808 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 14 



beneath the bark of logs lying in full sunlight may reach a degree fatal to insects.* 

 Craighead working with ash logs in several localities in the South observed sub- 

 cortical temperatures which exceeded air temperature by 60° (F?) depending upon 

 the locality, condition of the sky and angle of the sun's rays. On the basis of his 

 experiments he recommends the weekly turning of logs to protect them from wood 

 destroying insects. 



So far as some species of logs are concerned, the author has independently reached 

 similar conclusions from work conducted at the Minnesota Experiment Station. 

 But these experiments have also shown that, in northern latitudes at least, some logs 

 never reach the fatal temperature for insects even on the brightest days. Thus it 

 is essential that the factors influencing the subcortical temperature should be un- 

 derstood if this method of control is to be used intelligently. A more complete dis- 

 cussion of the factors influencing the subcortical temperature of logs will appear 

 in the 18th Report of the Minnesota State Entomologist. 



The results thus far obtained may be summarized as follows: 



1. In bright sunlight subcortical temperature on the upper side of moderately 

 thin barked logs often passes above a point fatal to insects. 



2. That this is not true of all logs is illustrated by certain thin barked Norway 

 pine logs, the subcortical temperature of which never exceeded 46° C during the 

 entire summer season of 1920. 



3. One of the primary factors influencing the temperature in logs is solar radia- 

 iion. The effect depends upon light intensit}^ solar altitude and the angle of in- 

 cidence of the sun's rays upon the log. 



4. The position of the log with reference to the sun's rays determines the pro- 

 portion of the log which may attain a high temperature. Logs lying east and west 

 will have heated only a comparatively narrow strip on the south side while almost 

 one-half of the log lying north and south may exceed the fatal temperature of in- 

 sects. 



5. The bark characteristics which affect log temperatures are : (a) Color. Dark 

 bark absorbes 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 dense structure and 

 therefore tends to hold down temperature, (d) Thickness. This tends to increase 

 insulation. 



6. In absence of solar radiation the subcortical temperature follows rather close- 

 ly the temperature of the surrounding air. 



7. The conduction of heat around a log is slow, but varies somewhat with the 

 different species. This results in the concentration of heat in limited areas. 



8. Air movement tends to increase radiation and therefore tends to reduce sub- 

 cortical temperatures. 



9. Evaporation of water from the surface layers of the bark, which often occurs 

 in the early morning or following rain tends to reduce the temperature beneath the 

 bark. 



10. Close proximity to other radiating or absorbing surfaces tends to stabilize 

 subcortical temperature. 



*F. C. Craighead (1920) Direct Sunlight as a Factor in Forest Insect Control. 

 Proc. Ent. Soc. of Wash., Vol. 22, pp. 106-108. 



S. A. Graham 



