I soon realized she was actually ovipositing, laying eggs, on these little plants. But what are these little plants? They don't look like the regular Asclepias curassavica milkweed plants that the Monarchs use at the arboretum.
My first thought was that this poor old, dying female was trying to put her last few eggs somewhere, anywhere, because she could no longer fly to where the proper host plants were.
Here's a shot of one of the little weeds next to my hand, for a size reference. The little white spots you see are not monarch eggs, but the white milky sap that oozed out when I picked a leaf.
So, hmmm.....maybe the butterfly knows something about this plant that I don't.
A little farther along on the same trail, I found these same plants supporting small colonies of yellow oleander aphids. This is the same aphid that commonly infests the milkweed plants (A. curassavica) that the monarch caterpillars love to eat.
So, I sought an ID from our ever-knowlegable plant curator, who quickly recognized it as Araujia sericifera, an invasive vine that is from the same milkweed subfamily: Asclepiadoideae. He said they must remove this pesky plant, or the vines would grow and grow until they overtake even the tallest trees in the arboretum. So don't expect to save it for the caterpillars. I didn't expect that, I assured him. I just wanted to find out if it really was a monarch host plant. Besides, so far I had only just seen one tattered old butterfly laying eggs on it. I hadn't seen any caterpillars...
Today when I went to collect a few of the plants that had eggs on them, I found a small caterpillar on one. I am going to try potting up a few of these weeds (taking care to make sure they don't escape and take over my whole back yard, of course) and try to see if a caterpillar will actually grow well on them. If they do, it would at least be another food source for the teachers who are always running out of milkweed for the caterpillars in their classrooms.