The Carlisle Mosquito Online

Friday, August 4, 2006


Giant ichneumon wasp

Name: Three species of giant ichneumon are common in the eastern U.S. Megarhyssa atrata is the largest. Mega means great; rhyssa means tail; and the species name, atrata, means clothed in black. This is a large black wasp with a great "tail." They look a bit fearsome but they won't bother you. The males just want to find females, and the females just want to find places to lay eggs.

When and where seen: I first heard about this wasp at a Boston Mycological Club meeting over a year ago. I didn't think of it again until June 11 when I was astride a beech log in Great Brook Farm State Park along the Tophet trail. I looked down on five alarmingly large and contorted insects. I moved carefully away. The insects stayed in place and I inched back towards them. They were giant ichneumons (pronounced like Rick Newman if his "R" had been stolen) and they were laying eggs. Four of the five wasps turned out to be Megarhyssa atrata and the fifth was Megarhyssa macrurus. If you spend a lot of time in the woods you can become fond of individual logs. On June 16 and 17 I revisited my ichneumon log and M. atrata was there again. On July 27 I was surprised to see egg-laying activity still going on.

Identifying characteristics: The first obvious feature of the giant ichneumon is the large size. The body is about an inch and a half long, the antennae add another inch, and the "tail," which is an egg-laying device called an ovipositor, is four or more inches. The head, the antennae, and the lower sections of the legs are yellowy-orange. The eyes are brown and the rest of the wasp is black. Sometimes you may see a large wasp fly by and just get a glimpse of long legs trailing below it and the extremely long ovipositor trailing behind it like an airplane with a banner. This would be a female giant ichneumon. The males are a little smaller and don't have the ovipositor.

Fatal attraction: Most ichneumon wasps are parasites of other insects. Giant ichneumons are highly specialized for the larvae of Tremex columba, commonly known as the horntail wasp. Here's why the Boston Mycological Club was interested. The female horntail has a pouch in which she carries spores of a wood-rotting fungus. When she lays her eggs in a damaged tree or log, she also deposits some fungal spores. The spores germinate and the fungus starts breaking down the wood. As the fungus consumes the lignin, it softens the wood for the horntail larvae to burrow into and consume the cellulose. That's a nice partnership, and the fungus also benefits from spore dispersal. But there's always trouble in paradise! Tell-tale pheromones from the fungus attract the giant ichneumon. If the fungus is present, chances are good that horntail larvae may also be there. The giant ichneumon walks slowly along the log tapping it with her antennae. When she detects larval activity she inserts her ovipositor into the wood and lays her eggs on or near the horntail larva. Her larva eats the horntail larva, slowly, and in a deliberate manner leaving the organs till last and thereby not killing the host until it is ready to pupate. The new adult giant ichneumon emerges from the log the following spring.
Megarhyssa macrurus (Photo by Kay Fairweather)

Egg-laying: The giant ichneumon is so focused on her egg-laying that it is possible to get very close and watch. When she has selected a spot, she stands tall like a house on stilts. She lifts her ovipositor high in the air and loops it over to get a straight-down angle on the log. In my observations, she was aiming for an existing hole. She drives the three-stranded ovipositor down into the log and pushes it as close as possible to a horntail grub. The whole process takes more than 30 minutes. Some of the wasps I watched struggled a bit in retracting their ovipositors. They don't always succeed. I found one hole with an ovipositor sans wasp.

Competition: At first I didn't understand why two species of giant ichneumons within the same square foot would be seemingly content in going after the same horntail population. It turns out that they are not direct competitors. The horntail grubs bore into the softened wood (thanks to the fungus) to different depths and then travel along the grain. Each species of giant ichneumon has a different length ovipositor and each likes to select larvae at the maximum distance it can reach.

References: Borror and DeLong's Introduction to the Study of Insects; Howard E Evans, Insect Biology; Leon Shernoff, Mushroom, the Journal of Wild Mushrooming, Winter 2005; Lloyd Eighme, Ichneumon Wasps;

Please feel free to write the Biodiversity Corner. The only requirements are that the topic is something that exists in the wild and was seen in Carlisle. If you don't want to write the column, send photos, sightings, or ideas for the column to Kay Fairweather at

2006 The Carlisle Mosquito