Pest aphids tend to have a wider host range than economically unimportant species.
Most aphid species are relatively host-plant specific, although they may occasionally be found colonizing certain plants or 'reserve hosts' with which they are normally not associated. Aphids range in size from 1.5 to 3.5 mm. Single species may have several color forms and variable shapes. The body is distinctly segmented and the head and prothorax are usually separate. On the back of the fifth abdominal segment, a pair of tube-like structures called 'siphunculi' or 'cornicles' are present on most aphid species.