PROTECTION FROM OTHER AGENCIES 315 



are constantly contributing to check the multiplication of 

 insects. Very frequently, however, there is a combina- 

 tion of favorable conditions resulting in the rapid devel- 

 opment of a given species of insect to a point where the 

 natural enemies can no longer keep them in check, and 

 an invasion is the result. 



In preventive control work, it is necessary to watch 

 the forest constantly for any indications of the spread of an 

 injurious insect which may cause an invasion. Where 

 the markets for wood and timber permit intensive silvi- 

 culture, all trees found to be infested are promptly re- 

 moved. The forest is kept clear of logs, tops, and other 

 slashings, not only to reduce the danger from fire, but 

 also to remove breeding-places for insects. In this way 

 the loss by desultory attacks of insects is reduced to a 

 minimum, and the chance for an outbreak is eliminated. 



Unfortunately the conditions in most of our forests 

 do not permit intensive forest management. In the ex- 

 tensive forests where forestry has not advanced much be- 

 yond the stage of mere protection, the first aim is not to 

 attempt to prevent the entire loss caused by death of an 

 occasional tree or group of trees, but to prevent a damag- 

 ing invasion. Even under our present conditions it is 

 entirely practicable to prevent large outbreaks of insects. 

 It is necessary to keep careful watch of the forest for any 

 general large development of insect work. As soon as it 

 is apparent that there is danger of an invasion, it is 

 necessary to apply immediately remedial measures, even 

 if it is necessary to incur considerable expense. 



