SCIENCE AND INDUSTRY. 
It was found that destruction by mechanical means had been freely 
tried, both by Governments and by individuals, but had proved too 
expensive for adoption on a large scale, though they are used to some 
extent in clearing valuable agricultural land. Destruction by chemical 
means had also been thoroughly investigated. It was found that the 
arsenious chloride method -was the cheapest, but even in this case the 
price imposed great restrictions upon its employment. Nor has destruc- 
tion by utilization proved commercially successful. Potash and paper 
making were two industrial uses to which the pear was to be put. 
But the pear still grows apace, and supports no industries. The pear 
cannot be eradicated by feeding it off to stock, and so there remains 
a 
Photo. by A. T. Clerk, Lands Dept., Brisbane. 
THE PEST PEAR (Opuntia inermis), showing dense pear in Brigalow Scrub. 
but the fourth line of investigation to be followed, viz., destruction by 
natural enemies. 
A few years ago the Queensland Government appointed a Commis- 
sion to visit all parts of the world to investigate the natural enemies 
of the prickly pear. Dr. T. Harvey Johnston, now Professor of 
Biology of the University of Queensland, and Mr. H. E. Tryon, Govern- 
ment Entomologist, undertook the search, and they were successful in 
discovering an insect, Coccus indicus, which was introduced into Queens- 
land, and has practically exterminated one species of pear, Opuntia 
monocantha, in every district where it was liberated. Unfortunately 
it will not attack Opuntia inermis, which is the pest pear. In view of 
the partial success due to the importation of Coccus indicus, and of a. 
general consideration of the whole question of plant parasitology, the 
46 
