|Black-and-white cocoon, 6mm long|
At least 15 species of this genus can be found in the UK and they parasitise the caterpillars of various butterflies and moths such as the Lymantriidae (tussock-moths), Pieridae (butterflies - whites and yellows) and Lycaenidae (butterflies - coppers, hairstreaks and blues). They do so by the female wasp laying an egg in the host caterpillar, piercing it with her ovipositor. Once hatched, the wasp larva develops inside its host which shrinks, becoming a hard, brittle shell - effectively mummified. When ready to pupate, the wasp larva spins a cocoon inside the larval skin (or in some cases the host skin splits and the wasp pupates outside) and this creates the black-and-white patchwork pattern - it takes about a month from the egg being laid to a new adult emerging.
I don't know which Hyposoter this is and I doubt that it's possible to tell from just a cocoon. However, one of the British species (H. ebeninus) has been filmed going through its life cycle, and this is certainly a candidate for the species here as the adults match ichneumons I have seen but not identified (though that isn't a reliable indiactor - they are a diverse and tricky group to ID), and parasitises the cabbage white butterfly (Pieris brassicae) which is common here. Again, this is not enough for an identification, but it is something I can look out for next season. In any case, I hope that's an interesting little ecological tale from a passing observation.
|Close-up of the Hyposoter cocoon showing silk threads attached to the host's mummified skin.|