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Detecting ant colonies inside walls: sound, frass, and moisture mapping

Carpenter ant colonies in wall voids are invisible from the outside. Here's how professionals locate them — and how homeowners can narrow the search before calling.

John Mercer·VP Operations & Lead Technician
8 min read

Signal 1: frass deposits

Frass is the most reliable colony indicator — see the full [carpenter ant frass identification guide](/guide/carpenter-ant-frass). In walls, frass emerges from pinhole kick-out holes in the drywall surface (often directly above a baseboard or at a ceiling-wall junction) and accumulates as a small mound on the baseboard, floor, or horizontal surface below. A fresh frass deposit that replenishes within 7 days after being swept up confirms an active colony at that location.

Signal 2: sound in walls

Large, active carpenter ant colonies in wall voids can be heard. The sound is a faint rustling or crinkling — sometimes described as the sound of crumpling paper — produced by workers moving through galleries. The best conditions for acoustic detection: a quiet house at night, press your ear directly against the drywall in the area of suspected activity. The sound is subtle and requires concentration to distinguish from building settling or plumbing. It is most reliably heard in rooms adjacent to exterior walls with known moisture issues.

Signal 3: moisture mapping

A pin-type moisture meter pressed against drywall registers the aggregate moisture of the paper face and any moisture migrating through from the framing behind. High readings (above 15-18% on drywall) indicate water in the wall cavity. Scanning the wall surface in a grid pattern — every 30 cm — and recording readings creates a moisture map that highlights wet zones. Carpenter ant colonies are almost always in or adjacent to the highest-moisture areas on this map. This technique requires only a $30-50 hardware-store pin meter and 20 minutes of systematic scanning.

Professional tools: borescope and thermal imaging

A borescope is a flexible camera on a thin tube — inserted through a small-diameter inspection hole (typically 12 mm) drilled in drywall at a suspected colony location. The camera view directly shows gallery structures, frass accumulation, and live ants if present. Borescope inspection is the definitive non-destructive method for gallery confirmation. Thermal imaging cameras detect temperature differentials in walls — a cavity with ant activity sometimes appears as a warm zone compared to adjacent areas. Thermal is a screening tool rather than a confirmatory one; borescope confirms.

The homeowner pre-inspection protocol

How to

Homeowner wall ant pre-inspection

How to conduct a systematic pre-inspection to identify suspected carpenter ant locations in walls before booking professional help.

  1. 1
    Walk the perimeter, exterior and interior
    Look for moisture stains, dark discolouration, soft wood, or any wood at or below grade. Note every location and photograph it. On the interior, note any ceiling stains, soft drywall, or swollen baseboards.
  2. 2
    Check for frass deposits
    Inspect all baseboards, windowsills, and ceiling-wall junctions in rooms adjacent to exterior walls. Check on top of attic insulation. Use a flashlight. Photograph every deposit you find.
  3. 3
    Acoustic scan
    On a quiet evening, press your ear against the drywall at exterior walls in every room, 30-60 cm above the baseboard and again at the top of the wall near the ceiling. Note any faint rustling.
  4. 4
    Moisture scan
    With a pin-type moisture meter, scan walls in 30 cm grid increments at exterior walls. Record readings above 12% on drywall. Mark the highest-reading areas with masking tape.
  5. 5
    Document and compile
    Create a simple floor plan sketch marking frass locations, acoustic detections, and moisture hotspots. Share this with the pest professional before they arrive — it significantly reduces inspection time and cost.

Frequently asked questions

Can I use a smartphone thermal camera to find ant colonies?+
Consumer thermal camera attachments for phones ($150-300) provide adequate resolution for spotting major temperature differentials but may miss smaller colony heat signatures. Worth trying as a screening tool, but professional-grade thermal cameras have significantly better sensitivity.
How many inspection holes does a professional typically drill?+
1-4 holes in target areas, typically 12 mm diameter. Holes are patched after inspection. Drilling multiple holes to map the colony extent is standard practice on large or ambiguous cases.
If I find frass in two separate walls, do I have two colonies or one?+
Possibly one colony with satellite nests, or two separate colonies. The distinction matters for treatment. Professional inspection to map the gallery extent is needed — the frass locations plus moisture map usually clarify whether it is a connected system.
Can I access the colony through the attic?+
Often yes — the attic is the most accessible inspection point for colonies in top-plate or rafter locations. A headlamp inspection of attic insulation surface and top plates is a good starting point before drilling any wall access holes.
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