Marine Wood Borers

Todd F. Shupe  |  9/27/2012 12:59:10 AM

Preservative-treated wood pilings in marine applications can be attacked by marine borers.

Shipworm damage to a wood piling.

The control of any marine borer depends on proper species identification. Wood can be maintained free of decay by submerging the wood in water and thus depleting the oxygen requirement for many wood-decaying organisms. This method prevents most insect injury but promotes injury by marine wood borers.

Wood placed in a marine environment, such as boat docks, must be treated with a marine preservative. Creosote is sometimes insufficient against some types of marine wood borers. In these cases, the wood must be protected with creosote and an inorganic arsenical in a process known as dual treatment. This is absolutely critical for the long-term integrity of the structure in these hazardous conditions.


Shipworms will attack any untreated wood submerged in salt water. The greatest injury is done to pilings and wooden boats. Untreated pilings may last less than a month in the Gulf parishes. The replacement cost of pilings destroyed annually by shipworms is tremendous. These animals prefer warm salt waters. More than one-half of the volume of a pile may easily be destroyed without any evidence of injury being apparent on the pile’s surface. Only by cutting into the piling can its condition be ascertained. The greatest damage in a piling usually occurs just above the mud line, although entrance holes may be found throughout the submerged area. Entrance holes about 1/16 inch in diameter are bored into the surface of the wood by the larvae.

Shipworm Control

Metal sheathing around a piling is effective but expensive and not very durable, unless it’s galvanized or made from noncorrosive metal. Cement casings are unsatisfactory because cracks develop as a result of expansion, contraction and wave action. Plastic and fiberglass coatings have been used with some success. A very successful method of protecting pilings against shipworms is impregnation with marine-grade creosote treatments of 20 to 25 pounds per cubic foot (pcf). Also, 2.5 pcf of chromated copper arsenate (CCA) or ammoniacal copper zinc arsenate (ACZA) may be used. Surface treatment is not sufficient, so the preservative must be pressure applied to obtain good penetration. Deep penetration is difficult if not impossible with many dense hardwoods. A softwood, such as Southern yellow pine, with a wide sapwood zone is preferred. Applying marine creosote is still the preferred treatment. Noncorrosive metal caps and coverings on pilings are beneficial because they protect against structural injury and entrance of shipworms.


Limnoria is the genus commonly known as gribbles, and they do considerable damage along coastal waters. They are confined to clear salt water and cannot endure fresh to turbid water. They can tolerate low temperatures. When these crustaceans attack a piling, their tunnels almost touch, so the thin walls between them are quickly worn away by wave action, leaving a new surface of wood ready for reinfestation.

Limnoria Control

The protection of wood from Limnoria attack is much more difficult than from shipworms. Some species are resistant to creosote, partially because of the deterioration and leaching of creosote in warm waters. The use of certain copper salts (chromated copper arsenate and ammoniacal copper zinc arsenate) provides effective protection from Limnoria attack. A heavy retention (2.5 pcf) of CCA or ACZA works well. Copper or tin salts, fungicides and insecticides, especially the chlorinated hydrocarbons added to creosote, have shown various degrees of effectiveness in preventing Limnoria attack, but their use is limited by environmental regulations.

In areas where severe attack by both types of marine borers is present, or if creosote-resistant species of Limnoria are present, use a dual treatment. This method calls for pressure treatment with the copper salt (1.0 to 1.5 pcf of CCA or ACZA) followed by a pressure treatment with marine-grade creosote (20 pcf). Movable wooden structures and boats can also be protected by an unbroken covering of marine paint. When borers have gained entrance to wooden vessels, they can be killed by running the boats into fresh water or dry dock for at least 30 days.

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