INSECTS AND FOREST MANAGEMENT — CRAIGHEAD 385 



of the beetles depends on their abundance, as they kill by gregari- 

 ous attack, introducing into the active sapwood fungi that cut off 

 conduction. When they become abundant their gregarious attack 

 will kill trees that are apparently in full vigor and which show no 

 characteristics of susceptible trees, but as the condition of the border- 

 line trees improves with increased moisture more bettles are required 

 to kill a tree, poorer broods develop, and the insects decrease in 

 abundance and consequently are of decreasing importance. 



This theory appears to be supported by evidence collected from 

 other insect outbreaks and drought periods in the more humid parts 

 of the United States. That such comparisons are not too far- 

 fetched is supported by the following recent statement by Pearson 

 and Wadsworth (1941) : 



Notwithstanding local variations in size and growth rate, ponderosa pine 

 behaves in much the same way wherever it is found. There is also more in 

 common between different species in widely separated regions than foresters 

 generally realize. The influence of light and shade and of root competition, 

 though differing in degree, is fundamentally the same whether one is dealing 

 with ponderosa pine in Arizona, with pines in the South, or the Lake States, 

 or with Douglas-fir in the Pacific Northwest. 



The southern pines (shortleaf, loblolly, and longleaf ) have from 

 time to time been subjected to sharp local droughts which have 

 killed large volumes of timber. Outstanding among these was the 

 summer drought of 1924 in Texas and Louisiana, when a deficiency 

 in precipitation of some 15 inches, occurring from June to January, 

 resulted in the destruction of over 100,000,000 board feet of timber. 

 Although Ips were prevalent in the dying trees, much of the timber 

 was very lightly infested. (St. George, 1925.) 



A few years later Gary (1932) reports a similar drought in 

 Florida and Georgia extending through 7 months, which destroyed 

 nearly as much pine timber. The southern pine beetle was notably 

 absent in these occurrences, as outbreaks are rare in longleaf and 

 slash pines, although this insect does attack these species. Personal 

 experience in both these cases was very convincing as to the greater 

 effect of drought and the secondary importance of the associated 

 insects. 



Blackman (1924) showed that the attack of the hickory bark 

 beetle was dependent on a moisture deficiency and that the attacking 

 insects were destroyed when normal rainfall was resumed. Craig- 

 head (1925b) analyzed a series of outbreaks of the southern pine 

 beetle in shortleaf pine and found that the same conditions held. 

 Subsequent observations have upheld this evidence and have per- 

 mitted issuance of warnings when precipitation has dropped below 

 normal. 



