Bug o’the Week – Therion Wasp

Greetings, BugFans,

The BugLady stood on a boardwalk in a wetland for about half an hour trying to photograph this amazing wasp as it dodged through thickets of sedges, ferns, and orchids, staying below knee-level, never landing.  This is the best picture she got.  She ID’d it as an ichneumon wasp in the genus Therion.

A quick review of Ichneumon wasps, since we haven’t been there for a while.  Ichneumonidae is a very large family indeed, with 25,000 known species worldwide (5,000 of them in North America) and as many as 75,000 additional species that may be waiting to be discovered!  They live everywhere they can find prey (except Antarctica), and there are so many species spread out over so many areas that the life stories of most are not known.

They range in length from 1/8th of an inch to 3 inches, and some are very colorful.  Many have long, slim abdomens that flare at the end, some tipped with impressive ovipositors https://bugguide.net/node/view/2001653/bgimagehttps://bugguide.net/node/view/1950499/bgimagehttps://bugguide.net/node/view/1828581/bgimage.  With a few exceptions, Ichneumons don’t sting, and you have to mess pretty seriously with those that do in order to get them to do it.  Their long antennae (often at least half as long as their body) have lots more segments than do those of other wasps, and many species have a white or yellow band midway along each https://bugguide.net/node/view/1924235/bgimage.  They’re tough to identify – the Field Guide to Insects of North America says that most of the photos submitted to be used in the book could not be identified, even by experts.

“Ichneumon” comes from a Latin word that was derived from an Ancient Greek word meaning “tracker,” so-named because of the way females hunt for other invertebrates.  The larvae of Ichneumons are parasitoids – sometimes on spiders, but mostly on insects that have complete metamorphosis (egg–larva-pupa–adult).

She lays her egg, often accompanied by a shot of venom, on or in her prey, usually the larva of a beetle, butterfly/moth, or another wasp; and many species of ichneumon target particular genera or groups of prey.  When it hatches, her larva starts feeding on the non-essential parts of its host, keeping it alive by leaving the vital organs for last.  Without ichneumons, the world would be overrun by caterpillars.

Adults feed on nectar, sap, and water (some species also eat pollen), and bugguide.net tells us that many adult ichneumons overwinter as adults under loose tree bark.

There’s a genus of Ichneumon wasps that is named Ichneumon, and the resulting confusion between genus and family has caused some Ichneumonid experts to refer to the family as Darwin wasps.  Darwin, along with his Victorian contemporaries, was so horrified by the parasitoid lifestyle that he famously said: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars.”

Most sources list 14 species in the genus Therion in North America (20 globally), and they can be difficult to tell apart, so the BugLady assumed her well-worn seat, way out on that taxonomic limb and guessed that this is Therion circumflexum (no common name).  She felt better about her guess when she found a 1973 paper by C. N. Slobodchikoff, in which he states that “It has recently been shown that all North American Therion can be considered as members of a single species Therion circumflexum,”and he refers to the rest of the named “species” as morphological types.  Apparently, it’s been re-thought since then.  There’s not a huge amount of information available about the species, but at least the contributors to bugguide.net have gotten better pictures of it than the BugLady did: https://bugguide.net/node/view/644490/bgpagehttps://bugguide.net/node/view/752769/bgimagehttps://bugguide.net/node/view/482314/bgimage.

Therion circumflexum (“circumflexum” is Latin for “bent about”) is in the subfamily Anomaloninae, and its posture in flight, with outstretched antennae and an elevated rear, is typical of the subfamily.  Weighing in at a little more than an inch long, it’s one of the larger members of the genus.  It’s found not only in North America but also in the UK, northern Europe, Asia, and even North Africa, and its habitat is listed as “oak-chaparral and willows.”

They often perch vertically, and they spend lots of time grooming – antennae first, then mouth, then legs and wings, and finally, abdomen.

After mating, females fly away, but males lie on the ground for a minute or two, abdomens curled up, wings and legs splayed, looking like dead wasps, before they, too, leave.  Females search for caterpillars during zig-zag flights and also on foot, in vegetation, using their antennae.  They target the larvae of moths and butterflies, and their diet is more catholic than that of many other ichneumons.  Females locate a potential host by smell, and then they sweep their antennae over its exterior, making sure that the caterpillar isn’t hairy or spiny and therefore difficult to deliver an egg into.  If it passes muster, she uses her short ovipositor to insert an egg.  The host larva may undergo its full (though weakened) development before the Therion larva delivers the coup de grace, and the ichneumon matures within the pupa of its dead host.

Nature red in tooth and claw.

Kate Redmond, The BugLady

Bug of the Week archives:
http://uwm.edu/field-station/category/bug-of-the-week/