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What Are Wood Wasps?

One of the more unusual insects you are likely to see is the wood wasp.  The terms wood wasp and horntail describe wood-boring insects in the families Siricidae, Xiphydriidae, Anaxyelidae, and Orussidea.  They are not wasps at all.  You are most likely to see these large insects that normally are attracted to and complete their life cycles in recently dead or dying conifer trees.  Timber, salvaged from fire damaged forests may be processed into infested lumber that can eventually lead to emergence of adult wasps in completed buildings.  These insects are nuisance invaders and are not harmful to humans.

Wood wasps are 1-inch in length, have an elongated, cylindrical body without a noticeable constricted “waist.”  They are often black or metallic in color.  They buzz while flying.  The female is large and has a long ovipositor, which is used only for egg laying and cannot be used to sting in defense.  Wood wasps and horntails can chew through wood, but they do not bite people.

A female wood wasp inserts her ovipositor nearly ¾ of an inch into wood of a weakened or dying tree and lays one to seven eggs.  Eggs hatch in three to four weeks, and larvae tunnel into the wood parallel with the grain.  The larva begins eating the softer wood (sapwood) just under the bark, moves into hardwood (heartwood) deeper in the trunk, and then returns to sapwood to complete it’s feeding.  The tunnel, or gallery, usually measures 10 to 12 inches in length when completed.  Larval feeding (associated with wood-decaying fungal growth) persists through development of four or five immature stages, taking between two to five years to complete.  Pupation takes place at the end of the gallery.  After fie to six weeks as a pupa, the adult emerges by chewing through about 3/4 of an inch of wood, leaving a round exit hole of 1/4 to 1/2 of an inch in diameter.

Wood wasp damage in buildings is more cosmetic than structurally compromising.  But the presence of the pest will frighten many customers.  Total numbers of insects emerging in any one house are usually limited to 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, non-ceramic 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.  So, wood wasps do not re-infest structures.

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.

Even though wood wasps can be a noisy, scary nuisance, they are not a threat.  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 re-infest harvested lumber.


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