Insect Damage 45 
on the soft cambium layer underneath the bark; the stem- 
borers hollow out twigs or make their abode in the trunks, 
feeding on the wood itself. 
Some do little or practically no damage; indeed, in the 
majority of cases only when an extraordinary development 
of an insect pest In any one season takes place, or when 
repeated invasions have to be endured, or when other acces- 
sory unfavorable conditions occur, need any apprehension 
for the life of the tree be felt. Such unusual developments 
of insect pests seem to take place periodically, when, due to 
specially favorable seasons or otherwise, the pest is favored, 
while natural enemies are reduced or less favored. 
The leaf-eaters of certain species are especially liable to 
this periodic excessive multiplication. 
If the defoliation is only partial and occurs at a time of 
the year when the foliage can still be replaced by the 
development of the dormant buds, outside of the temporary 
unsightliness, only a reduction in the season’s growth is the 
result. But, if the defoliation is complete, and, as happens 
with some species of leaf-eaters, more than one generation 
is developed in a season, repeating the destruction of the 
foliage as new leaves are formed and thereby preventing 
the assimilation of food materials, the tree may be weakened 
even to death. At least a repetition of the defoliation for 
two or possibly three seasons, according to the vigor and age 
of the tree, will be fatal. 
The new foliage, formed in the same season after the first 
defoliation, is apt to be smaller in size or abnormal in form 
and thinner, hence less effective. Sometimes, however, 
when a large number of buds have been destroyed, unusu- 
ally large and abnormally formed secondary leaves arise 
from the few remaining buds. In either case the nutrition 
of the tree is interfered with. 
