Insects and their world 



leaves in this way it takes about two years to recover and bear fruit 

 again. 



Levuana had been present for many years on one island only of the 

 Fiji group, called Viti Levu, and as a result it was not profitable to grow 

 coconuts on that island. Occasionally the moth would spread temporarily 

 to neighbouring islands, and by 1925 it was thought that there was a 

 very serious risk that it would eventually spread throughout the Fiji 

 group, where the growing of coconuts for copra was a flourishing, and 

 indeed a vital activity. The threat of possible ruin to the copra estates 

 was undermining the financial position of the islands. 



It appeared that on Viti Levu the moth Levuana was established 

 without any natural enemies that were capable of controlling it. The 

 method of attack, therefore, was to try to find some other place where 

 Levuana lived, and where it had enemies that might be introduced into 

 Fiji. Strangely enough, this particular moth could not be found any- 

 where else. The fact that it was uncontrolled in Fiji suggested that this 

 was not its natural home, and that it must previously have come from 

 some other group of islands where it is now extinct. 



In Malaya there were moths whose larvae had somewhat similar 

 feeding habits, and which were heavily parasitised by a Tachinid fly, 

 Ptychomyia remota. Infested larvae were transported to Fiji, and the 

 adult flies that emerged from them were placed in cages with Levuana 

 caterpillars, which they at once attacked. In less than three weeks a new 

 generation of parasites had emerged and for six months parasitic flies 

 were reared in this way in a laboratory, as well as being released in 

 batches at intervals. At the end of that time it was possible to collect 

 parasitised Levuana caterpillars on the coconut trees, showing that the 

 parasite had estabhshed itself. 



In a little over a year the Ptychomyia fly had exterminated the 

 Levuana moth completely from the islands. Then came the question 

 whether the fly could keep itself going by finding other food, or would 

 it die out in turn. This was tested by taking Levuana caterpillars from 

 the laboratory and exposing them on the trees, when they were at once 

 attacked and parasitised. Thus not only was the pest eliminated, but 

 there was an effective enemy waiting for it if any stray specimens should 

 ever turn up again in the coconut groves. 



A spectacular cure like this is very encouraging, but unfortunately 

 circumstances are seldom so favourable, and apart from a few similar 

 instances, biological control has not been an outstanding success. Either 

 the parasites do not flourish on the insect pest, or, having done so, they 

 spread to other hosts and become a pest in themselves. Biological 

 control appeals to a biologist, because it is precise and elegant compared 



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