THE entry of the prickly–pear into Australia dates from about 1787, when the species Opuntia monacantha was introduced from Bio de Janeiro. The object was to establish the cochineal industry in that land since prickly–pears constitute the host for this particular kind of insect. Some twenty–five other species of Opuntia have found their way into Australia, but their origins cannot be traced. All have become naturalized either as serious pests, minor pests or as garden escapes. The two major pest species in Australia are Opuntia inermis and 0. stricta. At one time landowners grew hedges of prickly–pear around their homesteads until they got out of hand and then the hedges were cut down. The rapidity with which these pests have increased is one of the botanical wonders of the world. Their original home is the coastal sector of Texas and Florida where the mean rainfall is 40–50 in. Yet in Australia the plants have adapted themselves to a very different environment and with a precipitation of only 20–30 in. annually. In 1900 an area of about 10,000,000 acres was affected in Queensland and New South Wales. The invasion advanced with such celerity that at the peak, in 1025, the affected area must have been greater than 60,000,000 acres: in some years the annual increase in infested territory exceeded 2,500,000 acres. The main distribution takes place by seeds, but every broken–off segment of the plant is liable to take root.