CHALCID PARASITES OF Al'STRALIAX SIlEEl' MAGGOT-FLIES. Ill 



that a parasite will readily attack its host under laboratory 

 conditions, wiU it do so on an extended scale where its host 

 is accessible in large numbers ? For example, Australencyrtus 

 giraulti will attack maggots -nith great avidity in the labora- 

 tory, but it is not known whether it would do so if given 

 the same opportunity on a large scale in nature. The only 

 way to test this Avould be to liberate the parasite in numbers 

 in a given district and see if it Avould establish itself. 



If any one of the factors be continuously unfavourable, 

 then the parasite cannot be of any very great importance. If 

 finally the parasite is to eradicate the host, not only would all 

 these conditions have to be very favourable, but a fourth 

 factor would have to be taken into account, i.e. the relative 

 rate of breeding of host species and parasites. It will now be 

 necessary to examine the effect of chalcid wasps on sheep-flies 

 along these lines. 



An immense amount of material for parasitizing is con- 

 stantly available to these wasj^s ; it follows, obviously, that 

 any parasite which is not common can destr6y only a very 

 small percentage of the available larvae or pupae ; if the 

 available material, however great in amount, is small compared 

 with what is not available, the economic importance of the 

 parasite is practically nil. It seems, as will be shown below, 

 that such is the case with those wasps Avhich attack the fly- 

 pupaj ; and such forms as Dirhinus sarcophagce, Hemilexomyia 

 abrwpta, Pachi/crepoideus dubius, and Paraspilomicnis froggatti 

 must be regarded as nothing more than entomologically interest- 

 ing. Their scarcity probably depends on the unfavourable 

 nature of the third factor, possibh^ also on the presence of 

 enemies, or on poor food supply. 



Spalangia muscidaruyn has been recorded as parasitizing 

 various Muscid flies to a great extent in Eidsvold ; it was also 

 occasionally found destroying blowflies in Roma and in 

 Brisbane, but there is as yet no evidence to show that it is of 

 any more practical use against the sheep maggot-fly pest than 

 such a form as Dirhinus sarcophagce. 



Of Nasonia hrevicornis much has been hoped ; by some 

 it has been regarded as the ultimate means of actually eradi- 

 cating, or at any rate controlUng, the blowfly. Taking into 

 consideration the above general remarks, it will be seen that 

 these claims are founded on no very firm evidence. Even 



