2 
BY THE INTRODUCTION OF PARASITES. 367 
In 1907 it was found that small colonies failed and subsequently larger colonies 
only were put out. “It may be, after all, that 40,000 individuals of Apanteles 
fulvipes are not enough to make one good colony.” 
Disease has been a factor, ie., a fungus disease in the brown-tail and 
a disease like “ flacherie” (silkworm disease) in the gipsy moth; but these 
are apparently ineffective 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 eftective, 
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 with numerous other control- 
ling agencies, results 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 ege-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 five-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. 
On 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 showmg the 
