14 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
far outweighs any little trouble we may be put to in providing 
for the wants of the birds. Both titmice and tree-creepers breed 
twice in a season, and produce six to ten young on each occasion. 
Each bird, it is calculated, destroys two or three hundred thousand 
insects annually, chiefly in the form of very minute eggs. Five 
thousand insects are often sufficient to completely denude an 
average-sized tree of its leaves. If such defoliation be repeated a 
second year, the probabilities are that the tree will die. The annual 
produce of one breeding-place, say twelve titmice, can destroy 
two and a half millions of insects each year, which would be 
sufficient to entirely denude five hundred trees of their leaves. 
The preventive measures which we have considered come from 
the side of the enemies of the insects, the other set is more 
directly due to the management, and may naturally be subdivided 
into the following heads :— 
1. The Establishment of the Wood.—Strong well-developed 
plants should be selected, as they are best able to withstand insect 
attack. In the case of danger from the attack of the pine weevil, 
only such young trees should be used as have been reared in 
comparatively open nursery lines, because plants which are 
much crowded when young have thin and delicate bark, and 
suffer very severely from this insect. Where a variety of trees 
thrive equally well, it is always advisable to cultivate mixed 
woods. This precaution is necessary, because, as a general rule, 
each species of insect prefers but one species, genus, or class of 
tree, and great calamities are only known in pure woods. It is 
scarcely necessary to say that great care must be exercised in 
inserting the plants in the ground, so that their growth may be 
interfered with as little as possible, for it almost always happens 
that the weakest plants are the first victims. One should also 
avoid as much as possible forming large compartments of wood 
of the same age. It is much safer to divide large woods into 
age-classes, because many insects confine their attack either to 
young or to old trees, but do not attack trees of all ages 
indifferently. 
2. The Tending of the Wood.—Thinning should be begun before 
there is any chance of the density of the crowding interfering so 
much with growth as to weaken the vitality of the trees, but, at 
the same time, care must be taken that over-thinning is avoided, 
otherwise fertility suffers, and the evil we wish to avoid is 
encountered. During the operations of thinning, all trees which 
