It might seem natural to expect Black Swallowtail Butterflies to emerge from Black Swallowtail Chrysalids. Given three identical crysalids, “made” by three identical swallowtail larvae, the odds are even better that three butterflies would emerge. I had three such potential packages awaiting delivery this week. Nature ain’t necessarily natural, however. Three large orange wasps emerged instead (see above).
My swallowtail larvae & chrysalids were parasitized by a species of wasp known as the Trogus Ichneumon Wasp. Ichneumons make their living off of other creatures and they specialize in certain types. The Trogus wasps zero in on swallowtail butterfly larvae and they appear to do un-natural things to their victims. In short, the wasp maggots develop inside the unsuspecting host and eventually kill them, eat them from the inside out, and then emerge from their hollowed out shells. All that is left of the original butterfly chrysalis is the empty casing with a massive hole in one side (see here). This is all perfectly natural.
It is estimated that 20%-40% of butterfly and moth pupae (crysalids in the case of butterflies) fall victim to parasites like the ichneumons. The process begins with an egg laid inside a young caterpillar by the female wasp. Ichneumons, the term means “tracker” in Greek, hunt down their hosts by using a finely developed sense of smell. They are attracted to the scent of leaves damaged by feeding caterpillars – deducing in their tiny wasp noggins that where damage exists the damagers can’t be far behind. Once located, the female inserts her needle-like egg laying tube (ovipositor) into the soft bodied host and deposits a single egg. She then resumes her search for fresh fare.
When the egg is laid, the female ichneumon adds a dose of Polydnavirus for good measure. This virus immediately infects the surrounding cells and essentially tells the caterpillar’s immune system to ignore this intrusion. The wasp maggot hatches and begins to eat all the non-essential internal parts of the swallowtail larva which, by the way, continues to grow as if nothing is wrong.
By the time the caterpillar enters into the chrysalis stage, it is more maggot than caterpillar (“twisted and evil”). Once settled into this stage, the wasp maggot is no longer bound to keep its host alive so it consumes the rest of the innards before entering into its own pupal stage. Two weeks later, the adult wasp emerges and exits via a custom made port hole (a closer view here).
In case you are wondering why the wasps in my hand are dead, I decided to freeze them for the benefit of recording their appearance. These are active beasts that don’t photograph well. I admit there might have been a little bit of come-uppance on my part as well. Sure, I was witness to a perfectly natural occurrence, but I admit to being slightly peeved and felt it was perfectly natural for me to claim the final word in this scenario.
[...] raising black swallowtail butterflies is not a walk in the park – Naturespeak has a post about a parasitic wasp that will lay eggs in the caterpillars – the caterpillars go about [...]
[...] raising black swallowtail butterflies is not a walk in the park – Naturespeak has a post about a parasitic wasp that will lay eggs in the caterpillars – the caterpillars go about [...]
Thanks for the great post. When this occured, we were clueless about what caused the anomoly. Three larvae were affected.
I’ve been waiting for a Black Swallowtail pupa to hatch, and today, it did. However, a Trogus Ichneuman wasp emerged (ID’d thanks to your great pics).
I have one larva left to pupate-is there any way to tell if it has been parasitized?
Jim:
No, as far as I can tell there is no external way to tell. My larvae looked fit as a fiddle before they pupated. It’s kinda like some kind of horrible lottery, I guess.