I look at milkweed everywhere I go. There is the off chance I'll find monarch caterpillars, though they're rare the last couple of years. But the plants are reliably infested with aphids, with the orange-yellow oleander aphid by far the commonest (photo right). This week I found leaves of common milkweed with the usual crowd of oleander aphids, and another I've never seen, photos below. I posted to bugguide.org for ID help, no answers. I asked Bob Dransfield, whose credentials include "UK Aphid identifier." He ruled out incidental polyphagous species, and said:
"... that leaves us with three milkweed specialists, namely Aphis asclepiadis, Aphis nerii and Myzocallis asclepiiadis. Neither of the Aphis species resemble your photos, but the elongate body shape strongly suggests Myzocallis. Bugguide http://bugguide.net/node/view/350341 describes Myzocallis asclepiiadis as follows: Pale green-yellow; later in the season, may have orange spots or be entirely orange.
The images on bugguide are unlike those I am seeing. Bob suggested that adults and nymphs may look quite different. In fact, the only alate I've seen (last pic below) is a better match for Myzocallis.