This is a genus of mostly grass-feeders, something in the ballpark of 20 species worldwide. In North America, it seems like most of the species are introduced from elsewhere, exceptions being Metopolophium palmerae (Hille Ris Lambers) in Colorado and Metopolophium arctogenicolens Richards from the Arctic. I have had some puzzling finds of Metopolophium in western North America that probably deserve further study. Two are mentioned below, one puzzle with each species of which I have photos.
Metopolophium dirhodum (Walker)
This is a very widespread and common pest of grains in temperate regions of the world. It is has capably invaded many natural systems in western North America, being common on several species of Rosa (its overwintering host), and many grasses from ocean shore to sub-alpine environments.
The puzzle I found regarding this species was a confirmed use of a rare native cliff-dwelling plant in Hells Canyon, Idaho: Glossopetalon nevadense (Crossosomataceae). The aphids were numerous and were just maturing to alate viviparae when I collected them on 30 April. I have quite a few photos of M. dirhodum because it inevitably is common in gardens.
Metopolophium frisicum Hille Ris Lambers
This species has not been confirmed from North America, although apparently there was a record by Börner of it somewhere on the continent (see Blackman and Eastop). I have now collected what seems, based on available keys and descriptions, to be M. frisicum in Washington, Oregon, and Colorado. When I’ve found it, it was feeding on a fine grass in the shade or in a wet cool hollow. With the morphology and habitat type fitting so closely the account by Hille Ris Lambers, I actually feel fairly confident about this identification. The localities where I found this aphid were partly natural, or might have appeared native, but they were heavily impacted by cattle grazing — situations in which the grasses are usually non-native (and also a situation in which I wish I knew grass taxonomy).