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Hi Anna,
The Pteromalus wasps depicted in the photo you posted are not attacking the pupa, it has already been infected at an earlier stage and the wasps are emerging from it through a small hole in the pupa. You must remove the pupa and destroy it quickly, as 100 plus wasps can emerge. The wasp infects the caterpillar at the stage of hanging in a ‘J’ and as soon as the caterpillar sheds its skin the wasp will inject its ovipositer into the soft pupa and deposit anything up to 150 eggs inside.
These hatch into grubs which then devour the contents of the pupa and then themselves pupate inside the host. The first adult chews a hole in the casing of the host pupa through which most of the following adults will emerge, mate immediately and then search for fresh pupae to infect. Once the pupal skin of the host has hardened the wasps are no longer interested in it. Pupae gathered from outside in the garden can be already infected, or one single female wasp can gain access to the butterfly house either by hitching a ride on clothing, simply by flying in when someone enters the butterfly house, or as in the castle of a castle as well – by hiding on the underside of a potted plant leaf when it is introduced. They can gain access through an extremely minute gap, but probably not the fine mesh on a castle. These wasps are destructive little demons and can wipe out a generation of admirals in a butterfly house in a very short time.
The safeguard is to place the admiral larvae that are of pupating size into the likes of a castle that will exclude entry of any of the critters, and allow the larvae to complete the pupation phase in safety.
They will also infect monarch caterpillars, but not to the same degree, and have no problem finding their way through most of the shade cloths and standard sreen mesh.
Unfortunately you are experiencing what most breeders contend with at some stage, and will be able to take measures to prevent it. Keep an eye on any hanging ‘J’ caterpillars for signs of the wasp, for they will patiently wait up to 2 days near the hanging caterpillar, or sometimes even on it, and are ready to do their deed when the skin is shed. Occasionally an admiral pupa will violently shake and swing when touched or disturbed, and it is a defensive move to try and shake off the likes of a wasp, usually to no avail though.
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