A Parasitic yet Useful Wasp

TRUE, the ichneumon wasp is strange looking, almost frightening. But why is it labeled parasitic? Because it usually propagates by laying an egg on or in the larva of another insect or a spider.

In North America there are more than 3,000 species of true ichneumon wasps. These wasps, in turn, are part of a large family of different types of parasitic wasps. Scientists estimate that worldwide more than 40,000 species belong to this insect family.

Ichneumon wasps vary in size from about an eighth of an inch to two inches [0.3 to 5 cm] long. Their slender, curved abdomen is longer than the head and thorax together. Ichneumons are different from stinger wasps in that they have longer antennae.

The most distinguishing feature of ichneumons is a needlelike tube at the end of the abdomen. Called an ovipositor, this egg-laying structure is often longer than the body. It is no thicker than a horsehair and has three threadlike strands that slide back and forth to force the egg down the tube.

How does the ichneumon detect the larva of a potential host? The female Megarhyssa wasp, a genus of ichneumon, has been observed tapping a tree with her antennae to pick up vibrations of a larva living an inch [2 cm] or more beneath the tree bark. Once she senses that there is a larva, she intensifies the tapping. Eventually, she begins to probe into the bark with the tube, as if drilling into it.

Observers have noted: “Once the tip of the wasp’s ovipositor touches the larva, a single egg is squeezed down the tube, to be planted next to, or on top of, the doomed host.” When the egg hatches, the new larva feeds on the host larva’s fats and body fluids. Then it spins a silken cocoon in which it will develop into an adult wasp. When the wasp reaches the surface of the tree, it is ready to plague a new generation of insects.

Although one may describe this insect group as ruthless parasites, ichneumon wasps serve an important purpose. Their larvae feed on insects that are harmful to food crops, including chinch bugs, boll weevils, codling moths, and asparagus beetles, to name a few. So ichneumons evidently control the proliferation of agricultural pests.

Even though ichneumons are very numerous, they are seldom seen by people because they generally feed, breed, and lay eggs in habitats seldom visited by humans. Ichneumons thus serve as another example of the diversity and balance of living things that man has yet to comprehend fully.

[Picture on page 24]

Ichneumon wasp preparing to lay its egg

[Credit Line]

Scott Bauer/Agricultural Research Service, USDA