Marine Borers: Teredo Worms and the Damage You Cannot See

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

Pile Driving & Piling Repair | Dock & Pier Repair The first time I really understood teredo worms was on a bulkhead job in St. Augustine, 2003. The lady said her dock felt soft underfoot. I put a sounding hammer on a support pile, and it sounded like a cardboard box. One tap. That pile came out in chunks. That's the thing about marine borers. They work from the inside out, so you don't see a thing until it's too late. I'm Ed. I've built docks, seawalls, and pile foundations along the Atlantic and Gulf coasts for thirty years. I've seen more than one pier go down because nobody could see past the cosmetic surface.

Meet the Teredo Worm: A Clam That Works Like a Drill

Call it a "worm" if you want, but that's nothing but a bad habit. Teredo navalis is a bivalve, a clam, that switched careers. A shellfish that eats wood. It starts life as a microscopic larva drifting in brackish water, then finds a piece of cellulose and burrows in. Go ahead and call it a worm. We all do. Once inside, it grows into a long naked mollusk with a little shell at the head that rasp away. The body lines its tunnel with a shell-like tube. You'll see white calcareous tubes if you split a pile. And it gets long. A foot or longer in southern waters. I've pulled spaghetti-looking things from a pile in Apalachicola. They eat the lignin and cellulose. They digest it. The tunnel keeps growing, and the pile slowly becomes a honeycomb. The outside stays intact, sometimes a shell of solid wood an inch or two thick. On a 12-inch pile, that might leave a five-inch core of solid wood, but the rest as a brittle network. It still looks fine. That's the enemy.

Why a Good-Looking Pile Can Be a Lie

I've been around contractors who buy a pickup load of creosote piles and drive them in. Then they check on them years later and see no damage. I've been that guy. The truth is, teredo damage is nearly invisible unless you listen to the wood. Take a sounding hammer, a heavy solid hammer with a short handle. Give the pile a sharp rap above the waterline. A solid pile rings with a crisp ring. A pile that's made of hollowed-out wood sounds dull. Thud. Not always, though. A lot depends on the plant, the grain, and how wet the wood is. So you take it a step further. Drill a test hole. Use a long bit, inch or inch-and-a-half. Drill from the outside through the center. The chips come back clean. Then the drill drops into a void. That's when you cuss.

How to Inspect for Damage Properly

Do it once a year, right after the worst storm season. Hurricanes can break off piles that were already gutted. But don't just look at the visible section. The worst damage is often between the low tide line and the mudline. If your piles are in the water, you can't see it from the deck. Start with soundings. Walk the whole perimeter. Mark every thud with paint. Then drill cores, a minimum of one for every stack at risk. You can use a hand drill with a long auger, or hire a diving crew to run a pneumatic drill underwater. For a small dock, a homeowner can do the hammer test, but for a pier or a house on pilings, pay for a professional. It's cheap compared to repair. Then inspect the cores. You'll see the tunnels, the cross-hatch pattern. If you see more than about a third of the cross-section compromised, replace it. Don't wait. The next storm you'll be hauling rubble.

Fix It or Replace It: Prevention and Repair

Once a pile has teredo worms, you can't kill them. Not shallow. Not without pulling the pile and steaming it. So treat this like the emergency it is. The only realistic fix is to take the load off the wormed pile and drive a new one beside it. Use a properly treated pile, rated for marine use. For new construction, specify pressure-treated wood per AWPA. That's the American Wood Protection Association standard. Use Koppers creosote for saltwater, or CCA if you want a clean look. ACQ is also fine but be careful with steel fasteners. The treatment matters more than the species. Get the retention levels right. Southern pine, creosote, retention of 9 to 12 pounds per cubic foot. Don't let a supplier sell you land-grade. Another option is to wrap existing piles with a fiber-glass jacket and fill the gap with epoxy, like the commercial marine guys do. I've used a product called Sea-Shield or similar. It works best for decay that's just begun. But for teredo, where you've got tunnels running all directions, I'd rather replace. Or call in a reputable marine pile repair company. One digression: I've seen people pour concrete around a damaged pile. Don't they understand they're just encasing a hollow stick? It will still fail. The only thing that does is delay.

Frequently Asked Questions

What's the first sign of marine borer piling damage? A dull sound when you tap the pile with a hammer. If the pile rings, you're fine. If it thuds, drill a test core. Also look for a few tiny white holes at the waterline—the burrowing holes—but those are often covered with barnacles. How long can a pile last with marine borers? Depends on the plant, the water temp, and the dose of larvae. In the warm Gulf, an untreated pine pile can be reduced to a shell in three to five years. Treated piles last decades, but only if the treatment is intact and the end cuts are coated. Can you prevent teredo worms without treating the wood? Short answer: no. Wrapping with a tough plastic or concrete can work above the mudline, but it traps moisture. It's better to buy certified AWPA marine piles with the right retention. That's the only way to guarantee a lifetime. Do teredo worms attack all piles? They avoid toxic wood, but once a pile is installed without treatment, they'll find it eventually. They are most severe in brackish or saltwater with temperatures above 50°F. In the Gulf, you expect them every summer.