The Distribution of the Pale Western Cutworm, Porosagrotis Orthogonia Morr.: A Study in Physical Ecology

Abstract

In a previous study, recently published by the Minnesota Experiment Station, the author (I923) has developed rather briefly the relations between weather factors and the life cycles of certain Noctuidae of the cutworm group, and shown that each species has a definite optimum soil moisture condition during the immature stages, which may be expressed in terms of temperature and precipitation by a characteristic curve. Such curves, which may properly be called Climograph Curves, as they are based on a modification of the climograph of Ball (i9io), were calculated for three species of Noctuids adapted to rather divergent environments. The climograph itself, a method of plotting which finds varied uses in climatic studies, has been recently discussed in American literature by Varney (I920) and Flanders (1922), to which articles the reader is referred for details. The method as used here is merely the plotting of monthly figures for temperature and precipitation on a graph for which these factors form the axes, connecting the points for successive months by a line showing the direction of the annual cycle. The present paper is a continuation of the earlier study, in which the climograph curve for the Pale Western Cutworm, Porosagrotis orthogonic Morr. (Lepidoptera Noctuidae) was first calculated; and deals with a recalculation of this curve, based on more accurate data, and with the application of the curve to determine the probable limits of the distribution of the species.