Yesterday afternoon I found a large monarch caterpillar on my still quite small Asclepias incarnata, planted last fall. He surely hadn't been hatched on it because none of the leaves had been eaten. But by early afternoon today, half of one of the three stems had been defoliated/eaten. In another day or so, the entire plant would have been defoliated--dead plant, dead caterpillar. Fortunately a friend who is an expert on monarchs said I could move it to my neighbor's A. syriaca. And so I did. I watched for a bit as he recovered from the move and began searching, I'm sure, for those tender incarnata leaves as opposed to the much tougher syriaca ones.
I'll check on him later, but the mystery remains--Where did he come from and how did he get to my plant?