258 SOME ASPECTS OF ECOLOGY 



expressing certain physiological conditions. Neither phase can be 

 readily engendered out of season by modification of temperature 

 or other physical influences, nor can they be easily dispelled and 

 activity reinduced by similar means. It is not proposed to deal 

 here with established rhythms in insect life, but to refer to climatic 

 factors in so far as they are correlated with fluctuations in the 

 abundance or scarcity of insects which recur at more or less 

 irregular intervals. 



It is a matter of common observation that there is a more or less 

 strongly marked fluctuation in the prevalence of certain species 

 of insects taken over a period of years. Probably few species are 

 exempt, in temperate regions at any rate, from such rises and falls, 

 and the literature of entomology contains frequent reference to 

 such occurrences. In the case of noxious insects, upon which 

 most observation has been concentrated, the majority occasion 

 what may be described as average damage during ordinary 

 seasons, while now and again mass outbreaks of variable duration 

 occur. Diverse causes have been attributed to such outbreaks, 

 but it needs to be emphasised how difficult it is in many cases to 

 definitely prove that any observed factor is actually responsible. 

 Much exact and prolonged observation is required before it will 

 be possible to discriminate between the direct effects of weather 

 upon insect prevalence, as distinct from its indirect influence, 

 operating through such potent biological factors as food-plants, 

 parasites, predators and disease organisms of various kinds. In 

 certain instances tolerably close correlation has been observed 

 between weather conditions and the prevalence of individual 

 pests. Examples of such phenomena will be found in text-books 

 by Wardle and other writers, and in the published proceedings of 

 the Empire Meteorologists' Conference ^ held in London in 1929. 

 It has been recorded, for instance, that the prevalence of low 

 temperatures, unaccompanied by snowfall, has a marked effect 

 in reducing the prevalence of the Colorado beetle in Manitoba : 

 low temperatures following a previous mild spell in early spring 



^ Conference of Empire Meteorologists, 1929. Agricultural Section. 

 I, Report ; II, Papers and Discussions. Each obtainable, price \s., post 

 free, from the Ministry of Agriculture and Fisheries, 10 Whitehall Place, 

 London, S.W.I. 



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