FACTORS INFLUENCING THE SUBCORTICAL 

 TEMPERATURES OF LOGS^ 



By S. a. Graham 



It has long been a matter of common observation among students 

 of xylophagous insects, particularly among those interested in bark 

 beetles, that the rate of insect development in logs varies greatlv in 

 different situations. We find frequent references to "heat prostration" 

 of beetles in thin-barked logs exposed to the sun. Likewise it has often 

 been noted that the time of brood emergence may vary by several 

 weeks between north and south exposures in the same locality. More- 

 over it is well known that in the North a species mav have but one 

 generation during a season, while in the South the saiue soecies may 

 have two or more generations. While this is, perhaps, best exempli- 

 fied by cambium dwellers the same may also be applied to all insects 

 in the log. 



These well-known facts indicate rather conclusivelv that the activi- 

 ties of insects in logs are strongly infiuenced bv the external factors of 

 their environment. Little can be found in literature regarding the ac- 

 tion of these factors as a whole, or their comparative influence upon 

 the insects beneath the bark. The most extensive piece of work con- 

 tributing to the solution of this problem is that of Hennings. who 

 conducted experiments with Ips typographus Linn, under controlled 

 temperature and moisture conditions. Unfortunately his work is 

 based only upon air temperature and air humidity, which, as will be 

 shown later, have only an indirect influence upon conditions within 

 the log. Thus Hennings observed "heat paralysis" at an air tempera- 

 ture of 24 degrees C, but the temperature within the log at that par- 

 ticular tiiue can only be a matter of conjecture. Undoubtedly it was 

 much higher. Hennings also noted that changes in the relative humid- 

 ity of the surrounding air influenced the rate of development and 

 also appeared to influence the fatal temperature of the insects under the 

 bark. Here again he was dealing with an important factor, but in an 

 indirect way. 



Other factors than temperature and humidity undoubtedlv play 

 an important role in the life of these insects. Hennings, Swaine, and 



1 I'nblishefl with the approval of the Director as Paper No. 253, <if the .Journal 

 Series of the Minnesota Agricultural Experiment Station. 



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