Tuesday, 26 February 2008

Update on a 2-year project

I thought you might be interested in an update on a series of observations that I have been making over the last couple of years: I have been observing an ichneumonid (parasitic wasp) ovipositing in Nematus pavidus (sawfly) larvae on Willow.

This is the start of the investigation in September 2006:

http://homepage.eircom.net/~hedgerow16/sept12.htm

Here's a close-up of her injecting:

http://homepage.eircom.net/~hedgerow16/sept13.htm (bottom picture)

And here's my discovery that she lays up to 4 eggs in each larva:

http://homepage.eircom.net/~hedgerow16/sept15.htm

Today I got (from Gavin Broad at the NHM, London) further fascinating information:

It turns out she's a Mesochorus sp. These are hyperparasites and only lay their eggs in larvae which have already been parasitised by other ichneumonids (or, indeed Tachinids - parasitic flies). Rather than being parasitic on the Sawfly larvae, Mesochorus larvae are parasitic on the primary parasite's larvae. So the photographs I have taken completely miss out an intermediate stage: the initial parasitisation. Given the time taken from the sawfly eggs hatching to my photographs being taken, the initial parasitisation must be very quick indeed. This is simply enthralling.

It might also explain the behaviour that I noticed being used when the hyperparasite was checking out the larvae. She banged her front feet down beside the head of the larva and appeared to be watching for the sawfly larva's normal defensive posture change. I previously thought she was checking to see if others like her had already paid a visit, in which case she would move on to another larva. It turns out she was checking to ensure that the larva had indeed been pre-parasitised, so that she could parasitise the primary parasite. Wonderful.

I'm very interested in species dependencies, and this one is four deep: Willow, Sawfly larva, primary parasite, hyperparasite.

As it happens, I saw my first ichneumonid of 2008 on Sunday, although I didn't realise it until I blew up the pictures on the PC. Here's the shot:


The photo contains 4 insects. The ichneumonid (top left), a wonderful crimson weevil (bottom right) and a copulating pair of Chrysomelid beetles (centre right). I can't go in any closer: each of the insects is less than 2mm long. The moving red dot of the weevil was all I could see with the naked eye. Now all I have to do is identify the weevil and the chrysomelids. It starts.

10 points for the plant. (Evil grin)

8 comments:

jenny said...

Is it groundsel? (But you can probably carry on grinning!)

I'd been following that thread, so its really interesting to find out more

Stuart said...

Not Groundsel. Interesting offer, though..;)

Gill said...

"It turns out she was checking to ensure that the larva had indeed been pre-parasitised, so that she could parasitise the primary parasite. Wonderful." You can say that again - what an incredible story. This does of course imply that the first parasite alters the sawfly larva's behaviour somehow, and fast.

Ragwort, maybe Oxford ragwort? I'm with Jenny, it looks like a Senecio. Do we get any habitat clues?

Stuart said...

It's that same earth wall, but that won't help you: both Common and Marsh (and the hybrid) are within feet of this location. I'd chosen Common, simply because I could see that the leaves had no big lobe at the end. The pic doesn't show that.

I've been thinking about the hyperparasite. Why doesn't she just lay her eggs in any sawfly larva? That would appear to give her a bigger choice of targets. I can only assume that her eggs are acceptable enough to the primary parasites to avoid rejection, but would be rejected by the sawfly larva. Which leads us on to the evolutionary sequence by which this triple dependency arrived.....now THAT will take some working out.

Gill said...

"Which leads us on to the evolutionary sequence by which this triple dependency arrived.....now THAT will take some working out."

How about the ich had already developed the idea of parasitising the first parasite - and then this species "decided" it was better to parasitise a sawfly larva than whatever it was doing before?

Stuart said...

Yes. It really has to be that way round. The Sawfly larva is completely incidental to the hyperparasite (although I bet it uses them to navigate by).

I did some more reading last night, and Mesochorus sp. lay into developing larvae of the primary parasite. These larvae must hatch very quickly inside the sawfly larva for this all to happen. But they must then develop very slowly or the sawfly larva would die too quickly.

The sawfly larvae are translucent, so I must get some under a 'scope this year.

Gill said...

"The sawfly larvae are translucent, so I must get some under a 'scope this year."

Ooh! yes! That sounds very exciting - maybe you can find three sorts:
- just sawfly
- sawfly with primary parasite
- sawfly with both parasites

...bit like a Russian doll, really - wonder if the others are transparent too?

Dragonstar said...

Your original post is fascinating - and then there's this resulting discussion! I'm learning so much. Can't wait for your further studies . . .