Each year about this time, the swamp milkweed acquires an infestation of yellow aphids. I don’t know where they come from or how they find the plants (chemical cues?), but they regularly appear just as the pods are elongating and maturing seed.
Aphis nerii, the yellow milkweed aphid, is a specialist that only feeds on milkweed species. Winged adults settle on the plant and then produce hundreds of asexual (non-winged) clones of themselves.
Usually I find the aphids just on the terminal stems and pods, but one plant had a congestion of aphids from the ground level, up the stem, to the tip of the pods.
It’s hard to believe the aphids get much nutrition from the woody part of the milkweed stem.
Aphids feed on milkweed sap by inserting their slender proboscis into the vessels that carry the sugar manufactured by the plant. Swollen yellow abdomens stick upright from the plant surface while their heads are down near the surface. When threatened by a potential predator (or human), they wiggle their butts in unison.
For more information on this interesting phenomenon, see my earlier post on how predators control the aphid population.