’Much maligned horntail stirs up a hornet’s nest

WHEN you have a reputation for knowing about wildlife, you get emergency calls.

The other day a neighbour phoned on her mobile because she was terrified to go into her house: there was an enormous hornet in her kitchen.

This was interesting. There are no hornets in Ireland. They are confined to Continental Europe and North America. But the giant Asian hornet is invading France and Spain. It is devastating honey-bee colonies and has killed two Frenchmen this year, after apparently arriving from China in a crate of pottery. Could it have arrived in Ireland? But when I bravely entered the neighbour’s kitchen, I was relieved the offending insect was not a hornet but a female wood wasp. The wood wasp, or greater horntail, is a scary insect. They are large and noisy, they come in yellow-and-black warning colours, and they have what looks like a vicious stinger on their rear end. But it’s all a bluff — they are harmless insects with a fascinating life cycle.

Male wood wasps, which are seldom encountered by people, are much smaller and less scary; they look like a different species. Both sexes have a single spine or ‘horn’ on their rear end, which is not a sting, though what it’s for seems to be a mystery. Females have a long ovipositor, which trails behind when they’re flying. This is a remarkable organ.

A mated female will fly around prospecting for a suitable tree or log in which to lay her eggs. She’s quite fussy and prefers softwood to hardwood or timber that’s dead or unhealthy. When she finds what she wants, she stretches upwards on her legs, turns her ovipositor at right angles to her body and begins to bore a hole with it. This hole usually penetrates a couple of centimetres into the wood.

If she’s not happy with the timber, she’ll move on and bore another test hole. But if she likes what she finds, she’ll squeeze a single egg down the inside of the ovipositor. She accompanies this with a drop of a fungus that she’s been cultivating in a gland close to the base of the ovipositor. She lays 50 to 100 eggs each in an individual hole.

The fungus attacks the timber, and after five or six weeks the egg hatches into a larva that starts feeding on the softened wood. The larva moves slowly through the timber in a tunnel, which gets bigger as it gets fatter, for several years. Then, it tunnels up close to the surface of the timber, spins a cocoon and pupates. When the pupa hatches into a winged insect, it bites its way out and flies off.

The fussy, egg-laying females are particularly fond of using logs of spruce or pine that have been felled by foresters. During the years that the larvae are tunnelling away, the logs often get taken to a saw-mill and turned into building timber. This means that when the adult emerges, it may find itself inside your kitchen, scaring the be-jasus out of people with its hornet mimicry.

The larva tunnels close to the surface of the timber to pupate to make it easier for the adult to bite its way to freedom. This precaution may not be necessary, because it takes a lot to stop a determined wood wasp. There’s even a case on record of one biting its way out of timber encased in sheet lead.

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