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Wasps, fungi and directional drilling

The first time you see a female Western Giant Ichneumon wasp flying around the Kimberley Nature Park, you will likely be a bit nervous about the length of the "stinger" trailing behind its abdomen. Fortunately for humans, the stinger is really an egg-laying device (ovipositor) -- these parasitic wasps have no interest in large mammals like us. They are, however, quite interested in the larvae of the horntail wood wasp.

This large and colourful ichneuomon was found on a tree in Forest Crowne just outside the park boundary.

Horntails have a fascinating life cycle that begins when a female uses its own ovipositor to drill into a conifer tree and lay an egg beneath the bark. As the egg is laid, the female also injects a small amount of Amylostereum (white rot) fungus that it carries around in a special gland. The fungus spreads throughout the wood, softening it for the developing larvae and providing it with a source of food as it carves a tunnel under the bark.

The ichneumon wasp preys on horntail larvae, and the female flies from tree to tree sniffing for the chemical signature of the fungus. When it finds some, it lands and uses its antennae to detect the vibration of a munching wood wasp grub. When a grub is located, the ichneumon unfurls the ovipositor curling around its abdomen and positions itself at the bark surface. This egg-laying device is a marvel of evolution, and can probe several centimetres into the side of a tree.

In this photo a female ichneumon is drilling into the tree, a process that can take up to 40 minutes.

Penetrating the hard bark and wood of a conifer is not easy, but the ovipositor (which has two sections called valves joined by a tongue and groove) uses a reciprocating motion to push its way in. One section moves slightly forward and then anchors in the wood with toothy projections, while the other section slides past it further into the trunk. That section then anchors, and the first pushes forward again. There is some evidence that the tip of the probe secretes enzymes that soften the wood, and it appears that the wasp may also be able to change the direction of the tip as it "drills" to better locate the prey.

Chemical analysis of the cuticle of the ovipositor has revealed that it is metal-reinforced with manganese atoms that increase its durability and hardness. Once the ovipositor enters the tunnel of the horntail larvae, an egg is delivered to the surface of the grub where it soon hatches and begins to feast. Eventually an adult ichneumon emerges from the tree to fly about and mate and begin the cycle again.

A second female has joined the first and started probing for its own horntail larvae.

Ichneumon wasps make up a large family of insects with about 100,000 species worldwide. The Western Giant Incheumon, or Megarhyssa nortoni, is one of the largest and showiest species found in the nature park. Interestingly, the unique design of the ichneumon ovipositor has inspired the creation of a new class of surgical probes that are steerable and minimally damaging to human tissues.

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