112 I'HOCEEDIXUS OF THE KOVAL S(!(IKTV OF (^I^EENSLANU, 



assuming that all the other factors are fa%()urable (which is, 

 however, not the case) it is evident that Xasonia cannot 

 possibly act as an efficient control, since the blowflies breed 

 not only faster but also more numerously. Suppose, for 

 example, we liberated a single female Nasonia and a single 

 gravid female blowfly, e.g. Pycnosoma, in a certain district 

 in which neither occurred. The blowfly would lay about 

 250 eggs, all of which, it may be assumed, would possibly 

 produce adults. Now Nasonia would parasitize these pupie 

 to such an extent that about 113 offspring would be produced 

 by it — not at the expense of 113 blowflies however, but of only 

 about 17 to 20, as already mentioned. Nasonia, in_ fact, 

 would be a much more useful insect if it laid only one egg per 

 pupa. Hence after the very first generation we would have 



113 chalcids and about 230 blowflies ; after the next few 

 generations the difference would become very great, and 

 if the flies had no other enemies they would soon overrun the 

 country. Even if we contrast the period (a) which elapses 

 between the de^position of eggs by the chalcid and oviposition 

 by the females which emerge from such eggs (oviposition 

 occurring 'within a few hours after emergence), with (b) that 

 occupied by Pycnosoma from its deposition as an egg until thp 

 resulting female is capable of ovipositing, it appears that the 

 above result \\'ill not be materially affected ; since (a) the 

 wasp period referred to, during midsummer in Brisbane, is 

 about 14 days, whereas {b) in the case of the blowfly it is from 

 14 to 16 days distributed as follows — about 9 to 10 days in 

 the immature stages (egg, larva, and pupa) and about 5 to 6 

 days after emergence before egg-la jang takes place. As an 

 effective control, then, Xasonia is of little value. To what 

 extent, then, does it act as a check ? This depends entirely 

 upon the ratio of the number of pupae to which it has access, 

 to the number of those which it cannot harm. Simple experi- 

 ments, such as enclosing a number of these chalcids in a jar 

 in the bottom of which was a thin layer of soil containing pupse, 

 showed definitely that the wasps would not dig into the ground, 

 It is necessary, therefore, to inquire as to the proportion of 

 pupae lying on the soil to those below it. The following simple 

 experiments were carried out during the summer of 1920-21 in 

 Brisbane, but it should be carefully noted that they wore 

 performed partly under laboratory conditions and partly 

 under natural local conditions, but not under sheep-district 

 conditions. It is reasonable to assume, therefore, that similar 



