Ichy Friday: on the evolution of parasitoid Hymenoptera

"Nature is so much worse than science fiction"

That's a quote from one of the students in my introductory entomology class, from earlier this week. That student was teaching about the Ichneumonidae - a hyper-diverse family of Hymenoptera, well known for their "parasitoid" behaviour. Yeah, the things they are do are quite, well, 'ichy' - and certainly as horrible as any science fiction movie (parasitoid, by the way, essentially means a 'parasite' which consumes/exploits its host, eventually killing it). As one example, there's a species that lives in the tundra of the Yukon territory, seeking out wolf spider females carrying egg sacs. The wasp female lays eggs into the egg sac of the wolf spider and the wasp larvae emerges and consumes the spiderlings. Ichy.  Our research has documented that in some parts of the Yukon, parasitism rates can be remarkably high (Bowden & Buddle 2012). Here's a photo (by Crystal Ernst) of the wasp peeking out where one would normally expect to see spiderlings.

Credit: C. Ernst. A wasp emerging where we would expect a spider: Gelis - a spider egg sac parasite

The student in my entomology course also showed this video - it too illustrates the rather incredible, yet somewhat grotesque, habits of some parasitoid wasps (um, not for the faint of heart).

By all accounts, the evolution of parasitism in the Hymenoptera has been remarkably successful. The Ichneumonidae alone are estimated to contain upwards of 60,000 species --> probably the most diverse family of organisms on the planet (Grimaldi & Engel discuss their diversity and evolution in great detail - interested readers should head there). And the Ichneumonidae are only one family in the Hymenoptera which are considered parasitoids.

Ok, back to our entomology lecture: the following question was posed: "what is the evolutionary history of this highly successful parasitoid behaviour?". Great question! And one (as usual) for which I did not know the answer. Thankfully two twitter friends, Miles and Laura, came to the rescue. Whitfield's (2003) paper is probably the most complete treatment of this question, and the answer seems to be the following: (I can't say it better than Whitfield, so I'm quoting him directly, here)

An earlier hypothesis on the origin of parasitism in Hymenoptera by Handlirsch (1906–8) seems to have hit the mark, however. Handlirsch noted the apparent morphological similarity between Siricoidea (horntails and woodwasps) and the putatively most primitive parasitic Apocrita, and the fact that the latter were often parasites of the former or of other wood-boring insects. He proposed a series of stages by which a gradual transition from wood-boring to ectoparasitism of wood-borers might have taken place. It is of interest in this regard to note that one relatively uncommon group of woodwasps, the Orussoidea, appears to have an ectoparasitic mode of life while still retaining much of the sawfly/ woodwasp morphology. (from Whitfield 2003)

So, stated another way, something that looked kind of like a wood wasp laid its egg near to a wood-boring host insect, and its larvae would have munched on that host instead of on plant tissue, and would have done so as an ecto-parasite (i.e., living outside of the host).

As Gauld (1988) argues, a primitive wasp, laying eggs in concealed plant tissue, could be putting its egg at risk due to the presence of other organisms (e.g., its future host) within the same tissue, so there could be a selective advantage in that egg being very, very close to that other arthropods... perhaps attached.... Gauld (1988) also argues that the switch from relatively semi-fluid plant tissues to semi-fluid insect tissues is a pretty easy one, from a physiological standpoint. So, voila: somewhere back in the Jurassic, natural selection favoured this parasitoid behaviour.

As a final note to this ichy post, the rather gruesome lifestyle of Hymenoptera parasitoids did not go unnoticed by Darwin: here's the quote:

"There seems to me too much misery in the world. I cannot persuade myself that a beneficent & omnipotent God would have designedly created the Ichneumonidæ with the express intention of their feeding within the living bodies of caterpillars..."

Charles Darwin, 1860


Bowden, J.J. and C.M. Buddle (2012). Egg sac parasitism of Arctic wolf spiders (Araneae: Lycosidae) from northwestern North America Journal of Arachnology DOI: 10.1636/P11-50.1

Gauld (1988). Evolutionary patterns of host utilization by Ichneumonoid parasitoids. Biol. J. Linnean Soc 35(4): 351-377.

Grimaldi, D and M. Engel 2005. Evolution of the Insects. Cambridge.

Whitfield JB (2003). Phylogenetic insights into the evolution of parasitism in hymenoptera. Advances in parasitology, 54, 69-100 PMID: 14711084


14 Responses to “Ichy Friday: on the evolution of parasitoid Hymenoptera”

  1. Sean McCann Reply | Permalink

    They are certainly ichy! For a fascinating read about someone with a passion for the family, check out Bernd Heinrich's "The Snoring Bird" http://amzn.to/I8KX2N
    Dr. Heinrich's dad was a fanatically devoted old-school taxonomist with the self-appointed Herculean task of cataloging and collecting Ichneumonids. The book brilliantly contrasts the old school natural historians with the "modern" advent of ethology.

    • Christopher Buddle Reply | Permalink

      Thanks Sean- for the comment. And, WOW... I shall add that book to my Christmas wish list - thanks for the link.

  2. Matt Bertone Reply | Permalink

    Sometimes I wonder about the semantics of "parasitoid". Do the Gelis larvae each inhabit one egg, or do a few larvae eat an entire clutch? I know that ensign wasps (Evaniidae) are considered egg predators, because one or a few larvae eat an entire cockroach ootheca contents, thus they feed on multiple individuals and are "predators". The same is often said of hunting wasps (Sphecidae s.l.) whose mothers provision each larva with multiple hosts.

    • Christopher Buddle Reply | Permalink

      Matt - great comment - thank you. I struggle with the semantics. Gelis larvae consume an entire clutch of spider eggs (and there are often many Gelis in a single egg sac - I think our record was 13 !!). So, fair enough... probably calling Gelis an 'egg predator' is much better terminology.

      • Matt Bertone Reply | Permalink

        It certainly is a gray area, especially if you think of an egg sac as a complete "unit" and the eggs to be simply the "stuff inside"; akin to muscles and fat bodies in the complete unit of a caterpillar.

  3. Adrian D. Thysse Reply | Permalink

    I second Sean's recommendation of The Snoring Bird: enjoyed it immensely.
    These parasitoid wasps are fascinating. Here is a sequence that I recorded this last spring with Copidosoma sp. parasitoid wasps emerging from Army Cutworm (Euxoa auxiliaris).(If you find it too long, skip to the end to see the scale of things..).


  4. Stephen Thorpe Reply | Permalink

    All very speculative! Depositing eggs on or in another organism, rather than in a well hidden spot, might actually result in a greater lielihood of the eggs being eaten (by predators of the host).

    • Christopher Buddle Reply | Permalink

      Thanks, Stephen, for the comment - yes some speculation, but given the behaviour and morphology of the Orussoidea, it does seem to make the 'most' sense. (Where's that time machine when you need one?)

  5. seo Barnet Reply | Permalink

    Hi there just wanted to gife you a quick heads up and let you know a few
    of the images aren't loading properly. I'm not
    sude why but I think its a linkinng issue. I've tried it in two different brokwsers and
    both show the same results.

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