What a mud tube is
A mud tube is a covered road. Subterranean termites cannot survive exposure to open air for long — they dry out — so to reach wood above ground they build an enclosed tunnel out of soil, saliva and droppings and travel inside it.
That constraint is the reason the sign is so reliable. The tube is not a nest and not damage; it is infrastructure, and no other household insect has the same problem to solve or builds the same answer to it.
What they look like
About the width of a pencil, brown to grey, with a slightly rough and crumbly surface. They run in fairly straight lines and are usually vertical, since the point is to get from soil to timber by the shortest covered route.
Where a tube crosses a smooth surface such as poured concrete it often flattens into a ribbon rather than a round tunnel. Where it has to cross an air gap it becomes a free-standing tube hanging from the timber above, which looks strange enough that people usually photograph it.
| Not this | How to tell |
|---|---|
| Mud dauber nest | Smooth, hard, built as rounded cells side by side rather than a long narrow tunnel |
| Water run-off staining | A flat smear on the surface with no raised walls; nothing to break away |
| Ant soil trails | Loose spoil heaped at an opening, not a roofed tunnel you can lift off |
| Old paint or render | Blisters and flakes follow the surface finish rather than running a consistent route |
Where to look
Termites choose shaded, damp, undisturbed surfaces. Search those first and you will find tubes far more often than by walking the perimeter in daylight.
The places worth checking, roughly in order of how often they turn up: the outside of the foundation wall on the shaded side of the building; inside the crawl space on piers and the underside of joists; the inside face of the foundation in a basement; behind stored boxes and against garage walls; and behind baseboards in ground-floor rooms.
Take a torch. Tubes on an unpainted concrete wall are close to the same color as the wall and are genuinely easy to walk past.
The break test — is it still active?
An abandoned tube looks much like a live one, and the difference matters. There is a simple test, and it takes three days and no equipment.
Remove a short section from the middle of one tube — an inch or two is plenty — and leave everything else alone. Note the date. Come back in two to three days and look at the gap.
If the gap has been sealed over with fresh soil, the colony is active and using that route now. If it is still open after several days, that particular tube may be abandoned — though it does not clear the building, because termites re-route and may simply be travelling elsewhere.
Break one section only. Knocking down every tube you find scatters the colony, causes it to re-route, and destroys the evidence an inspector would use to trace the infestation back to its entry point.
What finding one means
It means subterranean termites established a covered route between soil and structure, and it rules out drywood termites, which never touch soil and never build tubes.
It does not tell you how far the damage has gone. The tube is the road, not the destination — the feeding is inside timber somewhere beyond it, and the only way to know how much has been eaten is to sound the wood and look. See the six signs for what else to check while you are there.
What to do next
Photograph the tube before touching it, then run the break test on one section and photograph the gap. Two pictures taken three days apart are the single most useful thing you can hand an inspector.
Do not spray it. Repellent products applied to a tube push the colony away from the treated area rather than killing it, and the usual result is that the termites re-route somewhere less visible and keep feeding.
While you wait out the three days, fix what made the route attractive: the leak, the blocked gutter, the mulch banked over the foundation, the firewood against the wall.