290 Journal of Agricultural Research voi. xii. No. s 



Thus, the inevitable tendency of every individual opiine lan^a upon 

 hatching is to destroy every other parasitic larva in its domain, whether 

 it be one of its own kind or of another species. This would appear to be 

 an infallible instinct and one of great consequence in Hawaii in the 

 development of the opiine parasites now present. 



SUPPRESSION OF OPIUS HUMILIS BY DIACHASMA TRYONI AND D. 



FULLAWAYI • 



The parasites Diachasma tryoni and the closely related Diachasma 

 fullawayi, by virtue of this larv^al instinct, coupled with further en- 

 dowed superior body characters, have been responsible for the great 

 suppression of the parasite Opius humilis. The last-named species is 

 more prolific and hardy than either of the two others and is more gener- 

 ally efficient than both combined. By their association with the 0. humilis, 

 they have worked a detriment by reducing the total extent of parasitism 

 to a point below that to which it is capable of exerting alone. The evi- 

 dence of such suppression, gained from microscopical examinations of 

 fruit-fly larvae and pupae secured from various fruits in Hawaii during 

 191 6 and 191 7, may be expressed as follows: 



From April 16 to May 10, 1916, a dissection was made of 757 fruit-fly 

 pupae, freshly secured from coffee collected in the Kona coffee district of 

 the Island of Hawaii. From this total, 345 were parasitized by only 

 Opius humilis, 90 contained living larvae of Diachasma tryoni together 

 with dead larvae of O. humilis, 9 contained living larvae of O. humilis 

 together with dead larvae of D. tryoni, i was parasitized by only D. 

 fullawayi, 5 contained living larvae of D. fullawayi together with dead 

 larvae of O. humilis, 2 contained living Opius larvae together with dead 

 larvae of D. fullawayi, 57 contained living larvae of only D. tryoni, and 

 248 were not parasitized. Here it is seen that from the total of 757 pu- 

 paria, 106 cases of duplication in parasitism occurred in which a species 

 of Diachasma was found in the same puparium with one or more indi- 

 vidual larvae of O. humilis, and that in 95 of these the latter was killed 

 and the former survived. 



A further series of microscopical examinations of the contents of fruit- 

 fly puparia, freshly secured from coffee fruits in this same district in 

 January, 191 7, during the coolest part of the year, strongly confirmed 

 the results of the previous year. Six hundred and twenty-seven puparia 

 were opened and examined. Of these a total of 343 were parasitized by 

 Opius humilis alone, 67 were parasitized by only Diachasma tryoni, 2 con- 

 tained only larvae of Diachasma fullawayi, 129 contained living larv'ae of 

 D. tryoni together with dead larvae of O. humilis, 8 contained dead larvae 

 of D. tryoni together with living larvae of O. humihs, 4 contained living larvae 

 of D. fullawayi together with dead larvae of O. humilis, 2 contained dead 

 larvae of D. fullawayi together with living larvae of O. humilis, and 72 

 puparia were unparasitized. These results are again very significant. 



