Pencil-width dirt tunnels running up your foundation are the calling card of subterranean termites. Here's how to read them — and the one test that tells you if they're active.
If you've found thin tubes of dried mud running up your foundation, a pier, or a garage wall, you've found the signature of subterranean termites. Unlike drywood termites — which live sealed inside wood and leave frass — subterranean termites give themselves away with these earthen tunnels. Learning to read them tells you a lot about what's happening and how urgent it is.
What Mud Tubes Are
Subterranean termites nest in the soil and feed on wood above it, but they can't survive long in open, dry air. So they build protected tunnels out of soil, saliva, and droppings to travel between the ground and their food while staying humid and hidden from predators. Those tunnels are the mud tubes — usually about the width of a pencil, tan-to-brown, running in more or less straight lines up a vertical surface.
Where to Look for Them
| Location | Why to check |
|---|---|
| Foundation walls (interior + exterior) | The classic spot — tubes climbing from soil to wood |
| Crawlspace piers and subfloor | Protected, humid, and close to the framing |
| Garage walls and slab edges | Common entry where wood meets concrete near grade |
| Behind planters / against the house | Moist, hidden, easy for termites to build unseen |
They favor dark, humid, low-traffic areas, so the tubes you can see may be only part of the story — which is one reason a full inspection checks places you normally wouldn't.
The Break Test: Active or Old?
Here's the single most useful thing you can do with a mud tube — carefully break out a small section (an inch or so) and leave it. Then check back in a few days:
- If the gap gets rebuilt, the tube is active — termites are still using it, and you have a live infestation.
- If it stays broken and you see no repair, the tube may be old/abandoned — but that does *not* mean the property is clear, only that this particular tube isn't currently in use.
Either result is a reason to get an inspection; the break test just tells you how urgent. And don't simply knock all the tubes down and call it handled — destroying tubes doesn't touch the colony in the soil, which will just rebuild elsewhere.
What Mud Tubes Confirm (and Don't)
A mud tube confirms subterranean termite activity or history at that spot. It doesn't tell you the size of the colony, how many entry points exist, or how much damage has been done inside the wood — subterranean termites eat wood from the inside out, often leaving a normal-looking surface. Our post on how subterranean termites destroy homes covers how that hidden feeding works, and drywood vs. subterranean termites covers telling the two apart, since the treatment differs.
What to Do
Leave the tubes largely intact for the inspector (aside from a small break test if you want to gauge activity) — they're useful evidence of the travel path and entry points. Then get an inspection. Subterranean activity is typically addressed with a soil barrier treatment that targets the colony's route between ground and structure, which is a different approach than drywood treatment.
A free inspection confirms whether the tubes are active, locates the entry points, and identifies the right treatment — with a written report and no obligation. If you've also seen other signs, how to identify termite damage covers the full picture.