BIOLOGICAL CONTROL 360 



diseases, together with the depredations of those insects directly 

 dependent upon vegetation for their sustenance. From the 

 entomological viewi)oint, therefore, the problems of biological 

 control may be divided into those affecting insects and those 

 affecting plants. 



The economic significance of insect parasites and predators, 

 which live at the expense of other members of their class, has led 

 during the last forty years to their increasing practical utilisation 

 as natural agents in pest repression. 



Biological Control of Insect Pests 



Biological control of insect pests may be considered under 

 two categories : {a) introductions of parasites into countries 

 where they did not previously exist, and (b) the utilisation of the 

 indigenous parasites of a country. The remarkable developments 

 which the application of this method has undergone in recent years 

 are almost exclusively with reference to parasite introductions. 



The chief principles underlying the theory and practice of 

 biological control are outlined by Thompson (1930), and a general 

 text-book of the subject by Sweetman (1936) has recently appeared. 

 The mathematical theories vmderlying certain of these principles 

 have been discussed by several writers, and of these the most 

 recent are Nicholson (1933), H. S. Smith (1935), and Nicholson 

 and Bailey (1935). The last named have brought into prominence 

 the fact that if there is a balance in the population of a particular 

 insect some of the factors causing mortality must be dependent 

 upon host density and destroy a greater proportion of the 

 individuals of a specific host when the density of the latter is 

 high than when it is low. In the case of parasites and predators, 

 it is assumed that they search for their hosts at random, and it is 

 shown that the mortality which they cause will be dependent upon 

 host density. Since climate and other factors operate irrespective 

 of such density, parasites and predators are regarded as the main 

 factors bringing about balance. The reader who is specially 

 interested in this side of the subject is referred to these two 

 papers, which form a notable contribution to the theoretical 

 aspects. 



