vit] Introduction to Insects 95 
They often lay their eggs on the bands or sometimes 
underneath them, but even if the caterpillars hatch 
they cannot travel the long distance to the leaves 
where they usually feed. From this experiment it was 
decided that the damage done by these caterpillars 
could be to a large degree prevented, and this method 
of grease-banding trees is now largely used in com- 
mercial fruit-growing. In seeking remedies for con- 
trolling insect diseases we must always bear in mind 
the question of expense. The farmer grows his crops 
for profit, and if it costs more to control the insect than 
the value of the damage done such measures are of no 
use to him. 
One very annoying factor in dealing with insects 
is that they can sometimes travel considerable distances, 
so that a farmer who keeps them down on his own farm 
is often troubled by others coming from his neighbour’s. 
Insects may be roughly divided into two classes: 
(1) herbivorous, which feed on plants and cause dis- 
eases; (2) carnivorous, which feed on other insects 
and allied animals. 
The members of the second group are extremely 
useful as they keep down the numbers of those which 
attack plants, e.g. large numbers of green fly are 
devoured by the larvae of lady-birds. 
In nature both kinds live side by side in such a 
way that a sort of balance is set up so that no single 
species increases rapidly in numbers. In planting 
large areas with one particular crop man has upset 
this balance by providing the insects which live on 
this crop with an ideal feeding ground. Many herbiv- 
orous species are capable of living only on certain 
families or orders of plants. In nature this means 
