林木の生育におよぼす食葉性害虫の影響

Abstract

It seems that the defoliators have a great influence upon the growth of the forest trees. The leaf-eating insects, one group of the defoliators, feed on the leaves, assimilation parts, have a direct effect upon the growth of trees and enfeeble the vitality of trees. Moreover, they make trees ready to get other damages. The author investigated the decline of the growth which was caused by the abnormal decrease of needle of red pine mainly and the feeding quantity of pine caterpillar (Dendrolimus spectabilis Butler) on P. densiflora, the gypsy moth (Lymantria dispar Linne) on Quercus acutissime, and the camphor silk moth (Dictyoploca japonica Butler) on Castanea crenata. This report elucidated the effects of the feeding damage by pine caterpillar, one of the most injurious among the leaf-eating insects in red pine forest, upon the growth of red pine. Chap. 1 deals with the elongation of normal needles and the rest of cut needles of red pine and black pine and with artificial defoliation-experiments with red pine (P. densiflora), Mukunoki (Aphananthe aspera) and Enoki (Celtis sinensis var. japonica). Chap. 2 deals with the results of the individual breeding of pine caperpillar, gypsy moth and camphor silk moth feeding on P. densiflora, Quercus acutissime and Castanea crenata. Chap. 3 deals with the effects of the feeding damage of pine caterpillar upon red pine by artificial defoliation in the nursery and the examples in the field. These investigations were carried out in 1955 ~ 1961, and the results obtained from them were as follows: 1. Under the normal conditions in Kyoto, the elongation of red pine-needle began late in April or early in May, and the greater part of elongation was over early in August. When red pine-needle was cut in its elongating period, the rest (the part of leaf-sheath) began to elongate again but could not after it. The re-elongating length was influenced by cut time and always shorter than the normal elongating in the same term. It was the same with black pine. 2. If all the leaves of tree crown were defoliated, red pine happened to wither in the first defoliation, but Mukunoki and Enoki did not. Namely, red pine defoliated before late in July did not wither owing to the re-elongation of needles, however, in case of the defoliation from August to October, the defoliatd trees never failed to wither. 3. In case of red pine, defoliation had influence on the diameter growth immediately after it, but on the height growth it exerted a great influence in the next year. When the percentage of defoliation was below about 60%, the height growth of defoliated tree had smaller influence than the diameter growth. In case of defoliation at the end of growing period, it scarcely had influence on the height growth. 4. When all the needles were defoliated before late in July, the defoliated red pine went on growing. However, it had the smallest growth as compared with the normal. That is, the later red pine was defoliated, the worse its growth became. The growth percentage showed 40 ~ 60% on the diameter growth, 45 ~ 70% on the height growth and 40 ~ 60% on the stem growth as compared with the normal for two years after defoliation. Besides, these trees seemed to be affected by defoliation even in the third year. 5. When the percentage of defoliation was below about 40%, the growth percentage of its red pine showed 85 ~ 90% on the diameter and 90 ~ 100% on the height of the normal growth percentage for two years after defoliation, and in case of about 60%-defoliation, it showed 75 ~ 85% on the diameter and 75 ~ 100% on the height of the normal for two years after it. When defoliation was below about 60%, red pine seemed to be little influenced by defoliation in the third year after it, but it appears to be impossible that its pine can recover the decline of the growth occurred for two years by defoliation. 6. In consequence of artificial defoliation, it seems that the decrease of the growth was mainly caused by the diminution of the leaf-volume in the treatment year, but the growth in the next year was reduced because the quantity of respiration was too much compared with assimilation. 7. The total length ingested by pine caterpillar was about 50m per male, about 78m per female in red pine needles and about 8.6g and 13.6g in dry weight of needles. The feeding quantity at the last instar amounted to about 70% of total quantity. A larva ingested 2000 ~ 3000cm2 at male and 2700 ~ 4000cm2 at female in the leaves of Castanea crenata (camphor silk moth), and 700 ~ 1100cm2 at male and 1100 ~ 1800cm2 at female in the leaves of Quercus acutissime (gypsy moth). 8. The correlations between the feeding quantity and the amount of the frass were shown as follows: [Table omitted] 9. The number of the frass evacuated by a larva of pine caterpillar per day was from fourteen to seventeen except the fourth instar after the hibernation. 10. It was observed that pine caterpillar ingested twenty-eight foreign pine-species in Kyoto. From this fact, it seems to be possible that all species of pine-genus in Japan are infested with pine caterpillar. 11. The steady artificial defoliation like the feeding damage of pine caterpillar and the defoliation at once, if their percentage and period were the same, had similar influence on the growth of red pine. But in case of lower percentage, the effects of the former were less than the latter. 12. Judging that mean net-assimilation rate is 2.045 g/g•year, it is found that when assimilation parts is 30% of the tree, about 93%-defoliation of the tree crown makes the growth of red pine zero, and similarly 20%----about 89%, 10%----about 75%. In those cases, it seems that defoliated red pine will wither. In case of the first, red pine stand of one-hectare is infested with about 260, 000 pine caterpillars, in the second, about 250, 000 larvae/ha and in the third, about 210, 000 larvae/ha. If the closed red pine forest is infested with about 270, 000 pine caterpillars per hectare, all the needles seem to be defoliated during its larval stage. 13. When the growth is reduced to zero, it is supposed that a red pine of twenty years old is infested with about 60 pine caterpillars and that of fourty years old with about 230 on the second site.