Probably Tachinid Flies.

Thanks for the reply but no, I know it's not tachinid flies. Whenever we have a caterpillar that's been attacked by a tachinid fly, a string hangs down from the caterpillar or pupa, the fly larva crawls down the string, goes under the paper towel in the bottom of the container, and then within a short time we find the reddish-brown tachinid fly pupa in the bottom of the container under the paper towel. I've been raising Monarchs for 14 seasons and this is the first time I've had this problem, whatever it is. I wasn't really expecting a solution from anyone but was wondering if someone on here ever had this problem with the Monarchs before, that little "flies" would fly out of the container and there'd be dead cats inside the container.

I originally thought that this was some kind of a disease being passed around from one cat to another, but then when my daughter said she saw some kind of insects fly out of the containers, that made me suspicious that they were the cause of this huge die-off. It's very disturbing. Even though I don't like the containers not having air flow, the only thing I can think of doing besides disinfecting the containers is putting the solid lids on the tops of the containers instead of having screen for bugs to possibly get inside, even though I never knew before that that's possible.

Wish I could be of help. Hope you will figure it out and share. Not long ago I had some little black somethings identified here. The first answer was caterpillar fras, but it turned out pupa of wasps that go after caterpillars. They were really just weird looking things on a leaf that caught my eye. Seems there are so many things that can go wrong with caterpillars. Good luck! Here is a picture of the little specks. Doesn’t look like anything really, does it?

Gosh, there's a bunch of of them there, isn't there! I've never seen anything like that here. How did you know that they were wasp pupae? Did you see any come of them? Yes, there are so many things that get after Monarchs that supposedly only one or two out of a hundred hatched make it. I've seen people on here mention it various times, but I really don't know where the original data came from. It's why so many of us raise them in containers, to help their numbers. It's a ton of work for my daughter and me and we get little else done, but I guess we feel obligated to help the Monarchs since their numbers have decreased drastically over the years.

You are doing a great job! Sorry for the late reply. Someone on the insect forum knew what they were. The pictures I compared them to on the Internet matched. I walked by there today and took a picture. There are two that looked like flies. But maybe the one on the lower right is just coming out? It’s really hard to tell. They are maybe around 3 to 4 mm.