Natural Mosquito Populations Research Articles

Efficacy Of Transfluthrin Varies By Species And Placement In A Warm Temperate Mediterranean Environment

ABSTRACT Spatial repellents may protect personnel in delimited areas from incursion of vector or nuisance Diptera, such as mosquitoes and sand flies. We investigated the efficacy of resin diffusers impregnated with transfluthrin in protecting 2 simulated military field scenarios (partially opened tents and open bivouacs lacking shelter) from natural populations of Culex species mosquitoes and Phlebotomus species sand flies. Using a benchmark efficacy of 61% expected reduction from a recent literature survey, we found that transfluthrin protected bivouacs from sand flies (74.1% average reduction; P < 0.001, t(38) = 4.234) but not mosquitoes (13.1% mean reduction, not significant). However, treated tents were not sufficiently protected from sand flies (33.3% mean reduction, not significant), and mosquito numbers increased by an average of 89.3% (not significant). Spatial repellents such as transfluthrin can be highly effective, but efficacy may vary by species and placement, and they should be incorporated into integrated vector management with care.

Wolbachia infection in natural mosquito populations from Argentina.

The increasing spread of mosquito vectors has made mosquito-borne arboviral diseases a global threat to public health, leading to the urgent need for effective population control methods. Strategies based in the intracellular bacterium Wolbachia Hertig, 1936 are considered environmentally friendly, safe for humans, and potentially cost-effective for controlling arboviral diseases. To minimize undesirable side effects, it is relevant to assess whether Wolbachia is present in the area and understand the diversity associated with native infections before implementing these strategies. With this purpose, we investigated Wolbachia infection status, diversity, and prevalence in populations of Aedes albifasciatus (Macquart, 1838), Aedes fluviatilis (Lutz, 1904), and hybrids of the Culex pipiens (Linnaeus, 1758) complex from Argentina. Aedes albifasciatus and C. pipiens complex samples were collected in the province of Buenos Aires, and A. fluviatilis in the province of Misiones. Aedes albifasciatus was found to be uninfected, while infections with strains wFlu and wPip were detected in A. fluviatilis and hybrids of the C. pipiens complex, respectively. All strains were fixed or close to fixation and clustered within supergroup B. These finding provides valuable information on Wolbachia strains found in natural mosquito populations in Argentina that might be used in heterologous infections in the future or be considered when designing control strategies based on Wolbachia infection.

Gene Drive and Symbiont Technologies for Control of Mosquito-Borne Diseases.

Mosquito-borne diseases, such as dengue and malaria, pose a significant burden to global health. Current control strategies with insecticides are only moderately effective. Scalable solutions are needed to reduce the transmission risk of these diseases. Symbionts and genome engineering-based mosquito control strategies have been proposed to address these problems. Bacterial, fungal, and viral symbionts affect mosquito reproduction, reduce mosquito lifespan, and block pathogen transmission. Field tests of endosymbiont Wolbachia-based methods have yielded promising results, but there are hurdles to overcome due to the large-scale rearing and accurate sex sorting required for Wolbachia-based suppression approaches and the ecological impediments to Wolbachia invasion in replacement approaches. Genome engineering-based methods, in which mosquitoes are genetically altered for the modification or suppression of wild populations, offer an additional approach for control of mosquito-borne diseases. In particular, the use of gene drive alleles that bias inheritance in their favor is a potentially powerful approach. Several drives are frequency dependent, potentially giving them broadly similar population dynamics to Wolbachia. However, public acceptance and the behavior of released drives in natural mosquito populations remain challenges. We summarize the latest developments and discuss the knowledge gaps in both symbiont- and gene drive-based methods.

Vector competence of Aedes albopictus field populations from Reunion Island exposed to local epidemic dengue viruses

Dengue virus (DENV) is the most prevalent mosquito-borne Flavivirus that affects humans worldwide. Aedes albopictus, which is naturally infected with the bacteria Wolbachia, is considered to be a secondary vector of DENV. However, it was responsible for a recent DENV outbreak of unprecedented magnitude in Reunion Island, a French island in the South West Indian Ocean. Moreover, the distribution of the cases during this epidemic showed a spatially heterogeneous pattern across the island, leading to questions about the differential vector competence of mosquito populations from different geographic areas. The aim of this study was to gain a better understanding of the vector competence of the Ae. albopictus populations from Reunion Island for local DENV epidemic strains, while considering their infection by Wolbachia. Experimental infections were conducted using ten populations of Ae. albopictus sampled across Reunion Island and exposed to three DENV strains: one strain of DENV serotype 1 (DENV-1) and two strains of DENV serotype 2 (DENV-2). We analyzed three vector competence parameters including infection rate, dissemination efficiency and transmission efficiency, at different days post-exposition (dpe). We also assessed whether there was a correlation between the density of Wolbachia and viral load/vector competence parameters. Our results show that the Ae. albopictus populations tested were not able to transmit the two DENV-2 strains, while transmission efficiencies up to 40.79% were observed for the DENV-1 strain, probably due to difference in viral titres. Statistical analyses showed that the parameters mosquito population, generation, dpe and area of sampling significantly affect the transmission efficiencies of DENV-1. Although the density of Wolbachia varied according to mosquito population, no significant correlation was found between Wolbachia density and either viral load or vector competence parameters for DENV-1. Our results highlight the importance of using natural mosquito populations for a better understanding of transmission patterns of dengue.

Evolutionary adaptation under climate change: Aedes sp. demonstrates potential to adapt to warming.

Climate warming is expected to shift the distributions of mosquitoes and mosquito-borne diseases, facilitating expansions at cool range edges and contractions at warm range edges. However, whether mosquito populations could maintain their warm edges through evolutionary adaptation remains unknown. Here, we investigate the potential for thermal adaptation in Aedes sierrensis, a congener of the major disease vector species that experiences large thermal gradients in its native range, by assaying tolerance to prolonged and acute heat exposure, and its genetic basis in a diverse, field-derived population. We found pervasive evidence of heritable genetic variation in acute heat tolerance, which phenotypically trades off with tolerance to prolonged heat exposure. A simple evolutionary model based on our data shows that the estimated maximum rate of evolutionary adaptation in mosquito heat tolerance typically exceeds that of projected climate warming under idealized conditions. Our findings indicate that natural mosquito populations may have the potential to track projected warming via genetic adaptation. Prior climate-based projections may thus underestimate the range of mosquito and mosquito-borne disease distributions under future climate conditions.

Ontogenetic transfer of microplastics in natural populations of malaria mosquitoes in Western Siberia

AbstractThe uptake, accumulation, and ontogenetic transfer of microplastics (MPs) in bloodsucking mosquitoes of the genus Anopheles, vectors of helminth and protozoa parasites in humans and animals, were studied under laboratory conditions. In the experiment, 2‐μm‐diameter fluorescent polystyrene spheres were counted in mosquitoes of all stages, from larvae to adults. Larvae from a natural population readily ingested MPs with food, accumulating on average 3.9 × 106 particles per larva within 3 days. The concentration of MPs decreased significantly in mosquitoes during their transition from stage to stage. The average number of detected MPs attained 110 particles per pupa and 3.0 particles per adult. MP uptake by mosquitoes did not affect their survival, whereas the rate of metamorphosis in MP‐consuming mosquitoes was slightly higher. Our data confirmed that MPs can be transferred from feeding larvae to non‐feeding pupae and adult flying Anopheles mosquitoes from natural populations in aquatic ecosystems.

CRISPR-Cas9 and Cas12a target site richness reflects genomic diversity in natural populations of Anopheles gambiae and Aedes aegypti mosquitoes

Due to limitations in conventional disease vector control strategies including the rise of insecticide resistance in natural populations of mosquitoes, genetic control strategies using CRISPR gene drive systems have been under serious consideration. The identification of CRISPR target sites in mosquito populations is a key aspect for developing efficient genetic vector control strategies. While genome-wide Cas9 target sites have been explored in mosquitoes, a precise evaluation of target sites focused on coding sequence (CDS) is lacking. Additionally, target site polymorphisms have not been characterized for other nucleases such as Cas12a, which require a different DNA recognition site (PAM) and would expand the accessibility of mosquito genomes for genetic engineering. We undertook a comprehensive analysis of potential target sites for both Cas9 and Cas12a nucleases within the genomes of natural populations of Anopheles gambiae and Aedes aegypti from multiple continents. We demonstrate that using two nucleases increases the number of targets per gene. Also, we identified differences in nucleotide diversity between North American and African Aedes populations, impacting the abundance of good target sites with a minimal degree of polymorphisms that can affect the binding of gRNA. Lastly, we screened for gRNAs targeting sex-determination genes that could be widely applicable for developing field genetic control strategies. Overall, this work highlights the utility of employing both Cas9 and Cas12a nucleases and underscores the importance of designing universal genetic strategies adaptable to diverse mosquito populations.

Wolbachia and Asaia Distribution among Different Mosquito Vectors Is Affected by Tissue Localization and Host Species.

Microbial communities play an important role in the fitness of mosquito hosts. However, the factors shaping microbial communities in wild populations, with regard to interactions among microbial species, are still largely unknown. Previous research has demonstrated that two of the most studied mosquito symbionts, the bacteria Wolbachia and Asaia, seem to compete or not compete, depending on the genetic background of the reference mosquito host. The large diversity of Wolbachia-Asaia strain combinations that infect natural populations of mosquitoes may offer a relevant opportunity to select suitable phenotypes for the suppression of pathogen transmission and for the manipulation of host reproduction. We surveyed Wolbachia and Asaia in 44 mosquito populations belonging to 11 different species of the genera Anopheles, Aedes, and Culex using qualitative PCR. Through quantitative PCR, the amounts of both bacteria were assessed in different mosquito organs, and through metagenomics, we determined the microbiota compositions in some selected mosquito populations. We show that variation in microbial community structure is likely associated with the species/strain of mosquito, its geographical position, and tissue localization. Together, our results shed light on the interactions among different bacterial species in the microbial communities of mosquito vectors, and this can aid the development and/or improvement of methods for symbiotic control of insect vectors.

The challenge of assessing multiple stressors in freshwater ecosystems: nonintuitive interactions between pesticide exposure and larval crowding

It is widely recognized that populations of freshwater aquatic organisms are faced with a myriad of co-occurring stressors. These likely include manufactured chemicals, stressors due to climate change, habitat alterations, water quality parameters, etc. Importantly, these stressors are superimposed over “natural” stressors such as density of conspecifics. Density effects, in particular, are important and can result from resource competition or crowding; here we define crowding as high density but without resource limitation. Crowding has received less research attention despite its potential ecological importance and frequency of occurrence. In larval mosquitoes, for example, both physical and chemical components are important effects of crowding, which result in increased mortality, prolonged development, and reduced size. The objective of this research was to determine how different crowding conditions would affect subsequent insecticide sensitivity using the yellow-fever mosquito, Aedes aegypti. We hypothesized that stress due to crowding in the larval stage would increase insecticide sensitivity. Results showed that when larvae were reared at various crowding densities (without resource competition) but later exposed to a contact insecticide (permethrin) at equal densities they exhibited similar sensitivity. However, when larvae were reared at equal densities but exposed at various crowding densities there appeared to be a protective effect of crowding, as more densely crowded larvae were significantly less sensitive to the insecticide. Possible mechanisms for this protective effect were investigated. Induced detoxification enzymes do not appear to be a factor, but density-modified larval exposure is likely a mechanism. This research provides important insights into how mosquitoes may respond to control efforts as well as providing empirical recommendations on designing laboratory toxicity tests to better reflect ecological conditions in natural mosquito populations.

No detectable fitness cost of infection by cell-fusing agent virus in Aedes aegypti mosquitoes.

Aedes mosquitoes are well-known vectors of arthropod-borne viruses (arboviruses). Mosquitoes are more frequently infected with insect-specific viruses (ISVs) that cannot infect vertebrates. Some ISVs interfere with arbovirus replication in mosquito vectors, which has gained attention for potential use against arbovirus transmission. Cell-fusing agent virus (CFAV), a widespread ISV, can reduce arbovirus dissemination in Ae. aegypti. However, vectorial capacity is largely governed by other parameters than pathogen load, including mosquito survival and biting behaviour. Understanding how ISVs impact these mosquito fitness-related traits is critical to assess the potential risk of using ISVs as biological agents. Here, we examined the effects of CFAV infection on Ae. aegypti mosquito fitness. We found no significant reduction in mosquito survival, blood-feeding behaviour and reproduction, suggesting that Ae. aegypti is tolerant to CFAV. The only detectable effect was a slight increase in human attraction of CFAV-infected females in one out of eight trials. Viral tolerance is beneficial for introducing CFAV into natural mosquito populations, whereas the potential increase in biting activity must be further investigated. Our results provide the first insight into the link between ISVs and Aedes mosquito fitness and highlight the importance of considering all aspects of vectorial capacity for arbovirus control using ISVs.

Artificial light at night has species‐specific effects on oviposition behavior of mosquitoes

AbstractArtificial light at night (ALAN) is a pervasive and growing issue worldwide. ALAN disrupts the physiology and natural crepuscular and nocturnal behavior of organisms, with widely observed effects on insects. Mosquitoes (Diptera: Culicidae) are disease vectors that evaluate and select freshwater habitats as oviposition sites. This behavior has both ecological and epidemiological implications. However, it is unknown how ALAN affects mosquito oviposition. We compared oviposition rates in outdoor mesocosms exposed to light‐emitting diodes (LED, 3000 K, ca. 13 lux) with dark controls. We assayed the oviposition behavior of natural populations of mosquitoes by quantifying mosquito eggs (Culex restuans Theobald and Ochlerotatus japonicus Theobald) deposited in the experimental mesocosms over 7 days. Mosquitoes had species‐specific responses to ALAN. Mean cumulative Cx. restuans egg raft deposition was greater in control pools than in ALAN pools (21 vs. 10 eggs). We observed no response of Oc. japonicus, potentially reflecting the risk associated with the alternative oviposition strategies of the two species (eggs rafts vs. skip oviposition). Our results show that ALAN has species‐specific effects on organisms, thereby complicating our understanding of the behavioral and potential ecological and epidemiological effects of this novel anthropogenic stressor.

Collecting, Storing, and Hatching Aedes aegypti Eggs.

Laboratory study of natural populations of mosquitoes can play a key role in determining the underlying causes of variation in burdens of mosquito-borne disease. Aedes aegypti is the main vector of the viruses that cause dengue, chikungunya, Zika, and yellow fever, making it a high priority for laboratory study. Ae. aegypti eggs provide an ideal starting point for new laboratory colonies. Eggs can be collected using ovicups, which are small plastic cups lined with seed-germination paper and partially filled with leaf-infused H2O. Once collected, dry eggs will remain viable for months and can be safely transported long distances back to the laboratory as long as they are properly stored. This protocol provides step-by-step instructions for preparing for collecting, storing, and hatching Ae. aegypti eggs and has successfully yielded laboratory colonies from locations across both the native and invasive range of this species.

Mosquito densovirus significantly reduces the vector susceptibility to dengue virus serotype 2 in Aedes albopictus mosquitoes (Diptera: Culicidae)

BackgroundDengue virus (DENV) is a major public health threat, with Aedes albopictus being the confirmed vector responsible for dengue epidemics in Guangzhou, China. Mosquito densoviruses (MDVs) are pathogenic mosquito-specific viruses, and a novel MDV was previously isolated from Ae. albopictus in Guangzhou. This study aims to determine the prevalence of MDVs in wild Ae. albopictus populations and investigate their potential interactions with DENV and impact on vector susceptibility for DENV.MethodsThe prevalence of MDV in wild mosquitoes in China was investigated using open access sequencing data and PCR detection in Ae. albopictus in Guangzhou. The viral infection rate and titers in MDV-persistent C6/36 cells were evaluated at 12, 24, 48, 72, 96, and 120 h post infection (hpi) by indirect immunofluorescence assay (IFA) and real time quantitative PCR (RT-qPCR). The midgut infection rate (MIR), dissemination rate (DR), and salivary gland infection rate (SGIR) in various tissues of MDV-infected mosquitoes were detected and quantified at 0, 5, 10, and 15 days post infection (dpi) by RT-PCR and RT-qPCR. The chi-square test evaluated dengue virus serotype 2 (DENV-2) and Aedes aegypti densovirus (AaeDV) infection rates and related indices in mosquitoes, while Tukey's LSD and t-tests compared viral titers in C6/36 cells and tissues over time.ResultsThe results revealed a relatively wide distribution of MDVs in Aedes, Culex, and Anopheles mosquitoes in China and an over 68% positive rate. In vitro, significant reductions in DENV-2 titers in supernatant at 120 hpi, and an apparent decrease in DENV-2-positive cells at 96 and 120 hpi were observed. In vivo, DENV-2 in the ovaries and salivary glands was first detected at 10 dpi in both monoinfected and superinfected Ae. albopictus females, while MDV superinfection with DENV-2 suppressed the salivary gland infection rate at 15 dpi. DENV-2 titer in the ovary and salivary glands of Ae. albopictus was reduced in superinfected mosquitoes at 15 dpi.ConclusionsMDVs is widespread in natural mosquito populations, and replication of DENV-2 is suppressed in MDV-infected Ae. albopictus, thus reducing vector susceptibility to DENV-2. Our study supports the hypothesis that MDVs may contribute to reducing transmission of DENV and provides an alternative strategy for mosquito-transmitted disease control.Graphical abstract

Amphibian abundance is associated with reduced mosquito presence in human‐modified landscapes

AbstractThe impacts of landscape anthropization on mosquito abundance and diversity are already well studied, but the underlying ecological factors behind these effects are still poorly understood. One such underlying ecological factor may be related to the loss of amphibians in human‐modified landscapes, providing ideal habitats for several mosquito species. In this study, we conducted a mosquito and amphibian larvae survey in 77 ponds along a gradient of landscape anthropization in western Switzerland. We used structural equation models to investigate the direct and indirect effects of building footprints and several local pond parameters on mosquito abundance and composition through amphibian abundance. We highlighted that landscape anthropization increases the presence probability of mosquito larvae and the proportion ofCulex pipiensin mosquito communities. Our results also suggested that one of the ecological mechanisms underlying these effects is a reduction in amphibian abundance, limiting competition and predation experienced by mosquitoes in human‐modified landscapes. In addition, we showed that several local pond characteristics prevent the presence and abundance of mosquito larvae. Indeed, deep ponds, which promote amphibian abundance, limit the probability of larval mosquito presence, and large and old ponds have a lower larval mosquito abundance. Amphibians appear to be effective control agents of natural mosquito populations. Therefore, the development of measures to protect amphibians in human‐modified landscapes could help to reduce both mosquito‐related nuisances and the risk of emergence of vector‐borne diseases in humans and wildlife.

Fitness costs of Wolbachia shift in locally-adapted Aedes aegypti mosquitoes.

Aedes aegypti mosquito eggs can remain quiescent for many months before hatching, allowing populations to persist through unfavourable conditions. A. aegypti infected with the Wolbachia strain wMel have been released in tropical and subtropical regions for dengue control. wMel reduces the viability of quiescent eggs, but this physiological cost might be expected to evolve in natural mosquito populations that frequently experience stressful conditions. We found that the cost of wMel infection differed consistently between mosquitoes collected from different locations and became weaker across laboratory generations, suggesting environment-specific adaptation of mosquitoes to the wMel infection. Reciprocal crossing experiments show that differences in the cost of wMel to quiescent egg viability were mainly due to mosquito genetic background and not Wolbachia origin. wMel-infected mosquitoes hatching from long-term quiescent eggs showed partial loss of cytoplasmic incompatibility and female infertility, highlighting additional costs of long-term quiescence. Our study provides the first evidence for a shift in Wolbachia phenotypic effects following deliberate field release and establishment and it highlights interactions between Wolbachia infections and mosquito genetic backgrounds. The unexpected changes in fitness costs observed here suggest potential tradeoffs with undescribed fitness benefits of the wMel infection.

Fitness costs associated with a GABA receptor mutation conferring dieldrin resistance in Aedes albopictus.

Understanding the dynamics of insecticide resistance genes in mosquito populations is pivotal for a sustainable use of insecticides. Dieldrin resistance in Aedes albopictus is conferred by the alanine to serine substitution (A302S or RdlR allele) in the γ-aminobutyric acid (GABA) receptor encoded by the Rdl gene. On Reunion Island, dieldrin resistance was initially reported in natural Ae. albopictus populations sampled in 2008 despite the ban of dieldrin since 1994. To monitor insecticide resistance in Ae. albopictus on the island and to identify its drivers, we measured (i) the frequency of resistance alleles in 19 distinct natural populations collected between 2016 and 2017, (ii) fitness costs associated with dieldrin resistance in laboratory-controlled experiments, and (iii) the resistance conferred by RdlR to fipronil, an insecticide widely used on the island and reported to cross-react with RdlR. The results show a persistence of RdlR in Ae. albopictus natural populations at low frequencies. Among the measured life history traits, mortality in pre-imaginal stages, adults' survival as well as the proportion of egg-laying females were significantly affected in resistant mosquitoes. Finally, bioassays revealed resistance of RdlR mosquitoes to fipronil, suggesting that the use of fipronil in natura could select for the RdlR allele. This study shows that dieldrin resistance is persistent in natural mosquito populations likely as a result of combined effects between fitness costs associated with RdlR and selection exerted by cross-reacting environmental insecticides such as fipronil.

Vectors that spread malaria: introduction to the microbiome-science of mosquitoes

Amongst six main mosquito species, Anopheles minimus responsible for transmitting malaria in India. Anopheles minimus is predominant in India particularly northeastern regions and is most accountable for focal infection outbursts marked by an increase in Plasmodium falciparum diseases, and further deaths that can be directly linked to them. According to genetic analysis, Anopheles minimus (formerly species A) is the most common species occurring in India, involving the north-eastern provinces and the East-central Odisha state, out of the three species that make up the Minimus Complex, which is distributed throughout Asia. It is documented throughout the year and explains perennial transmission as shown by records of sporozoite infections. This species has been observed spawning year-round in slow-moving seepage water streams. It is also primarily endophilic and endophagic. Malaria is still a serious public health concern in many parts of the world, especially in tropical areas. It is a vector-borne infectious disease. The protozoan parasite Plasmodium, which causes malaria, is spread via the bite of female Anopheles mosquitoes carrying the infection. Insecticide-treated nets (ITNs) and indoor residual spraying are the major methods used in the control interventions aimed at mosquito vectors that have shown great success over the past 20 years (IRS). Unfortunately, natural mosquito populations are developing resistance to the conventional pesticides now employed in public health, which is impeding the efficacy of the existing vector management measures over the long run. Therefore, it appears vital to enhance vector control strategies through the creation of novel, environmentally friendly methods in order to reach the objective of eliminating malaria. Our review article provides background for describing the biology, information gaps, and possible public health danger of Anopheles viruses.

Malaria-Transmitting Vectors Microbiota: Overview and Interactions With Anopheles Mosquito Biology.

Malaria remains a vector-borne infectious disease that is still a major public health concern worldwide, especially in tropical regions. Malaria is caused by a protozoan parasite of the genus Plasmodium and transmitted through the bite of infected female Anopheles mosquitoes. The control interventions targeting mosquito vectors have achieved significant success during the last two decades and rely mainly on the use of chemical insecticides through the insecticide-treated nets (ITNs) and indoor residual spraying (IRS). Unfortunately, resistance to conventional insecticides currently being used in public health is spreading in the natural mosquito populations, hampering the long-term success of the current vector control strategies. Thus, to achieve the goal of malaria elimination, it appears necessary to improve vector control approaches through the development of novel environment-friendly tools. Mosquito microbiota has by now given rise to the expansion of innovative control tools, such as the use of endosymbionts to target insect vectors, known as “symbiotic control.” In this review, we will present the viral, fungal and bacterial diversity of Anopheles mosquitoes, including the bacteriophages. This review discusses the likely interactions between the vector microbiota and its fitness and resistance to insecticides.

Mathematical modelling to assess the feasibility of Wolbachia in malaria vector biocontrol

Releasing mosquitoes transinfected with the endosymbiotic bacterium Wolbachia is a novel strategy for interrupting vector-borne pathogen transmission. Following its success in controlling arboviruses spread by Aedes aegypti, this technology is being adapted for anopheline malaria vectors. However, antagonistic interactions between Wolbachia and naturally resident Asaia bacteria in malaria vectors have been demonstrated experimentally, potentially jeopardising Wolbachia biocontrol. We developed the first mathematical model accounting for interspecific competition between endosymbionts to assess the feasibility of this novel strategy for controlling malaria. First, Asaia prevalences among natural mosquito populations were compared with simulations parametrized with rates of Asaia transmission reported from laboratory studies. Discrepancies between projections and natural Asaia prevalences indicated potential overestimation of Asaia transmissibility in artificial laboratory settings. With parametrization that matches natural Asaia prevalence, simulations identified redundancies in Asaia’s many infection routes (vertical, sexual and environmental). This resilience was only overcome when Wolbachia conferred very high resistance to environmental infection with Asaia, resulting in Wolbachia fixation and Asaia exclusion. Wolbachia’s simulated spread was prevented when its maternal transmission was impeded in coinfected mosquitoes and the pre-control Asaia prevalence was beyond a threshold of 60–75%. This theoretical assessment highlights critical next steps in laboratory experiments to inform this strategy’s feasibility.

Why did theWolbachiatransinfection cross the road? drift, deterministic dynamics, and disease control.

Maternally inherited Wolbachia transinfections are being introduced into natural mosquito populations to reduce the transmission of dengue, Zika, and other arboviruses. Wolbachia-induced cytoplasmic incompatibility provides a frequency-dependent reproductive advantage to infected females that can spread transinfections within and among populations. However, because transinfections generally reduce host fitness, they tend to spread within populations only after their frequency exceeds a critical threshold. This produces bistability with stable equilibrium frequencies at both 0 and 1, analogous to the bistability produced by underdominance between alleles or karyotypes and by population dynamics under Allee effects. Here, we analyze how stochastic frequency variation produced by finite population size can facilitate the local spread of variants with bistable dynamics into areas where invasion is unexpected from deterministic models. Our exemplar is the establishment of wMel Wolbachia in the Aedes aegypti population of Pyramid Estates (PE), a small community in far north Queensland, Australia. In 2011, wMel was stably introduced into Gordonvale, separated from PE by barriers to A. aegypti dispersal. After nearly 6 years during which wMel was observed only at low frequencies in PE, corresponding to an apparent equilibrium between immigration and selection, wMel rose to fixation by 2018. Using analytic approximations and statistical analyses, we demonstrate that the observed fixation of wMel at PE is consistent with both stochastic transition past an unstable threshold frequency and deterministic transformation produced by steady immigration at a rate just above the threshold required for deterministic invasion. The indeterminacy results from a delicate balance of parameters needed to produce the delayed transition observed. Our analyses suggest that once Wolbachia transinfections are established locally through systematic introductions, stochastic "threshold crossing" is likely to only minimally enhance spatial spread, providing a local ratchet that slightly-but systematically-aids area-wide transformation of disease-vector populations in heterogeneous landscapes.