I've been watching a couple of milkweed plants by my house hoping to see Monarch caterpillars with no luck.The only thing I saw was some black stuff on the bottom of a few leaves and some small black ants around it. A few days later I looked and found one plant almost totally stripped down to the stem. See pics. There were a lot of caterpillars but I don't think they are Monarchs. A couple of days later the other plant was covered with the caterpillars. What do I have and should I do anything? Will the plants die? I've been trying to with natives or little maintenance plants.