|Photo by Ned Harris 2/8/2012|
Even on that ragweed of the gods, Ambrosia ambrosioides or Canyon Ragweed, you can find loads of galls. According to this site:
"Plants are often found that have numerous pale green warts on the leaf surface. These are caused by nearly microscopic gall mites (Eriophyidae). The mites feed on the plant tissue from the inside of the hollow blisters that are formed by the plant in response to secretions and irritations of the mites. Breaking open the galls will reveal the mites visible with a 20x magnifier."Now you know!