BY THE INTIJODUCTION Ol' TAIIASITES. 367 



111 1907 it was found that small colonies failed aud subse(.|ueiiUy large)- colonies 

 only were put out. " It may be, after all, that 40,000 individuals of Apaidelcs 

 fulvijjes are not enough to make one good colony." 



Disease has been a factor, i.e., a fungus disease in the brown-tail aud 

 a disease like " flacherie " (silkworm disease) in the gipsy moth ; but these 

 are apparently ineft'ective as checks. Studies were made of the parasites 

 of various indigenous Lepidoptera. Parasitism plays a different part in 

 every species, and while some are extensively parasitised and checked, 

 others have only a limited number of parasites none of which becomes 

 abundant. The authors conclude that no single parasite will be effective, 

 but that a sequence of different parasites will be necessary, attacking the 

 egg, larva and pupa consecutively. The problem they have to solve is the 

 establishment of such a complete sequence. The authors further distinguish 

 " catastrophic " from " facultative " control. In the former class of checks, 

 which are mostly climatic, such as storms, frosts, etc., the average percentage of 

 destruction remains the same, no matter how common or rare the pest may be ; 

 but a " facultative " check is one which becomes relatively more effective as the 

 pest becomes more abundant, thus operating to prevent undue increase. It is 

 further assumed that " each species of insect in a country where the conditions 

 are settled is subjected to a certain fixed average percentage of parasitism, which, 

 in the vast majority of instances and in connection witli numerous other control- 

 ling agencies, I'esults in the maintenance of a perfect balance." In order that 

 this balance may exist there must be at least one " facultative " check. 



Birds, and similar predaceous foes, are considered to exert no " facultative " 

 control whatever ; this is directly opposed to the present writer's view. 



The authors in fact give controlling agencies as : — 



(1) Catastrophic (storms, etc.) — killing a percentage only, irrespective of 



abundance. 



(2) Predatory (birds, etc.) — killing a gross number only in any year, 



irrespective of abundance. 



(3) Facultative — increasing automatically as the insect preyed upon becomes 



more abundant. 

 Calculations were made as to the amount of parasitism necessary to keep the 

 gipsy moth in check. It was estimated that, taking egg-masses for instance, 

 for every one that existed in the spring, six were laid in the autumn ; that is to 

 say, that in the United States, the rate of increase then was actually six-fold. 

 If therefore it was desired to keep the species down to an innocuous level it 

 would be necessary to kill iive-sixths of the total insects yearly, or 83 per cent. 

 In any case the death rate would have to be maintained at not less than 75 per 

 cent. 



The investigators were anxious to determine whether the gipsy moth 

 was controlled by parasites in other countries ; and if so, whether the 

 parasitism was anywhere so effective as to kill 75 per cent, of the insects. 

 ()n this point no information was available, and attempts were made to discover 

 it from the material collected elsewhere and sent to them. This was 

 unsatisfactory, so far as figures went, but the authors give tables showing the 



