IX. 



The pest pear of the Oahu, Hawaii, closely resembles the " Westwood 

 pear " of our own Slate. 



Since America is the home of prickly-pears, they were there studied by 

 us under their natural conditions. Still, species belonging to one region have 

 become naturalised, especially if useful, in parts of the North and South 

 American continents other than those to which they are indigenous. 



DESTRUCTION BY NATURAL ENEMIES. 



A. Rodents. 

 In the cactus regions of the United States and Mexico, there occur several 

 species of so-called " wood-rats," belonging to the genus Neotoma, which at 

 times cause great havoc to prickly-pear, utilising it as food to such an extent 

 sometimes as to locally exterminate it. Occasionally this action involves its 

 propagation. However, since they, as well as various other rodents, are not 

 restricted in their dietary, to cactaceous plants, their introduction to Queensland 

 would probably be attended with grave risks, and is therefore not recommended. 



• B. Insects. 



Prickly-pears were found to be little if at all adversely affected by insect 

 enemies in the Mediterranean region, the Canary Islands, and Hawaii, and only 

 to a slight extent in the West Indies and in most parts of South America 

 which were visited. 



In Ceylon, India, and South Africa, only one species of Opuntia 0. mona- 

 cantha, was controlled in this way, the agent being in each case a wild cochineal 

 insect. In the two first-named countries the result of the parasitism by Coccus 

 indicus had been so disastrous to the host plant that extermination had been 

 practically brought about, O. monacantha now being found only in relatively 

 few scattered situations, whereas it was formerly abundant, and in places so 

 prevalent that this insect was actually employed in its subjugation. In a few 

 isolated localities in both of these countries the wild cochineal is still exerting 

 its baneful influence. The other naturalised species in India {0. dillenii and 

 O. nigricans) and Ceylon {0. dillenii) were not attacked. 



In South Africa the effect was much less marked, this result being due 

 partly to the different specific action of the particular wild cochineal insect 

 found there {Coccus confusus capen^is), and partly to the presence of predatory 

 insects which controlled the spread of the coccid. The other species of Opuntias 

 found naturalised in South Africa were unattacked. 



The interest of the foregoing remarks lies in the fact that O. monacantha, 

 the plant victimised by this insect, is one of the Queensland prickly-pears, 

 whereas 0. dillenii, and also the South African pest pears, which were not affected, 

 are related to certain other of our pest pears, viz., the common pest pear, the 

 Gayndah pear, the Westwood pear, and 0. aurantiaca. It is unlikely that any 

 of these will be attacked by it, although they may have other species of Coccus 

 similarly related to them. 



As was to be expected, it was in the Western hemisphere that insect 

 enemies were found to be prevalent. Most information regarding them was 

 obtained in the United States, where a considerable amount of investigation 

 concerning prickly-pear enemies had already been carried out by the Bureau 

 of Entomology of the United States Department of Agricultiu-e, as well as by 

 certain other institutions. 



The most important insect enemies of cacti, known to them and to the 

 Commission as occurring in America, are certain beetles, moths, plant bugs, and 

 midges, all of which are restricted to the Cactaceae. 



