That climatic conditions exercise a far-reaching influence on the insect pests of agricultural crops is of course well known, but knowledge as to the exact relationships existing between insects and climatic factors is at present rather scanty, and is largely confined to the influence of temperature and humidity upon certain insects in certain regions. The recent Conference of Empire Meteorologists, 1929, emphasises this important connection between weather conditions and agricultural entomology, and shows the desirability of greater co-operation between the research meteorologist and the entomologist. As knowledge of the subject increases it may be possible to forecast outbreaks of certain insect pests with some accuracy, and by statistical analysis of climatic conditions to determine the probable extent of distribution of injurious insects, particularly perhaps of those inhabiting the soil. It is already possible, by comparing the effects of a succession of cold winters for example, to gain an idea of what insect epidemics may be expected in the ensuing season. Perhaps the most outstanding instance of the correlation of insect outbreaks with meteorological factors is the case of the American ‘pale western cutworm’ (Porosagrotis orthogonia Morr.). Uvarovda), quoting Cook(3), says: “Outbreaks of this cutworm have been found to depend on the total amount of rainfall in May, June and July of the preceding year; the critical amount of precipitation varies with the temperature and produces a definite critical soil moisture. If the moisture exceeds the critical amount, the number of cutworms decreases in the following year; if the moisture be less than the critical amount, the number of cutworms increases.” The heavy rain forces this species to leave its subterranean workings and emerge on the surface where it becomes an easy prey to its parasites.

The chief climatic factors influencing the distribution and development of insects are, temperature, moisture, wind and light, and the effect so far as is known of these factors upon certain insects will be briefly dealt with in the remainder of this chapter.

TEMPERATURE. Insects, during the various phases of their life cycles, may live through a range of temperature of about 500 F., and they are also able to withstand great extremes of temperature. This reaction to extremes of temperature, however, may differ in the same insect, apparently according to the physiological condition of the insect at the time.