unidentified yellow/black caterpillar with eggs

[Lemars]

Morning all

I was out walking yesterday and passed a water tower (in France) which was covered with hundreds and hundreds of leaf beetles and caterpillars. The leaf beetles were far more numerous, but there were 2 types (at least) of caterpillar, making their way to the top (if you zoom the - cropped, unfortunately - tower photo, every black dot is an insect. They were all round the tower and dense in the frames of the building).


Clearly, the sudden burst of sunshine had brought them out, but I'm curious about the caterpillars. Could anyone explain what's happening here please - and identify the caterpillars?
If they've emerged from these eggs, why are they still with them and why are the other eggs so small in comparison ? I'm sure it's my ignorance, but I can't quite make sense of it.

[The Woodman]

I get a few of these, Lemars. I think it is the parasitised larva of a Large White - Pieris brassicae. The ichneumon Pimpla instigator might be the culprit but that's a half guess.

[Dogghound]

It is a large white larvae, although the parasitic cocoons are from Cotesia rather than a pimplinid.

[Lemars]

Thank you both, but now could you please explain something else to me? There were several caterpillars higher up the tower - and climbing. They had no attached eggs and were moving freely.

If these are parasitic cocoons (and the caterpillars on them seemed fairly healthy) did the eggs come from the caterpillars? This probably seems like a silly question to you, but there are a lot of eggs there, to have emerged from one little critter.

Also, are those without eggs likely to end up the same way ?

Could you tell me what's happened, please?

C

[triops]

Cotesia (Apanteles) glomeratus is a parasitic (parasitoid) wasp which uses the Large White butterfly as its host. Eggs are laid in the caterpillar by the adult wasp and these larvae feed and grow inside the caterpillar, avoiding essential organs. The parasitoid larvae and the host caterpillar mature at the same time, the caterpillar wanders, usually upwards, and when the caterpillar stops and begins to pupate the larvae of the wasp emerge through the skin of the caterpillar and spin their yellow silk cocoons. Patches of cocoons (not eggs!) are where the dead or dying caterpillar has fallen off.

[Lemars]

Thankyou Triops, but does that suggest that the caterpillars seen with these cocoons are the host of those we're looking at ?

If so, they still seem very healthy. I got the impression they were still alive although those cocoons must have emerged a while ago. Do the caterpillars remain with the cocoons - but will eventually die? How long is this process - poor caterpillars !
If not, why would they be attracted to cocoons left by others ?

And I presume those caterpillars which are still climbing the wall will either pupate (and the cocoons will emerge) soon - or they've not been parasitised and will keep climbing and pupate as normal ?


I'm still a little hazy about this. I think it's the size of the cocoons which is confounding me !!
And having just read what I've written, I expect you're a little hazy about what I'm asking, but all info gratefully received.

C

C

[triops]

Using your picture as an example:
These cocoons have been spun by the larvae which emerged from this caterpillar. They will have taken just a few hours to spin their cocoons. The caterpillar is dead or very nearly so. Often after this sort of event the caterpillar just falls off, being weakened, light and almost empty. Most of the other caterpillars you saw will have been parasitised and will suffer the same fate. Hopefully a few will survive.

[Lemars]

Thank you Triops, for taking the time to explain - it's clear now.
I went back there yesterday and took a few more photos. I also found the remains of many caterpillars on the ground.

There was one batch of empty cocoons - they looked very dried out so maybe from last year?
The insects are hatchling wasps, I suppose? Nice to know who the culprits are.

And it was clearly all too much for the one caterpillar......................