active in wintertime when a nest is located in a heated area of the building.¹

10.1 Why winter sightings are diagnostic

Ants foraging indoors in a Winnipeg January are not ants that wandered in. Outdoor colonies are dormant. Activity at that time of year indicates a nest inside heated space, which substantially narrows the search.

A winter sighting is therefore more informative than a summer one, and it is the point at which the indoor nest is most reliably confirmed rather than merely suspected.

10.2 The general pattern

This is the same mechanism described in the urban ecology article published here for cockroaches and pharaoh ants. A heated building decouples the population inside it from the season outside, and the species behaves as though it were somewhere else entirely.

11. What an inspection should establish

The sequence follows from the biology rather than from where ants were seen.

Because carpenter ants are often associated with moist wood, the first step is to determine whether there is a moisture problem and repair it. More serious problems associated with moisture include mould growth and structural damage.³

11.1 The wider finding

That sentence contains something a pest control company should say out loud. A carpenter ant inspection that finds elevated moisture has found a condition that matters independently of the ants, because the same water that invited them supports mould and decay.

An operator who treats the ants and does not report the moisture has withheld the more important finding.

12. What actually resolves it

The published guidance is consistent and is mostly not chemical.

Replace rotted or water-damaged wood and eliminate wood-to-soil contact.¹

Eliminate leaky pipes and other moisture problems to avoid carpenter ants establishing nests.⁹

Remove dead stumps within about 50 feet of the house where practical, prune broken limbs from trees and seal holes in the trunk.¹

Remove stumps, wood piles and wooden objects near the home that might house parent colonies outdoors.⁵

Prune to eliminate plant-to-house contact.⁵

Bait where the parent colony is located but inaccessible, including in standing dead trees.⁵

12.1 The pattern

Five of those six are building maintenance and landscaping. Only the last is a pest control product.

That is the same conclusion the drain fly article reached for food premises, and it is worth being direct about the commercial implication: a company that resolves a carpenter ant problem properly sells less product than one that does not.

13. Limitations and open questions

The sources are extension and applied literature. Most are university extension publications,¹³⁶⁹ which are reliable but are syntheses rather than primary research.

One source is commercial. The moisture discussion drawing on Hansen and Klotz comes via a pest control company's material,² and we have not read the underlying text.

Species differ. Much of the detail concerns Camponotus pennsylvanicus. Other species have different habits, and colonies of some species such as C. vicinus may have multiple queens rather than one.⁷

Nothing here is Manitoba-specific. We have found no local research on carpenter ant species composition, prevalence or nesting behaviour in Manitoba housing, which given how common the complaint is locally is a gap of the kind this journal keeps encountering.

The damage severity question is unresolved. Our reconciliation in §9.1 of sources that characterise damage differently is our reasoning, not a reported finding.

We sell carpenter ant treatment. Section 9 argues the damage is usually less severe than the industry implies and §12 concludes that most of the fix is not a pest control service, which runs against that interest.

14. Conclusion

Carpenter ants excavate wood rather than eating it, leaving smooth sanded galleries and expelling coarse shavings through cut windows, which distinguishes them cleanly from termites.¹³⁸ Parent nests are associated with wood above fifteen per cent moisture content, a condition produced in buildings by rain, leaks and condensation.¹

The colony is split. The parent nest holds the queen, eggs and small larvae and usually sits outdoors where moisture is reliable. Satellite nests hold workers, larger larvae and pupae but never eggs, and are the ones found indoors. The queen always stays in the parent colony.¹³ With a foraging range exceeding a hundred yards,⁹ that parent colony may be on someone else's property.

So the ants in the wall are a satellite of a reproductive unit somewhere else, established because something in the building is wet. Treating the visible nest removes workers and leaves both the queen and the water. The moisture meter is a better first instrument than the sprayer, and the finding it produces matters beyond the ants, since the same water supports mould and decay.³

The damage, honestly stated, rarely compromises a building's structural integrity.⁹ What it reliably indicates is a moisture defect that was going to cost money regardless. The insect found it first.

References

1. Jones, S.C. Carpenter Ants. Ohio State University Extension, Ohioline fact sheet HYG-2063. Source for the excavation of galleries with mandibles without consuming wood, the two-nest structure of parent and satellite with the parent housing queen, eggs and small larvae and satellites holding larger larvae and pupae, the association of the parent nest with damp decayed wood above 15 per cent moisture content caused by rain, leaks and condensation, the three and a half to four year interval before swarmer production with mature colonies of roughly 2,000 to 4,000 individuals, winter inactivity except where a nest is in a heated area of the building, and the control guidance on replacing water-damaged wood, eliminating wood-to-soil contact, removing stumps within about 50 feet and pruning broken limbs. https://ohioline.osu.edu/factsheet/HYG-2063
2. Ants, Moisture, and Pest Control. Commercial pest control company article citing Hansen and Klotz (2005), The Biology of Carpenter Ants. Used for the reported preference of Camponotus pennsylvanicus for wood above 15 per cent moisture content in locations such as leaky roofs, poorly sealed windows and water-damaged crawlspaces, and for the observation that parent colonies require a moist primary nest often outdoors while satellite colonies may exploit drier indoor areas if connected to a moisture source. Note the commercial interest. https://www.colonialpest.com/2025/04/23/ants-moisture-and-pest-control/
3. Carpenter Ant Management, publication G1738. University of Nebraska Extension. Source for the ability to live in creosote-treated railroad ties given that the wood is not eaten, the smooth gallery appearance contrasted with mud-packed termite galleries, the preference for moist wood with occasional nesting in dry sound wood, the vulnerability of windows, roof eaves, decks and porches, the recommendation of a moisture meter for locating hidden moisture, the composition of satellite colonies as workers, larvae and pupae but never eggs with the queen always remaining in the parent colony, the usual outdoor location of the main colony, and the guidance to determine and repair moisture problems given the associated risks of mould growth and structural damage. https://extensionpublications.unl.edu/assets/html/g1738/build/g1738.htm
4. Camponotus. Bugwoodwiki. Source for the parent colony containing an egg-laying queen, brood and around 2,000 workers, average colony size of 3,000 to 6,000 workers for most species, nesting in existing cavities or soft rotting wood of higher moisture content, use of pre-existing voids such as curtain rods, hollow-core doors and wall spaces by smaller species, the variable diet including live and dead insects, honeydew, sweet substances, meats and fats, the statement that carpenter ants do not eat wood, and their ecological role preying on forest defoliators including budworms and tent caterpillars. https://wiki.bugwood.org/Camponotus
5. Carpenter ants here and there: Camponotus spp. Bug of the Week, University of Maryland. Source for the description of outdoor primary colonies in decaying stumps, fallen wood and hollow trees providing shelter for egg-laying queens, the role of plugged rain gutters, leaky pipes and water-soaked window sills in allowing invasion, the guidance to remove stumps, wood piles and wooden objects near the home, the observation that carpenter ants do not kill trees and colonised trees need not be removed, the use of baits to eliminate colonies in standing dead trees, and the recommendation to prune to eliminate plant-to-house contact. https://bugoftheweek.com/blog/2016/1/13/carpenter-ants-here-and-there-icamponotusi-spp
6. Carpenter ants, publication G7423. University of Missouri Extension. Source for the excavation along softer spring growth leaving harder summer grain as walls with openings cut between tunnels, the smooth sanded gallery appearance, the identification of satellite nests in structural wood as the greatest concern including roof trim, siding, rafters, joists, sheathing, decks, porches, steps, sills, subflooring, doors and window frames and hollow construction voids, and the observation that ants can move from decaying wood into sound lumber while enlarging the nest. https://extension.missouri.edu/publications/g7423
7. Carpenter ants. Texas A&M Urban Entomology. Source for nesting in live or dead trees, stumps, buried wood, firewood, fence posts, landscape timbers, tree holes, hollow doors, ceiling beams, roofing boards, attic insulation and voids above windows and porches, the attraction of timbers damaged by water leaks and the selection of damp soft decaying areas, the diet based mainly on honeydew from aphids, mealybugs and scales, the polymorphic worker caste, the single inseminated queen in C. pennsylvanicus colonies against multiple queens in species such as C. vicinus, and the note that indoor nests may be satellite colonies. https://urbanentomology.tamu.edu/urban-pests/ants/carpenter/
8. Summary: Carpenter Ant Biology. Hansen, L. Source for the excavation of runways and galleries with windows cut at intervals to expel sawdust, the smooth sand-papered gallery appearance as distinct from other wood damage, and the statement that carpenter ants cause serious damage when infestations occur in structures. https://faculty.sfcc.spokane.edu/lhansen/summarycarpenterartbiology.htm
9. Carpenter Ants and Their Control, fact sheet FS1101. Rutgers New Jersey Agricultural Experiment Station. Source for the foraging range exceeding 100 yards to search for food and establish satellite colonies, the targeting of structures surrounded by large trees, the statement that carpenter ant damage rarely compromises the structural integrity of a building, the damage caused to utility poles and urban shade trees with 75 per cent of New Jersey urban shade trees sampled in 1981 to 1982 infested, the consequent secondary infestation by other pests and pathogens, and the guidance to eliminate leaky pipes and other moisture problems. https://njaes.rutgers.edu/FS1101/