Two women approached me after a presentation at the Rotorua Library. They had a very large and healthy stinging nettle patch and were hopeful of raising admirals. Instead, they were being plagued by a parasitic wasp devastating the butterflies.
They told me that from the outside of the chrysalis it looks as though the caterpillar is changing into a butterfly but inside, the cells are instead feeding developing wasps.
Probably the most common parasitic wasp in NZ is Pteromalus puparum, which was introduced here in the 1930's as a biological control for the cabbage white butterfly. Females are shiny black and the males a metallic greenish bronze, usually smaller than the females. They can deposit anything up to 200 eggs inside a pupa or prepupa!
As the eggs hatch and grow they devour the contents of the pupa. The first adult chews a hole in the pupal case and on average 23 wasps will emerge. They mate immediately and start searching for fresh pupae.
If you want to see P. puparum at work, this video on YouTube is very informative.
Here is another parasitic wasp: Echthromorpha intricatoria. Also known as the cream-spotted ichneumon, the female waits for a butterfly caterpillar to pupate, and immediately injects its eggs into the soft chrysalis with its long ovipositor. It doesn't have a sting - that's the ovipositor you can see. And fortunately, with this wasp, it only lays one egg per pupa! This wasp, I believe, was self-introduced from Australia.
After I had left the Rotorua Library it occurred to me that whenever we come up against a pest and want to do something about it, the first important step is to learn more about it. Common names can be confusing and when someone tells you it's a fly, someone else will tell you its a wasp - or a bee. So the first step is to identify the enemy!
Then you need to think about ways of outwitting it. And there is no right way or wrong way - so much depends on your lifestyle, the size of your property, and your means. These were my thoughts.
They had told me they had a very large patch of stinging nettle. They were collecting pupae and bringing them indoors, raising them individually in jars... and had a huge collection of jars, but most of the pupae would be infected.
It was, of course, the nettle which was bringing the admirals into their garden. The admirals attracted the parasitic wasps. Effectively, what they were doing with their beautiful nettle patch was... breeding wasps. The problem would be solved if they eliminated the nettles...
So, why not plant nettles somewhere else in a parasite-proof area. It could be inside the house, or in a caterpillar castle (or similar) or a butterfly house - any area protected with parasite-proof netting. Then, when this was established, temporarily remove most of the nettle patch outdoors, just leaving sufficient which was easily accessible so they could search for admiral larvae and transfer them to the protected nettle.
Having nettles means that there is at least a chance for successful emergence of adults. Without nettles that opportunity is eliminated.
Presumably the life cycle of the wasp is about 30 days. So if they wait two months - two cycles - for the wasps to 'die out', it should give them a head start before wasps found their nettle patch again. The nettle of course would recover after a few months of being mown or covered.
They will never be wasp-free in the wild, but this method should boost their admiral numbers sufficiently.
I would be interested in feedback on this suggestion from those of you already raising admirals.