Horntails
Horntails, also called wood wasps or Siricids, highly resemble wasps in appearance and although they belong to the same order as wasps (i.e. Hymenoptera), they are actually a type of sawfly and neither the male nor the female bites or stings. Sawflies are so-named because of their saw-like ovipositor which the females use to cut into plants when laying their eggs. Horntail females lay their eggs singly–to a depth of as much as 0.75 inches–in either hardwood or coniferous trees, especially if the trees are weakened, dying, or have been recently cut. Each female can lay from 300 to 400 eggs in all. The resulting larvae chew their way into the sapwood and heartwood of the tree, and do not complete their larval stage for two to three years. They form their cocoon and pupate inside the tree as well, emerging as adults in late summer/early fall.
Although the larval tunnels of horntails can range anywhere from one to two feet in length, horntail numbers are rarely so great as to cause significant damage overall.
Additional Information: Horntails (Cornell) — see also the YouTube video showing a female horntail maneuvering her ovipositor into a tree.