The two side by side
One lives in the ground and commutes. The other lives inside the wood and never leaves. Almost every practical difference follows from that.
| Subterranean | Drywood | |
|---|---|---|
| Where the colony lives | In the soil, often well away from the building | Inside the timber it is eating |
| Needs soil contact | Yes — must return for moisture | No, none at all |
| Mud tubes | Yes — the defining sign | Never |
| Frass | Not visible; used in tube construction | Yes — hard pellets pushed out of kick-out holes |
| Colony size | Hundreds of thousands, into the millions | Small — often confined to a single piece of wood |
| Galleries | Soil-lined, dirty, follow the grain | Clean and smooth, cut across the grain |
| Usual swarm season | Spring, daytime, often after rain | Late summer to autumn, often at dusk |
| Where in a building | Low — subfloor, ground floor, anything near soil | Anywhere, including roof timbers and furniture |
Frass — the drywood signature
Drywood termites keep their galleries clean by pushing waste out through small openings. The result is a small heap of pellets that people almost always mistake for sawdust or spilled sand.
Under any magnification the pellets are clearly not sawdust: they are hard, roughly six-sided, and remarkably uniform in size — around a millimetre — with color varying from cream to dark brown depending on the wood. Sawdust is irregular, fibrous and soft.
Look above the pile rather than at it. Somewhere overhead there will be a kick-out hole, about the size of a pen tip, and finding it tells you which piece of timber is infested. Sweeping the pile away and checking whether it returns is a reasonable activity test.
Mud tubes — the subterranean signature
Subterranean termites face a problem drywood termites do not: they dry out in open air, and their colony is in the soil while their food is not. Their answer is the mud tube, a covered route of soil and saliva running from the ground to the timber.
Because it solves a problem drywood termites do not have, the sign is close to conclusive on its own. There is more detail — including how to tell an active tube from an abandoned one — on the mud tubes page.
The damage looks different too
If you can see inside a gallery, the two are easy to separate. Subterranean termites carry soil with them and pack it into their workings, so the galleries are dirty, often damp, and tend to follow the softer grain of the wood.
Drywood galleries are the opposite: clean, smooth-walled, sometimes almost polished, and cut across the grain rather than along it. There is no soil in them anywhere, because no soil ever came in.
Why the distinction changes the treatment
This is not a taxonomic detail. The two live in different places, so treatment has to reach different places, and the wrong approach fails quietly.
A subterranean colony is in the ground and may extend well beyond the building footprint. Whatever is done has to reach the soil around and under the structure, because the termites inside the wall are a small and constantly replaced fraction of the colony. Treating only the timber leaves the nest intact.
A drywood colony is entirely inside the wood, with no soil component to reach. Treatment targets the infested timber. Soil treatment around the foundation does nothing at all for a drywood infestation in a roof beam.
This is the most expensive mistake in the subject, and it is easy to make from the outside, because both leave hollow wood and both produce swarmers.
How to check, in order
Start at the bottom and outside. Walk the foundation on the shaded side looking for tubes, then the crawl space or basement on piers and joists. If you find a tube, you have your answer and it is subterranean.
If not, work upward looking for pellets — under exposed beams, on windowsills, on the floor of an attic, beneath wooden furniture. Pellets without tubes point to drywood.
Whichever you find, photograph it before you disturb it. A clear photo of frass beside something for scale, or of a mud tube on the wall it was built against, is worth more than any description when the time comes to have it confirmed.