It maybe assassin bugs. I just caught assassin bugs sucking the life out of a caterpillar this evening. After they deflate them, they turn black.
If you mow down your patch, you may be killing caterpillars that aren't sick, but healthy. I've been squishing and washing off the aphids. I also released a bunch of ladybugs, six times in the spring to early summer. We didn't get Monarchs until mid July.
Herndon, VA (USA)
Take care of the small things....