Population studies ofBrevicoryne brassicae (L.), its parasites and hyperparasites in England

Summary Studies on populations ofBrevicoryne brassicae (L.), its parasites and hyperparasites were carried out by actual counting in the sprouts field and by sticky and water traps.B. brassicae was found to be attacked by one primary parasite,Diaretiella rapae (McIntosh), which in turn is parasitized byAlloxysta brassicae (Ashm.),Asaphes vulgaris Walker,A. suspensus (Nees),Pachyneuron minutissimum (F?rster) andDendrocerus carpenterii (Curtis). The aphid population in the field was started by immigrant alates which were found flying too early to be synchronized with the sprouts plants. SimilarlyD. rapae was not synchronized with the aphids although many individuals could have been carried into a plot through parasitized immigrant alates, of which less than 30% were found parasitized. Because of high hyperparasitism (especially byA. brassicae)D. rapae was not able to maintain a high rate of parasitism to curb the aphid population growth. The maximum percentage mummies being 27.8%, while the maximum, percentage parasitism being 56.6% recorded only during the early 1974 season (mean=12.9%). The decline of aphid population from September onwards was largely due to the cold weather, Syrphid predation and occasionally fungal attack. The high rate of hyperparasitism byA. brassicae is attributed to its better synchronization withD. rapae. The mean percentage of parasite that emerged from mummies collected during 1973–74 wereD. rapae 31.3%,A. brassicae 64.3%,A. vulgaris andA. suspensus 4.3%,D. carpenterii 0.2% andP. minutissimum 0.1%.     

Age-specific survivorship analysis ofHeliothis spp. Populations on cotton

Summary Population dynamics ofHeliothis virescens (F.) andHeliothis zea (Boddie) (Lepidoptera: Noctuidae) eggs and larvae were studied for two years in a small plot of cotton,Gossypium hirsutum (L.). Due to morphological and ecological similarities, the pooledHeliothis population was considered for most of the analyses. Two generations ofHeliothis eggs and larvae were completed during each year. Stage recruitment was estimated for the eggs and larval instars 2–6, and recruitment variances were estimated by a Monte Carlo method. A modified form of the Weibull distribution was developed and used as a model to characterize survivorship curves for each of the fourHeliothis generations. A Type I survivorship curve (mortality rate increasing with age) was inferred for both Generation 1 (early season) data sets, whereas a Type II survivorship curve (mortality rate constant and thus independent of age) was inferred for both Generation 2 (late season) data sets. The shapes of the survivorship curves for the individualH. virescens andH. zea populations were inferred to be the same as those for the pooled populations. Analysis of the contributions of various factors toHeliothis stage-specific mortality indicated that natural enemies (predators and parasites) and the availability of food for larvae were responsible for between-generation differences in survivorship patterns.     

Competition between laboratory populations of green leafhoppers,Nephotettix spp. (Homoptera: Cicadellidae)

Summary Intra- and interspecific competition between laboratory populations of four green leafhoppers,Nephotettix spp. was studied in the laboratory under three different temperature regimes of 24°C, 27°C and 30°C. For the single-species population of the three tropical species, the equilibrium density increased as the temperature increased. On the other hand, for the temperature speciesN. cincticeps, the highest equilibrium density was at the intermediate temperature and the lowest at high temperature. Interspecific interactions between two tropical (N. virescens vs.N. nigropictus), a tropical and a temperature (N. virescens vs.N. cincticeps) and a rice-feeding and a grass-inhabiting (N. virescens vs.N. malayanus) Nephotettix species were also studied in the laboratory at the three temperature regimes. Temperature differentially affected the outcome of competition between twoNephotettix species. BetweenN. virescens andN. nigropictus, the latter was more successful over the former at low and intermediate temperatures, while the former was more successful at high temperature. BetweenN. virescens andN. cincticeps, the temperate species inhibited the growth of the tropical species at low temperature while the tropical species inhibited the growth of the temperate species at high temperature. At intermediate temperature, the population ofN. virescens persisted at a slightly higher density over the population ofN. cincticeps. Between the rice-feedingN. virescens and the grass-inhabitingN. malayanus, regardless of temperature the population density of the latter was greatly reduced and later became extinct while the population of the former continued its growth. These consequences of competition between twoNephotettix species conformed fairly well to those predicted by theLotka-Volterra model using demographic parameters specified for each species.     

Anopheles culicifacies giles: Adult ecological parameters measured in rural punjab province, pakistan using capture-mark-release-recapture and dissection methods, with comparative observations onAn. stephensi liston andAn. subpictus grassi

Summary Dispersal, immigration and emigration rates, horizontal and vertical survivorship and absolute population size were estimated for micropopulations ofAn. culicifacies, An. stephensi andAn. subpictus at a series of cattle sheds in rural Punjab Province, Pakistan, during November 1979 and May 1980 using capture-mark-release-recapture and dissection methods. Dispersal was temperature-related, with populations more vagile during May. Mean dispersal distance per individual was low for all species. More than 70% of all recaptures were taken at the point of release and the longest detected flight was 1250 meters. Horizontal survivorship was greater during November and was always less than vertical survivorship calculated from dissection agegrading data. Survivorship during the nulliparous period was greater than survivorship throughout total life, indicating the survivorship curve may be slightly sigmoid. Daily population sizes of endemic and immigrating females and males were calculated usingBailey's (1952) modification of the Lincoln Index, with the daily captures adjusted for immigration which was highest in May. Daily additions to the indoor resting population exclusive of immigrants were estimated using the method ofManly andParr (1968). The relationship of the present findings to malaria transmission and genetic control were discussed.     

Application of a marking-and-recapture method for the analysis of larval-adult populations of an insect,Nezara viridula (Hemiptera: Pentatomidae)

Summary Difficulty arises in applying marking-and-recapture methods to insects when the probability of recapture of marked individuals is changed with advancing age, either due to detachment of the mark by moulting (in the case of larvae) or to changes in their survival rate or their behaviour. A modification of the re-recapture method (Leslie et al., 1953) has been devised to analyze the capture-recapture data of the 5th-instar larvae and adults ofNezara viridula L. Estimation of the rate of moulting to the adult stage is made with the aid of additional information on larval survival. Migration rates of the larvae between the two halves of the census field is estimated byIwao's (1963) method. Through these analyses, the dynamic feature of the population during transition from the 5th instar to, adult is revealed. Several problems involved in the application of marking-and-recapture methods to insect populations are discussed. Contribution from the Entomological Laboratory, Kyoto University No. 392.     

Dynamics of age- and size-structured populations in fluctuating environments: Applications of stochastic matrix models to natural populations

Recent developments of the theory of stochastic matrix modeling have made it possible to estimate general properties of age- and size-structured populations in fluctuating environments. However, applications of the theory to natural populations are still few. The empirical studies which have used stochastic matrix models are reviewed here to examine whether predictions made by the theory can be generally found in wild populations. The organisms studied include terrestrial grasses and herbs, a seaweed, a fish, a reptile, a deer and some marine invertebrates. In all the studies, the stochastic population growth rate (ln λ _s ) was no greater than the deterministic population growth rate determined using average vital rates, suggesting that the model based only on average vital rates may overestimate growth rates of populations in fluctuating environments. Factors affecting ln λ _s include the magnitude of variation in vital rates, probability distribution of random environments, fluctuation in different types of vital rates, covariances between vital rates, and autocorrelation between successive environments. However, comprehensive rules were hardly found through the comparisons of the empirical studies. Based on shortcomings of previous studies, I address some important subjects which should be examined in future studies.     

Inconstancy of Taylor'sb: Simulated sampling with different quadrat sizes and spatial distributions

Summary Taylor's power law,s ² =am ^b , provides a precise summary of the relationship between sample variance (s ² ) and sample mean (m) for many organisms. The coefficientb has been interpreted as an index of aggregation, with a characteristic value for a given species in a particular environment, and has been thought to be independent of the sample unit. Simulation studies were conducted that demonstrate that the value ofb may vary with the size of the sample unit in quadrat sampling, and this relationship, in turn, depends on the underlying spatial distribution of the population. For example, simulated populations with hierarchical aggregation on a large scale produced values ofb that increased with the size of the sample unit. In contrast, for a simulated population with randomly distributed clusters of individuals, the value ofb eventually decreased with increasing quadrat size, as sample counts became more uniform. A single value ofTaylor'sb, determined with a particular sample unit, provides neither a fixed index of aggregation nor a complete picture of a species' spatial distribution. Rather, it describes a consistent relationship between sample variance and sample mean over a range of densities, on a spatial scale related to the size of the sample unit. This relationship may reflect, but not uniquely define, density-dependent population and behavioral processes governing the spatial distribution of the organism. Interpretation ofTaylor'sb for a particular organism should be qualified by reference to the sample unit, and comparisons should not be made between cases in which different sample units were used. Whenever possible, a range of sample units should be used to provide information about the pattern of distribution of a population on various spatial scales.     

Comparison of male adult population densities of the oriental and artocarpus fruit flies,Dacus spp. (Diptera: Tephritidae), in two nearby villages in Penang, Malaysia

Summary The populations of native male adult oriental fruit flyDacus dorsalis (Hendel) and artocarpus fruit flyD. umbrosus (F.) in two selected site (BU and SD) were estimated weekly by the capture-recapture technique using live traps baited with methyl eugenol. In BU where many varieties of fruit trees were grown, the estimated population densities ofD. dorsalis were between 980 and 3100 male flies per ha between May and July, 1984. During the same period, in SD where there were fewer number and varieties of fruit trees, the estimated population densities were between 300 and 1000 flies per ha. The estimated population densities ofD. umbrosus over the same period were between 570 and 1290 flies per ha in BU; and between 5 and 95 flies per ha in SD. Of a total 6828 markedD. dorsalis flies released only one fly (released 6 weeks earlier in BU) was caught in a different site.     

Continuous population estimates forDendroctonus frontalis Zimm. (Coleoptera: Scolytidae) occurring in infestations

Summary Infestations ofDendroctonus frontalis Zimm. are often observed to enlarge continuously by the colonization of new hosts in a pattern similar to a forest fire. This pattern of infestation growth presents unique problems in quantitatively estimating populations ofD. frontalis. Beetle populations on each infested tree in an infestation go through five processes: attack, oviposition, reemergence, survivorship, and emergence. These processes, which have been described mathematically in the literature, each take several days for completion. In order to follow the distribution and abundance ofD. frontalis throughout the course of development of a spot, we need a daily estimate of the number of beetles involved in each process on every tree. Since it is not practical to sample each tree daily, we developed a procedure whereby quantitative estimation procedures for within-tree populations were used in combination with the mathematical models for the life processes to produce a daily record of the number of adults successfully attacking trees, the number of eggs oviposited, the number of beetles reemerging, number of beetles surviving within the trees, and the number of beetles emerging. These daily estimates were then summarized for all trees in the spot for the duration of the infestation. The daily record of populations ofD. frontalis, used with information on infestation geometry, were suggested to be of value in describing and elucidating several important facets of population dynamics including dispersal patterns within infestations, between tree beetle loss (mortality), and time lags among the various population processes. The information reported can be used to develop simulation models of population dynamics or to validate existing models. Texas Agric. Experiment Stn. TA No. 14689.     

A model for analyzing insect stage-frequency data when mortality varies with time

Summary A model is developed for the analysis of insect stage-frequency data which may be applied to populations with age-dependent mortality. The analysis of stage-frequency data is divided into two steps. In the first step, the number of different mortality rates and their values are estimated. The second step provides estimates of developmental rates and variances for each developmental stage and in addition provides estimates of the number of recruits to each stage. The model may be used both in analysis and prediction of insect stage frequencies. Hence, in addition to estimating developmental and mortality rates from stage-frequency data, it may also be used as a simulation model for an insect population. The model is applied to two populations ofHemileuca oliviae Cockerell, a lepidopterous pest of New Mexico grasslands. The model identifies, in the two populations, different mortality rates that are related to plant productivity.     

A stochastic computer model for simulating population growth

Summary A model is described for investigating the interactions of age-specific birth and death rates, age distribution and density-governing factors determining the growth form of single-species populations. It employs Monte Carlo techniques to simulate the births and deaths of individuals while density-governing factors are represented by simple algebraic equations relating survival and fecundity to population density. In all respects the model’s behavior agrees with the results of more conventional mathematical approaches, including the logistic model andLotka’s Law, which predicts a relationship betwen age-specific rates, rate of increase and age distribution. Situations involving exponential growth, three different age-independent density functions affecting survival, three affecting fecundity and their nine combinations were tested. The one function meeting the assumptions of the logistic model produced a logistic growth curve embodying the correct values orr _m andK. The others generated sigmoid curves to which arbitrary logistic curves could be fitted with varying success. Because of populational time lags, two of the functions affecting fecundity produced overshoots and damped oscillations during the initial approach to the steady state. The general behavior of age-dependent density functions is briefly explored and a complex example is described that produces population fluctuations by an egg cannibalism mechanism similar to that found in the flour beetleTribolium. The model is free of inherent time lags found in other discrete time models yet these may be easily introduced. Because it manipulates separate individuals, the model may be combined readily with the Monte Carlo simulation models of population genetics to study eco-genetic phenomena.     

Population regulation of the euonymus gall midgeMasakimyia pustulaeYukawa andSunose (Diptera: Cecidomyiidae) by hymenopterous parasitoids

Summary Yearly population fluctuations ofM. pustulae were investigated at 19 sites in Kyushu. In sites where a platygastrid is the only parasitoid of the midge, the percentage parasitism was very low in the incipient stage of the outbreak of the midge populations. After the number of midges reached a peak, the midge populations declined as the percentage parasitism increased, and then the outbreak ceased. On the other hand, in several populations no outbreak was found and the percentage parasitism was constantly at a high level. Therefore, the immediate cause for the outbreak seemed to be a decline of the percentage parasitism. Like the midge, the platygastrid has one generation each year, and its females also emerge in spring to deposit their eggs within host eggs. The decline of the percentage parasitism seemed to be mainly affected by the time lag between emergence periods ofM. pustulae and the platygastrid. In the midge populations parasitized by both the platygastrid and a eulophid (Chrysonotomyia sp.), an extinction of the population was observed, resulting from parasitism by the latter,Chrysonotomyia sp. is polyphagous and multivoltine, and is a late parasitoid, as discussed byAskew (1975). When the density of the midges is very low, the platygastrid may leave the host eggs unparasitized, whileChrysonotomyia sp. may not, because the mature galls are conspicuous.     

A population study ofStenocranus minutus (Fab. ) (Hemiptera: Delphacidae)

Summary The seasonal and annual fluctuations in a population of the delphacid,Stenocranus minutus (Fab.) onDactylis glomerata L. were studied from March 1968 to September 1970. The study involved the comparison of several sampling methods to estimate the egg, nymphal and adult numbers and an investigation into causes of mortality within each stage. A population budget was made for the years 1969 and 1970.     

Selection for divergent emigration rates in laboratory populations ofMusca domestica

Summary High and low emigration rates through a laboratory system were selected for in populations of house flies (Musca domestica L.). Emigration consisted of movement of flies from on plastic box to another by way of a connecting tube. Selection was carried out by first dividing a wild population of flies into two lines and then selecting for movement from the box in one line and against movement from the box in the other line. The selection experiment was performed twice. In both experiments a statistically significant difference between the two strains was obtained in three to four generations of selection. In the second experiment, after 5 genrations the mean number of high emigration rate flies emigrating in 1 hour was 4.37 times the mean number of low emigration rate flies and in 24 hours was 2.81 times the number of low emigration rate flies. The second experiment was terminated after 6 generations, but the first experiment was continued for 35 generations. In this case, divergence ceased in roughly 15 generations. The possible relevance of the findings to laboratory population experiments in which spatial discontinuities are included is discussed. This study was supported by a research grant of the National Science Foundation (Environmental Biology GB 4567) to Prof. DavidPimentel. The author wishes to thank Prof.Pimentel for his aid and encouragement.     

Elimination ofSitotroga cerealella (Olivier) bySitophilus zeamais Motschulsky from mixed laboratory cultures on maize

Summary This paper describes a series of experiments conducted to determine whySitophilus zeamais Mots. andSitotroga cerealella (Oliv.) could not survive together in maize cultures in the laboratory. The effect ofS. zeamais on different developmental stages ofS. cerealella was investigated. The presence of adultS. zeamais slightly affected moth copulation, egg laying and moth eggs in a mixed culture, but large numbers of developing moths inside maize grains were killed by the adult weevil through feeding on the grains. The major cause of elimination ofS. cerealella byS. zeamais from mixed cultures was therefore found to be damage to the immature moths in grain and such moth mortality increased as the developing moths became bigger in the grains. A weevil: grain ratio of approximately 1.4∶1 was found to be the critical weevil density at which the moth disappeared from the mixed cultures.     

Studies on the basic components of the predation ofPhytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae)

Summary The basic components of the predation ofPhytoseiulus persimilis Athias-Henriot feeding upon eggs ofTetranychus urticae Koch were studied in an open system where the predator could disperse freely. The type of the functional response of the predator to the density of its prey was the same as that studied so far in a closed system, i.e.,Holling's Type 2. The search rate of the predator, however, was much lower in comparison with the result from a closed system. The oviposition of the predator per day was only weakly related to prey densities higher than 10 per leaf disc. But the emigration rate was inversely dependent upon the initial prey density up to 60 per leaf disc. The searching behaviour of the predator was influenced by both the web density spun byT. urticae and the density of the prey: the predator searched for its prey intensively only after it had contact with web. Mutual interference was observed in prey consumption, but not in the emigration rate. The emigration rate was largely dependent upon the prey density available per predator.     

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)     

Sibling and non-sibling cannibalism by larvae of a lady beetleHarmonia axyridisPallas (Coleoptera: Coccinellidae) in the field

Summary Sibling (cannibalism among siblings) and non-sibling cannibalism (cannibalism among non-siblings) were studied in a natural population of a lady beetle,Harmonia axyridis Pallas. Of all the eggs laid (n=2269), 24.76% (n=562) were killed by sibling cannibalism and 36.10% (n=819) were killed by non-sibling cannibalism. Sibling cannibalism occurred constantly and intensively in most egg batches throughout the entire oviposition period. On the other hand, non-sibling cannibalism was more intense in the middle and late oviposition periods, and when the egg batches were close to an aphid colony. This may be due to the high density ofH. axyridis larvae relative to aphid density in the middle and late oviposition periods and also the larvae searching intensively near an aphid colony.     

Functional response and searching efficiency inPseudogonatopus flavifemur Esaki and Hash. (Hymenoptera: Dryinidae), a parasite of rice planthoppers

Summary The functional response ofPseudogonatopus flavifemur E & H (Hym., Dryinidae) was investigated by offering hosts (brown planthopper) at densities ranging from 8 to 160 per cage. The response curve was found to be sigmoid, i. e.Holling's (1959) Type III curve. In experiments involving 310 hosts per cage distributed unevenly in 5 densities (10, 20, 40, 80 and 160 per hill), and a different female parasite density each time (viz. 1, 2, 4, 8 or 16 per cage), the behavioral response was described well by the “random predator equation” ofRoyama (1971) andRoger (1972), which is a convex exponential curve. The area of discovery (a) decreased with an increase in female parasite density (P), and the relationship was described by the equation: loga=−1.0099−0.3638 logP. There was an apparent increase in handling time per host as the number of female parasites increased. Superparasitism, a rare phenomenon under natural conditions, was often observed in the laboratory. The potential ofP. flavifemur as a biocontrol agent of the brown planthopper is discussed.