Density-dependent regulation of Aedes cantans (Diptera: Culicidae) in natural and artificial populations

Abstract.

• 1 Mortality and size variation (siphon length) in Aedes cantans larvae were examined in natural populations in northern England in 1989 and 1990.
• 2 Under crowded conditions, density-dependent competition led to reduction in the size of both larvae and adults and increased larval mortality.
• 3 Larvae were also maintained in cages in the field at different densities. Results paralleled those for the natural populations in the ponds; larvae maintained at high densities showed increased mortality and reduced size.
• 4 Possible density-dependent factors leading to mortality and size reduction include cannibalism and contact inhibition leading to food shortages.
• 5 The main density-independent factor contributing to larval mortality was habitat desiccation.

     

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.     

Regulatory processes and population cyclicity in laboratory populations ofAnagasta Kühniella (Zeller) (Lepidoptera: Phycitidae). III. The development of population models

1. Life table data for interactions between Anagasta kühniella and its ichneumon parasite Venturia canescens in two room ecosystems (A & B) have been analyzed in an attempt to explain and model each room situation. The life table data have been presented in the form of a graphical key-factor analysis, and have been further analyzed by an investigation of the density relationships between the different mortalities and the Angasta densities upon which the mortalities act.
2. In room A (1.2 gm food per container), the parasites were present throughout the interaction. Egg and early larval mortality (k₁) appeared to be directly density-dependent and was the sole stabilizing influence when introduced into the model for room A. The area of discovery of the parasite was relatively constant and its mean value was used to calculate parasitism (k₃) in the model. All other mortalities were density-independent and treated as being constant at their mean values. The model predicts a series of oscillations of decreasing amplitude which are somewhat similar to those observed in the Anagasta population during the early stages of the interaction. The observed mean densities of host and parasite were very close to those predicted.
3. In room B, the parasites were absent for the first 8 generations (1- 2gm food per container). Model B₁ covers this period and includes a direct density-dependent component describing changes in k₁, the remaining mortalities being constant. The observed mean densities approximate to the calculated densities. The parasites were present from the ninth generation and after the eleventh generation the food per container was increased to 7.2 gm. Model B₂ covers the period in room B from generation 11. The most important component of k₁ after the parasites were established is a delayed density-dependent one which appeared to be due to wounding of very small larvae by the probing activities of the parasites. Since the changes in k₁ could not be suitably predicted, the observed values were used in model B₂. This delayed component was not detected in room A due to the relatively small range of parasite densities in room A compared with the 600-fold change in densities in room B. The calculated area of discovery for the parasite population in each generation was found to vary inversely with searching parasite density, and this ‘interference relationship’ was used in the submodel for parasitism. Again, this relationship was not detected in room A due to the much smaller range of parasite densities there. Model B₂ gives oscillations in host and parasite populations arising from parasitism being a delayed density-dependent mortality. The correspondence with the observed oscillations is partly due to the actual k₁-values being used and partly because the submodel for parasitism adequately describes the observed changes in k₃. The tendency for these oscillations to decrease in amplitude is due to both the damping effect of parasite interference and the direct density-dependent component of k₁.

     

Population extinction and quasi-stationary behavior in stochastic density-dependent structured models

Density-independent and density-dependent, stochastic and deterministic, discrete-time, structured models are formulated, analysed and numerically simulated. A special case of the deterministic, density-independent, structured model is the well-known Leslie age-structured model. The stochastic, density-independent model is a multitype branching process. A review of linear, density-independent models is given first, then nonlinear, density-dependent models are discussed. In the linear, density-independent structured models, transitions between states are independent of time and state. Population extinction is determined by the dominant eigenvalue λ of the transition matrix. If λ ≤ 1, then extinction occurs with probability one in the stochastic and deterministic models. However, if λ > 1, then the deterministic model has exponential growth, but in the stochastic model there is a positive probability of extinction which depends on the fixed point of the system of probability generating functions. The linear, density-independent, stochastic model is generalized to a nonlinear, density-dependent one. The dependence on state is in terms of a weighted total population size. It is shown for small initial population sizes that the density-dependent, stochastic model can be approximated by the density-independent, stochastic model and thus, the extinction behavior exhibited by the linear model occurs in the nonlinear model. In the deterministic models there is a unique stable equilibrium. Given the population does not go extinct, it is shown that the stochastic model has a quasi-stationary distribution with mean close to the stable equilibrium, provided the population size is sufficiently large. For small values of the population size, complete extinction can be observed in the simulations. However, the persistence time increases rapidly with the population size. This author received partial support by the National Science Foundation grant # DMS-9626417.     

Predator-prey interactions and community structure: chironomids,mosquitoes and copepods in Heliconia imbricata (Musaceae)

Summary Evidence from both field observations and experimental work indicates that predation by larvae of a midge, Pentaneura n. sp. (Chironomidae), causes the low densities of mosquito larvae (Culicidae) found in the water filled bracts of Heliconia imbricata (Musaceae), microhabitats typically colonized by mosquitoes. This predation affects 2 species of mosquitoes, Wyeomyia pseudopecten, a resident species, and Trichoprosopon digitatum, a non-resident species. Predation keeps resident mosquito densities low while completely excluding the nonresident mosquito from the habitat. Both these effects of predation depend on the presence of an abundant alternative prey, an undescribed species of harpacticoid copepod found in the bracts. These copepod prey sustain chironomids when resident mosquito densities are low, permiting predator densities to remain high enough to exclude the non-resident mosquito. I discuss the evolutionary and ecological implications of predation structuring communities.     

Effect of habitat complexity on the predation of Buenoa fuscipennis (Heteroptera: Notonectidae) on mosquito immature stages and alternative prey

Notonectids are well‐known predators in aquatic habitats, where mosquito larvae, chironomids, and cladocerans constitute their main diet. Our purpose was to assess the effect of structural complexity on the predatory ability of Buenoa fuscipennis, a common predator in aquatic habitats of Buenos Aires city (Argentina). Buenoa fuscipennis showed type 2 functional responses in both the presence and absence of prey refuge and no differences in attack rate or handling time between refuge treatments. Regarding mosquito size classes, B. fuscipennis exhibited a significantly higher preference for 2^nd instar larvae and no predation on pupae. In the presence of mosquito larvae and alternative prey, B. fuscipennis preferred mosquitoes over chironomid larvae and adult cladocerans over mosquito larvae. No switching behavior was detected in our experiments. Habitat structure only slightly affected the predator´s consumption rates on mosquito larvae. Overall, preference for prey did not vary with the presence of refuge, except for the preference for mosquitoes over chironomid larvae, which was significantly decreased in the presence of refuge as a consequence of reduced predation on mosquito larvae. The results suggest that B. fuscipennis could efficiently control mosquitoes in structurally simple habitats where chironomids are the most abundant alternative prey but not in temporary pools where cladocerans are abundant.     

Nestedness patterns of container-dwelling mosquitoes: effects of larval habitat within variable terrestrial matrices

Distributions of mosquito larvae likely are a consequence of multiple factors, although two commonly studied factors (quality of the larval environment and the terrestrial matrix in which these habitats reside) have rarely and simultaneously been varied in the field to understand distributions of larvae. We monitored aquatic containers of two leaf detritus levels within a forest, prairie, and industrial habitat across five months to understand the temporal and spatial colonization of aquatic invertebrates in Northcentral Illinois, USA. Data were collected monthly on mosquito populations and the composition of other invertebrates colonizing containers. Overall, six species of mosquitoes colonized containers, with Culex restuans and Aedes triseriatus having the highest relative abundances. There were strong seasonal abundance patterns for these two mosquito species, with the dominant species changing over time in the forest habitat. The responses of other mosquito taxa were more variable, with abundances reflective of either the terrestrial matrix or larval habitat quality. High detritus containers supported the highest abundances of most species encountered, regardless of habitat. Non-mosquito taxa were less common numerically, but analyses suggested that some taxa, such as syrphid larvae, often co-occurred with mosquitoes. Nested subset analysis indicated communities were strongly nested, and that both habitat type and detritus level were important in explaining nested patterns of aquatic invertebrates. Our data show that both the larval habitat and the surrounding terrestrial matrix shape patterns of container mosquitoes, and that other container invertebrates vary in similar ways as mosquitoes. Handling editor: K. Martens     

Density-related mortality of the flour beetle,Tribolium confusum duval

Life tables were constructed to assess the relative importance of some factors causing mortality of Tribolium confusum and to gauge their response to increasing population density. Observations focussed on three population densities (100, 400 and 800 individual/8 g medium) from the egg to the adult stage. The medium was not renewed in order to maximize predatory interactions. Generation mortality at densities 100, 400 and 800 was 42%, 50% and 74% respectively, i. e. density-dependent. Mortality in the first 10 days was also density-dependent reaching a maximum of 27% at density 800: predation by small larvae on eggs seemed the principal causative factor. The overall pattern of larval mortality was density-independent. Data on the mortality of pupae and callows were ultimately consistent with an inversely density-dependent pattern. Apparently, only mortality occurring within the first 10 days was capable of population regulation.     

Aquatic microfauna alter larval food resources and affect development and biomass of West Nile and Saint Louis encephalitis vector Culex nigripalpus (Diptera: Culicidae)

Ciliate protists and rotifers are ubiquitous in aquatic habitats and can comprise a significant portion of the microbial food resources available to larval mosquitoes, often showing substantial declines in abundance in the presence of mosquito larvae. This top‐down regulation of protists is reported to be strong for mosquitoes inhabiting small aquatic containers such as pitcher plants or tree holes, but the nature of these interactions with larval mosquitoes developing in other aquatic habitats is poorly understood. We examined the effects of these two microbial groups on lower trophic level microbial food resources, such as bacteria, small flagellates, and organic particles, in the water column, and on Culex larval development and adult production. In three independent laboratory experiments using two microeukaryote species (one ciliate protist and one rotifer) acquired from field larval mosquito habitats and cultured in the laboratory, we determined the effects of Culex nigripalpus larval grazing on water column microbial dynamics, while simultaneously monitoring larval growth and development. The results revealed previously unknown interactions that were different from the top‐down regulation of microbial groups by mosquito larvae in other systems. Both ciliates and rotifers, singly or in combination, altered other microbial populations and inhibited mosquito growth. It is likely that these microeukaryotes, instead of serving as food resources, competed with early instar mosquito larvae for microbes such as small flagellates and bacteria in a density‐dependent manner. These findings help our understanding of the basic larval biology of Culex mosquitoes, variation in mosquito production among various larval habitats, and may have implications for existing vector control strategies and for developing novel microbial‐based control methods.     

Population density, phenotype and mortality in the grasshopper Chorthippus brunneus

ABSTRACT.

• 1 The patterns of mortality in laboratory populations of the grasshopper Chorthippus brunneus Thunberg are examined at a range of densities and at different stages of nymphal development with particular reference to the phenotypes of nymphs that die or survive.
• 2 Total mortality during nymphal development rises from density-independence at the lowest densities to approximately compensating density-dependence at the highest densities.
• 3 The greatest proportion of total mortality occurs in the first two instars. The stage broadly covering the second instar is the ‘key-stage’ determining adult density. The mortality during these stages is largely density-independent. There is relatively little mortality during stages broadly covering the final instar, but it is density-dependent and also plays a significant part in determining final adult density.
• 4 As the mortality becomes more density-dependent, either with density itself or with cohort age, the smallest individuals become increasingly disproportionately prone to mortality.
• 5 Males, which are smaller, are more susceptible to mortality than females, but their susceptibility relative to their size is less than that of females.

     

Keystone nonconsumptive effects within a diverse predator community

The number of prey killed by diverse predator communities is determined by complementarity and interference among predators, and by traits of particular predator species. However, it is less clear how predators' nonconsumptive effects (NCEs) scale with increasing predator biodiversity. We examined NCEs exerted on Culex mosquitoes by a diverse community of aquatic predators. In the field, mosquito larvae co‐occurred with differing densities and species compositions of mesopredator insects; top predator dragonfly naiads were present in roughly half of surveyed water bodies. We reproduced these predator community features in artificial ponds, exposing mosquito larvae to predator cues and measuring resulting effects on mosquito traits throughout development. Nonconsumptive effects of various combinations of mesopredator species reduced the survival of mosquito larvae to pupation, and reduced the size and longevity of adult mosquitoes that later emerged from the water. Intriguingly, adding single dragonfly naiads to ponds restored survivorship of larval mosquitoes to levels seen in the absence of predators, and further decreased adult mosquito longevity compared with mosquitoes emerging from mesopredator treatments. Behavioral observations revealed that mosquito larvae regularly deployed “diving” escape behavior in the presence of the mesopredators, but not when a dragonfly naiad was also present. This suggests that dragonflies may have relaxed NCEs of the mesopredators by causing mosquitoes to abandon energetically costly diving. Our study demonstrates that adding one individual of a functionally unique species can substantially alter community‐wide NCEs of predators on prey. For pathogen vectors like mosquitoes, this could in turn influence disease dynamics.     

Habitat heterogeneity and intraguild interactions modify distribution and injury rates in two coexisting genera of damselflies

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The change of the distribution pattern of the large weevil,Hyposipalus gigas Fabricius (Coleoptera,Rhynchophoridae) within a single generation. A preliminary note

The distribution pattern of adults of the large weevil, Hyposipalus gigas on logs of the Japanese black pine and that of larvae in pine stumps were examined by using the m^*−m method (Iwao , 1968). The distribution of adults on logs was contagious, probably due to their response to local heterogeneity of the environment. The larval distribution was also contagious, but there was no significant correlation between the number of larval bores and the stump size. The mortality of young larvae seemed not to be related with the number of larvae per stump, but it was related with the density per unit area of bark surface. There was some sign of inversely density-dependent or ‘all-or-none’ type mortaltiy in the late larval stage.     

Bacterial microbiota assemblage in Aedes albopictus mosquitoes and its impacts on larval development

Interactions between bacterial microbiota and mosquitoes play an important role in mosquitoes’ capacity to transmit pathogens. However, microbiota assemblages within mosquitoes and the impact of microbiota in environments on mosquito development and survival remain unclear. This study examined microbiota assemblages and the effects of aquatic environment microbiota on the larval development of the Aedes albopictus mosquito, an important dengue virus vector. Life table studies have found that reducing bacterial load in natural aquatic habitats through water filtering and treatment with antibiotics significantly reduced the larva‐to‐adult emergence rate. This finding was consistent in two types of larval habitats examined—discarded tires and flowerpots, suggesting that bacteria play a crucial role in larval development. Pyrosequencing of the bacterial 16S rRNA gene was used to determine the diversity of bacterial communities in larval habitats and the resulting numbers of mosquitoes under both laboratory and field conditions. The microbiota profiling identified common shared bacteria among samples from different years; further studies are needed to determine whether these bacteria represent a core microbiota. The highest microbiota diversity was found in aquatic habitats, followed by mosquito larvae, and the lowest in adult mosquitoes. Mosquito larvae ingested their bacterial microbiota and nutrients from aquatic habitats of high microbiota diversity. Taken together, the results support the observation that Ae. albopictus larvae are able to utilize diverse bacteria from aquatic habitats and that live bacteria from aquatic habitats play an important role in larval mosquito development and survival. These findings provide new insights into bacteria's role in mosquito larval ecology.     

Studies on the interference among larvae of the citrus leaf miner,Phyllocnistis citrella stainton (lepidoptera: Phyllocnistidae)

Observation of the citrus leaf miner, Phyllocnistis citrellaStainton suggested that contacts among larvae frequently occur and either of two larvae encountered is sometimes killed by the interference. A model to describe the competitive interactions in the process of interference was constracted with this species. The distribution of hatchling on an area was considered in the model. The model was based on that of encounters in randomly moving particles in two dimensions. First the time when first death occurs was calculated and n hatchlings were reduced to n−1. Further calculation was made to obtain the time that next death occurs. Such calculation was repeated until the number of larvae was reduced to 1 or the area of mine attained to the completed size. By the simulation based on the data observed on the citrus leaf miner, the numbers of survivors were calculated at different hatchling densities and on different leaves in size. Based on this simulation analysis it was suggested that the following characteristics observed in the present studies enable the maximul number of individuals to become adults.

1. When two hatchlings occur on a leaf, they tend to be separated by the midrib. This implies that female moths tend to reject to lay eggs on the area which has already contained the egg.
2. The larva sometimes avoids the contact immediately before it occurs.
3. The larval mortality caused by a contact is reduced with the larval growth. Since the frequency of contacts increases as the mine is enlarged, the change to high survival prevents the larva from high mortality caused by the increase of contacts.
4. The larval survival does not decrease in spite of the reduction of the area available for mining.

     

Mosquitoes and other aquatic insects in fallow field biotopes and rice paddy fields

Fallow field biotopes that develop from abandoned rice fields are man‐made wetlands that provide new habitats for various aquatic animals. Although consideration of such biotopes generally focuses on their positive aspects, this study evaluated the negative aspects of establishing fallow field biotopes with regard to mosquito breeding sites. To determine whether fallow field biotopes become breeding habitats for vector mosquitoes, we evaluated mosquito fauna in fallow field biotopes and adjacent rice fields. We found larvae of Anopheles lesteri, Anopheles sinensis and Culex tritaeniorhynchus (all: Diptera: Culicidae) in the biotopes. Although abundances of mosquito larvae in the biotopes and rice fields were statistically similar, mosquito abundances in rice fields increased dramatically in August when the water level reduced after the rainy season. The abundance and variety of the mosquitoes' natural predators were greater in biotopes than in rice fields because the former are a permanent and stable aquatic environment. A generalized linear mixed model showed a negative effect of predator diversity on mosquito larvae abundance in both habitats. Although fallow field biotopes become breeding habitats for vector mosquitoes, establishing biotopes from fallow fields in order to protect various aquatic animals, including mosquito insect predators, may help to control mosquito breeding.     

The influence of aquatic predators on mosquito abundance in animal drinking troughs in New Zealand

The occurrence and abundance of mosquito populations may be associated with the abundance of predators. We examined the relationship between aquatic predators and populations of mosquitoes in animal water troughs in Waikanae, New Zealand. We also investigated the effects of water volume and environmental factors (temperature, rainfall, wind speed, humidity, and pressure) in order to further understand factors influencing mosquito and predator populations. Logistic regression indicated that the presence or absence of mosquitoes was primarily affected by three factors: predator abundance, week of observation, and water volume. Pearson's correlation indicated that the presence of predators had a positive correlation with water volume (r²= 0.176, p< 0.05). Otherwise, the presence of mosquito larvae in water troughs was negatively correlated with water volume (r²=?0.159, p=0.022) and wind speed (r²=0.142, p=0.041). We established a translocation experiment in which predators or mosquitoes were moved between troughs in order to examine the prey survival rate after exposure to Anisops wakefieldi predators. The survival rate of mosquitoes was not significantly different, between 0–0.1%, irrespective of the number of predators translocated (1–9) or the initial mosquito density (20–70 larvae). Our results suggested that A. wakefieldi predators may have the potential to be a promising biological control tool for the control of mosquito populations by altering mosquito population dynamics.     

Wetland isolation facilitates larval mosquito density through the reduction of predators

• 1 Wetlands harbour high biodiversity and offer important ecosystem services, but they are also a habitat for mosquito larvae (Diptera: Culicidae), which are important disease vectors.
• 2 Isolation among remnant, or newly created wetlands and ponds, and their consequent density in the landscape, is a key factor that can influence a variety of food web processes, including effects on mosquitoes which are important prey to many predators.
• 3 We assess the impact of habitat isolation on the density of pond‐breeding mosquitoes (several Anopheles and Culex species) both directly and indirectly through the food web.
• 4 Results from structural equation modelling of survey data shows that larval mosquitoes are denser in ponds that are more isolated from one another, and that this result was primarily driven indirectly by a reduction of larval mosquito predators (e.g. predaceous insects and amphibians). Furthermore, results from a long‐term mesocosm experiment factorially manipulating isolation and predator reduction show that the effect of isolation on mosquito density was eliminated when predators were experimentally reduced.
• 5 It is concluded that metacommunity processes, both directly and indirectly mediated through predators, can play an important role in the local abundance of wetland breeding mosquitoes and possibly the diseases they spread.

     

Effects of Metarhizium robertsii on the bloodsucking mosquito Aedes flavescens and non‐target predatory insects (Odonata)

The effects of different concentrations and methods of treatment with Metarhizium robertsii Bisch., Rehner & Humber conidia on the non‐target aquatic dragonfly larvae Lestes sponsa Hansemann, Lestes dryas Kirby and Aeshna affinis Vander Linden and on the target bloodsucking mosquito larvae Aedes (O.) flavescens (Muller) were analysed. We found that dragonflies are significantly less susceptible than mosquitoes to the fungus. Larvae of L. sponsa larvae were more susceptible to wet conidia than dry conidia. However, the mortality of the air‐breathing larvae of A. affinis was significantly higher after treatment with dry conidia relative to aqueous suspension. The results help to minimize the negative effects of entomopathogenic fungi on non‐target predator insects under the control of mosquito larvae.