Prickly pears usually grow with flat, rounded segments that are amply armed with two kinds of spines: large, fixed spines and small, almost hairlike spines that easily penetrate skin and detach from the plant.
Prickly pears are the only type of cactus normally found in the eastern United States, and are the most cold-tolerant of the cacti. Species normally found in the eastern U. S. are:
A species of prickly pear (Opuntia stricta) was imported into Australia in the 1920s for use as a natural agricultural fence and quickly became a widespread weed, rendering four million hectares of farming land unproductive. The Cactoblastis moth, whose larvae eat prickly pear, was introduced in 1925 and quickly and almost wiped out the infestation. This case is often cited as a textbook successful example of biological pest control.