INSECTS AND FOREST IVIANAGEMENT — CRAIGHEAD 383 



govern the size of the cut. The "beetle system" will demand low- 

 volume removals at frequent intervals in most stands. Can we mod- 

 ify our usual cutting practices so as to remove more frequently the 

 susceptible crop of trees? Under favorable weather and beetle con- 

 ditions, as have recently prevailed, it seems possible to do this with 

 a light cut every 5 years or so. Under adverse conditions, such as 

 prevailed 10 to 15 years ago, it would have been necessary to remove 

 25 to 50 percent of many stands in a few years. 



This discussion pertains to overmature and usually understocked 

 stands. It does not consider the conditions that will prevail once 

 these stands have been removed and abundant reproduction occupies 

 the ground. Obviously the competition in these younger stands will 

 offer conditions favorable to group attacks by the bark engravers 

 {Ips) and the mountain pine beetle, even if the western pine beetle 

 is not abundant, and judicious thinnings will be required to prevent 

 the excessive opening of the stands by large group attacks, thus 

 bringing about local understockings or conversion of type to less 

 desirable species (Craighead, 1936; Eaton, 1941). Pearson and 

 Wadsworth (1941) recognized this need in their experimental work 

 with ponderosa pine on the Colorado Plateau and show preliminary 

 favorable results from thinnings by poisoning and pruning of crop 

 trees. 



This marking and removal of low-vigor trees has been recom- 

 mended and applied only to the east-side ponderosa types of California 

 and Oregon. In the west-side types of more vigorous growth, beetle 

 outbreaks are less important. They occur in definite short periods 

 and die out as suddenly as they develop. 



THEORIES OF BARK BEETLE SUSCEPTIBILITY 



As indicated in the preceding discussion, the susceptibility of cer- 

 tain trees to bark beetle attack has been recognized for a number of 

 years. Various explanations have been advanced for this suscepti- 

 bility. Some of these are published and others are developed in 

 reports and correspondence in the files of the Division of Forest Insect 

 Investigations. A thorough understanding of what makes these trees 

 susceptible to bark beetle attack would aid materially in the applica- 

 tion of preventive measures. 



Observations in the field have shown that mortality in the ponderosa 

 pine type varies tremendously from periods of low (endemic) loss 

 to periods of excessive (epidemic) loss; that during certain periods 

 many trees die with few or no insects present; that others may be 

 infested by wood borers only; and still others are attacked by bark 

 beetles wliich introduce blue-staining fungi into the sap wood, thus 

 cutting off the transpiration stream between the roots and the crown. 



