10 Facts about Parasitoid Wasps (Ichneumonidae)
Parasitoid wasps are among the most amazing animals. They have the most unusual biology, are hyper-diverse, and are truly beautiful. These were my thoughts as continue with the 'ten facts about…' series on Expiscor, and my former post-doc Laura Timms kindly agreed to contribute a post on 'ichs' --> her favourite arthropod group, and one for which she is an expert.
Ichneumonidae is a family of parasitic wasps, and one of my particular research interests. If you’ve never heard of a parasitic wasp before, think chestburster from Alien, but for insects. A more scientific definition is that parasitoid wasps lay their eggs inside or on top of other insects; those eggs then grow and develop by feeding on their host’s tissue, resulting in the eventual death of the host. If you want to know more, this book has a lot of great information. In the meantime, here are ten facts about my favourite group of parasitoids.
1. Ichneumonidae is one of the largest families of organisms in the world – it contains an estimated 100,000 species, more species than all vertebrates combined. If you’re not impressed by sheer numbers, the thing that I find truly amazing about all that species richness is the amount of variation in life history that has come along with all that diversification.
2. For example, ichneumonids parasitize all kinds of hosts. Lepidopteran caterpillars and pupae are particularly common as hosts, but all sorts of others also make the list including beetles, flies, sawflies, spiders, caddisflies, scorpionflies, lacewings – and there are even reports of an ichneumonid being reared from a pseudoscorpion.
3. If you’re ever looking to find some ichneumonid wasps, try rearing some lepidopteran pupae that you found outside; it won’t take long before you find that a parasitoid pops out instead of the butterfly or moth. This is actually why I got into ichneumonids while studying forest caterpillars.
4. Some ichneumonids with crazy parasitoid lifestyles are the ectoparasitoids of spiders. Adult females of these species attack juvenile spiders and lay an egg on their body. After that egg hatches, the parasitoid larvae remains externally attached to the spider – even as the spider sheds its skin through various moults.
5. Another group with a very cool life history are the semi-aquatic ichneumonids. Species of Agriotypus are ectoparasitoids of caddisfly pupae. Adult female Agriotypus crawl down plant stems to reach their hosts underwater. The wasps are covered with dense hairs that trap air around their bodies, allowing them to breathe and stay under for extended periods of time. The parasitoid larvae and pupae also have some interesting modifications for life underwater.
6. Don’t forget the hyperparasitoids – the parasitoids of parasitoids. Euceros species are ichneumonid hyperparasitoids with particularly complicated biology: 1) Adult female Euceros lay their eggs on pine needles near feeding sawfly larvae; 2) the parasitoid larvae hatches and grabs on to a passing sawfly; 3) the Euceros larva remains attached to the sawfly until the larva of a primary parasitoid emerges; and, 4) the Euceros then moves inside of the inside of the primary parasitoid and consumes it. Did you follow all that?
7. Some ichneumonids have an unusual latitudinal diversity gradient - their highest species richness occurs outside of the tropics. This finding doesn’t apply to every subfamily of Ichneumonidae, and there are still groups we don’t know enough about, but my own research indicates that it holds true for at least some subfamilies.
8. Although there are many very small ichneumonids, the biggest are the species in the subfamily Rhyssinae, parasitoids of wood-boring sawflies. You can sometimes spot these large wasps on tree trunks as they lay their eggs into their hosts – as you can imagine, it can take a long time to locate the host under the bark and then drill down far enough to get to it. Some great video footage of Megarhyssa macrurus ovipositing can be seen here.
9. This quote by Charles Darwin: “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 …” [from this letter to Asa Gray, 1860] is often brought up in discussions about how Darwin began to lose his faith while writing On the Origin of Species.
10. Finally, there can’t be many books that feature ichneumonids, but The Snoring Bird by Bernd Heinrich is on that list. Bernd Heinrich is known for his work in comparative physiology and behaviour and also for his popular science writing. This book focuses on what it was like for Bernd to grow up with his father, Gerd Heinrich, an entomologist who devoted his life to the study of Ichneumonidae. I highly recommend this book for all sorts of reasons, not the least of which is that it paints a fascinating portrait of a man who really loved ichneumonids.
A BIG thanks to Laura for contributing to this series! You can follow her on twitter, here.
Thanks also to Matt Bertone for the photo!