Monday, 18 August 2008

Further update on Ichneumonids and sawflies

In February, I posted about hyperparasitic Ichneumonids, and yesterday the story took a further amazing twist.

To summarise, the Ichneumonid I had been following was a hyperparasite, which was targetting the larvae of the primary parasite already inside the sawfly larva. Since then I have been searching for the primary parasite to complete the picture.

Yesterday I found a new Ichneumonid ovipositing in the sawfly larvae, but the ovipositing process was the most bizarre I've ever seen. This is a shot taken from below the leaf:


The Ichneumonid is under the leaf, with her abdomen curled round the edge of the leaf and ovipositing into the larva, which is above the leaf. I've seen these long-abdomened wasps before, but didn't know why the abdomen was so long. Now I know.

Just in case there's any doubt, here's the ovipositor in action:

This shot shows the Ichneumonid leaving the scene after her work was done:

The portion of leaf shown is about 8 cm. long.

You couldn't write a story like this.

5 comments:

Gill said...

You are now watching these larvae for the hyperparasite? :-) Absolutely fascinating.

"I've seen these long-abdomened wasps before, but didn't know why the abdomen was so long. Now I know." Indeed so, though why some wasps should develop long (flexible) abdomens and others super-long ovipositors on 'normal' abdomens is an interesting question.

Stuart said...

>though why some wasps should develop long (flexible) abdomens and others super-long ovipositors on 'normal' abdomens is an interesting question.

The 'why' is easy...long ovipositors are used where the host is unreachable, e.g. at the bottom of a composite seedhead, such as Thistle, Knapweed or Burdock. Short ovipositors are used when the host is out in the open.

What I'm puzzling about is: why the subterfuge in this case? Normally the Ichneumonid just wanders between the hosts looking for its next victim.

Another observation is that the ovipositor must have some type of sensor on it, since the antennae of the Ichneumonid cannot be used to aid egg placement in this configuration.

Gill said...

"What I'm puzzling about is: why the subterfuge in this case? Normally the Ichneumonid just wanders between the hosts looking for its next victim." to hide from the metaparasite (is that the right term?) perhaps....

Makes you wonder how long this sort of co-evolution has been going on - I suspect we'll find the wasps are an ancient group: with one brood a year it'd take millions rather than thousands of years wouldn't it? And many millions of unsuccessful experiments!

john cullen said...

Great macro photos Stuart really fantastic stuff. I was out the other day and thought I had photographed some Ichneumonids wasps mating but it turns out that they were sawflies. I found one of your posts which also helped me confirm it too. Great Blog...

Stuart said...

Thanks for that, John. The main difference between sawflies and ichneumonids is that the ichneumon has a narrow, wasp-like joint at the thorax-abdomen intersection, whereas the sawfly has a thick 'waist', more like a bee.