PART IV. 
numbers By that seldom failing and still more seldom understood 
process known as the Balance of Nature, is able to assume the 
proportions of a pest. : 
It is obvious that no crop can be grown without thus affording an 
abundant supply of food to any insects which may be able to feed on 
it; but there are other ways of controlling the food supply of a pest. 
It the crop is an annual one, it is often possible to enforce a close 
season, so that for some period. of the year the insect is theoretically 
unable to maintain itself. Again, comparatively few plant-feeding 
insects are omnivorous or even catholic in their choice of food, while 
conversely, comparatively few confine themselves rigidly to one 
species of plant and one only. The majority of insects have a well- 
founded sense of systematic botany, and feed on all the plants of 
perhaps one Natural Order, or of one, or it may be two or three genera. 
The different food plants sometimes affect the life eyele of the insect 
or its fertility, thus one Egyptian cotton pest (Oxycarenus hyalini- 
pennis, Costa) which breeds on all the Malvaceous plants found in 
Egypt, lays an average of twenty eggs when it feeds on cotton, thirty 
if it feeds on Hibiscus esculentus, and less than ten when its host 
plant is a species of Sterculia (5, p. 101)+ 
It is thus clear that it may well be an unending task to try to 
control a pest if it is left unmolested on wild alternative host plants 
which may be growing in the vicinity of the crop; and that the control 
of the food supply may often be perfectly practicable. and a necessary 
adjunct to other methods. 
(c) and (d) TeHE UriLiSATION,K OF PARASITIC AND 
PrepaTORY ENEMIES OF INSECT PESTS. 
Under natural conditions, it is very largely the Parasitic and 
Predatory enemies which almost invariably succeed in keeping the 
numbers of an insect within reasonable limits. Insects are extremely 
prolific, there are but few species which lay less than 50 to 100 eggs, 
while with many the number is to be reckoned in thousands. Yet, 
on the average, only two out of each generation survive to reproduce 
their kind. 
But when man has upset the balance that formerly prevailed, 
the story may be a very different one. Food is there in abundance, 
and the insect feeding directly on the plant gets a start over its 
enemies and increases out of all proportion. It is true that under 
certain circumstances, even without any assistance from man, these 
enemies may also. increase and in time control the pest. Thus in 
certain parts of the United States, the Hessian Fly (Cecidomyia 
destructor) is not reckoned to be serious, because as soon as its 
numbers increase to any extent, its parasites also increase sufficiently 
to prevent any appreciable damage being done (4, p. 257)t This is 
not however always the case, and as will be shown shortly, unaided 
natural control of the Coffee Mealy-bug in this country is almost 
certainly impossible. 
When the balance of nature has been upset by Man, it is 
therefore necessary for Man to do something artificially to restore that 
balance. 
The larger animals, such as birds, toads, .ete., which may be 
predators on an insect pest, can often be encouraged without much 
t See list of references on page 194. 
186