What are moisture ants?
Moisture ants is a common name for several related ant species in BC, primarily in the genera Lasius and Acanthomyops. They are medium-sized workers (3-5 mm), yellow-brown to dark brown, and are almost exclusively associated with decaying wood in structures — particularly crawlspaces, subfloor framing, and buried or ground-contact wood. The name comes from their requirement for wood that is not just moist but actively decaying: moisture ants are secondary invaders following wood-decay fungi.
Moisture ants vs carpenter ants: the key distinction
| Trait | Moisture ant | Carpenter ant |
|---|---|---|
| Size | 3-5 mm | 6-13 mm |
| Colour | Yellow-brown to dark brown | Black or red-and-black |
| Wood condition required | Actively decaying (fungal rot) | Elevated moisture (16%+) sufficient |
| Gallery appearance | Carton material (resin + soil) mixed in | Clean smooth tunnels |
| Typical location | Crawlspaces, subfloor, ground-contact wood | Wall voids, attic, any moist structural wood |
| Pest-control priority | Secondary — fix the rot | Primary — structural pest |
The diagnostic significance of moisture ants
Finding moisture ants is the biological equivalent of finding wood-decay fungi — both require the same underlying condition (chronic elevated moisture leading to structural decay). Treating the ants without addressing the rot and the moisture source only delays reinfestation. In many cases, if the decay source is eliminated and the wood dries out, moisture ants abandon the structure without treatment. They are following the decay, not causing it.
Common Metro Vancouver moisture ant locations
- Crawlspace floor joists and sill plates — especially in pre-1990 homes with no vapour barrier on the crawlspace floor.
- Deck posts in ground contact — common in Burnaby, North Vancouver, and Richmond where wood posts are set directly in soil.
- Subfloor under bathrooms — shower pan leaks and toilet wax-ring failures cause chronic subfloor wetting.
- Garage sole plates — ground-contact wood framing in attached garages often has moisture-ant activity.
- Buried landscaping timbers — railroad ties and pressure-treated landscape borders that have deteriorated.
Treatment approach
Moisture ant remediation — BC approach
The correct sequence for addressing moisture ants in a Metro Vancouver structure.
- 1Identify and document the ant locationPhotograph the ant activity and any associated wood damage. Note whether frass (clean wood shavings) or carton material (dark, resin-mixed debris) is present — carton material confirms moisture ants.
- 2Assess wood moisture and decayUse a pin-type moisture meter. Moisture ant locations almost always read 25%+ and often show visible fungal growth (white or brown rot). Document the extent of affected wood.
- 3Identify the moisture sourceWork backward from the wet wood: failed vapour barrier, inadequate crawlspace ventilation, plumbing leak, gutter discharge near foundation, ground-contact wood. The moisture source must be eliminated.
- 4Replace decaying structural woodWood that is actively decaying needs replacement — moisture ant treatment in deteriorated wood is futile. Coordinate with a contractor for subfloor, joist, or post replacement as needed.
- 5Install vapour barrier if absentIn crawlspaces without a vapour barrier, install 6-mil poly on the crawlspace floor. This alone often reduces relative humidity enough to halt decay progression in adjacent framing.
- 6Apply bait only as secondary measureAfter moisture repair and wood replacement, apply non-repellent ant bait to any remaining activity. Most moisture ant colonies self-relocate once the decay source is gone.