4 Farmers’ Bulletin 1169. 
intent on sharing in practically every one of his enterprises, it is 
inevitable that man should have to contest every foot of his ground, 
especially when he adapts nature to his needs, convenience, or comfort. 
EXTENT OF INSECT INJURY TO SHADE TREES. 
As the growing season advances all shade trees apparently other- 
wise perfectly healthy will be found on more or less close inspection 
to show cumulative signs of insect injury, particularly in their 
foliage. This is inevitable when we consider the great variety of 
plant-eating insects they harbor. In most cases the injury is too 
insignificant for treatment. But from this point on the other 
extreme is not infrequent, when, as a result of abnormal increase 
of some insect species, the injury may be so severe as to menace the 
very life of its favorite host tree. Indeed, instances on record are 
not uncommon in which entire rows of certain trees and most trees 
of a given kind in communities and regions have been killed by 
insect pests within a few seasons. Certain trees are more subject to 
attack and injury and by a greater variety of the seriously injurious 
insects than others, so that, as a result of a process of selection, such 
trees are either no longer planted or sparingly, especially where 
their serious insect enemies are dominant. On the other hand, some of 
the injurious shade-tree insects, like the white-marked tussock moth, 
are almost omnivorous, so that practically no deciduous shade tree is 
immune to their attack and very severe defoliation. 
PRIMARY AND SECONDARY INSECT INJURY. 
Insects injurious to shade trees may be roughly grouped in ac- 
cordance with the condition of health of the tree they normally 
attack. Certain species—and this is especially true of leaf-chewing 
insects—show a decided preference for perfectly healthy trees. 
Others—and this is the case with most wood and bark boring in- 
sects—can inhabit trees only when the health of these has been 
previously impaired, whatever the agency responsible for this. These 
more or less distinctive groups of insects are designated as primary 
and secondary, respectively. From the viewpoint of control it is as 
important to know to which of these an insect found on a tree 
belongs as it is to know whether it is injurious, beneficial, or indif- 
ferent. It is wasted effort to proceed against an insect, even if found 
injurious, if its presence is conditioned by previous or primary injury 
or death of the tree. It is the cause of this primary injury that must 
be discovered and dealt with. On the other hand, there are still a 
number of insect species that are primary under some conditions and 
secondary under others, so that they can not be definitely assigned 
to either group. 
