396 BIOLOGICAL CONTROL 



covered by the alfalfa weevil and Mexican bean beetle are already 

 of great extent. 



The problem of parasite introduction in continental areas is 

 discussed by Thompson (1928), who points out that the effects 

 which follow the establishment of foreign pests in such areas 

 constitute a clear demonstration of their escape from control. 

 The evidence indicates that the factor whose absence has allowed 

 of increase and spread is the parasitic factor. In any attempt 

 to establish equilibrium over so vast an area, he is of opinion 

 that we have to rely upon biological methods if permanent and 

 inexpensive control is to be hoped for. Under such conditions no 

 one parasite or predator is likely to effect appreciable control. 

 With the European corn-borer the parasitic enemies do not exist 

 all together in any individual zone yet discovered in Europe. 

 The composition of the parasitic fauna differs in every region, 

 and in view of the differences of climate obtaining over the great 

 area of North America affected by this insect, it is most probable 

 that only a limited quota of the introduced parasites will colonise 

 a given zone. In order to establish the best results, species of 

 parasites failing to become colonised should be reintroduced as 

 the corn-borer reaches new areas differing climatically from those 

 now infested. (Thompson and Parker, 1928.) In other words, 

 re-establishment of natural equilibrium appears to stand the best 

 chance of being accomplished on an ecological basis. 



Multiparasitism. The factor of a multiparasitism is discussed on 

 p. 322, and a well-known example is afforded by the parasites of the 

 Mediterranean fruit-fly (Ceratitis capitata Wied.) in the Hawaiian 

 Islands (p. 374). When first introduced, the Braconid Opius 

 humilis Silv. was the most efficient of the imported enemies of 

 this insect. In 1915 it parasitised 31-5 per cent, of all the Ceratitis 

 larvae developing in fruits about Honolulu ; since then its efficiency 

 has declined, and in 1923 it was as low as 4-1 per cent. In 1924 

 and 1925 its effectiveness increased, when it parasitised 14-5 per 

 cent, of the larvae each year. Among coffee cherries in Kona the 

 Opius parasitised from 59 to 97 per cent, of the fruit-fly in 1915, 

 but its effectiveness gradually declined, and in 1926 not a single 

 example was reared from the larvae from 18,955 coffee cherries 





