ダイコンサルハムシの分布 : 特に成虫の分散が次の世代の分布に及ぼす影響について

Phaedon brassicae BALY (Coleoptera : Chrysomelidae) is one of the most injurious insects to cruciferous vegetables in Japan : early in autumn the adults, which move only by crawling, invade into fields and eat radishes or Chinese cabbages there. That is the season when the vegetables begin to extend their seed leaves.
In the autumn of 1957, the writers made a series of observations upon the distribution of this species in a vegetadle field (Fig. 1) and at the same time a release experiment of the adults in plots arranged for the purpose (Fig. 5), at Nogi, Matsue City.
In the vegetable field the observations began with those upon adults on the 15th of September, the next day on which several adults had frrst been discovered, and continued day by day until the 1st of October. From these observations it was shown that some of the beetles having invaded into the field arrived rather sooner at the centre of the field, though the greater portion of them remained for a considerably long time at the edges of the field (Figs. 1 and 3). But late in September or early in October beetles were observed to be distributed rather evenly in the field.
In the process of these observations it was clarified that the adults did not display any regular diurnal activity on the ground (Fig. 2).
The distributions of eggs and larvae in the field were examined five times during the period from the 20th of September to the 8th of October (Fig. 4), and the result was that the distribution of eggs corresponded pretty well to that of adults at first eggs were found abundantly at the edges of the field and then they were distributed rather evenly all over the field. As for the larvae, their distribution was uneven, but particulars could not be ascertained, because their density was very low.
At 8 on the 21st of September, 400 beetles were released at the northeastern corner of the experimental plots, where radishes had previously been cultivated, and then their dispersal was observed twice a day (Figs. 5 and 6; Tab. 2). They not only moved along ridges, but also dispersed crawling across them : the area invaded by the insects spread fanwise from the release point. It is noteworthy that a few insects were discovered in the remotest plot from that in which they had been released as early as three days after.
The distributions of eggs and larvae in the experimental plots were observed four times on the 26th of September, the 3rd, 10th and 22nd of October, on the last of which the writers made an observation upon pupae in the soil, too (Figs. 7 and 8). On the 26th of September no larva was recorded, and the density of eggs was conspicuously high in the part near the release point. The more distant the place was from the release point, the lower the density became. In each of the other three observations it was noticed that the ratio of the density in any immature stage to that in the preceding stage remarkably differed among three parts of the plot as seen in Fig. 7A: the ratio of the larvae to the eggs, on the 3rd and 10th of October, and that of the pupae to the larvae, on the 22nd of that month, were both the highest in and around the release point, and the lowest in the remotest part from the release point (Fig. 8). It means that the average age of individuals distributed around the release point was the oldest and in the remotest place the youngest. And moreover, the total density of the eggs and the larvae or the larvae and the pupae did not so much differ among the three parts. These facts seem to proved that the beetles released gradually spread all over the plot, depositing their eggs.
Besides, it was clarified that there existed a tendency that eggs were concentratively laid on some particular young radishes, even if they were all alike growing (Tab. 3).