CANE SUGAR. 



one or more parasites, and it is due to the absence of the parasite that the 

 sudden outbreaks of insect damage are due. The control of insect damage is to 

 be obtained by the introduction, breeding and distribution of approved parasites. 

 A striking example of this method is afforded by the work of Koebele, Perkins, 

 and their associates in the control of the leaf hopper pest in the Hawaiian 

 Islands. This insect was introduced unaccompanied by the parasites which in 

 other places kept its development in check ; expeditions were sent to Fiji, 

 Queensland and elsewhere, parasites were obtained, bred and distributed, and 

 the leaf hopper was in a few years placed under control ; this achievement may 

 well be considered as a classic in economic entomology. The parasites found 

 to be the most effective were extremely minute hymenoptera, belonging to the 

 Calcididae, of which Paranagrus optabilis, P. perforator and Anagrus frequens 

 were the most valuable. These parasites lay their eggs on the eggs of the leaf 

 hopper, and this form of parasitization is considered the most effective. 



Very often cane pests are kept in check by naturally occurring parasites ; 

 thus a species of Chrysopa, belonging to the neuroptera, controls the ' spittle 

 fly,' Delphax saccharivora, in the West Indies. In Java, the detailed studies of 

 Zehntner, Yan Deventer and others have brought to light many parasites ; the 



* moth borers ' are thus controlled there chiefly through the agency of two 

 minute chalcid hymenoptera, Ceraphron leneficiens and Chaetosticha nana ; 

 other of the pests in Java (and doubtless elsewhere) are controlled by other 

 minute hymenopterous insects which may be well considered as amongst the 

 most beneficial of their kind. In Figs. 58 and 59* are given, after Yan 

 Deventer, a drawing of Ceraphron beneficiens, and the eggs of the 'Borer,' 

 Diatrcea striatalis, parasitized by this insect. 



Amongst other beneficial insects may be mentioned various * ladybirds ' 

 ( Coccinellida) to which the control of aphis and other 'plant lice' is largely due. 



It may also be mentioned that the planting of certain plants, e.g. Pigeon 

 Pea (Cajanus mdicus), Bonavist Bean (Dolichos lablab\ near to cane fields has 

 been stated to attract the hymenopterous insects parasitic on many cane 

 enemies. 



The question of pest control by parasites and predators is, however, a 

 most complicated one, and includes many factors grouped together in the term 



* struggle for existence? Thus there may be a major pest of the cane, A, and 

 a minor pest, B, which is also predatory on A. Amongst these may be intro- 

 duced a third form of life, C, parasitic on A, which in its turn is parasitized 

 by D, which also includes B among its hosts. It is only by the careful study of 

 the life history of the pests and of their parasites, and by taking into account all 

 conditions, that it can be determined if the introduction of such and such a 

 form of life is advisable ; the ill considered disturbance of the natural balance, 

 as instanced by the results of the introduction of the mongoose into 

 Jamaica, is unreservedly to be condemned. 



* See Frontispiece. 



136 



