THE SEQUENCE THEORY 401 



greater or lesser degree of influence from indigenous species will 

 usually have to be contended with in such projects. 



In insular areas, if due precautions be taken, the problem of 

 hyperparasitism is frequently capable of circumvention, and this 

 appears to be an important contributory factor in the successes 

 attained with parasite introductions under such conditions. 



The Sequence Theory of Parasitic Control. The sequence theory 

 of the application of parasitic control is due to Fiske, and its 

 practical importance has been stressed by Howard in various 

 publications (vide Howard, 1924). The theory applies especially 

 to the control of Lepidoptera, in which the component stages of 

 the life-cycle differ in activity, habit, seasonal appearance, and 

 other factors. In a state of nature such species are almost 

 invariably attacked by a succession of different parasites affecting 

 the eggs, larvae and pupae respectively. When, therefore, an 

 insect like the gipsy moth, for example, has become accidentally 

 established in another country the theory postulates the necessity 

 of reproducing as nearly as may be possible its original parasite 

 association. It is claimed that there exists a delicately adjusted 

 condition of equilibrium between the host and each of its parasites. 

 This involves a limitation of the activities of the latter by environ- 

 mental and other factors which preclude the percentage of the. 

 hosts it destroys from rising above a certain mean figure. The 

 theory maintains for this reason that a single species of parasite 

 is incapable of destroying as many of a host population as is 

 destroyed by a succession of different enemies. It is claimed 

 that it is only when a certain percentage of the eggs have been 

 destroyed that parasites of the larvae will be able to reduce a 

 given pest to a sufficient degree that the final pupal parasites will 

 bring the species measurably under control. According to Fiske, 

 the gipsy moth has a six-fold rate of increase per generation, and 

 consequently a total parasitism of 88-33 per cent, is necessary in 

 order to attain effective control. 



The theory is ably criticised by Thompson (1923), to whose 

 paper the reader is referred for the detailed arguments. He points 

 out that insufficient evidence exists to prove that a sequence of 

 parasites is essential to the biological control of a given pest ; 



