SOME ASPECTS OF SPRAYING AGAINST PESTS. 215 
For successful spraying, therefore, the first essential is to know 
what pest the plant is suffering from ; the second is a knowledge of 
the life-history of the pest. To illustrate the importance of this 
knowledge I will briefly give the life-history of one of the common 
apple pests, Aphis sorbi, or the rosy-apple aphis. In the winter it 
exists on the apple as little black shining eggs, laid chiefly on or near 
the leaf scars of the fruit spurs. Towards the end of April it hatches, 
and a purplish-green larva comes out, which immediately begins to 
feed on the under-side of the leaves. At first it does nothing but 
feed and moult and grow, but after a varying number of days it 
becomes fully grown, and is then purplish, mealy, and rather globular in 
form, and is known as a queen mother. At this point it begins to 
bud off little pinkish larvae, which immediately insert their probosces 
into the under-side of the leaves, causing extensive curling. This 
production of young takes place without the intervention of a male. 
The young, when fully grown, repeat the process, and so it goes 
on through several generations. Towards the end of June, winged 
individuals appear, and these no longer feed on the apple, but fly 
to another host-plant, so that towards the end of July the apple 
is free from this insect. In September, winged migrants from 
the second host-plant fly back to the apple and produce a genera- 
tion of winged males and wingless females. These females soon 
begin to lay their eggs on the apple twigs, and the life-history is thus 
completed. 
When one considers this life-history it is clear that from a sprayin g 
point of view one may divide it into four periods : 
(1) The egg stage. 
(2) The young larval stage, before there is much leaf-curling and 
before the queen mother has commenced to bud off young. 
(3) The viviparous stage, when the leaves are curled and active 
reproduction is going on. 
(4) The autumn stage, when males and females are present, but 
no leaf -curling is caused. 
Of these stages (1) and (3) do not lend themselves to treatment 
No wash has yet been produced that will kill aphis eggs, so that spraying 
in the winter against this pest has so far been a failure. The vivi- 
parous stage is equally difficult, owing to the excessive leaf -curl pre- 
venting penetration by a spray. This leaves two other stages in which 
the aphis is comparatively vulnerable. I do not propose to enter into 
a discussion here as to the relative advantages of spring and autumn 
spraying. There are difficulties in both, but the point I wish to bring 
out is that they both present a vulnerable stage in the insect's life- 
history, and that this knowledge enables us to attack it with the 
greatest prospect of success. The knowledge of an insect's life-history 
is thus an essential for success in its control. 
So far two essentials for successful spraying have been considered, 
namely identification of the pest and knowledge of its life-history. 
There is in addition a third, and that is a knowledge of the action 
