Scary Horntail Wasps

A common theme in many of our articles on BugInfo is that there is a place for all living creatures, and in some way they are important in their natural environment, whether as pollinators, predators, parasites, or recyclers. It is when they are removed from that natural environment in which they have lived for hundreds of thousands of years, and placed into a different environment where they do not belong, that major problems can develop. It is difficult for most people to have an extensive knowledge of the world of bugs, and that is the reason we hope to explain many of the things you can observe in and around your home so you can help to make the proper decisions on whether or not to have them eliminated.

An insect that shows up on a regular basis in homes, and which almost always is the cause of a great deal of concern for the occupants of that home, is the Horntail Wasp. This is a fairly large wasp in a family called the Siricidae, also known as Wood Wasps, and there are many different species of them in the United States. Sometimes they may even begin to show up in fairly large numbers in the home, flying around with their loud "buzzing" flight and banging into windows in their attempt to escape. If this happens to you, then a little bit of careful inspecting of the walls of the house may reveal some round, pencil-sized holes in the walls that were not there before, evidence that these insects have come from the wood your house is built with.

The Siricidae are given their common names for these reasons:

  • Wood Wasp - the larvae feed on dead wood
  • Horntail Wasp - the females have very long tails that are actually their stout egg-laying organs called the "ovipositor"

Wonderful - so how did it end up in my house?

The life of the wood wasp begins when a tree dies. This may be from old age, from disease, insect attack, a forest fire, or when someone cuts the tree down for lumber or firewood. Once a tree dies it's chemistry changes, and chemical signals are given off that alert the female wood wasp that a new food source for its larvae has been provided. She quickly seeks out the available tree and lays her eggs in the bark. Apparently these wasps only lay eggs on trees that still have their bark on them, and this is a key to the relative importance of these wasps in structures.

I have talked with people who have been firefighters, working in forests that are being destroyed by fire, and they have seen tree trunks still smoking, but with the large wood wasps flying onto them to lay their eggs. The process of recycling that recently killed tree begins very quickly, as Nature is anxious to get the nutrients back into the soil for re-use. The larvae of the wasp hatch from the eggs and bore into the inner wood of the tree, where they now live for one to three years, eating their way through the solid wood. When they have reached the end of their life as a larva they bore closer to the surface of the wood and transform to the stage called the pupa, and shortly after this the adult wasp emerges and chews its way out of the wood with its very strong jaws.

Now, imagine where the 2 x 4 boards in the walls of your home have come from. A tree is cut down and hauled to the lumber yard, where it may sit for some time before it finally is cut into lumber. Quite often lumber companies are even able to use trees that have been killed by fire or bark beetle attack, as the inner wood of the tree is still quite sound, even though the tree is dead. In this latter case the tree may very well already have been "infested" by the eggs of the wood wasp or by some of the many kinds of wood-boring beetles that we will discuss in another BugInfo article. The wasps also may have sought out the logs as they lay in the lumber yard, still with their bark on them, and the eggs could have been placed at this time. But, either way and unknown to the lumber company, the tree is now cut into lumber and the boards laid out to cure for awhile.

Much of the lumber may be "kiln dried" or "pressure treated" for more specific uses, and these processes may kill most of the hitch-hiking insects inside. However, a lot of the lumber is simply air-dried and the wasp larvae survive, soon to be sold and built into a new home or office structure, usually as the hidden wood within walls or attic areas. The larva moves through its normal cycle, perhaps even accelerated a little bit by the warmth of the walls, and the adult wasp emerges. Something is wrong though. The wasp has chewed out of the wood it grew up in, but for some reason it still has not found Light. So, it keeps chewing through whatever is in the way - stucco, sheetrock, layers of paint, wallpaper, ceramic tiles, linoleum - it all falls to the strong, chewing jaws of these large wasps, and the wasp finally sees the light that signals it is free to fly away and find a new tree for another generation, leaving you with a hole in your wall or floor.

So, what are some of the key points?

A major point to make is that the Horntail Wasps will not lay their eggs on lumber that has already been milled. They oviposit only on the bark of trees, and therefore the wasps cannot fly into your home and begin the infestation, nor can they emerge from the wood in your home and lay eggs back into it. Their presence in the wood of a structure always is the result of their being built in with wood that was already infested. So, the good news is that you are not faced with an perpetual problem if you happen to be the recipient of some unwanted wood wasps.

A second point is that the life cycle of the wood wasp generally is completed in under three years. Now, there can be some oddities in nature whereby insects go dormant for awhile and don't change to the adult stage for several years past a normal length of time, but almost always I have seen these wasps coming from the walls of houses that had been built within the last couple of years. If your home is older than that and you find a wood wasp inside it likely just wandered in from outside, and now desperately wishes it could return to the outside environment.

That's the Good News, what's the Bad News?

The bad news is that, potentially, there can be many dozens of the wasps growing within the wood of a single home. I witnessed this one time in a year when the wood wasp population seemed to be at an all-time high, and a lot of people were experiencing encounters with the wasps. One home I went to had had at least three dozen of the wasps emerge over a period of several weeks, leaving the holes in the walls of every room in the house. There had been some major kills of forest trees in the California mountains the year before, so it could well be that more "salvage" logging had taken place, using trees already with the wasp eggs in them. This has been the expressed opinion of one beetle expert with the University of California, that salvage logging could lead to increased beetle activity in homes, but again we'll explore that in a different article.

What should we do?

While I realize it is very difficult to ignore large, shiny, long-tailed and big-jawed, buzzing wasps flying around in your home, the very best course of action with respect to Horntail Wasps is to let the infestation run its course. There may only be one or two wasps in the entire structure, and it will be over and done with very quickly, or there could be dozens. There really is, at this time, no way to determine just how many of the larvae are still in the walls after you have seen the first couple of adults pop out, so demanding a guarantee that is has ended is really not possible.

Should you decide that you absolutely, positively must have the remaining larvae in the walls killed, so that you don't have to deal with any more adults or their holes in the walls, the only guaranteed method is to fumigate the entire structure, and this is an expensive process that requires you to vacate the home for several days while it is tented and fumigated with a material that can penetrate into the wood members of the walls and attic areas. Even this may be a little bit of a gamble, for there probably is not testing of the fumigants on insect pests that are of such minor importance, and how well the fumigant actually kills these large larvae may not be known.

The better action on your part is to let the life cycle of the wasps complete itself, vacuum up the wasps as they emerge or open the door and let them out, and then fill in the holes they have created and repaint the spot. The wood wasps are not stinging wasps, and therefore they really pose no threat to people or pets. Even though they have what looks like an enormous "stinger" sticking out their back end the device is used only for laying eggs, and has no defensive purpose. Also, even though they have strong jaws there is no reason whatsoever for them to bite people, unless you trapped it in your hand and it was attempting to escape. Since the life cycle is fairly short, within a few months you probably will be all done with the emergence of the adults, and you can put the hole putty and plaster of Paris away.

We need Nature's recycling crew, whether it is the wasps or the big beetles. Look for an article on The Kinds of Beetles in Your Walls on BugInfo, and you will find some more interesting information there. Without these kinds of insects returning dead plant material to the soil our environment simply would not function correctly. We really appreciate that you've taken the time to better understand the Horntail Wasps by reading this article, and that you will be better equipped to deal with them now.