Ichneumonids are parasitic wasps which lay their eggs directly into the larvae of butterflies, moths, flies and sawflies, although some are hyperparasites (laying their eggs into the larvae of primary parasites), and a few are paraparasites ( laying their eggs into the larvae of hyperparasites). The host is always eventually killed by the parasite, usually in the pupal phase. Most Ichneumonids lay a single egg in each host, although some lay multiple eggs, and there are over 1750 different species in UK and Ireland, ranging from 2-3mm up to the largest at 8cm long. Due to the fact that the various different families converge on a very small number of colour patterns, identification is very complex and always requires trapping, killing and microscopic investigation. There are very few people who have embarked upon the years of study required to investigate this group, so they are relatively poorly understood. If I had a spare life I would certainly give them a go, because I find them absolutely fascinating. Their form varies enormously, although they all have long antennae and a very narrow 'waist' between the thorax and the abdomen. The nature of the host can usually be determined by looking at various characteristics of the parasite: size, length of legs, length of ovipositor.
I photographed this beauty last night:
Based on the size (3 cm. from nose to tail) alone, this one requires a very large host, and it will almost certainly parasitise one of the larger moths, such as the Drinker or Northern Eggar. The larva of the Elephant Hawk Moth is parasitised by the similarly-sized black and white Ichneumon Amblyjoppa proteus. [http://homepage.eircom.net/~hedgerow14/may30.htm shows the emergence sequence]The next is a Macrocentrus sp., and the structure of the body is very carefully arranged: she has massively long antennae, very long rear legs and a very long ovipositor. These lay their eggs into fly and moth larvae hidden deep inside the seedheads of Thistles and Knapweed. The long legs are required to give her body enough clearance to spin the ovipositor 180 degrees towards her antennae.
You can see an action shot here.
This next one is smaller, and has short legs and a medium-length ovipositor. I suppose this will be parasitic on a small larva which is exposed on the surface, rather than hidden in a seedhead.
The configuration of this one bothered me for a while. It has extraordinarily long antennae, long legs and no visible ovipositor. So what could possibly be the reason for such a shape?
(hint, wasp males usually have long antennae, and male ichneumonids are rare).Just to complete the story from yesterday, here is the larva of the Sawfly Nematus pavidus, with its body curled in the usual defensive posture.
2 comments:
That's a fascinating page. "and a few are paraparasites (laying their eggs into the larvae of hyperparasites)." Yikes! How many levels deep does this go?!
"The host is always eventually killed by the parasite, usually in the pupal phase" - just the original host, or the various intermediates in the chain as well? It's mind-blowing! Especially when you consider that quite often the parasite in this sort of relationship manages to control the host's behaviour, probably by chemical signalling.
8cm is a real monster, even if that length includes a long ovipositor - what on earth does that parasitise? Have any made the jump to say slugs or worms as their hosts?
Is there any way of telling whether a wasp is a parasitic one or not just by looking at it?
>Yikes! How many levels deep does this go?!
3 levels is all I've read about.
>just the original host, or the various intermediates in the chain as well?
All hosts die, regardless of where they are in the chain. Last in wins. Keep in mind the host has to be allowed to develop to sufficient size to sustain the parasite, so by the time a parasitised parasite dies, it will have consumed much (or all) of the original host. Note that there is wastage in the recycling process, so the creatures get smaller the further they are up the chain.
>...the parasite in this sort of relationship manages to control the host's behaviour, probably by chemical signalling
There is evidence that the parasite can also change the host's pupation date and also the type of cocoon.
>what on earth does that parasitise?
The parasite is Rhyssa persuasoria and the host is the Giant Wood Wasp, Urocerus gigas.
Rhyssa persuasoria (persuasive burglar!) is photographed here:
http://homepage.eircom.net/~hedgerow4/aug01.htm
That's on Angelica, so you can get some idea of scale.
>Is there any way of telling whether a wasp is a parasitic one or not just by looking at it?
Depends on how close you look...there are microscopic differences which will key them absolutely. But in the field, I'd say yes again. Sadly, you'd have to know what all the other ones look like to be able to instantly discount them. But I reckon an Ichneumonid is identifiable as such almost without reservation. I suppose the real difficulty comes with some of the sawflies, because they can converge with Ichneumonids in terms of colour patterns. The only way to be absolutely sure of those is to flip them over and look at how the abdomen joins to the thorax. Thick join (like a bee)= sawfly, thin join (like a wasp) = Ichneumonid.
Tomorrow I'll confound things even further by bringing predators into the frame.
I should point out that it gets much more complex than indicated above. There are also shades of pseudohyperparasitoids that don't oviposit into larvae, but oviposit into cocoons.
A host I forgot to mention is the nymph of some beetles.
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