Pine Bar^ Beetles 



429 



BECAUSE BARK BEETLES are con- 

 stantly at work in pine forests thin- 

 ning, harvesting, and wiping out entire 

 stands of timber to make room for new 

 ones they destroy on the whole a vast 

 amount of commercially valuable tim- 

 ber. In the long run they are no threat 

 to forest perpetuation, but they do take 

 a tremendous toll of wood that we need 

 badly. In some pine areas, this loss oc- 

 curs as a slow but steady annual drain 

 of merchantable trees spread over a 

 long period of years. In other areas, the 

 losses are more spectacular because 

 they result from epidemic infestations 

 that kill a fairly high percentage of the 

 stand in just a few years. But regardless 

 of the rate at which they occur, these 

 beetle-caused losses affect directly the 

 potential lumber output of pine-pro- 

 ducing areas and indirectly the taxable 

 wealth and pay rolls of entire commu- 

 nities. Estimates based on surveys in 

 the major pine regions of the Western 

 States are that during the period from 

 1926 to 1946 the western pine beetle, 

 the Black Hills beetle, and the moun- 

 tain pine beetle killed over 50 million 

 board feet of pine. 



Just as important is the damage the 

 beetles cause to scenic and property 

 values in our parks and summer-home 

 areas. Mainly for that reason do peo- 

 ple lament the extensive outbreaks of 

 mountain pine beetle in lodgepole pine 

 and white pine stands of Yosemite, 

 Grater Lake, Mount Rainier, Glacier, 

 and Yellowstone National Parks. 



True, those forests will be replaced 

 in time by other forests of the same or 

 different types, but for many years 

 these ghost forests of white snags are 

 gaunt lessons of forest destruction 

 rather than forest preservation. 



Another bad feature is that the 

 snags left by the beetles increase fire 

 hazards. Vast areas of beetle-killed 

 lodgepole pine are a particularly criti- 

 cal fire menace; some of the worst 

 forest fires on record have occurred in 

 these bug-killed localities. 



THE CAUSES of epidemics of pine 

 bark beetles we do not fully under- 



stand any more than we understand 

 the reasons for grasshopper plagues or 

 influenza epidemics. We do know that 

 bark beetles can increase their popu- 

 lations at a lightning rate 10, 20, 

 even 500 to 1 in a single generation 

 but usually natural factors like para- 

 sites, predators, unfavorable weather, 

 or the lack of suitable food keep them 

 from doing so. When susceptible host 

 material is abundant, however, and 

 natural controls are ineffective, then 

 beetles reproduce to capacity, and an 

 outbreak occurs. 



Probably the most important factor 

 in building up beetle populations to 

 epidemic numbers is an abundance of 

 suitable breeding material. Just a for- 

 est of pine trees is not enough. The 

 beetles prefer certain trees that are in 

 a susceptible condition for attack. 

 Such trees are the ones that are 

 making poor growth or those that are 

 injured and weakened by fire, wind- 

 storms, and by other causes. Recently 

 felled trees are especially attractive to 

 the beetles. Weakened trees can offer 

 little resistance by pitch flow when the 

 beetles attack and bore through to the 

 inner bark. When their populations 

 are low, the beetles continually select 

 and thrive in those weak trees; when 

 the beetles find many such trees, they 

 usually multiply rapidly. Either nat- 

 ural causes or disturbances of forest 

 conditions brought about by man's 

 activities can cause an abundance of 

 this susceptible host material in pine 

 forests. 



Fire-weakened trees are often fa- 

 vored as breeding ground by certain 

 bark beetles, which then turn out 

 large populations that threaten sur- 

 rounding forests. To the extent that 

 man fails to control or is responsible 

 for forest fires, he can be charged also 

 with the pine bark beetle damage. 



Drought frequently weakens pine 

 trees and makes them susceptible to 

 attack. Defoliating insects also slow 

 tree growth, reduce vigor, and make 

 trees an easy prey to the beetles. Most 

 pine stands also become more sus- 

 ceptible as they reach maturity. 



