Last year I didn't know what these large round and white aphids were. I thought that they might be aphids that were transforming into the flying variety and naively I removed them. They are aphids that have been parasitized by wasps. The wasps lay eggs in them and the wasp larva eventually come out to parasitize more aphids. Now that the wasps are out and active I've found I haven't had to work to control the aphids anymore as the large flying aphids are getting picked off before they can multiply. The short message is don't squish these!