THE PESTS AND DISEASES OF THE CANE 149 



A little later this observation was almost exactly paralleled in Hawaii, 

 where a sudden localized epidemic due to the larva of another root-eating 

 beetle, Anomala sp., broke out. In this case the importation was from Japan, 

 but was not in cane consignments. An investigation in Japan located its 

 parasite, Tiphia sp., which was successfully introduced into Hawaii. In 

 this case again the presence of the pest was hardly known in its natural 

 habitat. 



The introduction of the parasites of pests is not unattended by a certain 

 amount of danger, as in some cases the parasite may itself become a pest, 

 or it may attach itself to a new host, and in any case to obtain the maximum 

 of effectiveness the parasite must be introduced without its hyperparasites 

 if any such exist. In most of the cases recorded above the parasites were 

 obligatory, that is to say they were so highly specialised that they depended 

 for their life cycle on the host and on the host alone. Their introduction 

 then was attended with no danger of the disturbance of the natural balance, 

 but rather tended to re-establish it. 



The classical instance of the disturbance of the natural balance with 

 lamentable effects is the introduction of the mungoose into Jamaica. This 

 animal was so effective that not only did it destroy rats, but also attacked 

 other animals, as well as poultry and birds. The result was a great increase 

 in the number of ticks. 



The control agents mentioned above fall into two main classes, predators 

 and egg-laying parasites. In the former class are to be included such controls 

 as the lady-birds that devour the scale insects of Californian fruit trees. 

 Birds are very useful in controlling the various " army " and " grass " worms, 

 and the control of these in Hawaii is largely due to the importation of the 

 " mynah " bird from India. In the West Indies swallows have been re- 

 corded as keeping down the numbers of the cane fly, Delphax saccharivora. 

 Amongst insect predators an elaterid has been observed preying on the larva 

 of the moth borer in Cuba, and Muir also observed an elaterid destroying 

 the grubs of the Hawaiian beetle borer. Ants also are valuable predators, 

 but they have such a wide range of food habits that often they live on both 

 pest and parasite. 



Generally predators fall far short of egg-laying parasites in effectiveness. 

 These fall into two groups, egg parasites, and larval parasites. In the former 

 class are to be found many minute hymenoptera, chiefly represented by 

 chalcidids and myramids. In the latter class are to be found both hymenop- 

 tera and diptera. The former are represented by small wasps, of which the 

 scolids afford many examples. The latter contain many species of tachinid 

 flies, which appear to be amongst the most valuable control factors. The 

 egg parasites oviposit directly in the egg masses of their hosts. The scolid 

 wasps usually sting their host in such a way as to paralyse, but not to kill 

 it ; at the same time an egg is deposited in the body of the host, which forms 

 the source of food of the parasite until it emerges as the perfect insect. 



The method of parasitization pursued by the tachinid fly, which controls 

 the Hawaiian beetle borer is very remarkable. The fly, which is both 

 oviparous and viviparous, oviposits at the entrance to a borer tunnel, and 

 apparently locates the presence of a borer grub by the sense of smell, since 

 it seldom oviposits at the entrance to an empty tunnel. The maggots on 

 emergence find their way to the grub, the body of which they enter. The 

 grub is not killed until it has finished completing its puparium, which is 

 then used for the purpose of pupation by the parasite. 52 



