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PART II. 



Notes on Parasites or Insects that have been introduced 

 from foreign countries to check or exterminate 

 injurious insects. 



Parasites, and their valuo and limitations in controlling injurious insects o! the 

 garden and orchard. 



of the most interesting problems in the study of economic entomology is 

 that of how far we can avail ourselves of the services of predaceous or useful 

 insects that devour the injurious species, by introducing them from other 

 countries to check or exterminate in an artificial manner native pests, or 

 foreign accidentally-introduced ones that have become pests in their adopted 

 home. 



The subject is such a fascinating one, that most people are apt to rush to 

 conclusions before the matter has been investigated from all points of view. 



After many years' study in Australia, both in the field and laboratories, 

 after information received personally from entomologists, horticultural com- 

 missioners, orchardists, and inspectors during my extended travels, and after 

 careful reading of the many reports, bulletins, and newspaper cuttings 

 issued, I propose to state my views on this problem, arid shall quote in 

 conclusion the opinions of some of the leading authorities on the question. 



It is a fact that, if it were not for the countless millions of parasites (the 

 majority of them so minute that their work is never observed) which swarm 

 in our gardens and fields, there would be such an overwhelming multitude of 

 caterpillars, grubs, aphides, and scale insects at work, that there would not 

 be a green thing on the face of the earth. Nature in this abundance of 

 natural checks has provided for this balance of power, and it is so maintained 

 under the ordinary natural conditions of the native forest and plains. Thus 

 probably not more than 5 per cent, of the millions of eggs laid ever reach 

 maturity and develop into the adult insect. 



Though there are many varieties of parasites, they can be broadly divided 

 into the two very distinct groups for our purpose. First, the predaceous 

 insects, parasites that feed directly upon the injurious insects, such as the 

 ladybird beetles (Coccindlid<x\ which may be called external feeders ; and, 

 next, the insects that, looking for a home for their young, deposit their eggs 

 within the body of the pest insect by means of an ovipositor (a hollow, 

 produced needle-like process at the tip of the body), and from which hatch out 

 the maggots that eat up their host, and finally pupate under shelter of its 

 skin or in little cocoons in its body. This class of parasites may be called 

 internal feeders, and comprise the immense army of microscopic wasps and 

 flies. 



From their size, colour, and activity tfie fact that ladybird beetles were 

 useful insects was well known by entomologists at a very early date, and in 

 Kirby and Spence's Entomology, published in 1816, the authors called 

 attention to the value of the common English ladybird beetle to the hop- 

 growers in devouring their great pest, the hop aphis, in the south of England, 



