Wood Wasps

What Are They?

Wood wasps found in the United States include various subspecies of the native horntail wasp, members of the family Siricidae, and the invasive sirex wood wasp, Sirex noctilio. Both species reside in areas across North America, with the native horntails remaining the more established and widespread of the two. Wood wasps, as their name implies, are characterized by nesting habits that utilize pines and other coniferous trees. With vast forests coupled with moderate climates, the area proves ideal for establishing wood wasp populations.

What Do They Look Like?

The male sirex wood wasp tends to have a metallic blue head and thorax, with a black and orange abdomen and all-black legs. The females appear similar but have orange legs. Female specimens also possess a spike-like protuberance at the end of the abdomen, which is used to protect the ovipositor, or the organ used to dispense eggs. The average length of the insect is around an inch and a half. Horntail wasps are typically cylindrical and blackish or blue with yellow and red markings. Both males and females have clear-colored wings, and while the female is typically larger, both adults are usually longer than one inch in length.

Where Do They Live?

Wood wasps like the horntail and sirex wood wasp build their nests in tunnels bored into dying or recently felled trees. The more stress a tree appears to be in, the more likely it may be home to a wood wasp nest. The insect only nests in trees and will not be found in any man-made wood structures, unless that structure was built using untreated infested wood.

Do They Sting?

Neither variety will sting or bite humans or other animals, but the insects are known to be noisy flyers, buzzing incessantly in flight.

What Does a Wood Wasp Eat?

Adult wood wasps feed on nectar, pollen, fruits, and the occasional dead insect. Wood wasp larvae feed on the softer wood (or sapwood) around the area where the queen delivered her eggs into the tree. Larvae also feed on a parasitic fungus that the queen injects around the egg when laid. This fungus eats away at tree cells and feeds hatching larvae as they burrow deeper into the wood..

Life Cycle

Eggs

Adult females emerge from overwintering during the months of July to September. Females lay up to 450 eggs in holes bored into dead and decaying trees, using the ovipositor as a tool to make the initial nests. Female wood wasps typically inject a mucus and fungus that are toxic to the tree in order to kill wood cells and enable larvae to eat their way deeper into the bore.

Larvae

Larvae take anywhere from ten months to over a year to grow, pupate, and then turn into adult wasps. Throughout the gestation period, larvae move back and forth from the sapwood to the harder, denser wood in order to exit the bores once pupation concludes.

Problems Caused by Wood Wasps

Wood wasps generally invade native pines and other conifer trees, while also using maples, elms, beech trees, and other deciduous trees as nesting sites. Since wood wasps make nests inside the trunk and branches, tree cells die from the toxic mucus used by the female to lay her eggs. The life cycle may last a year or more, so the deeper the wasp burrows, the more difficult it may become to detect and eliminate. Some types of trees wood wasps prefer are used as packing materials and generally used in home construction. With damage done minimally to the interior of the wood, the material becomes damaged cosmetically versus structurally.

Signs of Infestation

The first signs of a wood wasp infestation come when resin noticeably drips from damaged trees. This is a direct sign of females laying eggs. On the trunks of trees, round holes denoting “exit holes” can be seen. The foliage on the trees will begin to wilt and then die. When trees are harvested and not kiln dried, it remains possible for larvae burrowed into the wood to survive. If that lumber is used in home construction, when the larvae emerge as wasps similar exit holes will be seen in drywall and other homebuilding materials covering the untreated wood.

Prevention

Neither horntails nor other species of wood wasps sting or bite, so danger to humans and other animals is extremely minimal. When structural timbers are infested, the insect can be considered a nuisance more than anything else. Lumber must be kiln-dried to prevent continued infestation after installation in building constructions.

Repairing cosmetic damage after wood wasps emerge will solve final infestation problems, as wood wasps will not reuse the lumber. If the infestation becomes a nuisance, home owners may want to seek professional assistance in pest removal.