20 THE SCOLYTID BEJKXLiJib. 



each degree south, while at the same latitude there will be a difference 

 of about four days for each 400 feet difference in altitude — four days 

 later for each 400 feet higher, and four days earlier for each 400 feet 

 lower. Thus a difference of 7^ degrees of latitude at the same eleva- 

 tion would mean a difference of about thirty days in the begin n ing 

 of activity or any other event, depending on a given average tem- 

 perature, while a difference of 3,000 feet in altitude at the same 

 degree of latitude would cause an equal difference in these phenolog- 

 ical events. 



In the fall of the year the beginning of hibernation and other events 

 will be earlier northward and later southward at localities of the 

 same elevation, or earlier at higher altitudes and later at lower alti- 

 tudes in the same latitude. 



Of course there are exceptions to these rules, especially in regions 

 like that of California, where remarkably abnormal conditions as to 

 influence of altitude and latitude prevail, as also in the case of southern 

 and northern exposures, sandy dry soils, and wet clayey soils or bogs. 

 In such cases the estimates must be corrected so as to allow for three 

 or four days later for the beginning of activity, etc., under average 

 colder conditions, or three or four days earlier for average warmer 

 conditions. The best indication of the rate of difference between two 

 localities is found in the average difference in the dates of opening 

 of the buds or flowers of some indigenous species of forest trees com- 

 mon to both localities, and especially of a species of conifer subject 

 to the attack of a given Dendroctonus beetle. 



A knowledge of the facts relating to this principle is of especial 

 importance as a basis for recommending or executing beetle-control 

 policies, since success depends largely on a Icnowledge of the proper 

 time to "begin and end certain timher-cutting or harlcing operations for 

 the destruction of the iroods of the beetles. 



When, as is usually the case, the seasonal-history data have been 

 collected at different latitudes and altitudes within the range of the 

 species, the discussion under each species is based on a probable 

 average. But when the data have been collected in one locality the 

 discussion relates to that locality, and the probable differences are esti- 

 mated for other localities. While there is yet much to be determined 

 in regard to the rate of difference between different localities at the 

 same latitude or elevation in the same region and the influence which 

 different latitudes and altitudes exert on different species, sufficient 

 evidence is at hand regarding some of the species of this genus and the 

 regions occupied by them to warrant certain preliminary conclusions 

 as a basis for action and further study. 



