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 aJl 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 destro^-ed 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 tbe 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 larvae. 



