30 NINETEENTH REPORT STATE ENTOMOLOGIST OF MIN NESOTA—I922 
literature to heat paralysis, or estivation. Craighead’s article emphasizes 
the fact that solar radiation may raise the subcortical temperature above 
a point fatal to the insect inhabitants of the logs. He observed sub- 
cortical temperatures exceeding air temperature by 60°F. depending 
upon locality, condition of sky, and angle of sun’s rays. 
The present experiments have shown that the chief factors influ- 
encing subcortical temperatures may be summarized as follows: 
1. Solar radiation 
a. Light intensity 
b. Solar altitude 
c. Angle of incidence 
Character of bark 
a. Color 
b. Surface 
CSS truchire 
d. Thickness 
Air temperature 
Air movement 
Evaporation from bark surface 
6. Proximity to other radiating or absorbing surfaces. 
The highest temperatures recorded were observed beneath the 
dark colored bark of White pine. Frequently the noon temperature 
exceeded a point fatal to insects and several times reached 60°C. 
Under the bark of Norway pine, however, the temperature never was 
observed to exceed 44°C., which is several degrees below the fatal 
point of most subcortical insects. The fatal temperature for most 
insects is about 48°C., altho there is evidence that some bark beetles die 
at a somewhat lower temperature, whereas Chrysobothris is able to 
endure slightly over 50°C. Tables III and IV show the effect of degree 
of shade and the character of the bark upon subcortical temperature 
for three species of pine during June, July, and August. From these 
tables it is evident that the bark characteristics of the different species 
to 
ures 
and the amount of shading the logs receiverhave a very decided influ- 
ence upon the subcortical temperature. 
As a result of these experiments it appears that 1n many instances, 
even in northern latitudes, the weekly turning of logs as recommended 
by Craighead would be effective in destroying wood-boring insects. 
This is particularly true of logs with moderately thin dark colored 
bark. It is equally evident that the method can not be applied to all 
cases since some logs will remain below the fatal temperature of insects 
even on very bright days. 
