94 Introduction to Insects [CH. 
vulnerable; otherwise treatment is haphazard and 
may mean attacking the insect at a time when it 
is most resistant, or sometimes attempting measures 
which prove to be useless. 
The object of studying the life history of an injurious 
insect with a view of preventing damage is to find the 
weak periods in its life. We want to know the time 
and whereabouts of the insect when it is most easily 
kept in check. Knowing this we can find only by 
experiment the best means of controlling it. A know- 
ledge of the cultivated plants and weeds on which the 
insect can feed is useful as it enables us to starve the 
insect or drive it away by keeping these plants from an 
attacked field for a certain length of time. 
The life history tells us where the various stages in 
the insect’s life are passed, how long it lives in these 
stages, and when the change from one to another 
takes place. From this we can often decide when and 
how to attack the insect in order to keep it under 
control. 
In the case of the winter moth whose caterpillars 
eat the leaves of fruit trees the females have no wings, 
and when they want to lay their eggs on the young 
shoots they crawl up the trees. In England this takes 
place from about the second week in October until 
the middle of January. It was argued that if these 
females could be prevented from crawling up the trees 
the caterpillars would not be able to eat the leaves. 
An experiment was tried of putting a band of a sticky 
substance (as used on fly papers) round the tree trunks 
and keeping them sticky during the time that the 
female moths are active. It was found that when they 
reach the bands they stick and are unable to go farther. 
