I recently joined some work colleagues for a farewell celebration in the courtyard of one of Helena’s public houses along Last Chance Gulch. One of our party noticed that we were sitting under a deciduous tree that seemed to have been dead for several years.
Presently, we were visited by a very large wasp, which several in the group found quite disturbing.
I recognized it and knew that in spite of its formidable “stinger” this wasp is nonaggressive and harmless—at least to people. The four-inch-long stinger is actually an appendage to deposit eggs deep into dead wood, which has earned it the nickname of “stump-humper” wasp.
The roughly 4,000 species of Ichneumon wasps are all parasitic and they lay eggs either directly onto or even into their hosts. This wasp was probably parasitizing a boring grub in the dead tree.
I was later able to identify the species as probably Megarhyssa macrurus,
from this website.
The ovipositor appendage actually has three components. Two are sheaths that contain the actual ovipositor, which although flexible is able to penetrate deep into hard, dead wood. The sheaths remain outside the wood and perhaps assist in penetration. How the female wasp is able to locate its host larva and force her flexible ovipositor to the precise location remains a mystery, and is one of the great marvels of entomology.
Extremely sensitive antennae apparently help identify the sounds of the grubs working within the wood. These wasps can be highly specific as to the species of larvae they parasitize. The length of the ovipositor reaches into the depths that the target larvae prefer.
One insect researcher out of Purdue University has identified manganese metal on the extreme tip of certain species’ ovipositors, which may assist in penetrating the wood. Metal has also been found on the mandibles of recently transformed adults to assist in emerging from the wood.
A single egg is laid directly on a specific host larva. Upon hatching, the wasp larva begins feeding on the host throughout the winter, gathering all the nutrients it needs. In the spring the wasp larvae pupates, kills the host, and emerges as an adult.
Male wasps of these species lack the long ovipositor and emerge early where they wait on the dead wood, listening for emerging females. Copulation occurs as the female wasps emerge. The adults do not feed and live only long enough to mate and continue the cycle.
Oviposition is clearly energy-intensive and the insect is vulnerable to predation during this time. The female wasp opens a distal section of her abdomen that appears to assist in cocking the stinger to penetrate the hard material.
We witnessed two ovipositions by this wasp into the tree. Each episode lasted 15 to 20 minutes and was fascinating to observe.
According to legend, the great evolutionary theorist Charles Darwin’s faith in a benevolent God was shaken by the behaviors of the Ichneumonidae family of wasps. These wasps were almost certainly the inspiration for the “Alien” movies, and more recently the British television series, “Fortitude.”
One species that parasitizes the adult orb-weaving spider causes the arachnid host to weave a very atypical web. When the wasp larva emerges from the dying host spider’s abdomen, it is welcomed by a more durable modified web from which it will complete a cocoon and emerge as an adult. According to Newsweek’s report on the research, the wasp derails the “zombie” spider’s natural web weaving to better benefit itself.
For more video footage of these monstrous wasps, visit this site, courtesy of Wired Magazine.
Paul J. Driscoll, a native of Butte, is a public information officer for the Department of Environmental Quality in Helena. He is also a writer, editor, essayist and former editorial cartoonist. He lives in Montana City.