マツカレハ

Investigations were conducted both in the field and the laboratory to determine some of the more important ecological factors affecting the efficiency of Trichogramma dendrolimi Matsumura, Telenomus dendrolimi (Matsumura), Anastatus japonicus Ashmead, A. gastropachae Ashmead and Pseudanastatus albitarsis (Ashmead), hymenopterous egg parasites of the pine moth, Dendrolimus spectabilis Butler. Field studies were made in the Japanese black pine forest on the coast of Fukuoka district between 1960 and 1967, and the majority of materials for laboratory studies were also collected from this area. Some biological characteristic s of the five egg parasites, e.g., fecundity, sex ratio, length of developmental period, adult longevity, pattern of ovigenesis etc. were studied. Characteristics of host population in time and space, e.g., seasonal prevalence of host population and spatial distribution of host egg masses in the pine forest were also studied in connection with parasitism by the egg parasites. In addition, hyperparasitism, predation of adult parasites by ants, alternative hosts, interspecific competition between parasites and source of foods for adult parasites were investigated. Conclusions to be derived from the comparative study of these ecological factors affecting the efficiency of the five principal egg parasites of the pine moth as natural enemies can be expressed as follows. /Trichogramma dendrolimi/ This species is the most prolific of the egg parasites of the pine moth (the mean fecundity, 186). There is little variation in the sex ratio, and an extremely high proportion of females (86-88% females) is always recorded. This is probably due to its uniparental reproduction (deuterotokous parthenogenesis). The developmental period of this species is the shortest of the erg parasites of the pine moth (the mean developmental period in 28℃, 8 days). This makes it possible for the parasite to complete three generations per one generation of the host. The above-mentioned characteristics of the parasite indicate that its potential rate of increase is exceedingly high. Consequently, the parasite can respond rapidly to a greater increase in the host population during a host oviposition period. A high level of parasitization at the end of the host oviposition period in the first generation is primarily caused by a rapid increase in parasite populations. This species is pro-ovigenic, and the adult is short-lived (the mean longevity of females fed on honey in 28℃, 11 days). These characteristics are disadvantageous to the parasite in the case of the second host generation in which the host population density is usually very low. The short longevity of the adult females indicates that they can not survive in the period between the first and second host generations. These are the reasons why host eggs in the second generation are rarely attacked by the parasite every year. Although many lepidopterous insects are known to be hosts of the parasite, they are all the inhabitants of other types of forests than pine forests. The absence of alternative hosts in the period between the first and second host generations is another reason for a low level of parasitization in the second host generation. Since there are no overwintering host eggs in the pine forest, the initial parasitization in the first host generation is probably due to the parasites which happen to enter the pine forest from outside areas. It is, therefore, apparent that the absence of alternative hosts in the pine forest is an important factor limiting the efficiency of the parasite in both of the two host generations. A high degree of gregarious parasitism inherent in this species (mean number of parasites produced in each host egg, 15-29) decreases the efficiency of the parasitism, so that each female parasite can parasitize only nine host eggs on an average during its life span. Spatial distribution of host egg masses in the pine forest influences t he efficiency of this parasite; the more the large-sized egg masses are deposited in a pine tree, the heavier the parasitization of the egg masses occurs, because large-sized egg masses are more frequently deposited in taller pine trees especially in their higher crowns which correspond to the preferential range of the female parasite. Hyperparasitism by Pacheneuron sp. is very rare and is not an important factor limiting the efficiency of the parasite. /Telenomus dendrolimi/ In spite of the relatively low fecundity of this species (the mean fecundity, about 70), its potential rate of increase is considerably high, because the developmental period is so short (the mean developmental period in 28℃, 12 days) that two generations of the parasite can be produced per one generation of the host. Consequently, the parasite is able to increase in number rapidly and to attain to its high population density at the end of the host oviposition period. This species exhibits arrhenotokous parthenogenesis. Male progeny produced by unmated females may reduce the proportion of females in the parasite population. However, such a phenomenon rarely occurs in the field, and a great preponderance of females (75-86% females) is usually recorded there. This species is pro-ovigenic, but the adult females fed on honey can live long (the mean longevity in 28℃, about 50 days). The combination of these two desirable characteristics as control agents suggests a high power of host discovery of the parasite followed by a high efficiency of egg deposition. Furthermore, the great longevity of the female parasite suggests that she can survive in the period between the first and second generations of the host during which no host is available. For these reasons, the high adult parasite population, which has already grown at the end of the host oviposition period of the first generation, can attack host eggs in the second generation period, and a high level of parasitization often occurs in the host generation regardless of the host population density. High host populations are occasionally observed in the second host generation, while this host generation is usually produced partially in the study area. Such a great abundance of hosts would favour the efficiency of the parasite, because the parasite can cause a severe mortality of the pine moth eggs in the second generation irrespective of the host population density. In contrast to Tr. dendrolimi, this parasite tends to attack the host egg masses in the lower crown of a pine tree. Such a failure to fully occupy a host-inhabited area may reduce the efficiency of the parasite as in the case of Tr. dendrolimi, but it prevents the former from decreasing its efficiency by avoiding the multiparasitic competition with the latter. Because this species is characterized by a lower degree of gregarious parasitism than Tr. dendrolimi (mean number of the parasites produced in each host egg, 5-7) in spite of its low fecundity, each female of the former can parasitize a few more host eggs than that of the latter during her life. Hyloicus caligineus Butler, only one alternative host of this parasite in the pine forest, is always very scarce and seems of little importance except for a brief period of spring when no pine moth eggs are available in the forest. /Anastatus japonicus, A. gastropachae and Pseudanastatus albitarsis/ Some factors common to these three related species influence their efficiency. These species are all synovigenic, and the adults are long-lived ( the mean longevity of females fed on honey in 28℃, 53 days in A. japonicus, 45 days in P. albitarsis and 28 days in 1. gastropachae respectively). It follows from this that the relatively high fecundity (the mean fecundity, e.g., 153 or more in. A. japonicus) and the great longevity of these species are largely dependent on the availability of the source of foods for adult parasites such as nectar-bearing plants and honeydew-producing insects in the pine forest. Therefore, the great scarcity of these kinds of food sources in the pine forest is an important factor limiting the efficiency of these parasites, The synovigenic gravid females of these species are characterized by a phenomenon of ovisorption. Low host densities would be advantageous to these species because of conservation of reproductive material based on ovisorption. However, high host densities attained in a short time during the host oviposition period as in the first host generation are disadvantageous to these synovigenic species because of their preoviposition period and of limited egg deposition in a brief period. The long developmental period of these species (the mean developmental period in 28℃, about 20 days in A. japonicus and A. gastropachae and 16 days in P. albitarsis respectively) indicates that they are unable to have more than one generation during one generation of the host and can not respond rapidly to a remarkable change in the host density during the host oviposition period. This is the main reason why a large number of host eggs in the first generation are usually able to escape from the parasitization by these species. These species exhibit arrhenotokous parthenogenesis. Male progeny produced by unmated females may reduce the proportion of females in the parasite population. Furthermore, in the laboratory the proportion of male progeny of A. japonicus and A. gastropachae increases toward the end of the life of females. However, such phenomena rarely occur in the field, and a great preponderance of females (84% females in A. japonicus, 79-86% females in A. gastropachae and 75-79% females in P. albitarsis) is usually recorded there. These species are all solitary in the pine moth egg. Undoubtedly, this parasitic habit gives the parasitism a greater efficiency as compared with gregarious parasites, Tr. dendrolimi and Tel. dendrolimi. These species may live the whole year on only pine moth egg without alternative hosts as far as the study area is concerned. Neverthless, there is very little doubt that the absence of alternative hosts in the pine forest is reducing the efficiency of these species.