The names wood wasp and horntail describe several kinds of wood-boring insects in the order Hymenoptera, family Siricidae. Of greatest concern are the large, non-stinging wasps that normally are attracted to and complete their life cycles in newly dead or dying conifer trees. Timber salvaged from these trees can be processed into infested lumber. This can lead to adult wasps emerging in recently completed buildings or structures.
Although these insects are extremely annoying, they aren’t harmful to humans or structures. They attack only trees and won’t bore into wood in buildings or furniture.
The dozen species of wood wasps in California, Oregon, and Washington look similar. They are large insects, generally 1 inch or longer, and wasp like in appearance but have an elongated, cylindrical body without a noticeable constriction or waist. They often are black or metallic dark blue or combinations of black, red, and yellow. They make a noisy buzz when flying.
The male and female have a similar body shape, except the female is larger and has a long egg-laying apparatus (ovipositor) that can exceed her body length. The female can use her ovipositor only for egg laying; she can’t use it to sting in defense. Although these pests can chew through wood, they don’t bite people.
In terms of color it can be difficult to use a single descriptor, since the term wood wasp is used to describe a number of subspecies. The appearance can vary from blue to black or even red/brown. Their markings are typically red amber or yellow, and they usually have a thicker waist than most wasps. Ironically despite the non-stinging nature of these wasps, the females often look the most intimidating of all the species. This is because of their large ovipositor, an appendage used only for laying and placing eggs (it can sometimes be longer than their body).
A female wood wasp drills her ovipositor nearly 3/4 inch into the wood of a weakened or dying tree and lays 1 to 7 eggs. At the same time, she squirts in a fungus from her abdominal gland. She continues this process, laying up to 200 eggs.
Eggs hatch in 3 to 4 weeks, and larvae tunnel into the fungus-predigested wood parallel with the grain. Larvae are legless, cylindrical, whitish, and have a spine at the tip of their last abdominal segment. As they chew, larvae use a spine at the tip of their abdomen to help push themselves forward, through the wood. Larvae begin eating the softer wood (sapwood) just beneath the bark, following the fungus into the heartwood, then return to the sapwood to complete their feeding.
Larval feeding continues through 5 or more immature stages, which take at least a year and as many as 5 years in cooler climates to complete. The tunnel, or gallery, usually measures 10 to 12 inches long at completion.
The right conditions having been found and an egg discharged through the boring instrument, from this in due course issues a six-legged, whitish larva, which sets to work with capable jaws on the solid wood, beginning the excavation of a long tunnel that will occupy several years before the larva is full-grown. That stage reached, it spins a silken cocoon, and changes into a pupa which has all its limbs of maturity formed and folded beside its body.
Before making its cocoon, however, it takes the precautionary measure of advancing its tunnel close up to the inner bark, so that in its winged state it will have only to bite a way through this softer impediment to its liberty; not that it is now incapable of dealing with anything firmer. Like the caterpillar of the Goat-moth, it does not hesitate, if necessary, to make a way even through soft metal. There is a record of a Sirex-infested tree having been cut into rafters which were used in building a roof and covered with sheet-lead an eighth of an inch thick . One of the rafters contained a Sirex in either the larval or pupal stage; and when the adult insect sought its freedom, it found the way obstructed by the lead. It went right through, apparently finding lead not much more difficult to deal with than bark.
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.
Wood wasps do not sting, but they may cause other problems for the homeowner. Although they do not re-infest seasoned wood such as lumber, their long life cycle may create a situation whereby live larvae remain in logs that are sawed into lumber. When the larval and pupal stages are complete, adults are able to emerge inside the home.
Also, wood wasps may be introduced to the inside of the home when infested firewood is brought inside and not used for a while.
Should wood wasp control be required, contact your pest management professional for advice about what is required to eliminate a wood wasp problem.
Wood wasps as pests
Wood wasp damage in buildings is more cosmetic than structurally weakening. The total number of insects emerging in any one house is limited, usually fewer than a dozen. Emerging wood wasps can chew through just about any substance, and you can see their large exit holes in wallboard or plaster walls, hardwood floors, linoleum, carpeting, non-ceramic floor tiles, and other interior surfaces.
Horntails are seldom present in large enough numbers to cause significant damage. The larvae bore into the heartwood and sapwood of declining shade trees and may cause some weakening of the tree, making it more susceptible to wind breakage. Pigeon tremex attacks maple, elm, beech, sycamore, hickory and others. Horntails do not damage seasoned lumber or wood structures, although they may continue to develop and emerge as adults from green sawed construction grade lumber. In recent decades, an invasive non-native wood wasp, Sirex noctilio, has been found in New York and nearby states. Its primary hosts in North America are red pine, Scots pine, and related conifers. Because it has caused major damage to pines in plantations in the southern hemisphere worldwide, there is concern that this insect will become a problem in pine-growing regions of southeastern USA