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The Amazing Ichneumon

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Published on: May. 2, 2002

Last revision: Nov. 12, 2010

within an expandable sac at the end of its tail. In this pose, the wasp inserts its long, thin ovipositor into the dead wood. It is common for the wasp to penetrate more than 2 inches of wood to reach the horntail larvae.

A great deal of mystery surrounds the method used by the megarhyssa to penetrate wood with its ovipositor. Many scientists believe ichneumon wasps force their long ovipositors into a tree by following cracks and crevices in the wood.

A group of researchers have found some evidence that the tips of the ovipositors may be hardened with ionized manganese or zinc, like metal drill bits. Their studies are not yet complete, but their work suggests that the wasps could be using metal "tools" to drill into wood.

When the ichneumon female determines it has found its target, it begins laying eggs. The egg is forced through the inside of the small ovipositor and into the host larva. Experts believe the egg-laying process in the megarhyssa species takes from seven minutes to 5 1/2 hours depending upon the depth it must drill to locate the host larvae.

The ichneumon wasp develops from the larval stage to the adult stage inside the horntail tunnel. This development is called pupation. The larvae feed on the blood and soft tissue of the horntail wasp larvae. When the ichneumon wasp matures in early spring, it chews through the wood fiber to emerge outside.

Ichneumon wasps are truly a study in contradictions. They look frightening, but they are harmless to people. They are highly numerous, but seldom seen. They are great allies against insect pests, yet few people know about them. Females penetrate wood with tiny ovipositors, but scientists don't fully understand how. And while they are common in the world of insects, they are certainly "uncommon" to those of us who have observed their amazing behavior first hand.

Studying them has been a fascinating pleasure. I once found a broken ovipositor projecting from a log. Using a chisel, I followed the ovipositor 2 1/4 inches straight into the log. The ovipositor did not seem to follow a crack in the wood, but looked like it had been inserted directly into the tree. The ovipositor ended in a tunnel that appeared to be a larval tunnel of a horntail fly. I found dried insect parts inside the tunnel.

The following spring, the trunk of the dead tree where I observed the Ichneumon females laying eggs was riddled with holes where the mature ichneumons presumably exited to begin their life cycle anew.

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