Observation notes: Sunny & hot. On milkweed plant in my yard. Were first seen on a yellow leaf; by the time I returned with my camera, the leaf had fallen to the ground with only one caterpillar remaining. I picked up the leaf to move it up onto the plant to let it crawl onto an attached leaf when I realized the remainder had already crawled up the plant and were busy eating. The lone caterpillar crawled onto a fresh leaf; interesting to watch how it sensed/smelled/rejected the grape vine entwined with the milkweed.