vir] Introduction to Insects 97 
they would feed are destroyed. Dirty fields full of 
weeds and rubbish are ideal wintering places as they 
provide both food and shelter. The destruction of 
grasses and other weeds in headlands, hedges and ditches 
destroys many, and also gets rid of a favourite wintering 
place for others. The time of carrying out the various 
tillage operations affects the number of insects which 
are destroyed by such operations. Suitable rota- 
tions are very beneficial in keeping some pests in 
check. 
The date of sowing often decides whether the crop 
will be good or poor. Thus the frit fly maggot is only 
capable of entering young oat plants; early sown 
crops may therefore escape the attack altogether, 
whereas late sown crops may be rendered almost 
useless. It is often possible to sow at such times that 
the insect is capable of doing the least amount of 
damage. Artificial manures are also useful in hastening 
the plant through the stages when it is most readily 
attacked. 
Various substances known as insecticides have 
proved useful in killing certain forms which attack the 
shoots of plants. The kind to use depends on the way 
in which the insect feeds. Some, such as caterpillars, 
have biting mouths and feed by eating the leaves and 
shoots. The insecticide used in these cases is a poison, 
such as lead arsenate. This poison is sprayed on the 
plant so that on eating the insects are killed. Others, 
such as green flies, have sucking mouths and live by 
penetrating the tissues and sucking the juices. If a 
poison is applied on the surface of the plant it has no 
effect on them as they do not eat the surface layers. 
In this case an insecticide is used which kills the 
PF. 7 
