Feb. 4, 191S Interrelations of Fruit-Fly Parasites in Hawaii 295 



The relative proportion of females to males in these two species of 

 parasites is interesting. Of 26,975 individuals of Diachasma tryoni, 

 reared from material collected in the field in 191 6 and 191 7, 16,845, or 

 62.4 per cent, were males. Of 10,843 individuals of Opius humilis 

 similarly reared from material collected in the field in 191 6 and 191 7, 

 6,128, or 56.5 per cent, were males. Here, again, the advantage, if any, 

 lies with O. humilis. 



In view of these several facts relating to the comparative prolificness 

 of the parasites Opius humilis and Diachasma tryoni, it appears that the 

 former is superior to the latter species or to any of the other introduced 

 parasites. Thus, when a host larva is parasitized by both O. humilis 

 and D. tryoni, the latter survives, and in so doing produces an individual 

 less prolific than would have been the case had the O. humilis been per- 

 mitted to develop. This seems to point to a certain, distinct loss. If 

 one species of larval parasite when working alone parasitizes 60 per cent 

 of the host, and another species not strikingly different from the first and 

 working the same in every known respect parasitizes 40 per cent of the 

 host when working alone, there is no reason to assume that both com- 

 bined can exceed a parasitism of 60 per cent. All overlapping by the 

 species capable of only 40 per cent parasitism can only serve to reduce 

 the total effect to a point below 60 per cent of parasitism. 



CONCLUSION 



Sufficient evidence has been presented to prove the superiority of the 

 parasite Opius humilis over the other introduced fruit-fly parasites in 

 Hawaii and demonstrates the decided restraint operated over it by the 

 unfailing cannibalistic activities of the larvae of Diachasma tryoni in 

 particular and of the other parasites in part. Knowing the capacity of 

 O. humilis for parasitizing from 80 to 90 per cent of the larvae of the fruit 

 fly in favorable localities, such as the large Kona coffee belt on the island 

 of Hawaii, the writers here maintain that detrimental results to a certain 

 extent have arisen from the liberation in Hawaii of parasites other than 

 O. humilis that attack the larva of the fruit fly. The total parasitism 

 has simply been reduced in value to that of a parasite of secondary value. 



It is hoped that the present analysis of the interrelated activities of 

 the imported fruit-fly parasites in Hawaii may serve to stimulate greater 

 discrimination in the selection of parasites proposed for future intro- 

 duction. 



27808°— 18 5 



