I didn't see this activity when I had a few monarchs on my asclepias two years ago. I only saw them eat leaves and also "flag" them, as in this video: https://www.flickr.com/photos/isabel95/52298608878/in/photolist-2nFwmkG-2nWUcdr-2oUVR58-2hncQye-2nE6zTr-2nEhond-2nEkAXn-2nELxVC-2nFmpu4-2nFs3n5/