アワヨトウ及びその近縁種における密度依存的変異性の比較

抄録

It has been shown that the true armyworm, Leucania unipuncta, exhibits a conspicuous variation dependent upon the population density in which the larvae grew up (IWAO 1956). In this paper, the author made an attempt to discover whether there are any differences in the mode of response to density change between L.unipuncta and its related species, L. loreyi and L. placida. L. loreyi is a pest of sugar-cane and other graminaceous crops in tropical and subtropical countries, though it is less injurious than L. unipuncta. L .placida occurs in Japan and adjacent areas but causes no damage to the crops. Parallel breeding experiments were carried out under the constant temperature of 25〜28℃ and natural daylight with Bromus or maize leaf as food. Larval density varied as 1,5,10 or 20 individuals per vessel. In L. unipuncta the larvae reared in crowds bear fine black coloration in contrast with greenish yellow or reddish brown color of the larvae in isolation. The crowded larvae of L. loreyi are also somewhat darker than the isolated ones but they never become blackish even at very high density. L. placida does not change its color in response to density conditions (Figs. 1,2). The rates of larval and pupal development are accerelated in the crowded culture of L. unipuncta but retarded in those of the other two species (Fig. 3 ; Tables 1,4). The weight of pupae decreases with increasing density in all three species (Fig. 4 ; Table 3). In L. unipuncta the crowded larvae are more irritable and consume more food than the isolated ones, while in L. loreyi and L. placida such clear-cut differences could not be found between the larvae reared in crowds and in isolation (Table 2). In any species the length of preovipositional and ovipositional periods, fecundity and coloration of adult moth are not affected significantly by the density during larval period (Table 5). Longevity of the adult is, however, considerably prolonged in high-density type of L. unipuncta, but rather shortened in that of L. loreyi (Fig. 5 ; Table 5). Thus the mode of density-dependent variation observed in Leucania unipuncta is quite different from those of its related species, L. loreyi and L. placida, while it resembles in many respects the phase variation of swarming locusts. It seems likely that "phase" dimorphism is characteristic in those species which fluctuate their population density extremely owing to their habits and/or habitat conditions.