What Can Go Wrong
Raising caterpillars can be more difficult than just feeding it leaves and cleaning its poop. There are parasites, predators, and viruses that can wreak havoc with your herd. This is especially true when you have a very large amount of caterpillars.
One thing you may not think about is what can go wrong when raising caterpillars that you found in the wild. I know when we were kids if we found a caterpillar we didn't even think twice about diseases, parasites, etc.
I found one caterpillar that had odd brown splotches on it's back. They were kind of oily looking and I thought the caterpillar was just dirty. But because he looked a little odd I thankfully kept him separated from the others. However, I did have him in the same container as some of the milkweed I had collected, and if I could go back I would not have done that.
This caterpillar most likely has/had OE, which stands for Ophryocystis elektroscirrha. This is a protozoan parasite that spreads by spore. Microscopic spore. It causes the caterpillar to become mottled looking and can end up in either a dead caterpillar, a deformed butterfly, or a butterfly that can fly but will be smaller and weaker and will spread the spores as it flies around. Bad.
Let me explain about another parasite that can afflict these creatures before I tell you what to do if you find this occurring. There is a parasitic fly called the Tachinid Fly that will lay it's eggs behind the caterpillar's head by injecting them right through the skin. It then flies off and the caterpillar goes on munching away. Depending on what stage the caterpillar was at when infected it will either die and the larvae will come out of the caterpillar itself, or it will pupate and the larvae will drill their way out of the chrysalis and rappel their way down on long white threads. You can easily tell if a chrysalis is infected, but there is almost no way to tell if the caterpillar is unless it dies. Here is an infected chrysalis that I had:
There are drops of rain on the chrysalis in these photos. This was the second infected chrysalis I had. The other had the strings on it but it freaked me out so I threw it away before taking a photo!
Here is what to do if you have either of these problems occur:
You will need to destroy the insect/chrysalis. I know this is hard, and I have trouble killing ANYTHING especially something that I love and raised. For the one with OE I chose the method of putting it into a zip lock bag and putting it in the freezer. I had to wrap the bag in paper towel so I wouldn't have to look at the dead stuff after. Leave it in the freezer for a night or two and then make sure it is still sealed and throw it in the trash.
DO NOT PUT THEM BACK OUTSIDE!!! Do not put a diseased caterpillar, chrysalis, or butterfly back out where the parasite can infect others! Stop the spread before it starts. If you have a butterfly emerge poorly from the chrysalis with deformities PLEASE do not release it--you will potentially be harming future generations to come. They are insects, without a soul (even if they are beautiful), and live a maximum of 7-14 days anyway, even less if they cannot fly or drink nectar.
Prevention is also the best method to keep this stuff from spreading and to raise healthy adults. I want to share what I did wrong in the beginning and then what I realized I needed to do and will religiously do from there on out:
Here is the wrong way to store a ton of caterpillars that you found all over the place:
Just look at that jumbled mess!
Here is the correct way to store a ton of caterpillars:
Each caterpillar has his/her own tupperware container with airholes at the top. I bought the red topped ones at the Dollar General for about .50 each. Remember to poke the air holes from the underside so that there are no sharp edges if the caterpillar crawls up to the top, which they often do to molt. Jim Hedbor, a butterfly/moth enthusiast that I knew since I was a kid, suggested that I put a piece of paper towel in the bottom of each container to keep it more sanitary and make it easier to clean. I bought the paper towels that come in those "choose your size" packs and cut each one in half, which was the perfect size for the containers. I empty and clean each container three times a day since I am a little neurotic. I don't like to let them sit in their own filth for very long. If you are gone most of the day I suggest putting 5-6 milkweed leaves in the container so that the poop will fall to the bottom and they won't be trying to eat around it.
Watch & Wait
This caterpillar looked like it had OE but after observing it in quarantine for 24 hours, it may just have cut itself. I am continuing to watch it and keep it separate to ensure that it is indeed not infectious.
Quarantining is essential. Make sure you wash your hands thoroughly after handling any suspected contagious caterpillar or the leaves/container it is using. Discard or bleach and rinse the cage thoroughly once you have disposed of any sick caterpillars.
Here is a link to the 10 Most Common Monarch Caterpillar Diseases.
Here is a link to a wonderful resource for raising all kinds of butterflies!