Effects of crowding on life history parameters of four green leafhoppers,Nephotettix spp. (Homoptera: Cicadellidae)

The effects of intraspecific crowding on the different life history parameters of the three tropical and subtropical green leafhoppers, Nephotettix virescensStål ,N. nigropictusDistant and N. malayanusIshihara etKawase , and one temperate species, N. cincticepsUhler , were studied under 25±1°C using the susceptible Japonica rice variety “Nipponbare”. Crowding during the immature stages of the four species prolonged the nymphal developmental period and resulted to smaller adults with reduced fecundity and shorter life span. Similarly, crowding during the adult stage also shortened the life span of both sexes of adults and decreased the fecundity of females of the four species. The effect of crowding during the adult stage was more adverse than the effect of crowding during the immature stages. N. cincticeps seemed more sensitive to crowding even at lower densities whereas the three tropical species were sensitive mostly at higher densities.     

Biochemical genetic studies on wild populations of three species of green leafhoppers,Nephotettix, from peninsular Malaysia

Nine populations of three species ofNephotettix (Insecta: Hemiptera) from Peninsular Malaysia were analysed for nine enzymes comprising 11 loci. Nei's (Genetics 89, 583, 1978) genetic distance,D, betweenN. virescens andN. malayanus was 0.181, that betweenN. virescens andN. nigropictus was 0.283, and that betweenN. malayanus andN. nigropictus was 0.203. The genetic distance betweenN. nigropictus from rice plant and from the weed-grassL. hexandra at Universiti Pertanian Malaysia was 0.004 and their genetic identity was 0.996, thus indicating that this insect species feeds on both host plants. The proportion of polymorphic loci and the observed heterozygosities were higher inN. nigropictus, with a wider range of host plants, than inN. virescens andN. malayanus, restricted to rice andL. hexandra, respectively.     

Comparison of dispersal ability between the green leafhoppers,Nephotettix virescens andN. cincticeps (hemiptera: Cicadellidae) by tethered flight

Dispersal propensity of green leafhoppers was compared between a tropical species, Nephotettix virescens, and a temperate species, N. cincticeps. The flight ability was measured with tethered flight technique under laboratory conditions (25°C, 16L-8D). The pre-flight period was shorter and the flight duration was longer in N. virescens than in N. cincticeps in both sexes. No significant correlations were found between the flight activity and morphometric characters for either of the two species. The results suggested that dispersal propensity of N. virescens is higher than that of N. cincticeps.     

Estimating the survival rate and mean longevity for adults in a field population of the green rice leafhopper,Nephotettix cincticeps Uhler,by the application ofHokyo andKiritani’s method

Application of the female dissection method proposed byHokyo andKiritani (1967) was attempted in both 1968 and 1969 to estimate the daily survival rate and the mean longevity for the adult population of the green rice leafhopper, Nephotettix cincticeps, in a paddy field. The estimated mean longevity for females was far shorter than the physiological longevity of this species, ranging from 4 to 7 days with some variation between different generations. This could explain the remarkable discontinuity among successive generations which proved to form an important feature of the pattern of seasonal population changes of this insect. Between the two years, the estimated survival rate (and hence the mean longevity also) was negatively correlated to the estimated population size of adults that emerged in either of the two successive generations. This apparent density dependence suggests the possibility that the adult survival, including the effect of dispersal, plays some critical role in bringing about the remarkable population stability from year to year shown by the field population of N. cincticeps.     

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 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.     

Competitive interactions among tropical intertidal limpets

The keyhole limpet, Fissurella virescens Sowerby, and the pulmonate limpet, Siphonaria gigas Sowerby, are commonly found in intertidal rocky shores along the Pacific Coast of Costa Rica. They occur from mid-low to high intertidal levels and their abundances are inversely correlated. Fissurella virescens is more abundant in the mid intertidal zone, whereas Siphonaria gigas is more abundant in the high zone. A factorial experiment was conducted to determine the effects of density on survival and weight loss of each species in order to distinguish between the effects of intra- vs. interspecific competition. The experiment was carried out in the habitat of highest density of each species. Intraspecific competition was found to be important in regulating the population density of Fissurella. Intraspecific competition did not regulate population density of Siphonaria but caused a decrease in body weight at high densities. Interspecific competition between both species was weak.     

How to analyze long-term insect population dynamics under climate change: 50-year data of three insect pests in paddy fields

We can precisely predict the future dynamics of populations only if we know the underlying mechanism of population dynamics. Long-term data are important for the elucidation of such mechanisms. In this article we analyze the 50-year dynamics of annual light-trap catches of three insect pest species living in paddy fields in Japan: the rice stem borer, Chilo suppressalis (Walker) (Lepidoptera: Pyralidae); the green rice leafhopper, Nephotettix cincticeps (Uhler) (Hemiptera: Deltocephalidae); and the small brown planthopper, Laodelphax striatellus (Fallén) (Hemiptera: Delphacidae). We separate the long-term dynamics into two components by using locally weighted scatterplot smoothing: (1) the underlying dynamics of populations, and (2) the influence of the past changes in the environment. The former component is analyzed by response surface analysis and vector autoregression to evaluate the nonlinearity of density-dependence and the inter-specific influence of density, respectively. On the basis of these analyses, we perform the state-space model analyses. The state-space model selected by Akaikes information criterion indicates that the observed number of light-trap catches of C. suppressalis and N. cincticeps in summer increases with increasing temperatures in the previous winter. It also indicates that the influence of temperature is not carried over to the next year. We utilize the selected model to predict the impact of global warming on these species, by substituting the temperature predicted by a general circulation model.     

A comparative analysis on the distribution of nymphal populations of some leaf- and planthoppers on rice plant

The frequency distribution of the number of nymphs per hill of rice plant were analyzed for three species of rice leaf- and planthoppers, Nilaparvata lugens, Delphacodes striatella and Nephotettix cincticeps, based on the sampling data obtained during their last generations in the paddy field. For every species concerned, individual distributions were proved to be contagious and to fit well to the negative binomial distribution. Further, it was found that the value of negative binomial parameter k is so stable for same species that a single value of k is applicable for a series of counts with different means, whereas that k differs remarkably among different species: if the reciprocal of the weighted estimate of common k which is an adequate index measuring degree of contagiousness of the distribution, is compared among different species, it is higher in the order of Nilaparvata, Delphacodes and Nephotettix. The ecological and practical implication of constancy and heterogeneity within and among species was discussed respectively as to the value of parameter k.     

Identifying factors determining the altitudinal distribution of the invasive pest leafminers Liriomyza huidobrensis and Liriomyza sativae

Abiotic and biotic factors that change with altitude can influence the distribution of herbivorous insects. We examined factors influencing the distribution of the generalist leafminers Liriomyza sativae Blanchard and Liriomyza huidobrensis Blanchard (Diptera: Agromyzidae), two pests of agricultural crops, in the tropical Dieng mountainous area of Central Java, Indonesia. Liriomyza huidobrensis predominated at altitudes above 700 m a.s.l. and was the only species collected above 1 400 m a.s.l. In contrast, L. sativae predominated below 600 m a.s.l. and was not found above 1 200 m where the average temperature was 20.7 °C. Parasitoid diversity decreased with altitude, but parasitism did not change. The distribution of neither species was affected by parasitoids; L. huidobrensis predominant at high altitudes was preferred by Opius parasitoids common at these altitudes. Intra‐ and interspecific competition was detected in laboratory experiments where larval density was high, but led to coexistence rather than species displacement. No competition was detected in a field experiment when larval density was low. However, L. sativae failed to reproduce at the highest altitude, whereas L. huidobrensis established at all altitudes. Host composition varied with altitude and one host (faba beans) preferred by L. huidobrensis was common at high altitudes. By relating published data on the performance of the leafminer species to altitudinal temperature changes, we were partly successful in predicting the altitude at which the dominant species switched. Temperature plays an overriding influence on the altitudinal distribution of leafminers.     

Interspecific competition and coexistence of the two chrysomelids,Gastrophysa atrocyanea motschulsky andGalerucella vittaticollis baly (Coleoptera: Chrysomelidae), under limited food resource conditions

Field experiments were conducted in order to investigate the mode of exploitation of food resources and the mechanism of coexistence of mixed larval populations of the two chrysomelids,Gastrophysa atrocyanea andGalerucella vittaticollis, under limited food resource conditions. The larval survival rates seemed high enough to assure coexistence when hatchlings of the two species were released in 1∶1 and 1∶3 ratios on a host plant. However, the survival rate became almost nil for both species when a 3∶1 ratio was employed, suggesting asymmetrical interspecific competition. Wasted food consumption was much higher inG. atrocyanea larvae. The population ofG. atrocyanea seemed to be regulated more by intraspecific competition, while on the other hand, the population ofG. vittaticollis was considered to be more likely affected by the interspecific competition withG. atrocyanea, depending on the initial ratio and density of the two species.     

Warming at the population level: Effects on age structure,density, and generation cycles

The impact of climate change on strongly age‐structured populations is poorly understood, despite the central role of temperature in determining developmental rates in ectotherms. Here we examine the effect of warming and its interactions with resource availability on the population dynamics of the pyralid moth Plodia interpunctella, populations of which normally show generation cycles, a consequence of strong and asymmetric age‐related competition. Warming by 3°C above the standard culture temperature led to substantial changes in population density, age structure, and population dynamics. Adult populations were some 50% larger in warmed populations, probably because the reduced fecundity associated with warming leads to reduced larval competition, allowing more larvae to develop to adulthood. Warming also interacted with resource availability to alter population dynamics, with the generation cycles typical of this species breaking down in the 30° populations when standard laboratory diet was provided but not when a reduced nutrient poor diet was used. Warming by 6° led to either rapid extinction or the persistence of populations at low densities for the duration of the experiment. We conclude that even moderate warming can have considerable effects on population structure and dynamics, potentially leading to complete changes in dynamics in some cases. These results are particularly relevant given the large number of economically important species that exhibit generation cycling, in many cases arising from similar mechanisms to those operating in P. interpunctella.     

RNA interference‐mediated knockdown and virus‐induced suppression of Troponin C gene adversely affect the behavior or fitness of the green rice leafhopper,Nephotettix cincticeps

The green rice leafhopper, Nephotettix cincticeps, is a major rice pest in Southeast Asia and Southern China. Novel control strategies must be explored to control the rice pest. Behavior or fitness regulation of insect by modulating the Troponin C (TnC) may be a novel strategy in the comprehensive management of the insect pest. However, characterizations and functions of TnC, especially regarding effect of its RNA interference‐mediated gene knockdown on the behavior or fitness of N. cincticeps remain unknown. Here, we successfully cloned and characterized TnC gene from N. cincticeps (Nc‐TnC). We demonstrated that Nc‐TnC ubiquitously transcribed at all development stages and special tissues in adult insects, with relative higher levels at the adult stage and in the intestinal canal. Microinjection‐ or oral membrane feeding‐based transient knockdown of Nc‐TnC adversely affected the performance or fitness, such as the decreased survival, feeding capacity, weight, and fecundity of N. cincticeps. Furthermore, we revealed that the expression of Nc‐TnC was suppressed by its interaction with rice dwarf virus‐encoded nonstructural protein 10, which ultimately affected detrimentally the corresponding parameters of the performance or fitness of N. cincticeps. In conclusion, our data deepen understanding of Nc‐TnC functions during the development of and viral infection in N. cincticeps. It imply Nc‐TnC may serve as a potential target for N. cincticeps control in future.     

Life table analysis of the green rice leafhopper,Nephotettix virescens (distant) (Hemiptera: Cicadellidae), and efficient vector of rice tungro disease in asynchronous rice fields in Indonesia

Population dynamics of Nephotettix virescens was studied in 17 paddy fields transplanted at intervals of about 1 month in 1988–1990. The adult density was highest either in the immigrant or the 1st generation and sharply decreased to the 2nd generation. The survival rate of the 1st generation was lowest in the transition season when areal population density increased. Key factor analysis revealed that the nymphal and adult mortality of the 1st generation (kn) was the principal source of population fluctuations. No significant correaltion was found between kn and natural enemy density, natural enemy density/healthy egg density, or the precipitation during the nymphal period. On these bases adult emigration was suspected to be the key factor. Areal population build-up of N. virescens in the transition season was considered to occur as a result of increasing immigration to young stages of rice.     

BIOGEOGRAPHIC ANALYSIS OF THE ARMORED PLANKTONIC DINOFLAGELLATE CERATIUM IN THE NORTH ATLANTIC AND ADJACENT SEAS1

The distribution of the dinoflagellate genus Ceratium Schrank (Dinophyceae) in the North Atlantic and adjacent seas was studied by a combination of new observations on a large number of plankton samples collected from the northeastern Atlantic and North Sea, data from cruises off the east coast of North America and Caribbean Sea, and reports in the literature of the past 90 years. Seventy species were recorded, and their distribution was examined by several methods. Distribution maps were plotted for all species, and from these the ranges of temperature tolerance were derived. The 240 sets of data, which took the form of lists of species present in 5° latitude / longitude blocks obtained from the new work and the published material, were analyzed by clustering and ordination multivariate techniques using the programs Twinspan and Decorana. Analysis of the individual species showed that surface water temperature is the most important factor determining distribution and the number of species in a particular area. Warm water and /or low latitudes have many more species than cold waters and/or high latitudes. For example, at 5°N there are on average 23 species per block, whereas at 60° N there are only 8 species. On the basis of this work, the Ceratium species are divided into Group 1, Arctic-temperate species normally only found in water of less than 15°C; Group 2, cosmopolitan species, which are found virtually everywhere and are the species most likely to form blooms or “brown water”; Group 3, intermediate species, which extend into neither the coldest nor the warmest water; Group 4, temperate-tropical species, which have a lower temperature boundary of 5°–12° C; Group 5, warm-temperate-tropical species with a lower temperature boundary of 14°–15°C; and Group 6, tropical species, which are rarely found in water of less than 20° C. Analysis of the sample sites also confirmed the predominant influence of temperature, and the Atlantic Ocean was divided into four biogeographical zones of which the boundaries follow isotherms of surface water temperature. Zone 1 consists of the Arctic and subarctic area, with the southern boundary closely following the 10° mean annual temperature (MAT) line. Zone 2 is an intermediate or cold-temperate zone, of which the southern boundary follows the winter 10° C MAT isotherm or the similarly placed summer 15° isotherm. Zone 3 is the warm-temperate or subtropical zone, which is very broad. The southern boundary closely follows the 25°C summer isotherm. Zone 4 is the tropical zone, where water temperature is never likely to be much less than 23°C. These findings are discussed in relation to experimental work and environmental observations. We suggest that the genus Ceratium provides an excellent tool for defining ocean currents and temperature changes and may become of value in studies of global change.     

Zoophytophagy in the plantbug Nesidiocoris tenuis

1 The zoophytophagy of Nesidiocoris tenuis (Reuter) (Heteroptera: Miridae) was characterized in relation to prey availability and environmental factors by: (i) monitoring its population dynamics in tomato greenhouses; (ii) analysis of the influence of N. tenuis and whitefly density, temperature and humidity on the intensity of N. tenuis plant feeding; and (iii) laboratory assays under controlled conditions to determine the intensity of plant feeding in relation to prey availability, temperature and humidity. 2 A negative relationship was found between plant feeding and predated whiteflies in tomato greenhouses. Plant feeding was directly related to N. tenuis density and temperature and inversely related to whitefly density. The significance of prey availability and temperature was corroborated in laboratory assays. The intensification of plant feeding at low prey density indicates switching from zoophagy to phytophagy as prey become scarce. 3 Nesidiocoris tenuis showed a typical predator dynamic in relation to variance in prey density. Populations increased after whitefly outbreaks and decreased after whitefly had been depleted. The rapid decrease of N. tenuis populations after whitefly decreased, however, suggests that plants are a poorer nutrient source than whitefly for this species.     

A comparison of myocardial β-adrenoreceptor density and ligand binding affinity among selected teleost fishes

This study quantified the cell surface β-adrenoreceptor density and ligand binding affinity in the ventricular tissue of seven teleost species; skipjack tuna (Katsowonus pelamis), yellowfin tuna (Thunnus albacares), Pacific mackerel (Scomber japonicus), mahimahi (dolphin fish; Coryphaena hippurus), sockeye salmon (Oncorhynchus nerka), rainbow trout (Oncorhynchus mykiss) and an Antarctic nototheniid (Trematomus bernacchii). β-Adrenoreceptor density varied by almost fourfold among these species, being highest for the athletic fish: sockeye salmon among the salmonids and skipjack tuna among the scombrids. β-Adrenoreceptor density was lowest for the Antarctic icefish. β-Adrenoreceptor binding affinity varied by almost threefold. We conclude that there is a significant species-specific variability in myocardial β-adrenoreceptor density and binding affinity and these interspecific differences cannot be attributed to temperature even though intraspecifically cold temperature can stimulate an increase in myocardial β-adrenoreceptor density. Instead, we suggest that interspecifically myocardial β-adrenoreceptor density is highest in fish that inhabit tropical water. Accepted: 12 July 2000     

The relevance of life-history traits in the establishment of the invader <Emphasis Type="Italic">Eudiaptomus gracilis</Emphasis> and the extinction of <Emphasis Type="Italic">Eudiaptomus padanus</Emphasis> in Lake Candia (Northern Italy): evidence for competitive exclusion?

The niches invaded by exotic species are generally not entirely vacant, and one possible factor affecting the success of the invader to establish a permanent large population may be the ability of the former to outcompete native species. Eudiaptomus gracilis, which was not present in Italy before the 1980s, is becoming established in an increasing number of Northern Italian lakes, ultimately replacing the endemic E. padanus. Coexistence of the two species in Lake Candia lasted only 7 months, suggesting that species replacement was determined either by environmental changes or by strong competition. To assess whether the potential for interspecific competition existed and to identify species' traits which could explain the competitive superiority of E. gracilis, we examined field seasonal patterns, reproductive parameters and body size of the two species. E. padanus abundance was probably reduced by predation and parasitism, which favoured the displacement of the species shortly after invasion by E. gracilis. The temporarily underexploited niche provided an opportunity for the successful establishment of the invader. The reproductive patterns of the two species were found to be similar in most of their features, with the exception of a markedly larger clutch size and a smaller egg volume in the resident species. By contrast, the invader showed a higher adult:egg ratio and a lower death rate. Thus, despite the greater fecundity of E. padanus, the competitive success of the invader might be attributable to interspecific differences in developmental rates and/or juvenile mortality.     

Competition for a limited space in kleptoparasitic Argyrodes spiders revealed by field experiments

Most kleptoparasitic Argyrodes spiders rely exclusively on host spider webs for obtaining their food. Because their densities occasionally reach high levels within a restricted area, competitive interactions may be important for determining the density of these unique spiders. Here I used two Argyrodes species commonly found on webs of the large orb-web spider Nephila clavata to clarify whether inter- and intraspecific competition influences abundance and within-web distribution by using observational data and field experiment. Removing Argyrodes flavescens from the host webs induced a remarkably high immigration of that species while density on control webs remained almost at the same level, which is evidence for strong intraspecific competition. Larger individuals of A. flavescens were located more frequently at the capture area of the host webs where it is easy to access prey ignored by the host spider, and spiders immigrating into webs from which that species had been removed were smaller in body size, suggesting interference competition for space among conspecific kleptoparasites. Argyrodes bonadea increased in number on webs from which A. flavescens had been removed, and the increase was correlated with the number of A. flavescens removed. This finding is evidence for interspecific competition that is rarely reported in spiders. A multiple regression model including numbers of a conspecific parasite as well as web and body sizes of the host spider could not detect competitive interactions between species, suggesting the importance of experimental approaches. Received: May 22, 2000 / Accepted: December 1, 2000     

Temperature modulates the effects of predation and competition on mosquito larvae

1. Predation risk affects interspecific competition by decreasing foraging activity and relative competitive ability. Predation risk is determined by predators' prey choice and prey responses, both of which can be influenced by temperature. Temperature is especially important for larval prey and can result in a trade‐off between predator‐induced decreases in foraging activity and growth. Interspecific competition must also be examined in relation to intraspecific density‐dependent competition; weaker interspecific competition leads to coexistence of competitors. 2. This study explored how temperature (15 and 25 °C) could affect a focal species, larvae of the mosquito Culex quinquefasciatus, by examining prey choice in a shared predator (mosquitofish; Gambusia holbrooki) and the effects of predation risk on interspecific competition with Limnodynastes peronii tadpoles. Intraspecific density‐dependent competition in C. quinquefasciatus at these temperatures was also examined. 3. At 25 °C, G. holbrooki consumption of both C. quinquefasciatus and L. peronii increased; however, the effects of interspecific competition on mosquito survival did not decrease with L. peronii exposure to predation risk. The relationship between intraspecific density‐dependent competition and interspecific competition was temperature‐dependent, with competitive dominance of L. peronii at 25 °C. Male and female mosquitoes had different temperature‐dependent responses, indicating sex‐specific intrinsic responses to starvation and differential selection pressures. At 25 °C, females were susceptible to interspecific competition by L. peronii, while males were susceptible to intraspecific competition. 4. The use of competitors as biological controls has implications for mosquito disease transmission, and these results suggest that control effectiveness may be modified by climate change.