Search

Although classical biological controls aim to permanently minimize numbers of invasive pests using exotic natural enemies, few long-term studies have been conducted on this subject. In this study, we investigated the population dynamics of the chestnut gall wasp Dryocosmus kuriphilusYasumatsu (Hymenoptera: Cynipidae), and an introduced parasitoid, Torymus sinensisKamijo (Hymenoptera: Torymidae), over 40 years (1982–2023) at a site at which the parasitoid was first released (Tsukuba City, Ibaraki Prefecture) in Japan. Because the gall formation rate per 100 chestnut buds, an indicator of pest density, had decreased from 42.5 to 0.7% in the first 10 years after the release, the biocontrol program was deemed successful. However, further research showed fluctuations in the pest density over the next 30 years. It usually remained at low levels but occasionally increased dramatically, with three peaks exceeding the economic injury level (30%). Simultaneously, an indicator of parasitoid density (the number of parasitoids emerging from 100 galls × the gall formation rate) also fluctuated, in synchronization with the pest density indicator. These results suggest that T. sinensishas played an important role in suppressing D. kuriphilusat the releasing site over 40 years.

Based on the results of ecological surveys of Adoxophyes oranaand its natural enemies in apple orchards sprayed with the granulosis virus and control plots, we constructed working models to simulate the population dynamics in three different experimental plots; those treated with chemical insecticides, those with granulosis virus, and controls. The number of individuals killed by predators, parasitoids and by miscellaneous mortality factors could be calculated on the assumption of imperfect density relations; that is, relation of the number killed by each factor with the initial number of larvae was represented by a curve with an upper asymptote. We could estimate the proportion of virus infection using curves with upper asymptotes. Rate of increase from pupa to middle instar larvae of the next generation was subject to strong density-effect. Simulation we proposed in this paper suggests a possibility that a single spray of the virus at 1st generation can reduce not only the number of A. oranalarvae in the 2nd and 3rd generations but also the degree of fruits injured by this insect. Spray of chemical insecticide is considered to be ineffective in reducing the pest density and the degree of injury to low levels in subsequent generations, as compared with untreated plot, where the density of the 1st generation larvae is low.

Stabilizing mechanism in population of Malacosoma neustria testaceawas investigated in a central part of Japan based on eight year survey. Population fluctuation in each developmental stage in the experimental field was rather small, i.e., 5.9 times in egg and 85.0 times in female adult. Pupal weight negatively correlated with the densities of 5th instar larvae and prepupae (cocoons) and correlation coefficient was highly significant in females. Population density was stabilized by density-dependent dispersal of female moths in preovipositional period. Comparison between fecundity of emerged moths and that of actually oviposited ones in the experimental field suggested that density-dependent dispersal took place as the result of density-dependent size variation, i.e., small-sized female months have higher flying ability. This hypothesis was supported by the experiment in which flying ability of newly emerged female moth was measured. Similar stabilizing mechanism is expected to occur in semelparous or pro-ovigenic insects.

Stabilizing mechanism in population of Malacosoma neustria testaceawas investigated in a central part of Japan based on eight year survey. Population fluctuation in each developmental stage in the experimental field was rather small, i.e., 5.9 times in egg and 85.0 times in female adult. Pupal weight negatively correlated with the densities of 5th instar larvae and prepupae (cocoons) and correlation coefficient was highly significant in females. Population density was stabilized by density-dependent dispersal of female moths in preovipositional period. Comparison between fecundity of emerged moths and that of actually oviposited ones in the experimental field suggested that density-dependent dispersal took place as the result of density-dependent size variation, i. e., small-sized female moths have higher flying ability. This hypothesis was supported by the experiment in which flying ability of newly emerged female moth was measured. Similar stabilizing mechanism is expected to occur in semelparous or pro-ovigenic insects.

Intraspecies competition in a field population of Gregopimpla himalayensis(Hym.: Ichneumonidae) parasitic on the prepupae of Malacosoma neustria testacea(Lep.: Lasiocampidae) was investigated. The parasite oviposits the sufficient number of progeny (5 individuals/0.1 g dry weight of host) to exhaust a single host in a single attack. However, at the intensity less than 22–26 individuals/0.1 g d.w. of host, all individuals can emerge, i.e. density-dependent mortality does not occur. Within this range of intensity, survival of parasite larvae is guaranteed by diminution in body size and decreasing sex ratio. In contrast, total biomass of parasites showed a peak at 5 individuals/0.1 g d.w. of host at which a single host is exhausted. Above the intensity of 22–26, extraordinary minute individuals appeared and they died before maturation. If intraspecies competition play a role in regulation of G. himalayensispopulation in the field, the process is usually not through density-dependent mortality but through decreasing reproductive rate caused by decrease in the sex ratio, adult longevity and fecundity.