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October/November 2011

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Bluff and bluster!

The buzzing wood wasp is occasionally encountered in gardens, writes Dick Warner

Wood wasps, also called horntails, are pretty scary insects. They're large, they're fast and noisy fliers, they come in yellow and black warning colours and they have what looks like a ferocious stinger on their rear ends. But, although they're doing their best to frighten you off, they're completely harmless. 


I have been describing female wood wasps. Males aren't encountered very often because they lie low until the sun shines and then fly around in the tree tops looking for a female to mate with. They look like a totally different species and are much less scary. They are considerably smaller and lack the yellow and black wasp colours. Both sexes have a characteristic sausage-shaped body without the slim waist of a true wasp. And both sexes have a spike or horn on their tail, but it isn't a stinger - in fact its purpose seems to be a bit of a mystery. In addition the females have a needle-like ovipositor that trails behind them in flight.

This ovipositor is a remarkable organ. A mated female will fly around looking for a suitable tree branch or log in which to lay her eggs. She prefers softwoods to hardwoods and is particularly fond of dead or unhealthy wood. When she finds something to her liking she lands, stretches upwards on her legs and bends her ovipositor at right angles to her body. She then drills down into the wood for a considerable distance, usually a couple of centimetres.

Sometimes she doesn't like what she finds and flies off to do another test boring. But if she's satisfied she'll squeeze out an egg, down through the middle of the ovipositor and deep into the wood. At the same time she squeezes out a drop of a fungus that she's been culturing in a special gland near the base of the ovipositor. She'll do this maybe fifty to a hundred times until all her eggs are laid.

The fungus immediately starts to attack the wood around the egg which, after five or six weeks, hatches out into a whitish grub or larva. The grub feeds on the timber which the fungus has softened, slowly growing in size and slowly moving through the timber in a tunnel which increases in diameter as the grub gets bigger. In Ireland the grubs normally stay in the timber for two to three years. At the end of this period they tunnel up to a position near the surface of the timber and pupate inside a cocoon. The winged insect then bites its way to freedom through the remaining layer of timber or bark.

This is the stage at which wood wasps most commonly come into contact with humans. Female wood wasps are particularly fond of laying their eggs in logs that have been felled by foresters. While the grub is growing these logs often become structural timbers that are incorporated into buildings. When the next generation emerges it sometimes finds itself indoors.

And although the grub burrows close to the surface of the timber before it pupates to make it easier for the adult to chew its way free this doesn't appear to be absolutely necessary because the adults are very competent when it comes to escaping. There is a case on record where the timber in a building was encased in sheet lead and a wood wasp chewed its way out through the metal.

The tunnels that the grubs make are not very long and don't seem to weaken the timber to any significant extent, though they can look a little unsightly when it's sawn. But really this can be classified as a totally harmless and rather magnificent insect. There is more than one species of wood wasp in Ireland but the one I've been describing and the one you're most likely to encounter is the greater horntail. The wood-tunnelling grubs are a favourite food of woodpeckers so, with the recent return of the greater spotted woodpecker to Ireland, wood wasps may become a little less common.


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