42 Eighteenth Report State Entomologist of Minnesota — 1920 



2. That this is not necessarily true of all logs is illustrated by 

 certain norway pine logs which did not exceed 46 degrees C. during 

 the entire summer of 1920. 



3. The position of the log with reference to the sun's rays de- 

 termines the portion which will attain a high temperature. T-ogs lying 

 east and west will have only a comparatively narrow strip heated, while 

 almost half of a log lying north and south may exceed the tempera- 

 ture fatal for insects. 



4. Conduction of heat around the log is slow but varies some- 

 what in rate with dilferent species. 



5. ( )ne of the primary factors bringing about high temperaUires 

 in logs is solar radiation. The effect depends upon light intensity, solar 

 altitude, and the angle of incidence of the sun's rays. 



6. The bark characteristics which affect log temperatures are 

 (a) Color. Dark Ijark absorbs heat much more rapidly than light 

 colored bark. ( b ) Surface. Rough bark provides a larger absorbing 

 and radiating surface than smooth bark and gives higher temperatures 

 provided the angle of incidence is not great, (c) Structure. Scaly 

 bark is a better non conductor than bark of a uniform texture and 

 therefore tends to hold down the temperature. ( d ) Thickness, which 

 tends to increase insulation. 



7. In the absence of solar radiation the subcortical temperature 

 follows rather closely the temperature of the surrounding air. 



8. Air movement tends to increase radiation and therefore tends 

 to reduce the subcortical temperature. 



9. Close proximity to other radiating or absorbing surfaces tends 

 to stabilize the subcortical temperature. 



10. Evaporation of water from the surface layers of the bark, 

 which often occurs in the early morning or following a rain, tends 

 to reduce the temperature beneath the bark. 



