Randomness in the Fluctuation of Population Density of the Rice Stem Borer, <i>Chilo suppressalis</i>

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  • UTIDA Syunro
    Entomological Laboratory, College of Agriculture, Kyoto University

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Other Title
  • ニカメイガ個体数の長期変動に見られる偶然性
  • ガイチュウ コタイグン ノ チョウキ ヘンドウ ニ ツイテ ノ ケンキュウ 3
  • Studes on the Fluctuation of Population Density Observed in Several Insect Pests (3rd Report)
  • 害虫個体群の長期変動についての研究 (第3報)

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Abstract

Statistical analysis was made on the annual fluctuation of population density of the rice stem borer, Chilo suppressalis, from generation to generation using the records of number of moth captured by light trap at several different points in Japan. In Fig. 1, two examples of it obtained at Okayama and Siga are shown. Figs. 3 and 4 show the relation between the population density in a certain generation (n1) and the increase (or decrease) of the density from this generation to that of the next generation (n1-n2). If the fluctuation of population density is governed perfectly by chance, the points in the figure representing the relation between (n1) and (n1-n2) in each generation should be expected to fall on or near the diagonal line passing through abscissa at the point of mean density. As seen in these four examples on the rice stem borer, almost all of the points fall on or near the diagonal line excepting the some points representing the outbreak generation. The same is observed in the population fluctuation of a pine forest pest in Germany, Bupalus piniarius, and that of the wheat blossom midge, Sitodiplosis mosellana, in England. From this departure of the point, we can conceive that the population density at the retro-gradation stage of outbreak may increase without any restriction working in the normal year. It is further noteworthy that the point of post-outbreak generation falls on the line and thus it can be assumed that the outbreak comes to an end by the same mechanism regulating the population density in the normal year. In the normal year, the population fluctuates with certain width of departure of random source having a definite mean density of the equilibrium, being regulated by many compensating factors.

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