I was very excited when I discovered the huge crop of Eastern Black Swallowtail eggs a few weeks ago, but at the same time I also knew that that would mean having to deal with several caterpillars that wouldn’t make it all the way to adulthood. There can be many reasons caterpillars die: poison/insecticides, virus/bacterial infection, or just a genetic fault.
I took the photo above after changing the food supply and water for one of the enclosures. Those critters strip the rue and parsley pretty quickly, and many of their droppings end up in the glass. I also change the paper towels at the base of the enclosure, as it is also littered with caterpillar poop and pieces of the plants. I count at least 13 caterpillars in that shot; there are probably a few more hidden on the other side.
But when I clean out the enclosure, I often find several caterpillars on the floor of it, unresponsive. I remove them to a separate container, in case they are infected. A couple of days ago, I gathered 6 such sad critters from the two enclosures.
I puts bits of food in front of them to see if they start munching, a sign that they are still healthy. In the photo above, both #1 and #4 eventually “woke up” and began eating, so I returned them to the enclosure with their siblings.
#2 didn’t look very good at all. In the 15 minutes between when I placed them on the paper towel and when I took the photo, it had oozed a blackish liquid. It did twitch for a little while, but was soon dead.
#3 just looked deformed. I don’t know if it had some disease or a genetic fault, but it didn’t last long after this.
I had some hope for #5 and #6, as they are in the “J” shape that that swallowtail caterpillars form themselves before shedding their skin to become a pupa. They were smaller than pre-pupas typically are, but these little guys have surprised me before. I’ve had a few that laid on the bottom of the enclosure like this, and after a couple of days I’d look in on them and they had become chrysalises! They eventually emerged as perfectly normal adult butterflies, so I was hoping that that would be the case for these two. They were both wriggling from time to time, similar to the motion they make when getting ready to molt into a chrysalis.
Unfortunately, it was not to be. #5 turned dark and stopped moving in a few hours. #6 remained twitchy for 2 days before finally dying.
It’s always hard to see these tiny creatures hatch from eggs, and slowly grow bigger and bigger, only to die suddenly for no apparent reason. But it’s part of nature, and it helps to keep in mind that by keeping them away from predators and keeping them well-stocked with food, a much greater percentage do make it maturity. All you can do is your best to help them along.