24 NINETEENTH REPORT STATE ENTOMOLOGIST OF MINNESOTA—IQ22 
toward the clearing up of some of the controversies arising from con- 
flicting observations of authorities. He shows conclusively that the 
activities of insects are not governed by temperature alone but that 
humidity plays a very important modifying role. 
More recently the work of Pierce, Shelford and his students, 
Headlee, and others has further emphasized the importance of humidity. 
Huntington in his recent book has attempted to show the effect of 
temperature and humidity upon man. In the United States very little 
has been produced which bears directly upon the effect of physical 
factors of the environment upon forest insects. Hopkins (1919-20) 
in his attempt to formulate and apply the so-called “‘bioclimatic law’”’ 
to the activities of forest insects has taken a step in the right direction. 
Craighead (1920) working primarily with ash logs in several 
localities in the South, found that the temperature beneath the bark 
of logs lying in full sunlight frequently reached a point above the 
fatal temperature for wood-boring insects. On the basis of his findings 
he recommends the weekly turning of logs to protect them from injury 
by wood-borers. Even more recently Craighead (1921) has published 
the results of a series of experiments concerning the fatal temperature 
of the red-headed ash borer with special reference to the effectiveness 
of dry kiln temperatures in destroying these insects. He shows that 
the fatal temperature varies with the amount of moisture present. 
Studies of the nutritional requirements of insects have opened up 
a field of investigation. The food factor appears to be extremely impor- 
tant in regulating the activities of all insects, and particularly those 
that live upon such materials as wood. Baumberger (1919) has shown 
that Drosophila is very largely dependent for its successful develop- 
ment upon the microorganisms working in its food. In sterile media 
the development is very slow or even impossible, whereas in the presence 
of microorganisms normal development takes place. He refers to 
the work of Haberland (1915), who has shown that the protein content 
of trees is very low except in the cambium and phloem, while the 
carbohydrate content is high. As a result of this condition we find 
in sound green logs that the rapidly developing insects work in the 
cambium or in the outer sap wood, whereas the insects which penetrate 
the heart wood are usually much slower in their development. Some 
wood-boring species are unable to live in wood in the absence of the 
organisms producing decay. 
For the sake of brevity no attempt has been made in the above 
review to mention all the literature contributing to our knowledge of 
this subject. The only aim has been to give a general idea of the 
