MULTIPARASITISM 



397 



(Willard, 1926). Its decline has been traced to the fact that 

 0. humilis frequently occurs within the same individual host 

 larva, which is also harbouring either of the other Braconids, 

 Diachasma tryoni Cam, and D. fullawayi Silv. When this 

 happens the multiparasitism invariably results in the death of 

 the Opius. 



The view has been expressed that had the biology of these 

 parasites been fully studied prior to their liberation, their mutual 

 relations would have been disclosed and the desirability of liberat- 

 ing 0. humilis only, in the first instance, might have been apparent. 

 It is possible, therefore, that this single species of parasite might 



Fig. 94. Diagram representing multiparasitism, showing percentages 

 of hosts attacked and destroyed by Diachasma and Opius and 

 percentage of overlaps, which in most cases produced Diachasma. 

 From data of Pemberton and Willard. (After H. S. Smith, 

 Bull. Ent. Res., XX., 1929.) 



have proved more efficacious than the combined activities of the 

 four species already liberated. 



H. S. Smith (1929) has critically analysed this problem and 

 concludes that it is far from proven that the effects of the multiple 

 parasitism would have been less beneficial than the activities of 

 the Opius alone. He stresses the superior " balancing " effect of 

 several species of parasites over a single one in that they are not 

 all likely to suffer to the same degree from deleterious factors. 

 Several species would tend to stabilise a parasite population, 

 whereas a single species would tend to fluctuate in efficiency 

 annually. In Fig. 94 one phase of the problem is represented 

 graphically, and Smith claims that even if the 17 per cent, of the 



