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 wellfounded
 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 hyalinipennis,
 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.

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