46 Disease and Death of Trees — Generalities 
While some insects are general feeders, 7.¢., attacking all 
kinds or at least a number of tree species, insects, even more 
than fungi, are partial in selecting their hosts, 7.¢., given 
species of insects attack given species of trees. Besides, 
they differ much more in their life habits than fungi, hence 
they may not be dealt with as a whole, like the fungi, but 
it becomes more needful to know them specifically by name, 
appearance, and habit, with their life history, in order to 
apply judiciously measures for their reduction. Moreover, 
there are a number of enemies of the obnoxious insects 
found in the insect world, and it becomes necessary to dis- 
tinguish between friend and foe, in order not to destroy the 
natural enemies of the pest. 
We can here refer specifically only to the more common 
pests of ornamental and shade trees, leaving the vast num- 
ber of minor and of orchard pests to be studied in the specific 
literature. Fuller descriptions will be found in Chapter 
VI; here only the character of the damage in general is to 
be discussed. 
With regard to the manner in which the damage is inflicted, 
we can classify the injurious insects into a few groups, each 
group containing representatives of the various orders into 
which insects are scientifically classified. These groups 
may be called leaf-eaters, leaf-miners, leaf-rollers, leaf- 
suckers, gall-insects, bark-lice, bark-beetles and borers. 
The character of the damage itself may be threefold, 
namely, destruction of substance, occasioned by leaf-eaters, 
bark-beetles or borers; loss of sap occasioned by plant-lice; 
and formation of abnormal excrescences or galls, to which 
the plant is stimulated by various insects. All three kinds 
of damage or any two may be occasioned by the same pest. 
The leaf-eaters are mostly caterpillars of butterflies and 
especially of moths, which in feeding on the foliage generally 
