the western pine beetle 7 



Types of Infestation 



The collective attacks of western pine beetles in a stand or forest 

 are classified as either endemic or epidemic infestations, depending 

 on a quantitative measure of the trees killed and on whether the in- 

 fested trees grow primarily in groups or as single, scattered trees. 

 Under endemic conditions, when a low or normal population is pres- 

 ent, the western pine beetle is highly selective, attacking mostly single 

 trees of poor thrift or injured trees and green windfalls. Losses of 

 this nature constitute a normal expected annual mortality usually 

 amounting to less than 0.5 percent of the total green-pine volume of 

 the stand. Over a period of years under average timber conditions 

 endemic losses are balanced by annual growth. 



Under epidemic conditions, when an abnormally high beetle popu- 

 lation has built up under favorable circumstances, the beetles are 

 very much less selective with respect to the age and vigor of the trees 

 they attack. By sheer force of numbers they overcome single green 

 trees and large groups of normally healthy trees, causing losses 

 that average well over 0.5 percent of the stand volume. Occasionally 

 as much as 10 percent of a given stand may be killed in 1 year. In one 

 epidemic in Oregon over 200 trees were killed in one group by a 

 simultaneous attack of many beetles; 1,816 trees were killed in 1 year 

 on a single square mile, and instances of the killing of 1,000 or more 

 trees per square mile were not uncommon. Needless to say, losses of 

 such intensity are vastly greater than annual-growth replacement and, 

 unless checked by natural factors or artificial control, soon deplete a 

 forest. 



CONTROL 



Because the annual depredations of the western pine beetle have re- 

 mained at a high level for two or three decades, it is evident that the 

 control of this pest has been and still is very difficult. Once a pon- 

 derosa pine tree has been successfully attacked by the western pine 

 beetle, there is no known method by which it can be saved. Man, 

 therefore, must rely on natural factors and direct or indirect control 

 measures to protect valuable stands from excessive depletion. 



Natural Control 



The western pine beetle maintains, and in many years increases, its 

 population notwithstanding the combined effects of many natural 

 factors. Unfavorable climatic conditions, particularly winter air 

 temperatures lower than minus 20° F., maintained for a few days and 

 accompanied by low day temperatures, are periodically helpful in tem- 

 porarily checking outbreaks. Birds, especially woodpeckers, devour 

 vast quantities of immature beetles, and these birds, in search of food, 

 may nearly strip an infested tree of its bark (fig. 6). 



Predaceous clerid and ostomatid beetles are constantly associated 

 with western pine beetle infestations, preying on the mature and im- 

 mature stages of the beetle. Other predators and several species of 

 parasitic flies and wasps help to reduce populations. 



