The most common of 22 milkweed species in North America, Asclepias syriaca occurs throughout the midwest and east. Milkweeds are so named because nearly all of them exude a sticky latex when cut. The latex contains glycosides that protect against most insects, but Monarch caterpillers not only feed on milkweeds but become poisonous to their own predators because of it.
Identification: Plants are up to 3' (1 m) tall, rarely up to 6½' (2 m),
on an erect stem that is green or patched with dark red. Leaves are dull green, usually opposite, up to 8" (20 cm) × 3½" (8.9 cm). Hemispherical groups of pinkish purple flowers have a scent similar to that of violets or pansies.
The individual flowers are star-shaped, about ⅛" (4 mm) across, with five petals. Milkweed seed pods, and the thousands of silky parachutes that emerge from them in late fall, are unique. The pods are 2-4" (5-10 cm) in length.
A munching monarch caterpillar, and a newly hatched butterfly. Monarch
caterpillars are immune to the poisonous latex, and eat milkweeds exclusively. Photos courtesy of Heather A. Kent.
6/21/2010 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts ≈ 9 × 6" (22 × 14 cm)
Monarch butterfly caterpillars aren’t the only insects who dine on milkweed. These are oleander aphids (Aphis nerii) · 8/2/2012 · Nashua River Rail Trail, near Nashoba Hospital, Ayer, Massachusetts ≈ 4 × 6" (10 × 15 cm)