148 FROGliOPPEK BLIGHT OF SUGAR-CANE. 



Success then depends on the abili' v of these parasites to adapt them- 

 selves also to the new surroundings and on their being in turn compara- 

 tively free from destruction by hyperparasites. 



The latter condition is most likelj- to be fulfilled in a country with a 

 small favma, such as an island, particularly one well separated from the 

 mainland. 



Several cases are known in which great success has attended the 

 method. Thus a leaf hopper {Ferkinsiella saccharicida) was acci- 

 dentally introduced into Hawaii and threatened to destroy the sugar 

 industry of that island. The original home of the leaf hopper was found 

 to be Australia. A search was made there for parasites, those found 

 were introduced into Hawaii, and the leaf hopper ceased to be a positive 

 danger although still ranking as a considerable pest. 



In California the scale insect Iceria purchasi was accidentally 

 introduced and spread with enormous rapidity, but was successfully 

 reduced when a lady-bird beetle (Vedalia) was introduced from 

 Australia, the original home of the scale insect. 



More recently the hard-back beetle {I'liytalus smithi) was acci- 

 dentally introduced from Barbados into Mauritius, but its rapid spread 

 has been quite checked by the deliberate introduction of the wasp 

 Tiphia 2}araUela, which was known to^keep it in check in Barbados. 



On the other hand parasites have been introduced from all parts of 

 the world to combat the Gj'psy and Brown-tail Moths in the Kew 

 England States of U.S.A., yet they still remain a severe menace and spread 

 over a larger area each year. Other examples could be given of the 

 ailure of the method to produce appreciable results. 



In all the above cases we are dealing with a pest introduced into a 

 new country without its enemies, which have been left behind in the 

 country of its origin. In Trinidad we have quite a different condition. 

 The froghopper is an indigenous insect ; it has been here probably for 

 thousands of years and almost certainly long before the introduction of 

 the sugar cane. It has associated with it already a number of parasites 

 and hyperparasites, and the complexity of relationship between them is 

 indicated by the diagram in fig. 19. (p. 92j. 



Trinidad, though geogi-aphically an island, is biologically a part o 

 the South American continent and has its rich and varied fauna only 

 slightly reduced. Any new enemy introduced, before it can be the 

 slightest economic value, has therefore to be able to hold its own against 

 a number of hyperparasites, must be able to withstand the dry season 

 uninjured, and in addition must be able to breed with such rapidity that it 

 can overtake the froghopper. Against all these disadvantages, to be of 

 any practical value, it must be sufificiently powerful to alter the present 

 balance and prevent the froghopper from increasing beyond certain 

 narrow limits. 



A practical example of the results of the incessant struggle to 

 which a new parasite may be subjected is seen in the history of the 

 Mexican Bug introduced from Mexico in 1911 by Urich to destroy the 

 fioghopper. Many thousands of them were liberated in the cane fields 

 and not one was e\er recovered, in all probability owing to their exter- 

 mination by other predaceous insects, spiders, etc. 



However, the loss from the froghopper has probably averaged 

 Jt.50,000 per annum during the past fifteen years and with the increase 

 in the value of sugar it may be worth while, if the agricultural methods^ 

 now advocated fail to give relief, to take up once more the search for 

 parasites. 



