Aphids come in a variety of species. Colors vary by species but popular ones are either green, yellow, or white. A group called "woolly aphids" are covered in a white, waxy substance that resembles cotton or fuzz. All types of aphids pierce the plant at its leaves and stems. They digest plant juices and excrete a sticky, sugary 'honeydew' liquid which may attract other garden insects. Ants have been seen shepherding aphids and harvesting the sweet honeydew for themselves and their own colonies. The honeydew can also become black with mold and look like a layer of dark ash on the leaves and stems, ruining the beauty of any ornamental plants they inhabit.
Small populations are generally not a major problem for gardeners, however, they are really fast at reproducing. Large populations can take over a crop if not controlled. Many gardeners buy containers of live lady beetles (ladybugs) to spread over their garden in an attempt to curb population growth of aphids; success is varied as lady beetle may leave the area after release before making an impact. Recognizing and allowing fire beetles and parasitic wasps to visit the infested plants can also aid in reducing aphid numbers and subsequent plant damage. Chemical pesticides that specifically kill aphids also exist, but will likely require multiple applications because they may only be effective at certain life stages and colonies often consist of different developmental ages.