Abstract
To ascertain the effect of larval density on the characters of the larvae and pupae of the field populations of Mamestra brassicae, some observations were made in the cabbage fields at Hirosaki in 1959 and 1963. The results of the observations in 1959 indicate that the effects of larval density on the larval and pupal characters were the same as those observed on the laboratory population. In this case, however, it was also noticed that the density on a single plant fluctuates from time to time and the density dependent variation of characters did not correspond with the larval density on a single plant at the later season of the population fluctuation. The male larvae, especially the dark ones which might have been produced under the high density conditions, were presumed to have a strong tendency to migrate in the final instar. The larvae of all instars were collected throughout the whole breeding season in 1963 and the frequencies of color variants were determined. As shown in Fig. 6,the color variation was observed in the fourth, fifth and sixth (final) instars of the first (early summer) generation and in the third, fourth, fifth and sixth instars of the second (autumn) generation. The percentage of the dark larvae, which were considered to be produced mainly by the effect of high larval density and partly by the effects of low temperature and unsuitable food-plant (cf. HIRATA ; 1960,1962), was higher in the second generation than in the first. Within the same generation, it was also higher in later season than in the earlier. The results of measurements of the width of head capsule of the fourth and fifth instar larvae of the second generation in 1963 indicate that the dark larvae of the fifth instar had a slightly smaller head comparing to the pale ones.
