I have an extensive cacti collection of over 100 varieties. Some get an infestation of something I'm not sure what to call it. Others are immune. I have a picture, but don't know how to post it here. I think it may be called 'scale'. They are small, white, hard bugs(?) that are very difficult to get rid of, aside from picking them off one by one. Does anyone know what it could actually be or what to treat with? Bug sprays don't seem to work.