266 PROTECTION AGAINST INSECTS 



ating insects furnish examples, or to secure places to breed, 

 as is the case with certain bark beetles. More species of 

 insects occur in warm climates and at low elevations than at 

 high altitudes and cold climates. 



Within the last three decades attention has been called to 

 insect depredations because of the awakening of interest in 

 forestry. But extensive insect attacks are not of recent ori- 

 gin. Insects were always present in the forest and at inter- 

 vals developed sufficiently to cause widespread destruction. 4 

 Under ordinary conditions a given species of insect will be 

 found occurring in relatively small numbers, attacking single 

 trees or small groups of trees here and there throughout the 

 forest. When for some reason conditions become particu- 

 larly favorable for rapid multiplication, the insects may in- 

 crease enormously in numbers and extend their ravages until 

 a considerable portion of the timber over a large area is 

 attacked. 



Causes for such infestations existed in the original forest 

 long before the country was settled by man. Hopping, 8 on 

 page 2 of his " Manual of Insect Control for California," 

 states that: "In the past, epidemics of infestation were prob- 

 ably caused by high winds uprooting or breaking trees, light- 

 ning, fires, snowbreak, etc., or by favorable climatic condi- 

 tions. Man has now supplied other causes such as uncon- 

 trolled fellings, girdled trees, cull logs, limbs and brush left 

 over one season. Many of the brush openings in timber 

 supposed to be caused by fire are primarily the result of 

 epidemics of insect infestations." 



It is evident that the chances for insect outbreaks on a 

 large scale must be increased through the operation of the 

 new and additional causes which have followed in the train 

 of settlement, 'and that if such outbreaks are not to occur, 

 preventive or control measures must be undertaken. 



