Macrosiphum rhamni can be found on coffeeberry, Rhamnus californica, also known as California buckthorn. It is also known to have an alternate host plant, bracken fern.
According to Jensen, the eggs of this species can be found hatching around mid-February through March. Whilte these aphids will often migrate to their summer host, bracken fern, a small number of them (such as the aphids in these images found in June) may remain on coffeeberry through the summer.
They may be tended by more than one species of ant.
Resources
Jenson, Andy. Macrosiphum. AphidTrek. This is one of my favorite resources so far given the passion of the author for these aphids. <19 June 2017>
Macrosiphum rhamni wingless adult
Macrosiphum rhamni wingless nymphs and adults tended by ants