One of my native plant list serves has had these yellow aphids as a topic of discussion. They are non-native and are presumed to have arrived with the oleander that is omnipresent in S. California. They also like milkweed (Asclepias), which is what they are see on here. I also have Oleander in the yard, but it was surprising how quickly these guys arrived and spread on the milkweed. It was so fast that I thought for certain that I'd brought the plant home with an unseen infestation.
Treatment for the aphids is various among the list serve denizens. Everything ranging from "do nothing and let the insect predators get them" to insecticidal oils. Since this is a newly planted Asclepias fascicularis (Narrowleaf milkweed) that I want to give a maximal change of survival, I first tried crushing them, which was immediately satisfactory but led to a population rebound when I probably missed a few. I then tried isopropyl alcohol (dripped along the stem, since I had no sprayer) and that worked like a charm. No aphids at all some days later.
In the picture below the black hairs on the hindquarters are wing buds. (There are better pictures online. I added a rock to get my iPhone's autofocus to work better.) When the food source is depleted or the population too large, the aphids can fly to the next target. The various sizes are at various stages of development. These aphids are all clones of the mother: No males are reported to exist in nature.
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