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 aint 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 cant 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 caterpillars 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 dont 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.
Once Upon A Michigan 
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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.
Thank you SO MUCH for this article! I went into the kitchen to get breakfast and didn’t see my jar containing my two black swallowtail chrysali. Opened a cabinet door and there it was, with a wasp sitting on it, that promptly fell on my hand! I promptly screamed and shook my hand so it fell on the floor. From there, I scooped it up and took it outside where it very politely sat for photos.
I then spent an hour searching books and websites for an identification, with no luck. This evening, I went to look at the remaining chrysalis, and noticed the hole in other one. Aha! So, I started searching for parasitic wasps and it still took me half an hour to find your site.
I have photos, if you’d like, I can send them to you.
I am horrified that this sort of thing happens. (First year raising swallowtails, so naive!) But my sister warned me after I told her two of my chrysalis have been twitching around so much I’m sure my swallowtails are about to emerge but so far, no. Any idea whether this twitching may be related to being parasitized? Thanks!
I realize that I am responding to these posts one year later. However, I found this site because today I became acquainted with that hideous wasp you described. I raised two small caterpillars from when they were at about the 1st instar. One caterpillar died, but the other survived. This caterpillar did behave strangely. It began to roam before it had evacuated itself. Also, in certain instances it would become completely still as if it were going to molt but never did. I had a bad feeling about this caterpillar, but eventually it formed a pupae. 2 1/2 weeks later it formed one of those wasps. I did the same thing as the author did and took revenge on the wasp.
We found a caterpillar in our driveway and decided to keep it so my little ones could watch the metamorphosis. It has actually been months since the caterpillar turned into a chrysalis. I thought the caterpillar had died, yet I heard something flapping around today. I went and to my shock I found a wasp and a giant hole in the side of the chrysalis. After searching for what could have happened, I found your blog. Thanks for clarifying the great mystery!!
I have 5 of these caterpillars living on some parsley in my garden. Everyday my daughter and I go out to see how they are growing. Today however, I saw a wasp mounting one of them. It flew away and minutes later another wasp came and mounted a different one. I thought the wasps were eating the caterpillars. Thanks for this article. I now know the sad fate awaiting our little family of caterpillars.