Teredo Worms Ate Enough Wood to Rewrite Building Codes

By Ed Brannigan, Marine contractor. Reviewed by Amanda McCallister, editor.

Pile Driving & Piling Repair | Boat Lift Installation I pulled the piling out of the water and it came apart in my hands like a wet saltine. July 12, 1990, St. Simons, Georgia. The client's daughter had leaned on the dock rail and it gave way with a crack. I was the second contractor they called. The first guy blamed the sand. He wasn't wrong, but the real cause was underneath. I brought a diver down to inspect the entire length of a supporting pile. When we surfaced, we cut a six-inch section off the bottom. It weighed less than the coffee cup sitting on my boat.\n\nTeredo worms. The owner said "termites?" No. Termites eat wood from the outside. These things – they're clams, believe it or not – start as plankton, land on the wood, and drill in as larvae. Once inside, they grow for months, rasping out galleries parallel to the grain. The wood looks whole from the outside. On the inside, it's Swiss cheese with freckles. I've seen an eight-inch pile lose seventy percent of its cross-section and only show a tiny entrance hole. You could push your finger into the side of that pile. That dock collapsed three weeks later.\n\nThat job taught me more about marine building codes than any manual. After a decade of failures across the Gulf and Atlantic, the insurance industry and local building authorities started listening. Codes that once said "creosote-treated timber" now require specific treatment, no field cutting, and alternatives like concrete and steel for primary structure. Here's how the danger works, and how to build so you never have to watch a piling crumble again.

The Worm That Isn't a Worm

Teredo navalis is the species that costs us the most. It's a bivalve mollusk, meaning it belongs with oysters and clams, not with earthworms. An adult female sheds live larvae into the water. The larvae are tiny, maybe half a millimeter, but they have a foot and a shell with two boring plates. When they touch wood, they attach and begin rotating their shell to rasp away the surface. Within 24 hours they've bored a tunnel and begun a long, hidden life. Inside the wood, the animal stays soft and wormlike, and it can grow to a foot or more. It doesn't need to come out. It just sits there, converting cellulose into body mass and excrement. The tunnels are about the diameter of a pencil lead, so a single pile can host thousands of individuals without looking distressed. The problem is their reproduction. In warm coastal water, they breed all year. By the time you hear the first hollow sound, the colony is already well established.

Why the Codes Had to Change

In the 1950s, if you built a dock in St. Augustine with untreated southern yellow pine, you could expect maybe ten to fifteen years if you were lucky. Teredo was present but not always catastrophic. Then pressure-treating with creosote became common. That extended life to thirty or forty years. But builders would order 'treated' piles and then cut them to length, bore through them for bolts, and break the envelope of preservative. Saltwater wicks into those cuts, and teredo larvae find an opening. By the early 1980s, we were seeing entire docks fail in six to eight years. Insurance companies noticed because the claims were piling up. Municipalities started adding language to their marine codes requiring that piles meet AWPA UC4C and that any field cutting be coated with an approved end-preservative. Some counties now outright reject wood for the primary support members in high-salinity areas. The real change is that you can't just say 'treated timber' anymore – you have to prove the treatment is in the core. That's what the stamp says.

How to Spec Wood That Lasts

If you're building a dock or seawall in salt or brackish water, you want a pile that is stamped UC4C. UC4C is a heavy-load saltwater immersion rating. It calls for a higher retention of preservative, usually 0.60 to 0.80 pounds per cubic foot of ACQ or copper azole, or creosote if you're going that route. Ask the supplier for a certificate showing what was actually loaded. Don't accept 'ground contact' grade. That's UC4B or UC4A, and it will fail. When the wood arrives, it should have a visible dark green or brown color in the heartwood. If you cut or drill it after treatment, you have to coat the freshly exposed surface with a copper-based preservative that's approved for saltwater. Don't use linseed oil or paint – not the same. And use stainless steel fasteners; treated wood corrodes galvanized nails in short order. Composite decking is fine for top boards, but it's not structural. For the load-bearing piles, either UC4C wood, concrete, or steel.

The Inspection Habit

You don't need a degree to suspect a teredo problem. Tap every pile with a hammer and listen. A solid pile rings; a hollow one thumps. Push a screwdriver into the wood near the waterline. It should resist. If it slides in more than a quarter inch, you have damage. Also take a look at the waterline and the mud line – those are the easy entry points for larvae. Shoreline areas that are still and warm are the worst. I advise clients to inspect every six months during the first two seasons, then once a year. If you find a pile with more than twenty percent cross-sectional loss, you should sister it with a concrete pile or wrap it with a marine-grade epoxy or PVC boot. The boot should go down at least a foot below the mud line and up at least a foot above the highest high tide. That's not just for teredo – it also protects against fungal attack. But remember, a boot doesn't protect the inside if larvae entered before you installed it. So inspect, then cover.

Alternatives That Outlast Wood

Wood can work if you spec it correctly, but on certain projects it's not the best choice. Concrete piles are heavy and expensive, but they last decades with no marine borers. Steel piles with a protective coating and sacrificial anodes are also common, but the anodes need replacement. In areas with heavy ice or floating debris, concrete is safer. There's also a growing market for composite pilings made of recycled plastic and fiberglass. They're immune to any biological attack, but they flex more than wood and need proper design. The cost difference is substantial: wood is cheapest, concrete is roughly double, and composite runs three to four times the wood price. I quote clients by the linear foot, not per piece, so they can compare apples to apples. If you've ever seen a client leave their dock for two months and come back to a gangway leaning on a spaghetti pile, you know the wood savings isn't worth it in aggressive waters. Build for the worst and you'll get good years.

Frequently Asked Questions

Do teredo worms only live in saltwater? No, they're also found in brackish water. They need a certain salinity, but you'll find them in estuaries where the water is part salt. Any wood submerged in coastal water is a target. Can I treat the wood myself with a pesticide? Not effectively. The larvae bore in within hours of settling. You can't get a chemical inside already-infested wood. Your only line is to use properly treated wood or a physical barrier like a PVC boot. I have a wooden dock that's ten years old. Do I need to replace it? Maybe not. Tap the piles. If they sound solid, you can add a wrap. If any pile sounds hollow, have a contractor evaluate. Sometimes you can sister it with a concrete pile. Don't ignore it – teredo doesn't go away.