COXTKOLLIXC INSF.CTS IN LOGS 513 



preliminary note to call this matter to the attention of foresters. A 

 more extended paper, including tables and graphs, will appear in the 

 18th Report of the Minnesota State Entomologist. 



It has been found that in bright sunlight the temperature under the 

 bark of some moderately thin-barked logs often passes above a point 

 fatal to the insect inhabitants of the logs. For example, on July 3, 

 1920, a white pine log with rough bark 5 mm. thick reached a tempera- 

 ture of ()0° C. at 1 p.m.. which was 20° C. above air temperature in 

 the sun as registered by a white bulb thermometer. An excess of log 

 temperature over air temperature of from 15° C. to 20" C. was common 

 on bright days throughout the summer. Thus the subcortical tempera- 

 ture in white pine frequently exceeded 48° C, which is the fatal tem- 

 perature for most insects. 



On the other hand, not all logs reached this fatal temperature even 

 on the brightest days. Norway pine logs with bark 10 mm. thick never 

 exceeded 4(3° C. at any time during the summer. Even the Norway 

 pine logs with bark only 2 mm. thick did not exceed 48° C. at any 

 time. From this it is apparent that the control of insects in logs by 

 solar radiation is not applicable in all cases. It is therefore essential 

 that the factors limiting its application be understood if the niethod i<i 

 to be intelligently applied. 



Briefly these factors may be summarized as follows : 



1. To raise the temperature in logs above the fatal point for insects 

 it is necessary to have bright sunlight. Solar intensity depends upon 

 the clearness of the atmosphere and the altitude of the sun. 



2. The position of the log with reference to the sun's rays influences 

 the amount of heat absorbed by the log since the rate of absorption is 

 inversely proportional to the angle of incidence of the sun's rays upon 

 the log. The position also determines the proportion of the log which 

 will be raised to a high temperature. Only a narrow strip on the south 

 side of a log lying east and west will be exposed to the direct sunlight 

 while on a bright day the sun passing from east to west will strike 

 directly and heat a much larger proportion of the log lying north 

 and south. The direction in which the log is lying therefore determines 

 how far it should be turned each time. 



3. The bark characteristics which affect log temperatures are: (a) 

 Color ; dark bark absorbs heat much more rapidly than light colored 

 bark, (b) Surface; rough bark provides a larger absorbing and radi- 



