Stumphumping Horntail

On a sunny November Sunday, I photographed this wasp at Oaks Bottom Wildlife Refuge in Portland, Oregon. At first I wasn’t sure what it was, but it turned out to be a pigeon horntail. Here in Oregon, lumberjacks refer to these wasps as stumphumpers, or even stump*uckers. Great name for a wasp whose females drill into logs with a drill attached to their behinds.

Horntails, also known as wood wasps, are the misunderstood weirdos of the insect world. They look intimidating, like some kind of tactical wasp, with a long "stinger" jutting from their backside. But they don't sting. That tail is actually an ovipositor, a fancy term for the egg-laying drill bit that females use to bore into dead or dying trees. Inside, they deposit their eggs alongside a fungal cocktail that helps soften the wood for the larva that hatches. Basically, horntails are part carpenter, part fungus farmer.

Despite their frightening appearance, they're harmless to humans and disinterested in our picnics. The larvae tunnel slowly through wood for months, even years, living a life of quiet digestion before finally emerging as full-grown adults. Sometimes, a horntail emerges from furniture years after the tree was cut down. Imagine buying a bookshelf and finding a prehistoric-looking wasp crawling out of it like it just time-traveled through Ikea.

In Oregon and the Pacific Northwest, horntails show up in conifer forests and anywhere dead wood is left in peace.

Taxonomically, horntails belong to the family Siricidae, and they're closely tied to the health of forest ecosystems. While not flashy pollinators or charismatic megafauna, they play a role in breaking down dead trees and making way for new growth.

Meet the Pigeon Horntail

My stumphumper turned out to be a pigeon horntail, Tremex columba, also called the pigeon tremex. The name comes from Linnaeus, who gave it the species name columba, Latin for dove, back in the 1700s. Why a dove? Nobody seems to know. It looks about as much like a pigeon as a chainsaw looks like a teacup.

It is one of the biggest horntails in North America. A large female can stretch close to two inches, not counting the drill. She wears a rusty orange suit with yellow and black bands across her abdomen, orange legs, and a pale face, like a wasp dressed for Halloween. She is a hardwood specialist, drilling into dying and freshly dead maples, elms, beeches, oaks and other broadleaf trees, which makes the tangled bottomland woods of Oaks Bottom just her kind of neighborhood. Adults fly from late summer into fall, so a warm November afternoon is a good time to find one taking a stroll.

Not Quite a Wasp

Strictly speaking, horntails aren’t true wasps at all. They belong to the Symphyta, the sawflies and their kin, the oldest branch of the order that includes bees, ants and wasps. Look closely at a horntail and you’ll notice it has no “wasp waist.” Its thorax runs straight into its abdomen like a fuselage. Horntails skipped the corset. There are more than a hundred species of horntail around the world, most of them tied to the forests of the Northern Hemisphere.

The name is a bit of a misdirect, too. The “horn” in horntail isn’t the long drill. It’s a short, stiff spike at the tip of the abdomen that both males and females carry. Only the female gets the ovipositor, and she is the one lumberjacks are talking about when they call these insects stumphumpers. The males, meanwhile, spend much of their short adult lives up in the treetops, where hardly anyone ever sees them.

How to Drill a Tree With Your Rear End

The ovipositor isn’t a single spike. It’s a set of slender blades, edged with tiny teeth, that slide back and forth against each other like a pair of miniature reciprocating saws. The female braces her legs, arches her body, and works the blades into the bark and down into the wood, sometimes half an inch or more. It can take her many minutes to finish one hole, and she may drill several in the same tree.

It is not a job without risk. Now and then a female gets her drill stuck fast in the wood and can’t pull it out. Naturalists occasionally find them this way, dead at their post, still attached to the tree. It is the insect equivalent of dying at your desk.

Fungus Farmers

The fungus is the real secret of the horntail. Wood is hard to eat, and a horntail larva can’t digest it on its own. So the female carries spores of a wood-rotting fungus in tiny pouches at the base of her ovipositor. Each time she drills, she injects eggs and fungus together. By the time the larva hatches, the fungus has already begun breaking down the wood around it, and the larva eats its way through a tunnel of pre-digested timber. The pigeon horntail’s partner is a white-rot fungus called Cerrena unicolor, the mossy maze polypore, whose gray, fuzzy brackets you can find on dead hardwoods across North America. Next time you see one, there’s a fair chance a horntail planted it.

Some horntails have even learned to cheat. Females of the genus Xeris carry no fungus of their own. Instead, they lay their eggs in wood that other horntails have already inoculated, letting someone else do the farming. Every ecosystem has its freeloaders.

Horntails are also drawn to trouble. Many species home in on trees that are stressed, dying, or freshly burned, and the conifer horntails are often among the first insects to arrive after a forest fire. For a wood-borer, a scorched snag with its defenses down is an open invitation.

A Very Long Childhood

A horntail spends almost its entire life as a larva, a pale, cream-colored grub with a small spine on its tail end, chewing through the wood inside its private tunnel. Depending on the species and the wood, that can last one year, two, or more. When it is finally ready, it pupates just under the surface, then chews a perfectly round exit hole and flies off to spend a few short weeks as an adult. All that time in the dark, and the grand finale is a brief romance and, for the female, a lot of drilling.

Hunted Through Solid Wood

A larva buried deep in a log might seem safe, but horntails have their own specialists. The pigeon horntail’s nemesis is the giant ichneumon wasp, Megarhyssa, one of the most spectacular insects in North America. A female Megarhyssa trails an ovipositor that can be several times longer than her body, like a thread of black hair. She walks the trunk of a dead tree, sniffing out the scent of the horntail’s fungus, and when she finds a larva she threads that impossibly thin drill straight down through the wood and lays an egg on it. Her grub then eats the horntail grub. Nature, as usual, has no sentimental streak.

Conifer horntails face their own hit squad, from Rhyssa ichneumons to Ibalia, a parasitoid that takes a shortcut and sends its eggs down the holes the horntail mother already drilled. A rotting log may look quiet, but it is really a whole food web, complete with farmers, freeloaders and assassins.

Horntails of the Pacific Northwest

Oregon has several other native horntails, most of them working the conifers. The white-horned horntail, Urocerus albicornis, is a dark, glossy insect with pale antennae. Urocerus californicus is a larger, flashier species, and the blue-black Sirex horntails drill into firs and pines across the region. The pigeon horntail is the odd one out, a hardwood lover among conifer specialists.

Our native horntails are part of the cleanup crew in a healthy forest. Their European cousin is another story. Sirex noctilio, the European woodwasp, hitchhiked in shipping pallets and crates to Australia, New Zealand, South America and South Africa, where it has killed large numbers of plantation pines. It was found in New York in 2004 and has since spread across parts of the northeastern United States and Canada. One of the most effective defenses has been a parasitic nematode that sterilizes the female woodwasps, so that she drills into a tree and lays nematodes instead of eggs. Talk about a bad day at the office.

So if you see a stumphumper on a log this fall, let it be. It won’t sting you, it isn’t after your sandwich, and the dead tree it’s drilling into is exactly where it belongs.