185 



Pembekton (C. E.) & WiLLAED (H. F.)- Interrelations of Fruit-fly 

 Parasites in Hawaii. — JL Agric. Research, Washington, B.C., xii, 

 no. 5, 4tli February 1918, pp. 285-295, 4 plates. 



The recent introduction into Hawaii of four species of larval parasites 

 of the Mediterranean fruit-fly [Cemtitis capitata, Wied.) has shown 

 that, to quote Dr. Howard, " it is unwise and most unpromising to 

 attempt heterogeneous and miscellaneous importations of parasites 

 without careful study of the host-insect on its home ground and in its 

 natural en\aronment throughout the whole range of its existence, and a 

 similar biological study of its parasites and natural enemies under 

 such conditions." 



In 1913 two species of Opiine parasites of the fruit-fly were introduced 

 into Hawaii by Prof. F. Silvestri, Opius humilis, Silv., from South 

 Africa, and Diachasma tryoni, Cam., from Australia. In the course of 

 two years it was quite evident that 0. humilis was often parasitising 

 from 60-90 per cent, of all the fruit-fly larvae developing in coffee 

 berries. D. tryoni slowly but steadily increased and ultimately showed 

 a capacity for occasionally parasitising 50 per cent, or more of the 

 host-larvae. 



Examination of parasitised larvae showed that when, attacked by 

 both species of parasite, in the majority of cases 0. humilis was killed 

 and D. tryoni alone developed to maturity. This was due to the fact 

 that the newly hatched D. tryoni larvae are very active and well 

 protected ventrally by a thick mass of serosal material and provided 

 with very powerful hooked mandibles, while 0. humilis larvae are 

 sluggish, not so well protected ventrally, and have less powerful 

 mandibles. In the second instar both species lose their powerful head 

 armature and become helpless, enlarged and stiffened. Hence, if, as is 

 often the case, several eggs of these two species are deposited in the 

 same host, the last parasite to hatch stands the best chance of 

 destroying the others and maturing. 



Both these species, as well as D. fullaivayi, readily oviposit in the 

 same larva, showing no selection of parasitised or unparasitised larvae, 

 as many as 8 or 10 Opiine larvae being frequently found in the same 

 host. The mere deposition of 8-10 parasite eggs often causes the death 

 of the fruit-fly larva, and though all these eggs hatch out, the resulting 

 larvae die off in a short time. 



Cool weather retards the development of the Opiine egg, especially 

 that of D. tryoni, and this fact causes a seasonal rise and fall in the 

 effectiveness of 0. humilis. In the summer and autumn the ascendancy 

 of D. tryoni causes a great reduction in the abundance of 0. humilis, 

 which is reversed during the winter and spring by the reduced 

 activity of D. tryoni permitting a rapid increase in parasitism by 

 O. hionilis [see this Review, Ser. A, vi, p. 167]. 



The recent introduction of another fruit-fly parasite, Tetrastichus 

 giffardianus, Silv., has shown that this Chalcid is decidedly destructive 

 to any of the Opiines occurring with it in the same fly larva or 

 puparium. In oviposition, about 10 eggs are placed in the host at one 

 insertion of the ovipositor, whereas 0. humilis lays only one. Hence, 

 though soft, sluggish, devoid of cannibaHstic tendencies, and armed 

 with small, inconspicuous, blmit mandibles, it nevertheless survives 

 the Opiine larvae by sheer force of numbers. If the host-larva is already 



