On Apr 4, 2013, PTCGAZ from Black Canyon City, AZ wrote:
These are the bugs that suck the juices out of Opuntia and other types of cactus and can spread germs to them, that leaves them with the round dots on the pads. It doesn't kill the plant, but it can make them discolored. I have killed a few of them here in BCC. And btw they have been on Echinopsis, and a few others types of plants.