414 BIOLOGICAL CONTROL 



acres in Queensland and New South Wales, and in 1925 the 

 plant was stated to be spreading at the rate of a million acres a 

 year, but this increase is no longer being maintained. In the 

 main the affected areas embrace natural grazing country, where 

 the land is worth less than £3 per acre. Under such conditions 

 chemical or mechanical control is impracticable except in lightly 

 infested country. In their native terrain in North and South 

 America about 350 species of Opuntia are known, but none is a very 

 serious enemy, yet of the few kinds introduced into Australia, at 

 least four are to be regarded as major or minor pests. In America 

 insects, diseases, and other agencies keep the prickly pear within 

 reasonable bounds, whereas in Australia such natural controlling 

 factors are wanting, and there is little to check the reproduction 

 and spread of the pest. The Prickly Pear Board of Australia is 

 concerned with an attempt to bring about a condition of biological 

 equilibrium by the introduction of insects and plant diseases 

 likely to act as natural checks ; the control aimed at depends 

 upon the introduction of a complex of organisms working together 

 in destructive unison (vide Dodd, 1927, 1929). It needs to be 

 recollected that the two chief pest pears in Australia are Opuntia 

 inermis and 0. stricta, while several others are of minor importance. 

 This fact complicates the problem, for the reason that a particular 

 species of insect may prove effective against one kind of prickly 

 pear, and yet be of little value with respect to other kinds of those 

 plants. Officers of the Board have been engaged in studying the 

 insects affecting Opuntias in their native surroundings. They 

 have covered widespread cactus areas in North America, where 

 field stations have been set up, and have also visited South 

 America and the West Indies. In work of this character it is 

 important to study on the spot, not only the insects actually 

 attacking prickly pear, but also the natural parasites of such 

 insects. The exclusion of the parasitic forms from Australia is of 

 prime importance if their host insects are to multiply freely and 

 vigorously attack the prickly pear. At the Board's Station at 

 Urvalde, in Texas, extensive biological work has been prosecuted, 

 and the most promising cactus-feeding insects bred under caged 

 conditions ; furthermore, their life-histories have been worked out. 



