256 SOME ASPECTS OF ECOLOGY 



After becoming established first in the Gironde, it has steadily 

 extended its range, notwithstanding the application of measures 

 of repression. The result is that it has now spread over the 

 greater part of France and Belgium ; it has also reached the 

 Swiss frontier and entered the Saar region of Germany. In 1933 

 a small outbreak was discovered in the Tilbury-Gravesend district 

 of England, but measures taken by the Ministry of Agriculture 

 resulted in its eradication. It is, however, unlikely that England 

 will remain free from outbreaks indefinitely in view of the extent 

 to which the Colorado beetle is multiplying and spreading on the 

 continent of Europe. A species so prone to migrate will most 

 likely find a ready means of ingress afforded by cross-Channel 

 steamer traffic, even if it does not often succeed in crossing the 

 intervening strip of sea by flight. The insect now constitutes a 

 real and potential menace to potato cultivation over part of 

 Western Europe, and no climatic factor prevailing in this area 

 appears very likely to restrict its further dissemination (vide 

 Fryer, 1934, 1935). 



The foregoing examples have been selected in order to illustrate 

 the relationship of climate to the spread of insect pests. Although 

 emphasis has been laid upon the temperature factor, it scarcely 

 requires pointing out that the range of a given insect is governed 

 by a complex of climatic and other environmental influences. 

 Such a combination of factors may act in unison over the greater 

 part of the range of a given species, but when any one of them 

 approaches the limit of toleration that factor will be the major 

 controlling influence. 



With the advent of highly developed communications by land, 

 sea and air it has become no longer sufficient to take cognisance 

 only of the existing distribution of insects known to be pests : 

 the zones of their possible distribution cannot be wholly neglected, 

 at least by the applied entomologist. An investigation of the 

 climatic conditions prevailing over the area covered by the existing 

 range of a given species will enable certain general conclusions to 

 be drawn relative to its possible spread. The influence of the 

 individual factors upon that species can, to some extent, be 

 subjected to analysis by adequate laboratory experimentation. 



