If you object to wasps because they sting, you’re in luck! The largest family in the wasp world has only a few members capable of stinging, and it also includes some of the most colorful wasps and some of the most ecologically beneficial.
The family is called Ichneumonidae, its members are called ichneumon wasps, and by some estimates there may be more than 100,000 species of them in the world. It has been proposed as the most diverse family in the animal kingdom, so big that a large fraction of its members have not yet been described. North America hosts somewhere around 5,000 known species, according to an excellent summary of the family on Bugguide.net.
How many of those occur on the Vineyard is anybody’s guess. They’ve never been studied here, but I’d bet our species count is well into the hundreds at least. Ichneumons are among the last insects active in the fall, and the first in spring; in between, examples of these wasps can be found in virtually any habitat, and one of my projects for the coming year will be getting better acquainted with them. I’ve already found my first ichneumon of the year, an elegant caterpillar parasitoid named Ichneumon ambulatorius, on a late March outing into Correllus State Forest. This was almost certainly an individual that survived the winter, which quite a few ichneumons seem capable of doing.
Ichneumonidae is, in evolutionary terms, a relatively primitive wasp family, descended quite directly from the first Hymenoptera (that’s the insect order that contains wasps), which appeared about 250 million years ago. In fact, the better-known groups of stinging hymenoptera — what most of us think of when we think of wasps and bees — are an evolutionary branch stemming from the Ichneumons. As a group, female ichneumons generally possess an organ called an ovipositor, a harmless spike- or threadlike tube extending from the tip of the abdomen and used for laying eggs. In the stinging wasps (as well as in a few ichneumons), the ovipositor has become modified into a stinger.
Ovipositor variety is an area in which ichneumons truly excel. Members of this family are almost all parasitoids — that is, their larvae feed on and eventually kill an individual of a host species. Those hosts range from caterpillars to flies, to beetles; in fact, there are few if any insect groups immune to ichneumon parasitism. Each ichneumon is specialized for using a particular group of hosts, often just one particular genus or even species, and its ovipositor has evolved a shape specially designed for putting eggs on or into that host.
The results can be astonishing. In some members of the genus Megarhyssa, the ovipositor is literally inches long, several times the length of the wasp’s body. The reason? Megarhyssa lays its eggs on the wood-boring larvae of certain beetles, and does so by threading that ovipositor deep into a tunnel bored by a beetle larva. When the female wasp feels its ovipositor poke a beetle larva, she lays an egg, which will hatch into a grublike youngster that will consume and kill the beetle.
Because of this complex and somewhat grisly life history, ichneumons are important for the control of other insect populations, including many species regarded by humans as pests. To put it simply, these wasps are among the most beneficial of insects.
They’re also gorgeous. Many ichneumons have evolved orange, yellow, or white bands on their bodies, presumably mimicking the warning colors on their sting-equipped cousins. Some species are entirely bright orange. A lot of species have brightly colored legs, and on many species, one or more broad bands appear on the antennae (which are always long in this family).
Such gaudiness might suggest that ichneumons would be easy to identify, but think again: A large family that has been imperfectly studied, Ichneumonidae is one of the gnarliest identification messes to be found in the insect world. Given how many ichneumons there are, it’s no surprise that no truly comprehensive identification resources exist. And even if one did, an amateur entomologist would still struggle, because ichneumons only distantly related to each other can look virtually identical. Even specialists struggle with this family, depending on tiny anatomic details and wing-vein patterns to arrive at an identification.
So my usual modus operandi, taking photographs and then ID’ing the subjects by visual comparison to “known” photos on entomology websites, is only going to get me so far with this group. Of the ichneumons I’ve managed to photograph, I’ve been able to assign perhaps half to a subfamily; for only a few have I been able to assign a species name, and even those feel somewhat tentative.
But I’m undeterred. These wasps are attractive, diverse, important, and relatively easy to find; I intend to learn what I can about them.