Spined soldier bug nymphs
How do I get rid of the red nymphs without killing the monarch caterpillars?
Usually there is only one or two, so hand picking and relocating 50 or more feet away would work. But watch out, if there are dozens, then this may be another insect, milkweed bug nymphs which feed on the milkweed not the monarch caterpillars. Usually the milkweed bugs don't hurt the plants much.