The gruesome relationship between the Wood Wasp and Ichneumon Fly

This edited article about the wood wasp originally appeared in Look and Learn issue number 711 published on 30 August 1975.

The Wood Wasp and the Ichneumon Fly – the hunter and the hunted, by R B Davis

The flying insect settled on a fallen pine tree in the forest. Black and yellow in colour, it looked like a large hornet. Its name was urocerus gigas, commonly called the great horntail or wood wasp although it was not really a wasp but a kind of sawfly.

It was about 1 and a quarter inches (3cm) long and had what appeared to be a fearsome-looking sting at the end of its body – this was actually an egg-laying instrument, called an ovipositor.

The Wood Wasp was searching for a place to lay its eggs and after probing the bark for several minutes it finally decided on a suitable spot, bored into the wood with its ovipositor and deposited six eggs.

A month later, when the eggs had hatched and the grubs (or larvae) were feeding deep in the wood, another insect visitor arrived on the scene. This time it was a black, long-legged creature, an ichneumon called rhyssa persuasoria, which immediately began a close scrutiny of the bark, its long antennae waving and tapping the surface.

As it approached the spot where one of the grubs was located it became very excited and circled round and round, its whole body and wings quivering.

Eventually, deciding it was now ‘on target’ it unsheathed its long slim ovipositor and drilled down into the wood until the tip reached the larva below. After stinging the unsuspecting larva to partially paralyse it an egg was passed down the fine tube and placed on the body of the larva. The whole operation took about twenty minutes.

In two days the egg hatched and then began a rather gruesome episode. The tiny ichneumon larva roamed over the body of its host for a short time before it settled down to sink its prominent jaws into its unfortunate victim and began sucking its juices. At first, it concentrated on the non-essential organs so that its prey could continue to live and the supply of fresh food would last as long as possible.

On this diet the ichneumon larva grew rapidly and in only three weeks the wood wasp was just an empty shell with the parasite occupying the whole of the space it had previously occupied.

Both the wood wasp and the ichneumon are found in most parts of Britain where there are conifer woods.

How does the ichneumon manage to locate a wood wasp larva buried deep in solid wood? Is it smell, hearing or some sixth sense which we know nothing at all about? This remains one of nature’s most puzzling mysteries.

It is also remarkable that the slender and delicate-looking ovipositor is able to bore into the wood so efficiently.