Population dynamics of the green leafhopper,Nephotettix virescens Distant (Hemiptera: Cicadellidae) in synchronized and staggered transplanting areas of paddy fileds in Indonesia

The population dynamics of Nephotettix virescens, a vector of rice tungro virus disease was investigated in a synchronized transplanting area at Jatisari (1984–1986), West Java and in a staggered transplanting area at Sidan (1986–1988), Bali, Indonesia. The FARMCOP suction sampler was employed for population censuses of N. virescens and its natural enemies. The population growth pattern was affected by transplanting pattern: In the staggered transplanting area, the population density increased from the immigrant generation to the first generation, and sharply decrease thereafter, while in the synchronized transplanting area the population density often reached the highest peak in the second generation. The degree of contageousness in the spatial distribution of N. virescens was negatively correlated with population density of the immigrant generation.     

A simulation study of population interaction between the greenhouse whitefly,Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae) and the parasitoidEncarsia formosa Gahan (Hymenoptera: Aphelinidae) II. Simulation analysis of population dynamics and strategy of biological control

Simulation studies were performed to analyze factors affecting the population dynamics of the system with the greenhouse whitefly (Trialeurodes vaporariorumWestwood ) and the parasitoid Encarsia formosaGahan and to develop strategies for the introduction of E. formosa. The reduction of parasitization efficiency with an increase in parasitoid density promotes the stability of the system, which coincides with the prediction from current theory. The stability of the system is also shown to be promoted by the effect of host feeding. The population levels of the system are remarkably suppressed with an increase in searching efficiency and a decrease in host oviposition. The control effect of the parasitoids is enhanced when the number of parasitoids is divided among many introductions. An optimal time, an optimal density ratio of parasitoids to hosts and optimal densities of hosts and parasitoids exist in the introduction programme of parasitoids.     

Dynamics ofSitotroga cerealella (olivier) (lepidoptera: gelechiidae) andSitophilus zeamais motschulsky (Coleoptera: Curculionidae) populations in a small bulk of stored corn

The dynamics of Angoumois grain moth, Sitotroga cerealella (Olivier ), and maize weevil, Sitophilus zeamaisMotschulsky , populations breeding in a small bulk (initially 5.36 t) of shelled corn were studied over an 8-year period by monthly sampling. The weevil population showed wide fluctuations in density superimposed on a general decline with time. The moth population showed no upward or downward trend for the first 60 months, although it fluctuated widely. Following a decline that occurred between 56 and 60 months, the moth population fluctuated within a much narrower range, and there was a general decrease in density with time. The decline of the weevil population paralleled deterioration of the corn as did that of the moth population after ca 60 months, and the decline of both species probably resulted from increasing scarcity of suitable breeding sites. Both populations exhibited seasonal variation in density with minima in late summer and early fall, following periods of adversely high temperatures in the storage shed. The populations increased during the fall, leveled off or declined slightly during the winter months, and then increased to maximum levels in late spring or early summer. It thus appears that high temperatures had a greater adverse effect on the populations than low temperatures. The grain moth and the maize weevil both tended to be randomly dispersed at low population levels and moderately aggregated at intermediate and high levels, although the degree of aggregation was not correlated with population density when low population levels were considered separately, and the maize weevil showed a greater tendency for aggregation than did the grain moth. Analysis of individual samples at fixed points in time showed a conspicuous bias for negative correlation between numbers of the two species within sampling quadrats, suggesting a tendency for the two species to segregate within the grain mass. This process could have resulted from behavioral differences or from the destruction of one species by the other. Competitive displacement of the grain moth by the maize weevil has been demonstrated in laboratory experiments but has rarely been observed under natural conditions, and in our study the two species coexisted for 8 years in a relatively small grain bulk.     

Predation ofApochthonius minimus (Pseudoscorpionida: Chthoniidae) onFolsomia Candida (Collembola: Isotomidae) II. Effects of predation on prey populations

This study utilized the preference the pseudoscorpion, A. minimus, displays for certain sizes of the springtail, F. candida, in order to assess the population consequences of this selective predation. Although predation drove half of the treated collembolan populations to extinction, the populations which survived were not reduced in size and were in fact larger than most of the control populations (Figs. 1 and 2). During the first two weeks of the experiment, the predators apparently fed mainly on the original adults and the larger individuals of the second generation, thus increasing available space and improving the prey's chances for successful reproduction. The resulting demographic shift accounts for the significant decrease in mean length that occurred in the experimental populations without any accompanying significant change in their total numbers. Some effects of density, food, and age-structure on the fecundity of F. candida, acting in concert with the effects of size-selective predation, may lead to a variety of population consequences. In some circumstances, predation may cause increased production with decreased mean size, and no drastic change in standing crop. In different circumstances, a local population may become extinct. These results should further our understanding of the sporadic record of establishment and growth displayed byspringtail populations in the small, moist litter patches where they tend to aggregate.     

Acute and chronic toxicity of the pesticide methyl parathion to the rotifer Brachionus angularis (Rotifera) at different algal (Chlorella vulgaris) food densities

Methyl parathion is a commonly used insecticide in Mexico to eradicate insect pests. We evaluated the effects of this insecticide on rotifer B. angularis using both acute and chronic toxicity tests. Median lethal concentration (LC₅₀) of methyl parathion for B. angularis for a 24-h bioassay in the presence and absence of an algal diet was derived. Elevated LC₅₀ due to the survival of a greater number of test individuals in the presence of food was observed. Regardless of the toxicant concentration, population growth curves of the animals maintained at the low food level (0.75×10⁶ cells ml^-1) had a longer lag phase than those at the high food level (1.5×10⁶ cells ml^-1). Regardless of food level, an increase in the toxicant concentration in the medium resulted in decreased population growth. The lowest peak population density (50 ind. ml^-1) was observed at the highest toxicant concentration and the lower food level. The highest population density (200 ind. ml^-1) was observed in the controls at high food level. The rates of population increase per day (r) in the controls were higher (from 0.14 to 0.37 depending on the food level). Irrespective of food level, there was a decrease in the r values with increasing pesticide concentration in the medium. In order to detect the effect of population density on the growth rates in relation to the toxicant stress, we plotted the daily growth rate against initial density for the entire duration of the experiment. We observed the existence of a significantly inverse relation at all treatments except at the low food level and high toxicant concentrations (0.625 and 1.25 mg l^-1). We discuss the role of algae in the toxicity of methyl parathion to zooplankton.     

The role of sediment type in growth and fecundity of mud snails (Hydrobiidae)

Summary We test the hypothesis that body size and population density of the deposit-feeding gastropod, Hydrobia truncata, are greater in muddy than in sandy habitats as a result of faster growth on fine- compared to coarse-grained sediments. We refute this hypothesis using a combination of field measurements and laboratory experiments. Three out of three populations tested had higher maximal growth rates and two of three populations approached their asymptotic size more quickly on sand than on silt-clay fractions of natural sediment. Growth decreased with increasing snail density and was as high or higher on sand as on silt-clay at all densities. Two populations were more fecund on sand than on silt-clay, and fecundity of the third population was not affected by sediment type. We show that the smaller body sizes observed in snails from the sandiest habitat result from late recruitment of these snails, relative to the other populations.     

A population model for Dermanyssus gallinae (Acari: Dermanyssidae)

A model based on a time-varying distributed delay with attrition was developed for simulating the population dynamics of the chicken mite Dermanyssus gallinae (DeGeer, 1778). The model was parametrised according to cohort life table studies conducted under constant temperature conditions and was validated with two independent data set obtained in an experimental poultry house. The predicted mite densities and the stage-structure of the population corresponded to the values observed during experimental periods of 32, and 11 weeks, respectively. Temperature and a poultry house-specific density effect were determined to control the population development.     

Population dynamics of Ischnura E. elegans (Vander Linden) (Insecta: Odonata) with special reference to morphological colour changes,female polymorphism,multiannual cycles and their influence on behaviour

Laboratory and field experiments on adult I. elegans provide correction techniques for the estimation of population parameters based only on capture-recapture data. Thus it is demonstrated that male and female longevities are identical and that their sex ratio is 0.5 (= Male fraction). Longevity, measured at the water, erroneously appears to differ between sexes, and sex ratio is also biased. This bias is a function of population density, which causes a different distribution of males and females, matures and immatures, andro- and heteromorphic females, and is regulated by aggressive, territorial and mating behaviour of individuals. The female maturation period exceeds that of the male. Female polymorphism is an adaptation to population density, high density favouring andromorphs. This polymorphism is determined by single allelic autosomal inheritance with a sex-linked expression. In crowded populations, visual interactions between individuals cause their juvenile hormone titer to rise. This shortens maturation time, the period of morphological colour changes, and life span. This mechanism counteracts crowding, and synchronises maturation and development of sexual behaviour. In the course of their life, individuals undergo a number of colour changes. The development of the colours after eclosion coincides with spermato- and oogenesis. The morphological colour change at sexual maturity is due to neutralisation of waste products of the protein metabolism. The colour changes at old age are partly due to dehydration.     

Density and population estimate of gibbons (Hylobates albibarbis) in the Sabangau catchment, Central Kalimantan, Indonesia

We demonstrate that although auditory sampling is a useful tool, this method alone will not provide a truly accurate indication of population size, density and distribution of gibbons in an area. If auditory sampling alone is employed, we show that data collection must take place over a sufficient period to account for variation in calling patterns across seasons. The population of Hylobates albibarbis in the Sabangau catchment, Central Kalimantan, Indonesia, was surveyed from July to December 2005 using methods established previously. In addition, auditory sampling was complemented by detailed behavioural data on six habituated groups within the study area. Here we compare results from this study to those of a 1-month study conducted in 2004. The total population of the Sabangau catchment is estimated to be about in the tens of thousands, though numbers, distribution and density for the different forest subtypes vary considerably. We propose that future density surveys of gibbons must include data from all forest subtypes where gibbons are found and that extrapolating from one forest subtype is likely to yield inaccurate density and population estimates. We also propose that auditory census be carried out by using at least three listening posts (LP) in order to increase the area sampled and the chances of hearing groups. Our results suggest that the Sabangau catchment contains one of the largest remaining contiguous populations of Bornean agile gibbon.     

Seasonality in neotropical populations of Plutella xylostella (Lepidoptera): resource availability and migration

The present study aimed to verify (1) whether seasonal increases in neotropical populations of Plutella xylostella are directly provoked by regular influxes of migrants, and (2) whether temporal variation in food availability is the ecological process behind such predictable events. Over 3 years, plants that P. xylostella prefers were cultivated and irrigated in order to provide a continuous and abundant supply of food. Nevertheless, seasonal oscillations in the population of the herbivore still occurred. The hypothesis of seasonal availability of host plants could not explain the population pattern. In April, when the insect was practically extinct from the area, an artificial infestation (immigration) with 10,000 pupae established a precocious population. Therefore, the start of the natural cycles of population growth, during July–August, seems to be due to external factors, rather than an improvement in local conditions for resident individuals. In the beginning of the natural cycles, the increase in the density of adults significantly preceded the increase in immatures. Plutella xylostella does not diapause, and therefore immigration is the proximate cause of the seasonal population increases. Hypotheses about local factors are suggested to explain the decreasing phase of the predictable population cycles.     

Function of Prolonged Copulation in Nysius huttoni White (Heteroptera: Lygaeidae) Under Male-Biased Sex Ratio and High Population Density

Prolonged copulation as a response to sex ratio and population density has been demonstrated for a number of species but the functions behind it remain controversial. It is difficult to determine the function of prolonged copulations without examining the mechanisms of sperm transfer and storage, and fertilization success–copulation duration relationships. Here, we report our work on a polygamous seed bug, Nysius huttoni White (Heteroptera: Lygaeidae), where we manipulated the population density and sex ratio for a series of mating trials, quantified the relationship between copulation duration and ejaculate amount as well as reproductive outputs, and examined the sperm transfer and storage mechanisms. Our results suggest that males prolong copulations to increase their fertilization success relative to their rivals in response to high sperm competition intensity under male-biased sex ratio and high population density. The mechanism behind the positive correlation between copulation duration and fertilization success may be attributed to the characteristic structures of aedeagus and spermatheca in this insect, which allow direct transfer of sperm to a storage site of fixed size during copulation.     

Variation in predation pressure as a mechanism underlying differences in numerical abundance between populations of the poeciliid fish Heterandria formosa

We explored whether a variation in predation and habitat complexity between conspecific populations can drive qualitatively different numerical dynamics in those populations. We considered two disjunct populations of the least killifish, Heterandria formosa, that exhibit long-term differences in density, top fish predator species, and dominant aquatic vegetation. Monthly censuses over a 3-year period found that in the higher density population, changes in H. formosa density exhibited a strong negative autocorrelation structure: increases (decreases) at one census tended to be followed by decreases (increases) at the next one. However, no such correlation was present in the lower density population. Monthly census data also revealed that predators, especially Lepomis sp., were considerably more abundant at the site with lower H. formosa densities. Experimental studies showed that the predation by Lepomis gulosus occurred at a much higher rate than predation by two other fish and two dragonfly species, although L. gulosus and L. punctatus had similar predation rates when the amount of vegetative cover was high. The most effective predator, L. gulosus, did not discriminate among life stages (males, females, and juveniles) of H. formosa. Increased predation rates by L. gulosus could keep H. formosa low in one population, thereby eliminating strong negative density-dependent regulation. In support of this, changes in H. formosa density were positively correlated with changes in vegetative cover for the population with a history of lower density, but not for the population with a history of higher density. Our results are consistent with the hypothesis that the observed differences among natural populations in numerical abundance and dynamics are caused in part by the differences in habitat complexity and the predator community.     

Status and structure of the griffon vulture (Gyps fulvus) population in Crete

Griffon vulture (Gyps fulvus) population surveys were conducted during 1996–2002 in the island of Crete (Greece) to document population status and structure. Fieldwork was carried out during the breeding period when birds could be monitored in their colonies. Total population size was estimated at 379 individuals (range = 341–417) with adult birds comprising 63%. The breeding population was estimated at 141 pairs, which were distributed on an average in 23 colonies per year (range = 16–30) while the mean number of breeding pairs that laid eggs was 98 (range= 64–126). Crete thus supports the largest insular population of the species in the world and hosts 70–80% of the breeding population of the species in Greece. Population density was estimated at 6.9 individuals/100 km², 2.6 breeding pairs/100 km² and 1.8 nesting pairs/100 km². The average home range of an occupied colony (i.e., breeding group) was estimated at ca. 204 km² producing a theoretical foraging range of 8 km radius around the breeding cliff. No trends in the total number of individuals and breeding pairs appeared to exist, although significant differences in population size of individual colonies occurred between the years. The majority of the population was concentrated in small-sized colonies, which showed a low occupancy rate. The number of abandoned sites and the colonization of new ones could represent a shift of breeding pairs to alternative colonies provoked by local food abundance and conspesific attraction.     

Predicting extinctions: interspecific competition,predation and population variability in experimental Daphnia populations

We examine how interspecific competition and two types of size-selective predation affect population density, variability and persistence in laboratory cultures of two species of Daphnia, D. magna and D. longispina. When both species were analysed together, and for D. longispina alone, there were weak negative relationships between mean population density and population variability. Interspecific competition resulted in lower population densities and higher population variability. Extinct populations had lower densities and were also more variable than persisting ones. There was still an effect of population variability on extinction probability after the effect of density on population variability had been accounted for. Hence, the effects of population density and variability on population persistence were partly independent of each other. The effects of size-selective predation on population persistence were more species-specific and not directly related to density or variability. Since the effects of species interactions on persistence were large, we suggest that it is likely that population vulnerability analyses not incorporating effects of interspecific interactions are often misleading.     

Population density ofMonochamus alternatus adults (Coleoptera: Cerambycidae) and incidence of pine wilt disease caused byBursaphelenchus xylophilus (Nematoda: Aphelenchoididae)

Summary The seasonal occurrence ofMonochamus alternatus and newly weakened trees were investigated in aPinus thunbergii stand for 4 years. Adult beetles were present between June and September with a peak in their population occurring in early July followed by a decline then a period of about one month being in a steady number. The average number ofBursaphelenchus xylophilus (Nematoda), which is the causal agent of pine wilt disease, within beetles decreased as the season advanced. Pine trees newly weakened byB. xylophilus appeared between June and October, especially from August to October. The proportion of weakened or killed trees was directly proportional to the average beetle density per tree from June to August.     

Population structure and survival in a solitary wasp (Microbembex cubana: Hymenoptera,Sphecidae, Nyssoninae)

Summary This paper documents population structure in a solitary wasp, Microbembex cubana (Hymenoptera, Sphecidae, Nyssoninae), on a small (15 km²) Bahamian island. A relatively isolated portion of this population was studied April–June 1985. The population comprised small aggregations of territorial males and nesting females. Individuals of both sexes were with rare exceptions faithful to a home aggregation during their one- to two-month adult lifespans, conducting all reproductive activities there. Individuals from different aggregations, however, mixed daily during these activities: feeding on nectar, hunting for provisions and retiring to clustered sleeping burrows. Significant variation occurred among the nine breeding aggregations in size, density, sex-ratio (which was on average 2:1 in favor of males) and survival (which was 0.93–0.99 per individual/per day and which was not higher for females than for males). Aggregations retained the same characteristics for longer than the life expectancies of individuals in them.Factors affecting reproductive success and survivorship in M. cubana are complex: they are apparently only partially overlapping between males and females and subject to spatial variation. Patterns in the data suggest several hypotheses about how behavior, morphology and habitat interact to shape population processes. I propose that aggregations arise and are characterized by considerable behavioral inertia because individual M. cubana use conspecifics as sources of information on resource quality. Because M. cubana occurs in secondary habitats, individuals retain flexibility in responding to better opportunities for reproduction, but this population exhibits more viscosity than reported for other ground-nesting solitary Hymenoptera.     

Population dynamics of Panonychus osmanthi (Acari: Tetranychidae) on two Osmanthus species

Panonychus osmanthi is a non-diapausing species of spider mite that superficially resembles P. citri. It infests Osmanthus species, which are evergreen roadside and garden trees. The population dynamics of P. osmanthi were studied on Osmanthus aurantiacus and O.×fortunei during a three-year period. Seasonal changes in P. osmanthi populations were fundamentally the same in each year, although their density differed greatly from year to year. TheP. osmanthi population was bimodal, with one peak in spring (May–June) and another in winter (November–January). Populations abruptly declined after the spring peak. Predators showed a delayed density-dependent response to changes in spider mites from spring to summer, whereas in autumn and winter, predators were few because they had entered diapause. To determine the effect of predators on the rapid decline of spider mites just after the spring peak, the predators were removed by treating the trees with a synthetic pyrethroid. As a result, spider mite density did not decline after the spring peak and remained at a high level during the June-August period when spider mite density is usually very low. This suggests that predators play an important role in the drastic decline of P. osmanthi density just after the spring peak. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variation in demographic and fine-scale genetic structure with population-history stage of <Emphasis Type="Italic">Hemerocallis taeanensis</Emphasis> (Liliaceae) across the landscape

Fine-scale genetic structure (FSGS) in plant populations is expected to be influenced by variation in demographic processes across space and over time. I chose Hemerocallis taeanensis (Liliaceae), a perennial herb with a rapid population turnover, to quantify how demographic structure and FSGS change with a population’s history (i.e., density). Nonaccumulative O-ring statistic and spatial autocorrelation analysis (kinship coefficient, F _ij ) were used to quantify spatial patterns of individuals and FSGS in four populations belonging to two population stages (expansion and maturation) in west-central Korea. The O-ring function revealed that significant aggregation of individuals occurs at short spatial scales during the earlier stage of population expansion, which reflects restricted seed dispersal around maternal individuals. However, this pattern disappears as the population density increases during population maturation, probably due to a high population density. Significant evidence of FSGS was found in two populations at the stage of population expansion (Sp, a statistic which describes the rate of decrease of pairwise kinship with distance, was 0.018 and 0.029). The results show that most seeds fall around maternal plants when initially established colonists proliferate at suitable microhabitats. In contrast to this, much lower Sp values (−0.003 and 0.004) were estimated for two populations at the stage of population maturation, which may result from the overlapping of seed shadows due to high adult density. All of these results demonstrate considerable variation in within-population demographic and genetic structures of H. taeanensis with respect to population temporal stage across the landscape.     

Biology ofhyphantria cunea drury (lepidoptera : Arctiidae) in Japan. IX. population dynamics

Fluctuations in population density of Hyphantria cunea in Japan are characterized by a gradation-like pattern. Analysis of the life table data taken from two stations during eight successive generations showed that (1) mortality during egg and early larval stages was density-independent, (2) mortality during later larval stages was inversely density-dependent, and (3) mortality during prepupal and pupal stages was density-independent. Thus the overall mortality process from egg to adults eclosion was inversely density-dependent. The inverse density-dependence in mortality process during later larval instars was mainly attributed to the ‘escape’ (VOÛTE, 1946) of H. cunea populations from the predation pressure of polyphagous predators such as birds and Politses wasps. This inverse density-dependence was considered to be a cause of gradation-like fluctuation. Field collection of egg-masses showed that the mean number of eggs per egg-mass, which was believed to be a good representation of mean fecundity, varied from 425 to 1050 during 4 years. Density-dependent reduction in the mean number of eggs per egg-mass was demonstrated, and this reduction was a factor regulating the population density. Assuming fixed sex ratio and survival rate of adults, a preliminary population model was constructed. The number of eggs laid in the survey station could be predicted well by the model based on the number of eggs laid in the previous generation, in 9 out of 13 cases. An attempt to apply a model of the same type to mimic the fluctuation of abundance (peak number of larval colonies per tree) on road-side trees suggested another density-dependent process, that is, insecticide application by man. Discussion was also presented on the causes responsible for the turn of population trend from decreasing to increasing in the 1st generation of 1968.     

The population dynamics of the large pine aphid,Cinara pinea (Mordv.)

The large pine aphid, Cinara pinea lives exclusively on Pinus species, where it feeds on the foliated shoots of the current and previous year. The paper describes the development of a computer model designed to simulate the aphid's population dynamics on saplings in the controlled environment of the laboratory, i.e. in the absence of natural enemies. The model was able to account for about 80% of the variation in aphid numbers within and between trees over a three month period. Sensitivity analysis revealed that the number of pine aphids is limited primarily by nymphal emigration, the operation of which is sensitive both to density and to plant quality as reflected in aphid growth rates. Of secondary importance are changes in reproduction acting through increased reproductive delay, again a result of altered growth rates and adult size. Development, too, has an important secondary influence. Contrary to expectation and conventional belief, however, alate production proved to be of negligible importance, either in limiting or regulating population numbers. Alatae are produced in too few numbers and for too short a period to significantly alter the pattern of population change.