Now that spring is—at last—in the air, so too are the insects! One group about to take to the skies that you may not have encountered in the Arboretum is the Ichneumonidae, a family of parasitoid wasps. Approximately 65,000 described species of wasps engage in parasitoidism—25,000 of which are Ichneumons—almost half the total known species in this order. Parasitoids differ from parasites in that their consumption of a host always results in that host’s death. 

We have two species of ichneumon wasps in the Arb: the black giant and the long-tailed giant ichneumonid wasp. These species have bodies of about two inches long, and ovipositors (egg-laying instruments) that can reach lengths of up to five inches. But fear not! These wasps are harmless to humans, and do not even posses a stinger. Most parasitoid wasps are quite beneficial, as they oviposit into and kill the larvae of a variety of species, including numerous agricultural pests. 

Both of our species parasitize the larvae of the wood wasp, a common denizen of dead or dying deciduous trees. Females are able to detect the movement of wood wasp larvae through the wood, and drill their ovipositors into the tree to inject eggs into the larvae. Ichneumonid larvae develop over a few weeks, consuming their host and emerging as an adult by early summer. 

Female ichneumon wasps do not have to seek out natural fissures or holes in the wood to insert their ovipositors. When the females begin “drilling” into the wood, an abdominal membrane inflates, pushing the ovipositor into the tree. The internal surface of this membrane is comprised of a thin layer of tissue that secretes an enzyme that “digests” wood. This enzyme softens the wood around the ovipositor allowing the wasp to push the filament into formerly hard tree tissue. Despite this enzyme, a successful oviposition and extraction can take up to 45 minutes, during which time the wasp is extremely vulnerable to predation. If you look carefully at the trunk of a dead tree you can often find protruding ovipositors of ichneumons that met an untimely end at the hands of some lucky bird.