Summary: Horntail wasps are another insect with an intimidating sounding name and appearance, but are actually quite harmless. The short barb on their rear end is not a stinger, but a remnant of their larval stage. Female horntail wasps have a long stinger-like ovipositor on their tails used to lay eggs.
Horntail wasps are loud sounding, ferocious looking insects that are also known as wood wasps because they lay their eggs under the bark of dead or dying conifer trees. Fire damaged pine, spruce, and fir trees are common targets for a female horntail wasp looking to lay its eggs. Trees that have been damaged by drought, disease, other insects, or recently chapped down for lumber are other sites conducive for the egg-laying wasps.
The larvae of a horntail wasps looks like a caterpillar or grub. They are milky white and feed on the softwood and hardwood under the bark of a tree. They have a short spike on the end of their body that helps them to push through the wood while they chew foot-long and quarter inch wide tunnels. The larvae spend two to five years feeding inside the wood before chewing their way to the surface and emerging as adults.
Adult horntails are about an inch long, with the short spike on their back. They are black or a metallic dark blue color and have yellow or reddish brown spots on their abdomen. Females also have a very long ovipositor on their abdomen which looks like a long stinger.
When the horntail wasps emerge as adults they can become a nuisance to people. On occasion, fire-damaged trees are salvaged for use in building homes. The outside of the trees are burnt, but the wood inside is still usable. Horntail wasps often lay their eggs in trees that have been recently killed by fire or other causes. When these trees are used to build homes the wasp eggs incubating inside the wood continue to develop. After their two to five years the wasps emerge inside the home much to the surprise of the homeowners. A pencil-sized hole is left in the wall from where the wasps emerge. The horntail wasps have such strong jaws that they can still chew their way through plastic siding, sheetrock, or even soft metal.
Sometimes there will be many wasps that emerge all at once, filling a house with buzzing that can frighten children and parents alike. The emerging wasps leave many small holes looking like a shotgun was discharged inside. Your friends will be wondering who was taking target practice in the den.
What is impressive about horntail wasps is that they are able to lay their eggs deep into the wood of a tree where they will be protected from most predators. However, there are still some threats to the larvae. Woodpeckers like to snack on the horntail grub, and there is a parasitic wasp called an Ichneumon which lays its eggs onto horntail wasp grubs.
The most impressive physical feat is the female horntail's ability to pierce the hard bark and wood of a tree. She squats on a log and sticks her ovipositor into it, wiggling and pushing until it has reached the desired depth. This process takes about ten minutes and it has provided the horntail wasps with the nickname of œstump humpers. The female repeats this behavior several times, laying eggs in different locations throughout a log.
Horntail wasps are not a threat to humans and pest control is not necessary even when they turn up indoors. The worst that they can do is damage wood by feeding on it as a larvae and tunneling out when they are an adult. This damage is rarely severe even in large infestations. The horntail wasps do not lay eggs into pressure treated, kiln fired wood, or wood that has been treated with a borate like Boracare. They also will not lay eggs into wood that is stripped of bark. For these reasons it is highly unlikely that horntail wasps will continue to infest a house after they have emerged from their tunnels. Let them escape out of a door or window. Fill up the holes with plaster of Paris, or wood putty and paint over it. You will barely notice a hole if you do it right. Once all of the horntail wasps have emerged they will not return.