SHADE T REE IN S E CTs 
have developed which will attack and kill living trees, and in a few 
cases species which will attack and kill or seriously injure trees in 
apparently good health. In this they are very similar to the para- 
sitic and saprophytic fungi. Many of them cannot cause injury 
until they get started through a wound, or until the vitality of 
the tree has been weakened by some other cause. 
12. Among the borers you will find a smaller per cent. which 
can successfully attack a tree in perfect health than in any of the 
other classes of the injurious insects. In most cases the eggs are 
laid upon the bark, and when the minute larva crawls out of the 
shell it begins to burrow into the tree. The entrance is so small 
that it is hard to find, and the larva is quickly buried where it can 
carry on its destructive work unobserved. However, if the tree 
is in vigorous health, when the robber reaches and injures the cam- 
bium layer, this starts a flow of sap that drowns out the intruder. 
In some cases the healing is sufficiently rapid so that the burrow 
is closed faster than the larva can make it, and the pest is buried 
in a grave of its own digging. 
13. Anything tending to check the growth of the trees makes 
it easier for the borers to become established, and, if sufficiently 
abundant, to cause serious injury. Under the right conditions, a 
few dying trees can produce borers in such vast numbers that 
they cannot find enough dead or dying trees for their food; con- 
sequently, they must attack healthy trees or perish. 
14. Under city conditions, for this reason, you will meet with 
serious injury done to shade trees by species that would be harm- 
less under forest conditions, owing to the adverse conditions under 
which these city trees must struggle for an existence. Keep this 
thought in mind, therefore, both when working with the trees and 
when studying the literature on insects. Many of these species 
with which you will have to deal have, up to this time, been studied 
only from the forester’s standpoint. 
15. Remember, that city trees are very largely deprived of the 
natural protection afforded the forest trees by the birds and other 
natural enemies of the insects. Remember, too, that city trees 
must frequently struggle against insufficient sunlight, food and 
water. Their roots are also frequently buried beneath asphalt 
pavements and cement sidewalks, which keep away the fresh water 
and the invigorating gases brought down with the rain. City. 
trees are also subjected to the severe slashing of the contractors 
Page 4 
