Insect Parasites 129 
—the natural enemies of preying insects — the excessive 
development of the pests may be checked. 
When we speak of insect pests we mean an excessive 
development of destructive kinds, for it will probably never 
be possible to exterminate them entirely, and all our meas- 
ures can have in view only a partial extirpation, or keeping 
them in check. Under natural conditions the equilibrium 
between the forces of nature is disturbed only occasionally 
and for a short time; an excess of insects preying on trees 
may develop without a corresponding increase in their en- 
emies, but, if not interfered with, the “boom” is quickly 
followed by a “panic”: the enemies increase and over- 
development is checked. When man interferes, his methods 
sometimes unwittingly prevent the natural decrease or reduc- 
tion of the pest. 
For instance, there are certain fungus diseases attacking 
caterpillars, and parasitic insects laying their eggs into them. 
By destroying the caterpillars or the cocoons, these natural 
enemies are destroyed at the same time, so that the very 
method of combating it favors the pest. Properly, there- 
fore, an investigation as to the need or desirability of apply- 
ing remedies, of spending energy and money, should precede 
the attempt. It is often not necessary to make the effort, 
although in our “‘unnatural” conditions of street and lawn 
trees, the reliance on such assistance by nature may be 
misplaced. 
In addition to the enemies in the animal and plant world, 
there are inimical weather conditions, which may hold a 
developing insect pest in check. For example, early frosts 
in the fall may kill large numbers, very severe winter frosts 
decimate them, while rainy seasons and wet, open winters 
are also inimical, particularly in developing fungus diseases 
in the larve, 
