The common milkweed, Asclepias syriaca L., is remarkably adapted for cross pollination by insects. Its pollen sacs (pollinia) are often found attached to the appendages of bees, wasps, butterflies, and other insects that visit milkweed for its nectar (Judd, 1955; Matheson, 1951 ; Muller, 1883).
In the summer of 1966 and 1967 I collected numerous pierid and nymphalid butterflies associated with milkweed plants in Michigan in order to examine them for their pollen-carrying capability. Species of butterflies collected were Colias interior Scudder, C. eurytheme Boisduval, Pieris rapae (L.). and Speyeria aphrodite (Fab.). These insects were taken while feeding on or flying near milkweed plants between 3 July and 22 July each year--the period when milkweed was in full bloom--in Crawford, Montmorency, and Oscoda Counties, Michigan.
Wilson, Louis F.
"The Capability of Some Butterflies as Carriers of Common Milkweed Pollen,"
The Great Lakes Entomologist, vol 2
Available at: https://scholar.valpo.edu/tgle/vol2/iss1/10