Inter- and intra-population variations in diapause attribute of the two-spotted spider mite,Tetranychus urticae Koch, in Japan

Summary Populations of the two-spotted spider mite,Tetranychus urticae Koch collected from various localities and from various host plants in Japan showed wide variations in diapause attribute. Diapause percentages at 18°C/9L15D varied from nearly 100% in the north to 0% in the south-west. At intermediate latitudes the mites showed wide inter-population variations. Populations on herbaceous hosts in vinyl- or glass-houses gave significantly lower incidence of diapause than those on roses and deciduous fruit trees. Presence of winter hosts and better host quality under protected environments seemed to favour non-diapausing mites. The temperature threshold for diapause expression also varied widely among local populations. Northern populations consistently had higher and less variable thresholds than populations at intermediate latitudes with thresholds between 15 and 18°C. Inbred lines derived from a population in Kyoto exhibited a wide variation in diapause percentage at 18°C. These results show that diapause inT. urticae is a quantitative threshold trait and that populations in central Japan consist of a variety of genotypes with different diapause traits. This might provide a genetic source for adaptation to local and temporal variations in environmental conditions.     

Coexistence and weak amensalism of congeneric gall-forming aphids on the Japanese elm

Closely related species of gall-forming aphids are often associated with a single host species. SixTetraneura species coexist on the Japanese elm,Ulmus davidiana, in Sapporo, northern Japan. This paper describes the probabilities of coexistence on macro- and microgeographic scales (i.e., on host trees and host leaves) and examines whether coexistence with conspecific or heterospecific galls on leaves or shoots has any effect on the fecundity of each aphid species using multiple regression. A Monte Carlo simulation showed that the frequency distribution of the numbers of species on individual host trees differed significantly from that expected from the null model. There were significantly positive or negative associations between species.Tetraneura radicicola andsorini always coexisted with other species on trees they infested. Multiple regression revealed that the coexistence of conspecific or heterospecific galls on individual leaves had no influence on the fecundity ofradicicola andsorini, but had a negative influence on that of sp. O. On average,radicicola andsorini produced a smaller number of offspring in galls than sp. O, and obviously consumed only a small part of resources available on the galled leaves. Evidence available suggests that although amensalism does arise between sp. O and other species, its influence in not so strong as to exclude sp. O competitively from theTetraneura community.     

Simulation of the interactions of predatoryTyphlodromus mites with the European red mite,Panonychus ulmi (Koch)

Summary Interactions ofTyphlodromus mites and their prey, the European red mite, in orchard settings were examined through computer simulation. In particular, the consequences of very slow dispersal by the predators, compared with the weather-dependent higher rates of prey dispersal, were elucidated. In simulations of the interactions of the predator and its prey, both dispersal and temperature strongly affected the available supply of juvenile prey, and thus could determine whether the predators on an individual tree survived or perished.     

A ten-year study of population dynamics of citrus pests in the pesticide-reduced orchard

Summary To determine the effectiveness of a pest management system that uses pesticides at a low level, we censused populations of seven insect pests and three plant diseases from 1980 to 1989 in a citrus orchard which was managed with a pesticide-reduced and pesticide-free protocols, in Wakayama Prefecture, Japan. In the orchard, we controlled the pests solely by spraying petroleum oil once a year from 1980 to 1985, by not spraying insecticide in 1986, and spraying only germicide in 1987. The arrowhead scale population remained much lower level at which citrus trees begin to wither when petroleum oil was sprayed, whereas it rapidly increased and began to wither trees during the years without spraying petroleum oil. The Indian wax scale population rapidly increased only during a 2-year period and then declined. Although the infection level of the sooty mold was unusually high on the trees where the density of the scale was high, this pathogen did little damage to citrus trees. The population of the red wax scale gradually increased throughout the census period but did not reach the level at which citrus trees begin to be damaged. Other pests also caused negligible damages to citrus trees. The results indicated that the pesticide-reduced pest management system which was proposed by Inoue and Ohgushi (1976, 1977) is valid for the prevention of the outbreak of the arrowhead scale, and that is has the sufficient control efficiency for the other pests in citrus orchards. Contribution to the ecological studies of scale insect 3.     

Dynamics of buried seed population and seedling cohorts of two dominant weeds in a hill agroecosystem of the humid subtropics of India

Dynamics of the buried seeds and plant population of two dominant weeds, viz.,Emilia sonchifolia (Linn.) DC. andRichardsonia pilosa HBK were studied in the crop fields of Meghalaya, north-east India during radish and maize cropping and intervening fallow periods. The total buried seed population ofR. pilosa was always larger than that ofE. sonchifolia, but the germinable fraction was invariably greater in the latter. A major portion (39–41%) of the viable (germinable+dormant) seed population in both weeds was confined to the surface soil layer (0–5 cm). The viable seed population ofE. sonchifolia peaked during April, while that ofR. pilosa showed two peaks (during August and December). The survival pattern and half-lives of seedling cohorts showed, some differences in the two weed species, but both being summer annuals, their populations behaved in a similar manner by showing higher seedling recruitment (K) and survivorship (p) rates in the summer crop (maize) than in the winter crop (radish). However, the density of plants that could attain adulthood was significantly higher inE. sonchifolia thanR. pilosa which might have resulted in greater seed input of the former to the soil leading to its greater abundance in the crop fields. Supported by the University Grants Commission, New Delhi (Grants No. F. 3-37/87 SR II)     

Phenological studies on the wing dimorphism of a semi-aquatic bug,Microvelia douglasi (Heteroptera: Veliidae)

Summary Seasonal changes in the percentage of macropterous individuals in natural population ofMicrovelia douglasi in permanent habitats were investigated in Shimane, western part of Japan. Macropterous forms of 5th instar nymphs occurred most frequently from late June to early July. However, the seasonal appearance of macropterous adults differed between habitats and years. There were significant effects of density, temperature, photoperiod and food availability on the percentage of macropters under laboratory conditions. The percentage of macropterous forms generally increased with density, but the mode of the response was modified by temperature, photoperiod and food availability. When insects were reared under long day photperiod (16L8D) with abundant food (0.5–1.0 g/container/day), the percentage of macroptery varied most among the different density treatments at the intermediate temperature (24°C) and the difference was reduced at higher temperature (28° and 32°C). Effects of temperature were small under the short day photoperiod (12L12D). Under 12L12D photoperiod sensitivity of density responses were higher at 32°C and lower at 24°C than those under 15L8D photoperiod. When the insects were supplied with food every other day, the density response became obscure. Ecological significance of environmental responses in the wing morph determination was discussed in relation to the heterogeneous environment of water surface.     

Stabilization of the population of a parasite on fruit by the population of a cleptoparasite

Summary When there exist two species such that one is a parasite on fruit and the other exploits the parasitized fruits, they must compete for a limited resource with each other. The relation betweenDacus cucurbitae andAtherigona orientalis is an example of such a situation. We raise a question whether the population of a parasite on fruit can be stabilized by the existence of the cleptoparasite of the parasite on fruit. The changes in their population densities are represented as a differential equation with time delayed parameters, which is deduced from the context of life histories of the two species. An index representing degree of overlapping of generations (g) is defined as an average oviposition period devided by the average preoviposition period, and the value is assumed to be the same in the two species. The stability of the system is classified by three parameters: the reproductive rate of the parasite on fruits (R), the survival probability of it through competition against the cleptoparasite (p), and the generation overlapping index (g). For small values ofg, e.g. less than some 0.5, the stability is determined mainly by a product ofRp: the values larger than 1 result in no equilibrium and infinite increase of both species, the values near 0 lead to large amplitude oscillations, and the intermediate values make stable equilibria or regular small oscillations. Asg takes the larger values, the stability region in the space (p, R) occupies the larger area. The model presented here is well adjusted to the fluctuating pattern of the population ofD. cucurbitae on Okinawa Is., and would also be applied to analysis of both hyperparasitisms and inquilin. This study was supported partly by Science Research Fund (No. 574092) from the Ministry of Education, Japan.     

Reproductive compatibility between populations of the citrus red mite,Panonychus citri (McGregor ) (Acarina: Tetranychidae)

Summary Reproductive compatibility was studied among populations of different types of the citrus red mite,Panonychus citri (McGregor), i.e., the diapausing type from pear (DP), the non-diapausing type from citrus (C) and that from pear (NP). Copulation was also observed between mates of different types (DP and C). Only in crosses between C ♀ and DP ♂, was copulation occasionally broken off prematurely; duration of copulation varied considerably between pairs and the average duration was much shorter than that between DP ♀ and C ♂ and than that in crosses between mates of the same types. No F₁ adult females were produced at all from crosses between DP and C, showing that there was a complete reproductive isolation between the two types. There was a significant reciprocal difference in the egg hatchability and survival rate of immatures in the F₁ progeny; mortality in the eggs and that in the immature stage were significantly higher in crosses between DP ♀ and C ♂, as compared to those in the reciprocal cross. This suggested that fertilization may have occurred in the former cross, whereas it seemed that eggs were not fertilized in crosses between the C ♀ and DP ♂. The NP was compatible with the C, whereas it was completely incompatible with the DP. Thus, there was a complete reproductive incompatibility between the diapausing and non-diapausing type ofP. citri. This work was supported in part by Grant-in-Aid No. 60560049 from the Ministry of Education, Science and Culture, Japan.     

Population dynamics ofMalacosoma neustria testacea (Lepidoptera: Lasiocampidae): Stabilizing process in a field population

Summary Stabilizing mechanism in population ofMalacosoma neustria testacea was 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. Contribution Ser. A, No. 64 from Fruit Tree Research Station, Ministry of Agriculture and Forestry, Japan     

Distribution patterns of and sampling plans forTetranychus urticae Koch (Acarina: Tetranychidae) on roses

Summary I sampledTetranychus urticae Koch (Acarina: Tetranychidae) regularly from four rose gardens in Kyoto and Nara Prefectures in 1988–1990. When mite density was low,T. urticae showed an uneven vertical distribution, being more abundant in the lower third and absent from the upper third of the plants. Mite density was less variable within than between plants, suggesting that a sampling plan which includes more leaves from different plants rather than from different levels of a plant is favourable. Spatial distribution ofT. urticae was nonrandom and followed the negative binomial distribution. In addition, both the Taylor’s power law and the Iwao’s patchiness regression described the distribution well. An empty-sample method for estimating mite density from the proportion of empty sampling units was developed. Sampling plans for determining the sample size required to reach a predetermined precision level, based on this method and by directly counting the mites, were designed. The counting method was more accurate than the empty-sample method. However, when the time factor was taken into account, the latter was more favourable, because it was faster than the former at a density range of 1.5–300 mites leaf⁻¹, which was most commonly encountered in the field.     

Shoot growth dynamics and size-dependent shoot fate of a clonal plant,Festuca rubra, in a mountain grassland

The relation of the within-season and between-season patterns of shoot growth were compared in a clonal grass with long-lived shoots,Festuca rubra, in a mown mountain grassland. The growth rate of shoot length from spring to summer in a year was almost constant for each shoot irrespective of spring shoot length each year. The annual shoot growth rate from spring to spring was negatively correlated with the shoot length in the first spring. Shoots of different length and age therefore tended to converge over time to a population of identical shoot size, suggesting an equalizing effect of growth pattern on size structure. Shoot size (shoot length and number of leaves) influenced the fates of shoots. Larger shoots showed an increased incidence of both flowering and formation of intravaginal daughter shoots and a decreased incidence of death in the subsequent time period. The fates of shoots were independent of their age. Although the negatively size-dependent springto-spring annual shoot growth rate acted to decrease shoot size variation, the remaining variation within the shoot population was still sufficient to generate different fates of shoots. These fates were not related to the previous life history of individual shoots. There was a significantly positive effect of the shoot size at initiation on its life expectancy. This was mainly attributable to the positively size-dependent survival rate of shoots in the early stage (<1 year old) of shoot life history. Later on (> 1 year old), shoot size had little effect on the survival rate of shoots. Once small young shoots have survived this early stage (< 1 year old) in life history, they can grow vigorously, little affected by competition regardless of shoot size, and converge to a stable size structure of shoots of similar size. Only shoot size in the early stage ( < 1 year old) of life history is important for the persistence of a shoot population.     

One-host one-parasitoid system: Population dynamics of a zygaenid mothPryeria sinica Moore in an undisturbed habitat

Summary The population dynamics ofPryeria sinica was investigated in an undisturbed area in 1976–1979. We analyzed the process stabilizing the local population by the life table approach for immature stages and the mark-recapture method for the adult stage. Females usually layed about 130 eggs in an egg-mass. The shape of the survivorship curve was convex and was characterized by a relatively low mortality in the egg and larval stages and by a relatively high mortality in the prepupal and pupal stages. The low mortality in the early stage seemed to be not only due to the peculiar life cycle of this species (larvae develop in early spring when natural enemies are not active) but due to their protective nest-webs, larval warning coloration and repellent smell. The high mortality after cocooning was caused by severe parasitization byAgrothereutes minousubae. The number of adult in the population varied by 2.10-fold, which was less than that of other gregarious moths. The life table data and field observations suggest that adult female dispersal would have acted as a stabilizing factor, andA. minousubae as a conditioning factor on the dynamics of the moth population.     

Oviposition site selection and avoidance of additional mating by females of the dragonfly,Cordulia aenea amurensis Selys (Corduliidae)

Summary Females of an odonate species in which oviposition sites overlap with mating sites may adopt one or more of the following strategies when they lay eggs except when they ‘trade’ mating for access to suitable oviposition sites or for services (guarding, etc.) provided by males: (1) ovipositing at hidden places; (2) ovipositing at a time when males are neither patrolling nor watching; (3) indicating non-receptivity by a behavioral display. The density of ovipositing females of the dragonfly,Cordulia aenea amurensis Selys which was studied between 1970 and 1983 at a pond (H?rai-numa, Sapporo, Hokkaido) had a high negative correlation with the distance from ‘entrance’ (a part of shore at which the arrival of most adults seems to have occurred). On the other hand, oviposition was rarely observed at a sector being distant from entrance in spite of the inference that larval survivorship was probably high at this sector. Most females oviposited among emergent vegetation in which approach of males to them was difficult, and they scarcely traveled across the open water in search of oviposition sites. Therefore, most females of the population studied were considered to adopt the first strategy. The second and third strategy were not adopted by the population studied. Finally, the influences of some environmental factors and traits possessed by a species on the adoption of these tactics or on the execution of the ‘trades’ were discussed. Ecological studies ofCordulia aenea amurensis Selys, VII. This work was supported in part by a Grant-in-Aid for Special Project Research on Biological Aspects of Optimal Strategy and Social Structure from the Japan Ministry of Education, Science and Culture.     

Famine Mortality in Nineteenth-Century Japan: The Evidence from a Temple Death Register

Economic and demographic historians who have studied Japan's early modern period argue that preventive checks to fertility were the primary cause of Japan's stationary population in the eighteenth and early nineteenth centuries, and that the role of ‘positive’ checks was negligible. This paper presents evidence and a claim that mortality crises – famines in particular – also played an important role in checking population growth during this period. It analyses data from the death register of Ogen-ji, a Buddhist temple in the Hida region of central Japan. These data provide a remarkably detailed picture of the short-term demographic consequences of Japan's last great famine, the Tenpō famine of the 1830s. ‘Normal’ mortality patterns, by age and sex, are compared with patterns of mortality during the famine. Mortality of males rose considerably more than that of females, with the greatest rise occurring among young boys aged 5–14 and adult men aged 30–59. A surprising finding was that mortality at ages 0–4 rose relatively little, in part a consequence of a marked fall in the number of births during the famine. The Tenpō subsistence crisis was not the sole cause of population stagnation in the Ogen-ji population, but it was a prominent feature of the ‘high mortality regime’ that this population experienced during the eighteenth and nineteenth centuries.     

Lifetime reproductive success in reproductively suppressed female voles

Summary A population of the grey red-backed vole,Clethrionomys rufocanus bedfordiae, was investigated on a 1 ha control grid and a 1 ha grid on which the voles were fed within a 2.1 ha outdoor enclosure in Hokkaido, Japan by live trapping from 1984 to 1986, for testing the Reproductive Suppression Model of Wasser and Barash (1983)-females can optimize their lifetime reproductive success by suppressing reproduction when future conditions for the survival of offspring are likely to be sufficiently better than present ones as to exceed the costs of the suppression itself. Age at the first pregnancy more varied in a higher density population on the experimental grid and females could be classified into the early and the late reproductive type in two generations (A: females born from February to June 1985; B: females born from September to November 1985). Lifetime reproductive success (the number of pregnancies, the number of successful litters, and the number of offspring) was not different between the early and the late reproducing females. The late reproducing females lived for longer periods than the early reproducing females, so that the loss by delayed start of reproduction was compensated for by a longer life span. Life span was not different between offspring of the early and the late reproducing females. These facts supported the Reproductive Suppression Model.     

Life tables and population parameters of the green leafhopper,Nephotettix cincticeps (Uhler), in the southwestern district of Japan with special reference to the first generation on foxtail grass

Summary The three year (1988–1990) life tables ofNephotettix cincticeps were constructed, and the daily survival rate and longevity of female adults were estimated by Hokyo and Kiritani’s (1967) method for the overwintering and the first-generations on the foxtail grass in Okayama, southwestern Japan. The life tables and the population parameter values estimated were compared with those in the other generations on rice. The FARMCOP suction sampler was employed to survey the population density. The durations of pre-ovarial maturation of female adults of the 1st generation on foxtail grass and rice seedling were similar. Longevity of adults of the overwintering and the first generations which emerged on the wild host was longer than that of the other generations (2nd and 3rd generations) on the rice plants. Fecundity of females decreased successively as the generation proceeded and it became lowest in the final 3rd generation. Only about 3.5 percent of first-instar nymphs of the 1st generation emerged as adults in the fallow field. The survival rate of nymphs on foxtail grass was always lower in comparison with that on rice plants. However, the survival rates of nymphs on foxtail grass and rice seedling were not significantly different from each other under laboratory conditions. In the fields, senescence of foxtail grass occurred in the midst of nymphal period of the 1st generation. The survival rate of nymphs on foxtail grass decreased with the increasing in the nymphal density. Abundance of spiders during the 1st generation was higher than that in the early stage of rice plants.     

Population dynamics of the whitebacked planthopper,Sogatella furcifera (Hemiptera: Delphacidae) with special reference to the relationship between its population growth and the growth stage of rice plants

A population census was conducted to describe the effects of the growth stage of rice on the population dynamics ofS. furcifera, in particular, on immigration, seasonal abundance, population growth rate, and wing-form expression. The number of immigrants was highest on rice plants 17 to 30 days after transplanting (DAT), which suggested that immigrants prefer to settle or remain more on rice plants at the tillering stage (approximately 20–30 DAT). Population growth rate from immigration to the 1st generation decreased with an increase in rice plant age. In contrast, population growth rate from the 1st to 2nd generation was not influenced by rice plant age and was negatively density-dependent. The percentage of macropters (flyers) was positively related to the growth stage of rice when rice was in the vegetative or early part of the reproductive stage, and reached 100% at about 10 days before heading (booting stage). Threafter, most adult females molted into macropters regardless of population density. The roles of host plant age and crowding effect on the population dynamics ofS. furcifera are discussed.     

Origin and evolution of endemic plants of the Bonin (Ogasawara) Islands

The Bonin Islands are typical oceanic islands, located at the western part of the North Pacific Ocean and approximately 1,000 km south of mainland Japan. This archipelago consists of about 20 small islands. Although floristic diversity is low due to the small area and limited environmental diversity, the Bonin Islands harbor unique endemic flora as in other well-known oceanic islands. This paper presents a brief summary of the results obtained from recent studies on the endemic flora of the Bonin Islands. The results are reviewed in relation to the four stages of the evolution of endemic flora in the oceanic islands; migration, establishment, enlargement and diversification. The ancestors of the flora originated mostly from tropical and subtropical Southeast Asia or mainland Japan by rare events of long distance dispersal. The proportion of bird-dispersed species is relatively high as for other oceanic islands. Genetic data sets obtained from allozyme variation in some endemic species suggest that migration occurred several million years ago and genetic diversity is correlated with current population size. At the time of establishment, self-compatible plants are expected to have an advantage. However, the percentage of dioecious plants is relatively high. This is partly due to evolutionary changes from hermaphroditic ancestors to dioecy which occurred in two genera in the Bonin Islands. In addition, there are some examples of evolutionary changes from herbaceous ancestors to woody endemics. Adaptive radiation is found in some genera, although the number of congeneric endemic species is less than five. Studies of allozyme variation inPittosporum, Symplocos andCrepidiastrum showed that genetic identity is generally very high between congeneric species in spite of their distinct morphologies. This result suggests that divergence of these species occurred rather recently and distinct morphological differences are based on a limited number of genetic changes.     

A population dynamic model of Batesian mimicry

Summary A population dynamic model of Batesian mimicry, in which populations of both model and mimetic species were considered, was analyzed. The probability of a predator catching prey on each encouter was assumed to depend on the frequency of the mimic. The change in population size of each species was considered to have two components, growth at the intrinsic growth rate and carrying capacity, and reduction by predation. For simplicity in the analyses, three assumptions were made concerning the carrying capacities of each population: (1) with no density effects on the mimic population growth rate; (2) with no density effects on the model species; and (3) with density effects on both species. The first and second cases were solved analytically, whereas the last was, for the most part, investigated numerically. Under assumption (1), two stable equilibria are possible, in which both species either coexist or go to extinction. Under assumption (2), there are also two stable equilibria possible, in which either only the mimic persists or both go to extinction. These results explain the field records of butterflies (Pachliopta aristolochiae and its mimicPapilio polytes) in the Ryukyu Islands, Japan.     

Some beneficial effects of aggregation in young larvae ofPryeria sinica moore (Lepidoptera: Zygaenidae)

Summary The effects of group size on the survival and development of young larvae ofPryeria sinica Moore were investigated by laboratory and field experiments. Under laboratory conditions, about 20% of isolated larvae died of unsuccessful feeding in the first instar, however, larvae survived successfully in aggregations of four or more individuals. In the field, larvae emerge in early spring and wait for new leaves to open before feeding. In this period, the larger the group size of hatchlings the survival rate became higher. The nest-web spun by hatchlings was considered to play an important role in protecting them from desiccation. In the period that larvae began to feed on leaves, more than 36 larvae are necessary to aggregate for the successful establishment of feeding groups. The nest-web played an important role also in the establishment of feeding group. However, the natural group size of the first instar larvae was larger than the minimum group size to spin a sound nest-web in the field experiment. On the other hand, in later stage, larvae in a large group did not have an excess advantage in survival or developmental rate over larvae in a small group. It was found that the experiments on survival and developmental rates could not explain the reason that this species maintain large compact groups in the most part of larval period. The study was partially granted by the Ministry of Education (No. 374205).