キャベツ畑でのモモアカアプラとダイコンアブラとの個体群成長について II

抄録

(1) In the spring of 1958,a series of observations were made on the populations of Myzus persicae and Brevicoryne brassicae every fourth or fifth day in a cabbage garden, at Nogi, Matsue City, in which the aphid populations had already been observed once in 1956. A bulbed cabbage selected for observation was, for convenience sake, divided into the following three parts : (A) was the central part in which the leaves bulbed up ; (C) was the outer part in which the undersurface of the leaves faced directly toward the soil ; (B) was the part between (A) and (C). From each of Parts B and C, two leaves were cut off at random, and the whole individuals of M. persicae and B. brassicae living on them were collected and preserved in alcohol. In the case of B. brassicae, the number of its colonies on each leaf was recorded in advance of the collection. (No observation was made on Part A where few aphids had been found). Then the developmental stages of the aphids collected were individually determined by their antennal length and other characters (Figs. 1 and 2). Aphids mummied in consequence of being parasitized were taken into glass tubes for obtaining parasites from them. (2) The following results were the same as from the observation in 1956 : (a) The habitat segregation of these two species on a plant : in M. persicae, the population growing in Part C was always larger than that in Part B, while in B. brassicae it was conspicuously smaller (Table 1). (b) A time lag between M. persicae and B. brassicae in their coming into the garden : in the former, many of adults were apterae, the progeny of alates which had come flying into the garden, even at the beginning of the observation, while in the latter it was not until the end of May that apterae surpassed alates in number (Table 1). (3) B. brassicae increased in number much more rapidly than M. persicae, and at last on the 3rd of June, attained about eight times the number of the latter (Table 1 and Fig. 3). (4) In either species, the ratio of adults to the whole individuals collected was rather stable every observation day and its value, with a few exceptions, was about 10 per cent (Fig. 4). (5) It was assumed that the aphid populations increased exponentially on and after the 16th of May, and values of the coefficient of their growth were calculated (the slope of the straight lines drawn in Fig. 5). In the case of B. brassicae, the value obtained from Part C was much higher than that from Part B. The main reason for it, the writer thinks, was that in the latter half of May, as seen in Table 1,more than half of the adults living in Part B consisted of alates, most of which perhaps had been already aged and very low in their fecundity, while aduits found in Part C were mainly apterae, many of which were supposed to be vigorously producing their progeny because it had not been long after their coming to maturity. Either of values of the coefficient concerning M. persicae was lower than that concerning B. brassicae in Part B. This may be explicable if we assume that the mobility of the former lowers its productivity and urges it to move out from its population. (6) The number of colonies of B. brassicae on cabbage leaves increased in such a way as the number of the aphids forming them did(compare Fig. 7 with Fig. 3). The average number of aphids in a colony tended to be smaller in Part B than in Part C (Fig. 7). (7) Seven species of parasites and hyperparasites were obtained from mummied aphids gathered during the period of this observation ; it seemed that the fauna of parasites relating to B. brassicae was poorer than that relating to M. persicae (Table 2). At any rate, it was clear that the parasites which had not much increased as late as the middle of May failed to control the aphid populations effectively.