Resin or pitch pockets are of common occurrence in the wood of larch, spruce, fir, and especially of longleaf and other hard pines.  They are due to accumulations of resin in openings between adjacent layers of growth.  They are more frequent in trees growing alone than in those of dense stands.  The pockets are usually a few inches in greatest dimension and affect only one or two growth layers.  They are hidden until exposed by the saw, rendering it impossible to cut lumber with reference to their position.  Often several boards are damaged by a single pocket.  In grading lumber, pitch pockets are classified as small, standard, and large, depending upon their width and length.

INSECT INJURIES[37]

[Footnote 37:  For detailed information regarding insect injuries, the reader is referred to the various publications of the U.S.  Bureau of Entomology, Washington, D.C.]

The larvae of many insects are destructive to wood.  Some attack the wood of living trees, others only that of felled or converted material.  Every hole breaks the continuity of the fibres and impairs the strength, and if there are very many of them the material may be ruined for all purposes where strength is required.

Some of the most common insects attacking the wood of living trees are the oak timber worm, the chestnut timber worm, carpenter worms, ambrosia beetles, the locust borer, turpentine beetles and turpentine borers, and the white pine weevil.

The insect injuries to forest products may be classed according to the stage of manufacture of the material.  Thus round timber with the bark on, such as poles, posts, mine props, and sawlogs, is subject to serious damage by the same class of insects as those mentioned above, particularly by the round-headed borers, timber worms, and ambrosia beetles.  Manufactured unseasoned products are subject to damage from ambrosia beetles and other wood borers.  Seasoned hardwood lumber of all kinds, rough handles, wagon stock, etc., made partially or entirely of sapwood, are often reduced in value from 10 to 90 per cent by a class of insects known as powder-post beetles.  Finished hardwood products such as handles, wagon, carriage and machinery stock, especially if ash or hickory, are often destroyed by the powder-post beetles.  Construction timbers in buildings, bridges and trestles, cross-ties, poles, mine props, fence posts, etc., are sometimes seriously injured by wood-boring larvae, termites, black ants, carpenter bees, and powder-post beetles, and sometimes reduced in value from 10 to 100 per cent.  In tropical countries termites are a very serious pest in this respect.

MARINE WOOD-BORER INJURIES

Vast amounts of timber used for piles in wharves and other marine structures are constantly being destroyed or seriously injured by marine borers.  Almost invariably they are confined to salt water, and all the woods commonly used for piling are subject to their attacks.  There are two genera of mollusks, Xylotrya and Teredo, and three of crustaceans, Limnoria, Chelura, and Sphoeroma, that do serious damage in many places along both the Atlantic and Pacific coasts.