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Pest Information

Wood Wasps and Horntails

 

The common names "wood wasp" and "horntail" are used to describe several kinds of wood-boring insects (families Siricidae, Xiphydriidae, Anaxyelidae, and Orussidea). Of greatest public interest are the large, nonstinging wasps that normally are attracted to and complete their life cycles in recently dead or dying conifer trees. Timber, salvaged from drought-stricken or fire-ravaged forests, may be processed into infested lumber that can eventually lead to emergence of adult wasps in completed buildings. Even though these insects are extremely annoying, they are not harmful to humans or structures.

Many species of wood-boring beetles, especially those in the family Buprestidae (flatheaded or metallic wood borers [87K]) or the family Cerambycidae (which includes long-horned beetles [70K] and roundheaded wood borers), feed on live trees that are old or weakened or fire- or insect-killed trees but do not attack harvested lumber. They cause problems, however, when they emerge from wood in newly constructed buildings, leaving small circular or oval exit holes [68K] in the wood.

Insects that will invade and damage structural and decorative wood and furniture include the powderpost beetles, deathwatch beetles, and false powderpost beetles, carpenter ants, carpenter bees, and termites.

IDENTIFICATION
The dozen species of wood wasps in California, Oregon, and Washington have a similar appearance. They are robust insects, generally 1 inch or more in length, wasplike in appearance but have an elongated, cylindrical body without a noticeable constriction or "waist." They are often black or metallic dark blue in color, or with combinations of black, red, and yellow. They make a noisy buzz when flying. The male and female have a similar body shape, except that the female is larger and has a long egg-laying apparatus (ovipositor), which can exceed her body length. The ovipositor is used only for egg laying and cannot be used to sting in defense. Although wood wasps and horntails can chew through wood, they do not bite people.

LIFE CYCLE
A female wood wasp inserts her ovipositor nearly 3/4 inch into wood of a weakened or dying tree and lays one to seven eggs. Eggs hatch in 3 to 4 weeks, and larvae tunnel into the wood parallel with the grain. As it chews, the larva uses a spine at the tip of its abdomen to help push it forward, through the wood. The larva begins eating the softer wood (sapwood) just under the bark, moves into hardwood (heartwood) deeper in the trunk, then returns to sapwood to complete its feeding. The tunnel, or gallery, usually measures 10 to 12 inches in length at completion. Larval feeding (associated with wood-decaying fungal growth) persists through development of four or five immature stages, taking at least 2 years or as many as 5 years to complete. Pupation takes place at the end of the gallery. After 5 or 6 weeks as a pupa, the adult emerges by chewing through about 3/4 inch of wood, leaving a round exit hole 1/4 to 1/2 inch in diameter.

DAMAGE
Wood wasp damage in buildings is more cosmetic than structurally weakening. Total numbers of insects emerging in any one house are limited, and usually less than a dozen. Emerging wood wasps can chew through just about any substance. Their large exit holes may be seen in wallboard or plaster walls, hardwood floors, linoleum, carpeting, nonceramic floor tiles, and other interior surfaces. Even if male and female wood wasps had the opportunity to mate in the building, the females would not be stimulated to lay eggs in dry, finished lumber. Wood wasps do not reinfest structures.

MANAGEMENT
Wood wasps are likely to occur anywhere that infested timber is used for construction. Even though salvaged timber is adequate for restricted, lower grade construction purposes (such as studs and subflooring), it is not valuable enough to warrant kiln-drying. Kiln-drying or vacuum fumigation of lumber is the only effective way to kill wood wasp larvae that have survived milling operations, but treatment is costly. Fumigation of milled lumber in boxcars, under tarpaulins, and in standing buildings has not been successful.

Even though wood wasps can be a noisy, scary nuisance, they are not a threat to anyone or anything. Waiting out the life cycle and repairing cosmetic damage is about all that can be done in an infested building. Once emerged, wood wasps will not reinfest harvested lumber.

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