96 Introduction to Insects [CH. 
that they often have to travel long distances in search 
of food, which in many cases they may be unable to 
find. In fields where we grow plants suitable for certain 
insects these can get food without searching for it and 
are thus enabled to increase rapidly. In this connection 
it must be remembered that insectivorous birds are 
useful in checking the increase. 
Man has further upset the balance of nature by 
introducing foreign insects capable of living on our 
plants, e.g. the American woolly aphis. These are 
often capable of enormous damage as the insects, 
birds and animals which prey.on them in their native 
country may not be present here. The introduction 
of the enemies of imported pests has in some cases 
proved extremely useful. Our present system of 
marketing also helps the spread of diseases from one 
part of the country to another. 
Carnivorous species are capable only of keeping 
the herbivorous form within certain limits, and as man 
has favoured the increase of injurious insects he must, 
if he wishes to prevent them doing a considerable 
amount of damage, employ some means to reduce 
their numbers. A single insect is not capable of doing 
much damage, but it reproduces itself at an enormous 
rate and gives rise to numbers which collectively can 
often render crops almost useless. This rate of repro- 
duction is necessary in nature because of the numerous 
factors which lead to the death of so large a percentage 
of the total number of eggs laid. 
In endeavouring to keep injurious insects under 
control it is better to aim at prevention rather than at 
cure. Clean farming prevents a large number of insects 
from living through the winter as the weeds on which 
