314 THE PRINCIPLES OF HANDLING WOODLANDS 



insects injurious to trees. Ordinarily the effect is to ex- 

 tend an invasion rather than to be the primary cause of 

 an outbreak. 



There is also a close relation between insects and 

 fungous diseases affecting trees. The burrows in the 

 bark and wood often afford entrance for the fungi, and 

 hasten the decay of the tree. Then, too, trees weakened 

 by disease may be attacked the more readily by certain 

 insects. 



Injury to Reproduction. Insects affect the reproduc- 

 tion of the forest chiefly by attacking seed. Undoubt- 

 edly natural reproduction is often retarded because a 

 large proportion of the seed is destroyed by insects. 

 Young seedlings are also subject to attack by certain in- 

 sects. 



Control of Insects. In the systematic control of for- 

 est insects, provision must be made, first, for prevention 

 of a possible outbreak, and, second, for attacking an in- 

 vasion. 



The principles are comparable to those used in fight- 

 ing fires. In fire protection the first aim is to reduce the 

 causes of fires and to prevent them from being started. 

 In the same way in insect control, it is essential to pre- 

 vent the multiplication of insects to a point where an in- 

 vasion may be developed. If there is an invasion, then 

 very drastic measures must be used to stamp it out, just 

 as in the case of a dangerous forest fire. 



The "enemies of insects are very numerous. Birds, 

 parasitic insects, parasitic fungi, and unfavorable seasons 



