INSECT ENEMIES OF WESTERN FORESTS 



133 



Figure 57. — A bark-beetle-infested ponderosa pine stand in California. Light 

 areas show trees killed in one year by the western pine beetle. 



from one to two generations annually in the northern part of the 

 range and from two and one-half to four generations in the south- 

 ern portion, where activity continues almost without interruption 

 throughout the year. 



Woodpeckers, clerid beetles, and ostomatid beetles are import- 

 ant natural enemies of this insect, though its abundance is more 

 often determined by climatic influences and the resistance of the 

 host tree. Prolonged winter temperatures of —20° F. and lower 

 have been found to cause heavy brood mortality. Rapid, vigorous 

 tree growth increases host resistance and discourages epidemics. 



This bark beetle has been most successfully controlled through 

 sanitation-salvage logging, by which high-risk trees are removed 

 from the stand and utilized for lumber, thus depriving the beetles 

 of susceptible host material. Direct control may be recommended, 

 particularly for parks and recreational areas, when epidemics 

 appear to be developing, and for maintenance control. Direct con- 

 trol measures consist in felling the infested trees, peeling, and 

 burning the bark late in the fall, or in winter or early spring. 

 Such control work has been successful in reducing infestations 

 during critical periods, but cannot be relied on to eliminate them 

 and must be repeated until natural control factors become oper- 

 ative. 



The southwestern pine beetle (Dendroctonus barberi Hopk.) at- 

 tacks ponderosa pine in southern Colorado, southern Utah, Ari- 

 zona, and New Mexico. A preference is shown for trees over 6 

 inches in diameter and for those that, owing to drought or other 

 causes, are in a weakened condition. It is most frequently found 

 attacking mature, slow-growing trees on the lower fringe of pine 

 growth and trees exposed on rocky ridges and dry southern slopes. 

 This insect and the pattern of its galleries can scarcely be dis- 



