Stage-dependent predation on competitors: consequences for the outcome of a mosquito invasion

A difference in larval mosquito size allows a biocontrol agent to target the invasive species

As the global temperature rises in the coming decades, Aedes albopictus is expected to invade and establish in South East England, where Culex pipiens is currently the most common native mosquito species. Biocontrol measures that use local cyclopoid copepods against Ae. albopictus may be compromised if the copepods prefer alternate Cx. pipiens prey. In this study, I assessed the predation efficiency of Megacyclops viridis copepods against Ae. albopictus larvae from France and larvae that hatched from egg rafts of Cx. pipiens collected in South East England. The experiments were conducted at 15 and 25°C, which are representative of present and future summer temperatures in South East England. Ae. albopictus larvae that survived the course of the experiment in the predator-absent controls were significantly smaller than Cx. pipiens larvae that survived in the absence of predation. The background mortality of Cx. pipiens larvae increased with the 10-degree increase in temperature, and the smaller size of surviving Cx. pipiens larvae at 25°C, relative to survivors at 15°C, suggests that larger Cx. pipiens larvae were more likely to die at the higher temperature setting. Across all experimental treatments, the ratio of copepod body length to mean prey length, based on larval lengths of survivors from the corresponding predator-absent controls, was a significant predictor of the copepod's predation efficiency. Adding temperature setting to the predation efficiency model as a predictor did not improve model fit. Within the mixed prey treatments, the predation efficiency of M. viridis was 34.5 percentage points higher against Ae. albopictus prey than against Cx. pipiens prey. The higher predation efficiency that M. viridis exhibited against invasive Ae. albopictus prey, likely due to the smaller size of these larvae, supports the future use of M. viridis as a biocontrol agent in the United Kingdom.

Predation yields greater population performance: What are the contributions of density- and trait-mediated effects?

1. Population responses to extrinsic mortality can yield no change in number of survivors (compensation) or an increase in number of survivors (overcompensation) when the population is regulated by negative density-dependence. This intriguing response has been the subject of theoretical studies, but few experiments have explored how the source of extrinsic mortality affects the response. 2. This study tests abilities of three functionally diverse predators, alone and combined, to induce (over)compensation of a prey population. Larval Aedes aegypti (Diptera: Culicidae) were exposed to predation by Mesocyclops longisetus (Crustacea: Copepoda), Anopheles barberi (Diptera: Culicidae), Corethrella appendiculata (Diptera: Corethrellidae), all three in a substitutive design, or no predation. 3. The number of survivors to adulthood, female size and development time, and a composite index of performance (r') were analysed. Predator treatment did not have a significant effect on total number of survivors, nor on number of males, suggesting mortality by predation was compensatory. Predation significantly affected number of female survivors, with a trend of more females produced with predation, though no post hoc tests were significant. Predation significantly increased female development rate and r' relative to no-predator control. 4. A sensitivity analysis indicated that the change in the number of female adults produced was the largest contributing factor to the differences in r' among cohorts. While predation did not significantly increase overall production of adults, it did release survivors from density-dependent effects sufficiently to increase population performance. This study provides an empirical test of mechanisms by which predation may yield positive effects on a population of victims, a phenomenon predicted to occur across many taxa and food webs.

Host food quality and quantity differentially affect Ascogregarina barretti parasite burden, development and within-host competition in the mosquito Aedes triseriatus

Host condition depends in large part on the quality and quantity of available food and heavily influences the outcome of parasite infection. Although parasite fitness traits such as growth rate and size may depend on host condition, whether host food quality or quantity is more important to parasite fitness and within-host interactions is poorly understood. We provided individual mosquito hosts with a standard dose of a gregarine parasite and reared mosquitoes on two food types of different quality and two quantities. We measured host size, total parasite count and area, and average size of parasites within each treatment. Food quality significantly influenced the number of parasites in a host; hosts fed a low-quality diet were infected with more parasites than those provided a high-quality diet. In addition, we found evidence of within-host competition; there was a negative relationship between parasite size and count though this relationship was dependent on host food quality. Host food quantity significantly affected total parasite area and parasite size; lower food quantity resulted in smaller parasites and reduced overall parasite area inside the host. Thus both food quality and quantity have the potential to influence parasite fitness and population dynamics.

Quantifying the Influence of Larval Density on Disease Transmission Indices in Culex quinquefasciatus, the Major African Vector of Filariasis

Larval crowding is one of the abiotic factors affecting biological fitness in mosquitoes. This study aims at elucidating, quantitatively, the influence of more larval crowding on aspects of fitness in Culex quinquefasciatus mosquito. To this end, day-old larvae of the species were reared in 4 density regimens equivalent to 1 larva in 1.25, 2.5, 5, and 10 mL of distilled water. Developmental indices, adult fitness indices, and accumulation and utilisation of teneral reserves for metamorphosis were determined at these density regimens. The results revealed varying significant negative effects of larval density on all fitness indices measured for the species. The study also revealed high utilisation of teneral reserves for metamorphosis at high larval densities. The information generated will be useful in making informed-decisions in allocating scare resources for vector control, although field trials are advocated to establish these laboratory findings.

Know Your Enemy: Effects of a Predator on Native and Invasive Container Mosquitoes

We tested the effect of the native container-dwelling predator Toxorhynchites rutilus on the codominant container-dwelling mosquitoes: native Aedes triseriatus and invasive Aedes japonicus. We established two predator treatments (predator, no predator) by removing T. rutilus from all containers, and stocking T. rutilus larvae (1/3.5 liters) in the predator treatment. Removal and stocking was repeated every 3 d and established significantly different predator abundances in both large and small containers. Repeated-measures analysis of variance (ANOVA) on standard samples showed larvae+pupae/liter of A. japonicus was greater without versus with predation, and this difference increased across samples. In contrast, repeated-measures ANOVA showed larvae+pupae/liter of A. triseriatus was statistically indistinguishable for predation treatments and was greater in small versus large containers. Thus, predation reduced invasive A. japonicus while having no detectable effect on A. triseriatus larvae and pupae. A final destructive census of pupae showed that predation reduced pupae/liter of both species, but this effect was greater and more consistent across container sizes for A. japonicus. Predator effects on abundances were not products of the nonlethal effect of predator avoidance by ovipositing females, as T. rutilus presence did not lead to reduced egg inputs by either Aedes, nor by Aedes spp. as a group. Effects of predation thus are best explained by differential success of developing larvae due to the greater lethal effect of T. rutilus on A. japonicus than on A. triseriatus. Thus, this system is consistent with the hypothesis that native predators can limit success and potential impacts of invasive mosquitoes.

Effects of Cohabitation on the Population Performance and Survivorship of the Invasive Mosquito Aedes albopictus and the Resident Mosquito Aedes notoscriptus (Diptera: Culicidae) in Australia

The presence of Aedes albopictus (Skuse) in the Torres Strait of northern Australia increases the potential for colonization and establishment on the mainland. However, there is a possibility that native species that occupy the same habitats may influence the population performance of Ae. albopictus, potentially affecting the establishment of this species in Australia. Cohabitation experiments were performed with the endemic Aedes notoscriptus (Skuse), which has been found occupying the same larval habitats as Ae. albopictus in the Torres Strait and is the most widespread container-inhabiting Aedes species in Australia. The influence of environmental factors and cohabitation between the two species was examined using different climates, food resource levels, food resource types, and species densities. Survivorship proportions and a population performance index (λ') were calculated and compared. The consequences of increased Ae. notoscriptus densities were reduced survivorship and λ' for Ae. albopictus. Despite this, the mean λ' of Ae. albopictus and Ae. notoscriptus was consistently ≥ 1.06, indicating both species could increase under all conditions, potentially due to increasing conspecific densities negatively affecting Ae. notoscriptus. The outcomes from this study suggest that the preexisting presence of Ae. notoscriptus may not prevent the establishment of Ae. albopictus in Australia.

Can Horton hear the whos? The importance of scale in mosquito-borne disease

The epidemiology of vector-borne pathogens is determined by mechanisms and interactions at different scales of biological organization, from individual-level cellular processes to community interactions between species and with the environment. Most research, however, focuses on one scale or level with little integration between scales or levels within scales. Understanding the interactions between levels and how they influence our perception of vector-borne pathogens is critical. Here two examples of biological scales (pathogen transmission and mosquito mortality) are presented to illustrate some of the issues of scale and to explore how processes on different levels may interact to influence mosquito-borne pathogen transmission cycles. Individual variation in survival, vector competence, and other traits affect population abundance, transmission potential, and community structure. Community structure affects interactions between individuals such as competition and predation, and thus influences the individual-level dynamics and transmission potential. Modeling is a valuable tool to assess interactions between scales and how processes at different levels can affect transmission dynamics. We expand an existing model to illustrate the types of studies needed, showing that individual-level variation in viral dose acquired or needed for infection can influence the number of infectious vectors. It is critical that interactions within and among biological scales and levels of biological organization are understood for greater understanding of pathogen transmission with the ultimate goal of improving control of vector-borne pathogens.

Interspecific Competition between Aedes albopictus and A. sierrensis: potential for Competitive Displacement in the Western United States

The Asian tiger mosquito, Aedes albopictus, was first detected in North America twenty five years ago. It utilizes water-holding container habitats as immature development sites, and has rapidly spread throughout the eastern United States. Aedes albopictus has occasionally been detected in the western United States, but until recently no established populations of A. albopictus were reported. The western tree-hole mosquito, Aedes sierrensis, is the most common tree-hole mosquito throughout the western United States, and is expected to more frequently encounter A. albopictus. In this study, competition between A. albopictus from the eastern United States and A. sierrensis from the western United States was tested in order to better understand the potential for either competitive displacement of A. sierrensis by A. albopictus or competitive resistance of A. sierrensis to A. albopictus. Varying densities of each species were reared with limited resources in a response surface design. Consistent with a prior study, we found that A. albopictus was clearly a superior larval competitor than A. sierrensis. Aedes sierrensis λ' (finite rate of increase) decreased with increasing A. albopictus density, but in contrast, A. albopictus λ' actually increased with increasing A. sierrensis density; a result that was not reflected by individual fitness parameters. These results indicate that A. sierrensis will not be an effective barrier to A. albopictus invasion into tree-holes in the western United States.

Species-specific non-physical interference competition among mosquito larvae

Individuals of different sex, size or developmental stage can compete differently and hence contribute distinctively to population dynamics. In species with complex life cycles such as insects, competitive ability is often positively correlated with larval developmental stage. Yet, little is known on how the development and survival of early-instars is influenced by interference from late-instar larvae, especially at low densities when exploitative competition is expected to be negligible. Furthermore, the specificity and mechanisms by which interference competition operates are largely unknown. We performed two complementary experiments aiming to quantify the competitive effects of late instar Ochlerotatus caspius on early instar larvae at low densities and under high resource supply rate. The first experiment examined the net effect of interference by 4(th) on 1(st) instar O. caspius larvae, relative to the effect of 1(st) instars on themselves. The second experiment examined the effect of species-specific, non-physical interference competition (i.e., cage larvae) by 4(th) on 1(st) instar O. caspius larvae at low or high densities. Specifically, we compared the responses of O. caspius larvae raised in the presence of caged con- or hetero-specific, Culiseta longiareolata, with that of larvae in the empty-cage control group. As expected, interference from late instar larvae had a net negative effect on the development rate of first instars. In contrast, the presence of caged con-specifics (non-physical interference) accelerated the development rate of O. caspius, however, this pattern was only evident at the low density. Notably, no such pattern was detected in the presence of caged hetero-specifics. These results strongly suggest the existence of species-specific growth regulating semiochemicals.

Predation resistance does not trade off with competitive ability in early-colonizing mosquitoes

The tradeoff between colonization and competitive ability has been proposed as a mechanism for ecological succession, and this tradeoff has been demonstrated in multiple successional communities. The tradeoff between competitive ability and predation resistance is also a widely-described phenomenon; however, this tradeoff is not usually postulated as a cause of ecological succession. Early successional species that arrive before predator colonization could be either (1) less vulnerable to predation than their successors, by virtue of being poor competitors (direct competition-predation tradeoff); or (2) equally or more vulnerable to predation, because they normally colonize ahead of predators in succession and therefore are not evolutionarily adapted to avoid predators that they rarely encounter (no competition-predation tradeoff). To test these alternative hypotheses, we established water-filled containers in an oak-hickory forest. We allowed half of the containers to be naturally colonized by early-successional Culex mosquitoes, mid-successional Aedes mosquitoes, and the mosquito predator Toxorhynchites rutilus. In the other half of the containers, we prevented Aedes colonization via systematic removal of Aedes eggs, but allowed Culex and T. rutilus to colonize. The numbers of mature Culex larvae and pupae, and later the total number of Culex, were significantly greater in containers where Aedes had been removed, which suggests that Culex are competitively suppressed by Aedes. Toxorhynchites rutilus abundance and colonization rate were unaffected by the removal of Aedes, and densities of both Culex and Aedes decreased significantly with T. rutilus abundance in both treatments. In-laboratory bioassays showed that Culex were significantly more vulnerable to predation by T. rutilus than were Aedes. These data are consistent with the hypothesis that Culex and Aedes demonstrate a direct colonization-competition tradeoff, and are inconsistent with the hypothesis of a direct competition-predation tradeoff.

Demographic consequences of predators on prey: trait and density mediated effects on mosquito larvae in containers

Predators may affect prey population growth and community diversity through density mediated lethal and trait mediated non-lethal effects that influence phenotypic traits of prey. We tested experimentally the roles of thinning the density of prey (lethality) in the absence of predator cues and density and trait mediated effects (lethality + intimidation) of predatory midge Corethrella appendiculata on competing native and invasive mosquito prey. Predator-mediated reductions in prey and density reductions in the absence of C. appendiculata resulted in lower percent survivorship to adulthood and estimates of the finite rate of increase (λ′) for invasive mosquito Aedes albopictus relative to that of controls. In most instances, thinning the density of prey in the absence, but not in the presence, of C. appendiculata cues resulted in lower survivorship to adulthood and λ′ for native mosquito Aedes triseriatus relative to that of controls. Together, these results suggested trait mediated effects of C. appendiculata specific to each species of mosquito prey. Release from intraspecific competition attributable to density reductions in the absence, but not in the presence, of C. appendiculata enhanced growth and lengthened adult lifespan relative to that of controls for A. albopictus but not A. triseriatus. These results show the importance of predator-mediated density and trait mediated effects on phenotypic traits and populations of invasive and native mosquitoes. Species-specific differences in the phenotypic responses of prey may be due, in part, to longer evolutionary history of C. appendiculata with A. triseriatus than A. albopictus.

Larval environmental stress alters Aedes aegypti competence for Sindbis virus

OBJECTIVE

To evaluate how stress at the larval stage alters adult mosquito performance and susceptibility to viral infection.

METHODS

We used a model system consisting of Sindbis virus (SINV) and the yellow fever mosquito Aedes aegypti. Larvae were either reared under optimal conditions (control) or exposed to one of four types of stressors; suboptimal nutrients, starvation, elevated temperature, and a low dose of the insecticide malathion and adult females were fed SINV infectious blood meal. Differential expressions of stress, immune-specific and detoxification genes was measured in fourth instar larvae (HSP70, HSP83, cecropin, defensin, transferrin and CYP6Z6) and 3-day-old females (cecropin, defensin, transferrin) to identify plausible molecular mechanisms associated with mosquito response to stress.

RESULTS

There were stress-specific variations in mosquito performance (survival, development time, female size), but all stressors had a consistent effect of significantly increasing susceptibility to viral infection and dissemination relative to the controls. Three genes were up-regulated in fourth instar larvae exposed to temperature stress (cecropin, defensin and CYP6Z6) compared to single genes in suboptimal nutrient (cecropin) and malathion (transferrin) stress treatments and down-regulation of all the six genes in starvation treatments. In adult samples, transferrin was up-regulated in all but starvation treatments while defensin was up-regulated in starvation and temperature stress treatments.

CONCLUSIONS

Stress during larval development may cause alterations in adult mosquito phenotype and immunity that can increase their susceptibility to pathogens.

The effects of resource type and ratio on competition with Aedes albopictus and Culex pipiens (Diptera:Culicidae)

The introduction of Aedes albopictus (Skuse) in the United States has been associated with declines in abundance of resident mosquito species, presumably because of resource competition, as larvae of Ae. albopictus have been illustrated as superior competitors under certain resource conditions. We evaluated the hypothesis that varying the type and ratio of two food resources (Foxtail grass: American elm) alters the competitive outcome of Ae. albopictus and Culex pipiens (L.). We measured survivorship, development time, size, and adult longevity, and estimated the population growth index (A') of populations raised both alone and in equal number with the interspecific competitor, across five ratios of the two food resources. Competition was asymmetric with Ae. albopictus, the superior competitor across all resource treatments; however, the competitive advantage Ae. albopictus had over Cx. pipiens was reduced as grass became the predominant resource. With elm as the predominant resource, the population growth index (A') for both Ae. albopictus and Cx. pipiens was lower in intraspecific and interspecific competition treatments, respectively. The treatments also impacted adult life history, as life spans of both Ae. albopictus and Cx. pipiens varied when they emerged from larval conditions with different resource and competition treatments. We discuss the possible differences in the two species' efficiencies in exploiting the two resource types. Despite some resource conditions alleviating the competitive effects of Ae. albopictus on Cx. pipiens, competition remained asymmetric; thus, additional mechanisms are likely operating under field conditions when the two species coexist.

Global Climate Change and Implications for Disease Emergence

The early consequences of global climate change (GCC) are well documented. However, future impacts on ecosystem health, and on the health of humans, domestic animals, and wildlife, are much less well understood. Evidence of increasing frequency of extreme weather events (the 2003 trans-European heat wave, extended droughts in Australia and South America), of geographic changes in vector-borne disease (bluetongue and hanta viruses emerging in northern Europe, dengue virus expanding in central and northern America), and of altered animal behavioral responses (changes in bird migration patterns and fishery numbers) warrants action. To make valid choices, however, practitioners and decision makers must understand what is known about GCC and what is only theory. There will be a multitude of microbial, vector, and host responses to climate change, for example, and not all organisms will respond similarly or across equal time scales. Unfortunately, for many organisms and ecosystems the scientific community has a relatively poor understanding of current effectors and balances, making it problematic to describe the current situation, let alone to validate future predictions. The need for enhanced basic research and systematic surveillance programs is obvious, but putting such programs into place is daunting. However, the threats are real and fast approaching. What is done in the next few years may be decisive, whether for the good or the ill of all.

Your worst enemy could be your best friend: predator contributions to invasion resistance and persistence of natives

Native predators are postulated to have an important role in biotic resistance of communities to invasion and community resilience. Effects of predators can be complex, and mechanisms by which predators affect invasion success and impact are understood for only a few well-studied communities. We tested experimentally whether a native predator limits an invasive species' success and impact on a native competitor for a community of aquatic insect larvae in water-filled containers. The native mosquito Aedes triseriatus alone had no significant effect on abundance of the invasive mosquito Aedes albopictus. The native predatory midge Corethrella appendiculata, at low or high density, significantly reduced A. albopictus abundance. This effect was not caused by trait-mediated oviposition avoidance of containers with predators, but instead was a density-mediated effect caused by predator-induced mortality. The presence of this predator significantly reduced survivorship of the native species, but high predator density also significantly increased development rate of the native species when the invader was present, consistent with predator-mediated release from interspecific competition with the invader. Thus, a native predator can indirectly benefit its native prey when a superior competitor invades. This shows the importance of native predators as a component of biodiversity for both biotic resistance to invasion and resilience of a community perturbed by successful invasion.