PPDL Picture of the Week for
October 3, 2011

The Incredible Ichneumon

Timothy J. Gibb, Insect Diagnostician, Entomology, Purdue University

Every so often, if we take the time, we have the unique opportunity to witness one of nature's wonders. A home owner recently submitted the accompanying photographs taken of several giant ichneumonid wasps on a recently cut maple stump in the act of parasitizing wood boring insects deep inside the log. He had taken the time to watch and ponder upon a truly awesome phenomenon.

Female ichneumon wasps possess a very long flexible ovipositor (the egg laying/people stinging organ in many wasp species). In this species these long hair-like tails are several inches in length and - as you can see in the photos- can be bent up around the rear end of the wasp and slowly forced straight down into the hard wood of the tree.

Just how the wasp locates this host larva and how it forces its long flexible ovipositor into solid wood are two of the great marvels in entomology. Even Charles Darwin pondered on the spectacular achievements of the ichneumon wasp as he developed his early ideas about nature and evolution.

We have since learned that the female locates her potential host by sensing the vibrations of the moving larva deep within solid wood using her antennae in a tapping motion much like a carpenter might search for studs within a hollow wall. Once located, she drills her ovipositor into the solid substrate to the exact depth where it just touches the wood-borer. She then injects a small egg through the long hollow ovipositing tube and places it precisely on the back of the wood borer. Like other parasitiod wasps, the egg hatches and the resulting larva enters into and devours its host from the inside.

How a female is able to drill with her apparently soft ovipositor into solid wood is equally marvelous. Recent research has found metal (ionized manganese or zinc) concentrated in the extreme tip of some species' ovipositors. This may explain, in part, how such an incredible phenomenon can occur. Interestingly, high metal concentrations are not limited to the female's ovipositor. Hardened mandibles of the developing adult are also found to be possessed with metals and it uses these to chew itself back out of the wood once its development is complete.

Nature is full of such stories, if we take the time to ponder and observe. Thanks for sending the photos of the incredible ichneumon. We are all rewarded.

Click image to enlarge

Fig. 1. Ichneumonid wasps

Fig 1. Ichneumonid wasps. Note the length of the ovipositor (up to 4 inches).

Fig. 2. Wood boring ichneumonid wasps

Fig 2. Ichneumonid wasps inserting ovipositors into infested wood.

 

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service