Open field release of genetically engineered sterile male Aedes aegypti in Malaysia

Effects of a blood-free mosquito diet on fitness and gonotrophic cycle parameters of laboratory reared Anopheles gambiae sensu stricto

BACKGROUND

The current rise of new innovative tools for mosquito control, such as the release of transgenic mosquitoes carrying a dominant lethal gene and Wolbachia-based strategies, necessitates a massive production of mosquitoes in the insectary. However, currently laboratory rearing depends on vertebrate blood for egg production and maintenance. This practice raises ethical concerns, incurs logistical and cost limitations, and entails potential risk associated with pathogen transmission and blood storage. Consequently, an artificial blood-free diet emerges as a desirable alternative to address these challenges. This study aims to evaluate the effects of a previously formulated artificial blood-free diet (herein referred to as BLOODless) on Anopheles gambiae (An. gambiae s.s.; IFAKARA) gonotrophic parameters and fitness compared with bovine blood.

METHODS

The study was a laboratory-based comparative evaluation of the fitness, fecundity and fertility of An. gambiae s.s. (IFAKARA) reared on BLOODless versus vertebrate blood from founder generation (F0) to eighth generation (F8). A total of 1000 female mosquitoes were randomly selected from F0, of which 500 mosquitoes were fed with bovine blood (control group) and the other 500 mosquitoes were fed with BLOODless diet (experimental group). The feeding success, number of eggs per female, hatching rate and pupation rate were examined post-feeding. Longevity and wing length were determined as fitness parameters for adult male and female mosquitoes for both populations.

RESULTS

While blood-fed and BLOODless-fed mosquitoes showed similar feeding success, 92.3% [95% confidence interval (CI) 89.7-94.9] versus 93.6% (95% CI 90.6-96.6), respectively, significant differences emerged in their reproductive parameters. The mean number of eggs laid per female was significantly higher for blood-fed mosquitoes (P < 0.001) whereas BLOODless-fed mosquitoes had significantly lower hatching rates [odds ratio (OR) 0.17, 95% CI 0.14-0.22, P < 0.001]. Wing length and longevity were similar between both groups.

CONCLUSIONS

This study demonstrates the potential of the BLOODless diet as a viable and ethical alternative to vertebrate blood feeding for rearing An. gambiae s.s. This breakthrough paves the way for more efficient and ethical studies aimed at combating malaria and other mosquito-borne diseases.

Aedes aegypti Controls AE. Aegypti: SIT and IIT-An Overview

The sterile insect technique (SIT) and the incompatible insect technique (IIT) are emerging and potentially revolutionary tools for controlling Aedes aegypti (L.), a prominent worldwide mosquito vector threat to humans that is notoriously difficult to reduce or eliminate in intervention areas using traditional integrated vector management (IVM) approaches. Here we provide an overview of the discovery, development, and application of SIT and IIT to Ae. aegypti control, and innovations and advances in technology, including transgenics, that could elevate these techniques to a worldwide sustainable solution to Ae. aegypti when combined with other IVM practices.

Bioinformatic and literature assessment of toxicity and allergenicity of a CRISPR-Cas9 engineered gene drive to control Anopheles gambiae the mosquito vector of human malaria

BACKGROUND

Population suppression gene drive is currently being evaluated, including via environmental risk assessment (ERA), for malaria vector control. One such gene drive involves the dsxFCRISPRh transgene encoding (i) hCas9 endonuclease, (ii) T1 guide RNA (gRNA) targeting the doublesex locus, and (iii) DsRed fluorescent marker protein, in genetically-modified mosquitoes (GMMs). Problem formulation, the first stage of ERA, for environmental releases of dsxFCRISPRh previously identified nine potential harms to the environment or health that could occur, should expressed products of the transgene cause allergenicity or toxicity.

METHODS

Amino acid sequences of hCas9 and DsRed were interrogated against those of toxins or allergens from NCBI, UniProt, COMPARE and AllergenOnline bioinformatic databases and the gRNA was compared with microRNAs from the miRBase database for potential impacts on gene expression associated with toxicity or allergenicity. PubMed was also searched for any evidence of toxicity or allergenicity of Cas9 or DsRed, or of the donor organisms from which these products were originally derived.

RESULTS

While Cas9 nuclease activity can be toxic to some cell types in vitro and hCas9 was found to share homology with the prokaryotic toxin VapC, there was no evidence from previous studies of a risk of toxicity to humans and other animals from hCas9. Although hCas9 did contain an 8-mer epitope found in the latex allergen Hev b 9, the full amino acid sequence of hCas9 was not homologous to any known allergens. Combined with a lack of evidence in the literature of Cas9 allergenicity, this indicated negligible risk to humans of allergenicity from hCas9. No matches were found between the gRNA and microRNAs from either Anopheles or humans. Moreover, potential exposure to dsxFCRISPRh transgenic proteins from environmental releases was assessed as negligible.

CONCLUSIONS

Bioinformatic and literature assessments found no convincing evidence to suggest that transgenic products expressed from dsxFCRISPRh were allergens or toxins, indicating that environmental releases of this population suppression gene drive for malaria vector control should not result in any increased allergenicity or toxicity in humans or animals. These results should also inform evaluations of other GMMs being developed for vector control and in vivo clinical applications of CRISPR-Cas9.

The optimal strategy of incompatible insect technique (IIT) using Wolbachia and the application to malaria control

For decades, techniques to control vector population with low environmental impact have been widely explored in both field and theoretical studies. The incompatible insect technique (IIT) using Wolbachia, based on cytoplasmic incompatibility, is a technique that Wolbachia-infected male mosquitoes are incapable of producing viable offspring after mating with wild-type female mosquitoes. While the IIT method experimentally ensured its effectiveness in several field works, the failure of female mosquito population control by replacement owing to the accidental contamination of Wolbachia-infected female mosquitoes has been a concern and an obstacle in implementing the IIT method in nature. In this study, we develop a population-based IIT mathematical model using cytoplasmic incompatibility and evaluate the effectiveness of the IIT method in scenarios where contamination is present or absent. In addition, by extending the model to assess the disease infection status of the human population with malaria, we evaluate the optimal release strategy and cost for successful disease control. Our study proves that IIT could be a promising method to control mosquito-borne diseases without perfect suppression of vector mosquito population regardless of contamination.

Release of sterile Aedes aegypti mosquitoes: chilling effect on mass-reared males survival and escape ability and on irradiated males sexual competitiveness

In the sterile insect technique, it is important to measure the impact of mass-rearing and handling of sterile males to allow a successful control of the target wild population. This study evaluates the effect of pre-release chilling on the survival, escape ability, and sexual competitiveness of male Aedes aegypti. To determine survival and escape ability, mosquitoes were chilled at 4 °C using four different treatments of either one exposure (25 min) or two consecutive exposures (25 + 25 min, 25 + 50 min, 25 + 100 min). For sexual competitiveness, two different treatments were evaluated, chilling for 25 min once and twice. Results showed that the longest exposure to chilling caused a significant reduction of survival time, from 67 to 54 days. Escape ability was reduced by the first chilling from 25 to 7% and with the second chilling, it was reduced from 30 to 24% in the control to 4.9, 2.0 and 0.5% for 25, 50 and 100 min, respectively. Sexual competitiveness index was reduced from 1.16 in the control, to 0.32 and - 0.11 for treatments involving one and two chilling periods, respectively. It is recommended to increase the chilling temperature and reduce the exposure time to reduce the harmful effects on sterile males.

Transcriptomics analysis of ethanol treatment of male Aedes aegypti reveals a small set of putative radioprotective genes

Introduction: Sterile Insect Technique (SIT) is based on releasing sterilized male insects into wild insect populations to compete for mating with wild females. Wild females mated with sterile males will produce inviable eggs, leading to a decline in population of that insect species. Sterilization with ionizing radiation (x-rays) is a commonly used mechanism for sterilization of males. Since irradiation can cause damage to both, somatic and germ cells, and can severely reduce the competitiveness of sterilized males relative to wild males, means to minimize the detrimental effects of radiation are required to produce sterile, competitive males for release. In an earlier study, we identified ethanol as a functional radioprotector in mosquitoes. Methods: Here, we used Illumina RNA-seq to profile changes in gene expression of male Aedes aegypti mosquitoes fed on 5% ethanol for 48 hours prior to receiving a sterilizing x-ray dose, compared to males fed on water prior to sterilization. Results: RNA-seq revealed a robust activation of DNA repair genes in both ethanol-fed and water-fed males after irradiation, but surprisingly few differences in gene expression between ethanol-fed and water-fed males regardless of radiation treatment. Discussion: While differences in gene expression due to ethanol exposure were minimal, we identified a small group of genes that may prime ethanol-fed mosquitoes for improved survivability in response to sterilizing radiation.

A Systematic Review of Mathematical Models of Dengue Transmission and Vector Control: 2010-2020

Vector control methods are considered effective in averting dengue transmission. However, several factors may modify their impact. Of these controls, chemical methods, in the long run, may increase mosquitoes' resistance to chemicides, thereby decreasing control efficacy. The biological methods, which may be self-sustaining and very effective, could be hampered by seasonality or heatwaves (resulting in, e.g., loss of Wolbachia infection). The environmental methods that could be more effective than the chemical methods are under-investigated. In this study, a systematic review is conducted to explore the present understanding of the effectiveness of vector control approaches via dengue transmission models.

The size of larval rearing container modulates the effects of diet amount and larval density on larval development in Aedes aegypti

Mass-rearing of mosquitoes under laboratory conditions is an important part of several new control techniques that rely on the release of males to control mosquito populations. While previous work has investigated the effect of larval density and diet amount on colony productivity, the role of the size of the container in which larval development takes place has been relatively ignored. We investigated the role of container size in shaping life history and how this varied with density and food availability in Aedes aegypti, an important disease vector and target of mass-rearing operations. For each treatment combination, immature development time and survival and adult body size and fecundity were measured, and then combined into a measure of productivity. We additionally investigated how larval aggregation behaviour varied with container size. Container size had important effects on life history traits and overall productivity. In particular, increasing container size intensified density and diet effects on immature development time. Productivity was also impacted by container size when larvae were reared at high densities (1.4 larva/ml). In these treatments, the productivity metric of large containers was estimated to be significantly lower than medium or small containers. Regardless of container size, larvae were more likely to be observed at the outer edges of containers, even when this led to extremely high localized densities. We discuss how container size and larval aggregation responses may alter the balance of energy input and output to shape development and productivity.

Close Kin Dyads Indicate Intergenerational Dispersal and Barriers

AbstractThe movement of individuals through continuous space is typically constrained by dispersal ability and dispersal barriers. A range of approaches have been developed to investigate these. Kindisperse is a new approach that infers recent intergenerational dispersal (σ) from close kin dyads and appears particularly useful for investigating taxa that are difficult to observe individually. This study, focusing on the mosquito Aedes aegypti, shows how the same close kin data can also be used for barrier detection. We empirically demonstrate this new extension of the method using genome-wide sequence data from 266 Ae. aegypti. First, we use the spatial distribution of full-sib dyads collected within one generation to infer past movements of ovipositing female mosquitoes. These dyads indicated the relative barrier strengths of two roads and performed favorably against alternative genetic methods for detecting barriers. We then use Kindisperse to quantify recent intergenerational dispersal (σ=81.5-197.1 m generation^-1/2) from the difference in variance between the sib and the first cousin spatial distributions and, from this, estimate effective population density (ρ=833-4,864 km^-2). Dispersal estimates showed general agreement with those from mark-release-recapture studies. Barriers, σ, ρ, and neighborhood size (331-526) can inform forthcoming releases of dengue-suppressing Wolbachia bacteria into this mosquito population.

Insect repellents: An updated review for the clinician

Malaria, Zika virus, West Nile virus, Dengue fever, and Lyme disease are common causes of morbidity and mortality around the world. While arthropod bites may cause local inflammation and discomfort, a greater concern is the potential to develop deadly systemic infection. The use of insect repellents (IRs) to prevent systemic infections constitutes a fundamental public health effort. Cost effectiveness, availability, and high efficacy against arthropod vectors are key characteristics of an ideal IR. Currently, numerous IRs are available on the market, with N,N-diethyl-3-methylbenzamide (DEET) being the most widely used. DEET has an excellent safety profile and remarkable protection against mosquitoes and various other arthropods. Other Environmental Protection Agency-registered IR ingredients (eg, permethrin, picaridin, IR3535, oil of lemon eucalyptus, oil of citronella, catnip oil, and 2-undecanone) are alternative IRs of great interest because some of these ingredients have efficacies comparable to that of DEET. These alternative IRs possess low toxicity and favorable customer experiences in use (eg, cosmetically pleasant, naturally occurring). This review summarizes the currently available Environmental Protection Agency-registered IRs, including their origins, mechanisms of action, side effect profiles, and available formulations. This review will enable the clinician to select the best IR option to meet a patient's needs and provide the greatest protection from arthropod bites and their sequelae.

The Lesser of Two Evils: Application of Maslahah-Mafsadah Criteria in Islamic Ethical-Legal Assessment of Genetically Modified Mosquitoes in Malaysia

The release of over 6,000 genetically modified mosquitoes (GMM) into uninhabited Malaysian forests in 2010 was a frantic step on the part of the Malaysian government to combat the spread of dengue fever. The field trial was designed to control and reduce the dengue vector by producing offspring that die in the early developmental stage, thus decreasing the local Aedes aegypti population below the dengue transmission threshold. However, the GMM trials were discontinued in Malaysia despite being technologically feasible. The lack of systematic studies in terms of cost-benefit analysis, questionable research efficacy and safety-related concerns might have contributed to the termination of the field trial. Hence, this research aims to evaluate the feasibility of GMM release in Malaysia by using a holistic approach based on an Islamic ethical-legal assessment under the maslahah-mafsadah (benefit-risk) concept. Three main strategies based on the maslahah-mafsadah concept approach have been applied: 1) an evidence-based approach, 2) an impact-based approach and, 3) a priority approach. The analysis concluded that GMM could be categorised as zanniyyah (probable). GMM is a promising alternative for dengue control, but many issues must be addressed before its widespread adoption.

The main component of the scent of Senecio madagascariensis flowers is an attractant for Aedes aegypti (L.) (Diptera: Culicidae) mosquitoes

Aedes aegypti (L.) (Diptera: Culicidae) is one of the main vectors of arboviruses, including dengue, Zika, and chikungunya. It almost exclusively inhabits urban areas. Both sexes feed on plant carbohydrates, although for males, this is their only food source. In the case of floral nectars, mosquitoes locate plant sugar sources assisted by volatile compounds. In this work, we found that the floral scent of Senecio madagascariensis elicited a behavioral response in males; therefore, we focused on identifying the volatiles emitted by these flowers. The terpenes (±)-α-pinene, β-pinene, sabinene, and phellandrene and 1-alkenes 1-undecene, and 1-nonene were identified. To determine which compounds are bioactive, pure synthetic lures were assessed using an olfactometer. Only the main compound 1-nonene was an attractant for males. Since our goal was the introduction of synthetic floral-based attractants in toxic sugar-baited traps, we formulated 1-nonene in solid paraffin and stearin matrices to obtain a controlled release system. The bioassay with a toxicological end point showed that the incorporation of a feeding attractant to the toxic sugar trap increased overall mortality. These results suggest that it is possible to use plant volatile compounds or flower cuttings as male Ae. aegypti attractants to improve the efficacy of baited traps.

Phillips GC, Greene KK, Poletto M, Sperry BD, Warner SA, Rose NR, Frandsen GK, Verza NC, Gorman KJ, Matzen KJ. New self-sexing Aedes aegypti strain eliminates barriers to scalable and sustainable vector control for governments and communities in dengue-prone environments

For more than 60 years, efforts to develop mating-based mosquito control technologies have largely failed to produce solutions that are both effective and scalable, keeping them out of reach of most governments and communities in disease-impacted regions globally. High pest suppression levels in trials have yet to fully translate into broad and effective Aedes aegypti control solutions. Two primary challenges to date-the need for complex sex-sorting to prevent female releases, and cumbersome processes for rearing and releasing male adult mosquitoes-present significant barriers for existing methods. As the host range of Aedes aegypti continues to advance into new geographies due to increasing globalisation and climate change, traditional chemical-based approaches are under mounting pressure from both more stringent regulatory processes and the ongoing development of insecticide resistance. It is no exaggeration to state that new tools, which are equal parts effective and scalable, are needed now more than ever. This paper describes the development and field evaluation of a new self-sexing strain of Aedes aegypti that has been designed to combine targeted vector suppression, operational simplicity, and cost-effectiveness for use in disease-prone regions. This conditional, self-limiting trait uses the sex-determination gene doublesex linked to the tetracycline-off genetic switch to cause complete female lethality in early larval development. With no female progeny survival, sex sorting is no longer required, eliminating the need for large-scale mosquito production facilities or physical sex-separation. In deployment operations, this translates to the ability to generate multiple generations of suppression for each mosquito released, while being entirely self-limiting. To evaluate these potential benefits, a field trial was carried out in densely-populated urban, dengue-prone neighbourhoods in Brazil, wherein the strain was able to suppress wild mosquito populations by up to 96%, demonstrating the utility of this self-sexing approach for biological vector control. In doing so, it has shown that such strains offer the critical components necessary to make these tools highly accessible, and thus they harbour the potential to transition mating-based approaches to effective and sustainable vector control tools that are within reach of governments and at-risk communities who may have only limited resources.

Parentage Assignment Using Microsatellites Reveals Multiple Mating in Aedes aegypti (Diptera: Culicidae): Implications for Mating Dynamics

The mosquito Aedes aegypti is the primary vector of the dengue, yellow fever, and chikungunya viruses. Evidence shows that Ae. aegypti males are polyandrous whereas Ae. aegypti females are monandrous in mating. However, the degree to which Ae. aegypti males and females can mate with different partners has not been rigorously tested. Therefore, this study examined the rates of polyandry via parentage assignment in three sets of competitive mating experiments using wild-type male and female Ae. aegypti. Parentage assignment was monitored using nine microsatellite DNA markers. All Ae. aegypti offspring were successfully assigned to parents with 80% or 95% confidence using CERVUS software. The results showed that both male and female Ae. aegypti mated with up to 3-4 different partners. Adults contributed differentially to the emergent offspring, with reproductive outputs ranging from 1 to 25 viable progeny. This study demonstrates a new perspective on the capabilities of male and female Ae. aegypti in mating. These findings are significant because successful deployment of reproductive control methods using genetic modification or sterile Ae. aegypti must consider the following criteria regarding their mating fitness: 1) choosing Ae. aegypti males that can mate with many different females; 2) testing how transformed Ae. aegypti male perform with polyandrous females; and 3) prioritizing the selection of polyandrous males and/or females Ae. aegypti that have the most offspring.

Population replacement gene drive characteristics for malaria elimination in a range of seasonal transmission settings: a modelling study

BACKGROUND

Gene drives are a genetic engineering method where a suite of genes is inherited at higher than Mendelian rates and has been proposed as a promising new vector control strategy to reinvigorate the fight against malaria in sub-Saharan Africa.

METHODS

Using an agent-based model of malaria transmission with vector genetics, the impacts of releasing population-replacement gene drive mosquitoes on malaria transmission are examined and the population replacement gene drive system parameters required to achieve local elimination within a spatially-resolved, seasonal Sahelian setting are quantified. The performance of two different gene drive systems-"classic" and "integral"-are evaluated. Various transmission regimes (low, moderate, and high-corresponding to annual entomological inoculation rates of 10, 30, and 80 infectious bites per person) and other simultaneous interventions, including deployment of insecticide-treated nets (ITNs) and passive healthcare-seeking, are also simulated.

RESULTS

Local elimination probabilities decreased with pre-existing population target site resistance frequency, increased with transmission-blocking effectiveness of the introduced antiparasitic gene and drive efficiency, and were context dependent with respect to fitness costs associated with the introduced gene. Of the four parameters, transmission-blocking effectiveness may be the most important to focus on for improvements to future gene drive strains because a single release of classic gene drive mosquitoes is likely to locally eliminate malaria in low to moderate transmission settings only when transmission-blocking effectiveness is very high (above ~ 80-90%). However, simultaneously deploying ITNs and releasing integral rather than classic gene drive mosquitoes significantly boosts elimination probabilities, such that elimination remains highly likely in low to moderate transmission regimes down to transmission-blocking effectiveness values as low as ~ 50% and in high transmission regimes with transmission-blocking effectiveness values above ~ 80-90%.

CONCLUSION

A single release of currently achievable population replacement gene drive mosquitoes, in combination with traditional forms of vector control, can likely locally eliminate malaria in low to moderate transmission regimes within the Sahel. In a high transmission regime, higher levels of transmission-blocking effectiveness than are currently available may be required.

Estimating Aedes aegypti (Diptera: Culicidae) Flight Distance: Meta-Data Analysis

The yellow fever mosquito, Aedes aegypti, is a growing global concern. An ecological parameter necessary for vector control is the dispersal, or flight range, of mosquito vectors. Information on mosquito dispersal supports identifying optimal sampling and control strategies to limit the invasion of adult mosquitoes. Attempts around the world to understand the movement and flight range of Ae. aegypti have used a variety of techniques, but there has been little agreement regarding flight distances of Ae. aegypti, specifically the average linear distance Ae. aegypti travel in their natural environment. To generate a more robust estimate of Ae. aegypti flight distance, we conducted a meta-data analysis with the aims 1) to provide a flight distance measurement and 2) investigate how mosquito flight range can be affected by study design and climatic factors. Published studies were retrieved from public databases and reviewed for mean distance traveled (MDT) or maximum distance traveled measurements of Ae. aegypti. Linear regression was used to assess potential relationships between Ae. aegypti flight distance and factors pertaining to climate, degrees of urbanization, and study design. MDT estimates were pooled from 27 experiments to calculate a weighted MDT of 105.69 m. This study addresses the average flight distance of Ae. aegypti with the intention of informing vector control programs in Ae. aegypti prevalent regions of the world.

Modeling the impact of genetically modified male mosquitoes in the spatial population dynamics of Aedes aegypti

The mosquito Aedes aegypti is the primary vector of diseases such as dengue, Zika, chikungunya, and yellow fever. Improving control techniques requires a better understanding of the mosquito’s life cycle, including spatial population dynamics in endemic regions. One of the most promising techniques consists of introducing genetically modified male mosquitoes. Several models proposed to describe this technique present mathematical issues or rely on numerous parameters, making their application challenging to real-world situations. We propose a model describing the spatial population dynamics of the Aedes aegypti in the presence of genetically modified males. This model presents some mathematical improvements compared to the literature allowing deeper mathematical analysis. Moreover, this model relies on few parameters, which we show how to obtain or estimate from the literature. Through numerical simulations, we investigate the impacts of environmental heterogeneity, the periodicity of genetically modified male releases, and released genetically modified males quantity on the population dynamics of Aedes aegypti. The main results point to that the successful application of this vector control technique relies on releasing more than a critical amount of modified males with a frequency exceeding a specific critical value.

Elimination of a closed population of the yellow fever mosquito, Aedes aegypti, through releases of self-limiting male mosquitoes

Establishment of novel mosquito control technologies such as the use of genetically engineered insects typically involves phased testing to generate robust data-sets that support its safe and effective use as a vector control tool. In this study, we demonstrate the ability of the transgenic self-limiting OX513A Aedes aegypti strain to suppress a wild type Ae. aegypti population in an outdoor containment facility in India. OX513A is a genetically engineered Ae. aegypti strain with a repressible dominant self-limiting gene. When male adult OX513A mate with wild female adults, a single copy of the self-limiting gene is inherited by all the progeny, leading to death of >95% of progeny during larval/pupal development. A wild-type population of Ae. aegypti was established and stabilized during a 14 week period in five paired field cage units, each consisting of control and treatment cages, followed by weekly releases of OX513A male adults to suppress the target population. The successive introductions of OX513A male adults led to a consistent decline in wild type numbers eventually resulting in the elimination of Ae. aegypti from all treated cages within 10 to 15 weeks of release. This study demonstrates that Ae. aegypti elimination may be a realistic and achievable target in relatively isolated environments.

Aedes aegypti L. species is the primary vector responsible for transmission of the dengue virus worldwide including chikungunya, yellow fever and Zika virus. The experiment presented in the manuscript represents a study undertaken to demonstrate suppression of the wild type Ae. aegypti population in large outdoor field cages with natural exposure to the environment (physically-contained field cage facility) by sustained releases of male adults of OX513A Ae. aegypti strain. This investigation is a phase-2 contained study as per the World Health Organization guidelines for evaluation of genetically modified organisms and was recommended by the Indian regulatory board. This experiment demonstrates suppression of wild type Ae. aegypti population by sustained releases of OX513A male adults in a contained facility. The prospect of the project is to demonstrate and implement the technology for controlling/suppression of the Ae. aegypti vector in the open field environment.

The relevance of studying insect-nematode interactions for human disease

Vertebrate-parasitic nematodes cause debilitating, chronic infections in millions of people worldwide. The burden of these so-called 'neglected tropical diseases' is often carried by poorer socioeconomic communities in part because research on parasitic nematodes and their vertebrate hosts is challenging and costly. However, complex biological and pathological processes can be modeled in simpler organisms. Here, we consider how insight into the interactions between entomopathogenic nematodes (EPN), their insect hosts and bacterial symbionts may reveal novel treatment targets for parasitic nematode infections. We argue that a combination of approaches that target nematodes, as well as the interaction of pathogens with insect vectors and bacterial symbionts, offer potentially effective, but underexplored opportunities.

Osorio A, Valle J, Angulo-Kladt R, Gómez-Simuta Y, Fernández-Salas I, Dor A, Williams T. Comparison of Ground Release and Drone-Mediated Aerial Release of Aedes aegypti Sterile Males in Southern Mexico: Efficacy and Challenges

Sterile males of Aedes aegypti were released once a week for 8 weeks to evaluate the dispersal efficiency of ground and aerial drone release methods in a rural village of 26 Ha in southern Mexico. Indoor and outdoor BG-Sentinel traps were placed in 13−16 houses distributed throughout the village. The BG traps were activated 48 h after the release of the sterile males and functioned for a 24 h period following each release. Over the 8-week period of simultaneous ground and aerial releases, an average of 85,117 ± 6457 sterile males/week were released at ground level and 86,724 ± 6474 sterile males/week were released using an aerial drone. The ground release method resulted in higher numbers of captured males (mean = 5.1 ± 1.4, range 1.1−15.7 sterile males/trap) compared with the aerial release method (mean = 2.6 ± 0.8, range 0.5−7.3 sterile males/trap) (p < 0.05). Similarly, the prevalence of traps that captured at least one sterile male was significantly higher for ground release compared to the aerial release method (p < 0.01). The lower numbers of sterile males captured in the aerial release method could be due to mortality or physical injury caused by the chilling process for immobilization, or the compaction of these insects during transport and release. However, aerial releases by a two-person team distributed insects over the entire village in just 20 min, compared to ~90 min of work for a five-person team during the ground release method. Ground release also resulted in higher aggregations of males and some villagers reported feeling discomfort from the presence of large numbers of mosquitoes in and around their houses. We conclude that modifications to the handling and transport of sterile males and the design of containers used to store males are required to avoid injury and to improve the efficiency of aerial releases for area-wide SIT-based population suppression programs targeted at mosquito vectors of human disease.

Mark-release-recapture experiment in Burkina Faso demonstrates reduced fitness and dispersal of genetically-modified sterile malaria mosquitoes

Every year, malaria kills approximately 405,000 people in Sub-Saharan Africa, most of them children under the age of five years. In many countries, progress in malaria control has been threatened by the rapid spread of resistance to antimalarial drugs and insecticides. Novel genetic mosquito control approaches could play an important role in future integrated malaria control strategies. In July 2019, the Target Malaria consortium proceeded with the first release of hemizygous genetically-modified (GM) sterile and non-transgenic sibling males of the malaria mosquito Anopheles coluzzii in Burkina Faso. This study aimed to determine the potential fitness cost associated to the transgene and gather important information related to the dynamic of transgene-carrying mosquitoes, crucial for next development steps. Bayesian estimations confirmed that GM males had lower survival and were less mobile than their wild type (WT) siblings. The estimated male population size in Bana village, at the time of the release was 28,000 - 37,000. These results provide unique information about the fitness and behaviour of released GM males that will inform future releases of more effective strains of the A. gambiae complex.

Release of genetically-modified sterile mosquitoes is a potential method of malaria control but has yet to be tested in the field. Here, the authors perform a mark-release-recapture experiment and show that genetically-modified mosquitoes have reduced survival and dispersal compared to wild-types.

Monitoring Needs for Gene Drive Mosquito Projects: Lessons From Vector Control Field Trials and Invasive Species

As gene drive mosquito projects advance from contained laboratory testing to semi-field testing and small-scale field trials, there is a need to assess monitoring requirements to: i) assist with the effective introduction of the gene drive system at field sites, and ii) detect unintended spread of gene drive mosquitoes beyond trial sites, or resistance mechanisms and non-functional effector genes that spread within trial and intervention sites. This is of particular importance for non-localized gene drive projects, as the potential scale of intervention means that monitoring is expected to be more costly than research, development and deployment. Regarding monitoring needs for population replacement systems, lessons may be learned from experiences with Wolbachia-infected mosquitoes, and for population suppression systems, from experiences with releases of genetically sterile male mosquitoes. For population suppression systems, assessing monitoring requirements for tracking population size and detecting rare resistant alleles are priorities, while for population replacement systems, allele frequencies must be tracked, and pressing concerns include detection of gene drive alleles with non-functional effector genes, and resistance of pathogens to functional effector genes. For spread to unintended areas, open questions relate to the optimal density and placement of traps and frequency of sampling in order to detect gene drive alleles, drive-resistant alleles or non-functional effector genes while they can still be effectively managed. Invasive species management programs face similar questions, and lessons may be learned from these experiences. We explore these monitoring needs for gene drive mosquito projects progressing through the phases of pre-release, release and post-release.

Control of sheep flystrike: what's been tried in the past and where to from here

Flystrike remains a serious financial and animal welfare issue for the sheep industry in Australia despite many years of research into control methods. The present paper provides an extensive review of past research on flystrike, and highlights areas that hold promise for providing long-term control options. We describe areas where the application of modern scientific advances may provide increased impetus to some novel, as well as some previously explored, control methods. We provide recommendations for research activities: insecticide resistance management, novel delivery methods for therapeutics, improved breeding indices for flystrike-related traits, mechanism of nematode-induced scouring in mature animals. We also identify areas where advances can be made in flystrike control through the greater adoption of well-recognised existing management approaches: optimal insecticide-use patterns, increased use of flystrike-related Australian Sheep Breeding Values, and management practices to prevent scouring in young sheep. We indicate that breeding efforts should be primarily focussed on the adoption and improvement of currently available breeding tools and towards the future integration of genomic selection methods. We describe factors that will impact on the ongoing availability of insecticides for flystrike control and on the feasibility of vaccination. We also describe areas where the blowfly genome may be useful in providing impetus to some flystrike control strategies, such as area-wide approaches that seek to directly suppress or eradicate sheep blowfly populations. However, we also highlight the fact that commercial and feasibility considerations will act to temper the potential for the genome to act as the basis for providing some control options.

Evaluation of Gene Knockouts by CRISPR as Potential Targets for the Genetic Engineering of the Mosquito Culex quinquefasciatus

Culex quinquefasciatus mosquitoes are a globally widespread vector of several human and animal pathogens. Their biology and behavior allow them to thrive in proximity to urban areas, rendering them a constant public health threat. Their mixed bird/mammal feeding behavior further offers a vehicle for zoonotic pathogens transmission to people and, separately, poses a threat to the conservation of insular birds. The advent of CRISPR has led to the development of novel technologies for the genetic engineering of wild mosquito populations. Yet, research into Cx. quinquefasciatus has been lagging compared to other disease vectors. Here, we use this tool to disrupt a set of five pigmentation genes in Cx. quinquefasciatus that, when altered, lead to visible, homozygous-viable phenotypes. We further validate this approach in separate laboratories and in two distinct strains of Cx. quinquefasciatus that are relevant to potential future public health and bird conservation applications. We generate a double-mutant line, demonstrating the possibility of sequentially combining multiple such mutations in a single individual. Lastly, we target two loci, doublesex in the sex-determination pathway and proboscipedia, a hox gene, demonstrating the flexibility of these methods applied to novel targets. Our work provides a platform of seven validated loci that could be used for targeted mutagenesis in Cx. quinquefasciatus and the future development of genetic suppression strategies for this species. Furthermore, the mutant lines generated here could have widespread utility to the research community using this model organism, as they could be used as targets for transgene delivery, where a copy of the disrupted gene could be included as an easily scored transgenesis marker.

Dengue fever and insecticide resistance in Aedes mosquitoes in Southeast Asia: a review

Dengue fever is the most important mosquito-borne viral disease in Southeast Asia. Insecticides remain the most effective vector control approach for Aedes mosquitoes. Four main classes of insecticides are widely used for mosquito control: organochlorines, organophosphates, pyrethroids and carbamates. Here, we review the distribution of dengue fever from 2000 to 2020 and its associated mortality in Southeast Asian countries, and we gather evidence on the trend of insecticide resistance and its distribution in these countries since 2000, summarising the mechanisms involved. The prevalence of resistance to these insecticides is increasing in Southeast Asia, and the mechanisms of resistance are reported to be associated with target site mutations, metabolic detoxification, reduced penetration of insecticides via the mosquito cuticle and behavioural changes of mosquitoes. Continuous monitoring of the status of resistance and searching for alternative control measures will be critical for minimising any unpredicted outbreaks and improving public health. This review also provides improved insights into the specific use of insecticides for effective control of mosquitoes in these dengue endemic countries.

Mark-release-recapture of male Aedes aegypti (Diptera: Culicidae): Use of rhodamine B to estimate movement, mating and population parameters in preparation for an incompatible male program

Rapid advances in biological and digital support systems are revolutionizing the population control of invasive disease vectors such as Aedes aegypti. Methods such as the sterile and incompatible insect techniques (SIT/IIT) rely on modified males to seek out and successfully mate with females, and in doing so outcompete the wild male population for mates. Currently, these interventions most frequently infer mating success through area-wide population surveillance and estimates of mating competitiveness are rare. Furthermore, little is known about male Ae. aegypti behaviour and biology in field settings. In preparation for a large, community scale IIT program, we undertook a series of mark- release-recapture experiments using rhodamine B to mark male Ae. aegypti sperm and measure mating interactions with females. We also developed a Spatial and Temporally Evolving Isotropic Kernel (STEIK) framework to assist researchers to estimate the movement of individuals through space and time. Results showed that ~40% of wild females captured daily were unmated, suggesting interventions will need to release males multiple times per week to be effective at suppressing Ae. aegypti populations. Males moved rapidly through the landscape, particularly when released during the night. Although males moved further than what is typically observed in females of the species, survival was considerably lower. These unique insights improve our understanding of mating interactions in wild Ae. aegypti populations and lay the foundation for robust suppression strategies in the future.

Incompatible insect techniques for controlling populations of the dengue vector, Aedes aegypti, utilize the mating biology of adult male mosquitoes to achieve suppression through a sterilization process. As the study of Ae. aegypti control has typically focused on adult female mosquitoes, knowledge on male movement, survival and mating interactions in the field is lacking. Here we undertook several mark-release-recapture experiments on adult male Ae. aegypti in Innisfail, Australia, and measured important biological parameters. For the first time in large field experiments, we employed rhodamine B as a marker that when fed to adult males, identified both marked males and the wild females they mated with. We observed males moving further through the landscape, but surviving for a shorter period, than previous measurements undertaken on females in a field setting. A high proportion (~40%) of unmated females suggests individuals are constantly available for mating. As such, sterile male strategies may need to release at regular intervals to achieve effective population suppression. The unique insights provided by this study will assist in designing future sterile male field interventions.

Mathematical modeling of genetic pest management through female-specific lethality: Is one locus better than two?

Many novel genetic approaches are under development to combat insect pests. One genetic strategy aims to suppress or locally eliminate a species through large, repeated releases of genetically engineered strains that render female offspring unviable under field conditions. Strains with this female-killing characteristic have been developed either with all of the molecular components in a single construct or with the components in two constructs inserted at independently assorting loci. Strains with two constructs are typically considered to be only of value as research tools and for producing solely male offspring in rearing factories which are subsequently sterilized by radiation before release. A concern with the two-construct strains is that once released, the two constructs would become separated and therefore non-functional. The only female-killing strains that have been released in the field without sterilization are single-construct strains. Here, we use a population genetics model with density dependence to evaluate the relative effectiveness of female-killing approaches based on single- and two-construct arrangements. We find that, in general, the single-construct arrangement results in slightly faster population suppression, but the two-construct arrangement can eventually cause stronger suppression and cause local elimination with a smaller release size. Based on our results, there is no a priori reason that males carrying two independently segregating constructs need to be sterilized prior to release. In some cases, a fertile release would be more efficient for population suppression.

Laboratory Oviposition Choice of Aedes aegypti (Diptera: Culicidae) From Kenya and Gabon: Effects of Conspecific Larvae, Salinity, Shading, and Microbiome

The mosquito Aedes aegypti (L.) is the primary vector of several arboviruses. Mosquito control and surveillance are essential to restrict disease transmission, the effectiveness of which depends on our understanding of the mosquito's behaviors, including oviposition. Previous studies have identified a variety of oviposition cues. However, most of these studies involved only Ae. aegypti outside of the species' native range, Africa. Populations outside Africa differ in their genetics and some behaviors from their African counterparts, suggesting possibly different oviposition preferences. Within Africa, Ae. aegypti can be found in both ancestral forest habitats and domestic habitats. The African domestic populations may represent an intermediate state between the forest and the truly domesticated non-African populations. Comparing mosquitoes from these three habitats (African forest, African domestic, and non-African domestic) might provide insight into the evolution of oviposition behavior. In this study, I examined the oviposition choices of multiple Ae. aegypti colonies from all three habitats in laboratory settings. I applied a two-choice assay to test four oviposition cues: the preexistence of conspecific larvae, salinity, shading, and microbiome. A subset of African colonies showed similar oviposition choices as their non-African counterparts, whereas the rest show little response to the factors tested. Within the African colonies, oviposition choices of the domestic colonies were significantly different from the forest colonies in most experiments. Yet, their preferences were not always intermediate between that of mosquitoes from the other two habitats. Collectively, this study adds to our understanding of Ae. aegypti oviposition, especially in previously understudied African populations.

A knockout screen of genes expressed specifically in Ae. aegypti pupae reveals a critical role for stretchin in mosquito flight

Aedes aegypti is a critical vector for transmitting Zika, dengue, chikungunya, and yellow fever viruses to humans. Genetic strategies to limit mosquito survival based upon sex distortion or disruption of development may be valuable new tools to control Ae. aegypti populations. We identified six genes with expression limited to pupal development; osi8 and osi11 (Osiris protein family), CPRs and CPF (cuticle protein family), and stretchin (a muscle protein). Heritable CRISPR/Cas9-mediated gene knockout of these genes did not reveal any defects in pupal development. However, stretchin-null mutations (strn^Δ35/Δ41) resulted in flightless mosquitoes with an abnormal open wing posture. The inability of adult strn^Δ35/Δ41 mosquitoes to fly restricted their escape from aquatic rearing media following eclosion, and substantially reduced adult survival rates. Transgenic strains which contain the EGFP marker gene under the control of strn regulatory regions (0.8 kb, 1.4 kb, and 2.2 kb upstream, respectively), revealed the gene expression pattern of strn in muscle-like tissues in the thorax during late morphogenesis from L₄ larvae to young adults. We demonstrated that Ae. aegypti pupae-specific strn is critical for adult mosquito flight capability and a key late-acting lethal target for mosquito-borne disease control.

Is Malaysia Ready for Human Gene Editing: A Regulatory, Biosafety and Biosecurity Perspective

Gene editing platforms have revolutionized the field of genetics with a direct impact on the public health system. Although there are apparent benefits, it is often accompanied by public debates over its uncertainties and risks. In the Malaysian context, modern biotechnology has raised questions about how to best govern gene editing in regulations, biosafety, and biosecurity. Even though standards and guidelines on stem cell and cell-based therapies have been developed, there are no appropriate legal frameworks available for gene editing yet. Nevertheless, biosafety regulations were established to balance promoting biotechnology and protecting against their potential environmental and human health risks. There is also a need to address the potential of genetically modified organisms (GMOs) as bioweapons. Numerous frameworks from several international organizations may provide valuable input in formulating documents on gene editing. By establishing comprehensive guidelines, legal policies, and standards to tackle the challenges and risks associated with gene editing, Malaysia can successfully apply this modern technology in this country.

Genetic pest management and the background genetics of release strains

Genetic pest management (GPM) methods involve releasing modified versions of a pest species to mate with wild pests in the target area. Proposed for a wide range of applications in public health, agriculture and conservation, most progress has been made with pest insects. Offspring of the released modified insects and wild pests carry the modification-which might be transgenes, artificially introduced Wolbachia or genetic damage from radiation, for example-but they also carry a complete haploid genome from their laboratory-reared parent, as well as one from their wild parent. Unless these F₁ hybrids are completely unable to reproduce, further mating will lead to introgression of DNA sequences from the release strain into the wild population. We discuss issues around strain selection and the potential consequences of such introgression. We conclude that such introgression is probably harmless in almost all circumstances, and could, in theory, provide specific additional benefits to the release programme. We outline population monitoring approaches that could be used, going forward, to determine how background genetics may affect GPM. This article is part of the theme issue 'Novel control strategies for mosquito-borne diseases'.

Wing geometry and genetic analyses reveal contrasting spatial structures between male and female Aedes aegypti (L.) (Diptera: Culicidae) populations in metropolitan Manila, Philippines

The population genetic structure of Aedes aegypti (Linnaeus, 1762) has been studied in order to understand its role as an efficient vector. Several studies utilized an integrative approach; to combine genetic and phenotypic data to determine its population structure but these studies have only focused on female populations. To address this particular gap, our study compared the population variability and structuring between its male and female populations using phenotypic and genetic data from a highly-urbanized and dengue-endemic region of the Philippines, Metropolitan Manila. Five mosquito populations comprised of female (n = 137) and male (n = 49) adult mosquitoes were used in this study. All mosquito individuals underwent geometric morphometric (26 landmarks), and genetic (11 microsatellite loci) analyses. Results revealed that F_ST estimates (genetic) were 0.055 and 0.009 while Q_ST estimates (phenotypic) were 0.318 and 0.309 in in male and female populations, respectively. Wing shape variation plots showed that male populations were distinctly separated from each other while female populations overlapped. Similarly, discriminant analysis of principal components using genetic data revealed that male populations were also distinctly separated from each other while female populations showed near-overlapping populations. Genetic and phenetic dendrograms showed the formation of two groups in male populations but no groups in female populations. Further analysis indicated a significant correlation (r = 0.68, p = 0.02) between the genetic and phenetic distances of male populations. Bayesian analysis using genetic data also detected multiple clusters in male (K = 3) and female (K = 2) populations, while no clusters were detected using the phenotypic data from both sexes. Our results revealed contrasting phenotypic and genetic patterns between male and female Ae. aegypti, indicating that male populations were more spatially structured than female populations.

Adequacy and sufficiency evaluation of existing EFSA guidelines for the molecular characterisation, environmental risk assessment and post-market environmental monitoring of genetically modified insects containing engineered gene drives

Advances in molecular and synthetic biology are enabling the engineering of gene drives in insects for disease vector/pest control. Engineered gene drives (that bias their own inheritance) can be designed either to suppress interbreeding target populations or modify them with a new genotype. Depending on the engineered gene drive system, theoretically, a genetic modification of interest could spread through target populations and persist indefinitely, or be restricted in its spread or persistence. While research on engineered gene drives and their applications in insects is advancing at a fast pace, it will take several years for technological developments to move to practical applications for deliberate release into the environment. Some gene drive modified insects (GDMIs) have been tested experimentally in the laboratory, but none has been assessed in small-scale confined field trials or in open release trials as yet. There is concern that the deliberate release of GDMIs in the environment may have possible irreversible and unintended consequences. As a proactive measure, the European Food Safety Authority (EFSA) has been requested by the European Commission to review whether its previously published guidelines for the risk assessment of genetically modified animals (EFSA, 2012 and 2013), including insects (GMIs), are adequate and sufficient for GDMIs, primarily disease vectors, agricultural pests and invasive species, for deliberate release into the environment. Under this mandate, EFSA was not requested to develop risk assessment guidelines for GDMIs. In this Scientific Opinion, the Panel on Genetically Modified Organisms (GMO) concludes that EFSA's guidelines are adequate, but insufficient for the molecular characterisation (MC), environmental risk assessment (ERA) and post-market environmental monitoring (PMEM) of GDMIs. While the MC,ERA and PMEM of GDMIs can build on the existing risk assessment framework for GMIs that do not contain engineered gene drives, there are specific areas where further guidance is needed for GDMIs.

Evaluation of Rearing Parameters of a Self-Limiting Strain of the Mediterranean Fruit Fly, Ceratitis capitata (Diptera: Tephritidae)

The Mediterranean fruit fly (medfly), Ceratitis capitata, is a significant pest of stone and pome fruit that causes considerable economic losses worldwide. Current control is primarily based on insecticides, which are often mixed with protein baits. Oxitec has developed a self-limiting medfly strain (OX3864A) that demonstrates conditional female-specific mortality in the early life stages. Sustained release of OX3864A males offers medfly control, which should lead to substantial economic benefits in area-wide programmes. In the current study, the optimum quantities of mature and immature stages of the strain are assessed under semi-mass production. Moreover, the rearing and quality control limitations related to the production of this strain are provided. The data here demonstrate that the egg hatch rate can reach >85% under optimum rearing conditions. However, this depends on the number of pupae loaded in a cage and their ages. The suggested pupal density ranges between 14,000 and 18,000 pupae per cage to provide optimum egg production. In parallel, the embryo densities of 1.25-1.5 mL/kg larval Tet+ diet are recommended for strain propagation, while embryo densities of 1.25-2.0 mL/kg larval Tet- diet are suggested for male-only pupal production.

Attraction Versus Capture II: Efficiency of the BG-Sentinel Trap Under Semifield Conditions and Characterizing Response Behaviors of Male Aedes aegypti (Diptera: Culicidae)

Aedes aegypti (L.) is an important vector of viruses causing dengue, Zika, chikungunya, and yellow fever and as such presents a serious threat to public health in tropical regions. Control programs involving 'rear and release' of modified male Ae. aegypti are underway and require effective trapping methods for surveillance of both the released insects and the impacted wild mosquito population. The BG-Sentinel trap (BGS) is widely used in Ae. aegypti surveillance but its level of efficiency, that is, what proportion of the mosquitoes encountering the trap are captured, is unknown. This is especially true for male mosquitoes, the behavior of which is incompletely understood. We tested the efficiency of two versions of the BGS for capturing male Ae. aegypti under semifield conditions with and without CO2 and a human skin odor mimic lure and with these baits combined. A navy-blue BGS trap emitting CO2 and a human skin odor mimic captured 18% of the released male Ae. aegypti, with a capture efficiency of 9 % (of the total encounters with the trap). Male Ae. aegypti had multiple encounters with the BGS that did not result in capture; they crossed over the trap entrance without being captured or landed on the sides of the trap. Swarming behavior around the BGS was also recorded, even when only a visual cue was present. Understanding male Ae. aegypti behaviors during an encounter with the BGS can inform improvement of trap design and therefore capture efficiency for surveillance in control programs.

Using spatial genetics to quantify mosquito dispersal for control programs

Background

Hundreds of millions of people get a mosquito-borne disease every year and nearly one million die. Transmission of these infections is primarily tackled through the control of mosquito vectors. The accurate quantification of mosquito dispersal is critical for the design and optimization of vector control programs, yet the measurement of dispersal using traditional mark-release-recapture (MRR) methods is logistically challenging and often unrepresentative of an insect’s true behavior. Using Aedes aegypti (a major arboviral vector) as a model and two study sites in Singapore, we show how mosquito dispersal can be characterized by the spatial analyses of genetic relatedness among individuals sampled over a short time span without interruption of their natural behaviors.

Results

Using simple oviposition traps, we captured adult female Ae. aegypti across high-rise apartment blocks and genotyped them using genome-wide SNP markers. We developed a methodology that produces a dispersal kernel for distance which results from one generation of successful breeding (effective dispersal), using the distance separating full siblings and 2nd- and 3rd-degree relatives (close kin). The estimated dispersal distance kernel was exponential (Laplacian), with a mean dispersal distance (and dispersal kernel spread σ) of 45.2 m (95% CI 39.7–51.3 m), and 10% probability of a dispersal > 100 m (95% CI 92–117 m). Our genetically derived estimates matched the parametrized dispersal kernels from previous MRR experiments. If few close kin are captured, a conventional genetic isolation-by-distance analysis can be used, as it can produce σ estimates congruent with the close-kin method if effective population density is accurately estimated. Genetic patch size, estimated by spatial autocorrelation analysis, reflects the spatial extent of the dispersal kernel “tail” that influences, for example, the critical radii of release zones and the speed of Wolbachia spread in mosquito replacement programs.

Conclusions

We demonstrate that spatial genetics can provide a robust characterization of mosquito dispersal. With the decreasing cost of next-generation sequencing, the production of spatial genetic data is increasingly accessible. Given the challenges of conventional MRR methods, and the importance of quantified dispersal in operational vector control decisions, we recommend genetic-based dispersal characterization as the more desirable means of parameterization.

Genetic engineering and bacterial pathogenesis against the vectorial capacity of mosquitoes

Mosquitoes are the main vector of multiple diseases worldwide and transmit viral (malaria, chikungunya, encephalitis, yellow fever, as well as dengue fever), as well as bacterial diseases (tularemia). To manage the outbreak of mosquito populations, various management programs include the application of chemicals, followed by biological and genetic control. Here we aimed to focus on the role of bacterial pathogenesis and molecular tactics for the management of mosquitoes and their vectorial capacity. Bacterial pathogenesis and molecular manipulations have a substantial impact on the biology of mosquitoes, and both strategies change the gene expression and regulation of disease vectors. The strategy for genetic modification is also proved to be excellent for the management of mosquitoes, which halt the development of population via incompatibility of different sex. Therefore, the purpose of the present discussion is to illustrate the impact of both approaches against the vectorial capacity of mosquitoes. Moreover, it could be helpful to understand the relationship of insect-pathogen and to manage various insect vectors as well as diseases.

Genetic breakdown of a Tet-off conditional lethality system for insect population control

Genetically modified conditional lethal strains have been created to improve the control of insect pest populations damaging to human health and agriculture. However, understanding the potential for the genetic breakdown of lethality systems by rare spontaneous mutations, or selection for inherent suppressors, is critical since field release studies are in progress. This knowledge gap was addressed in a Drosophila tetracycline-suppressible embryonic lethality system by analyzing the frequency and structure of primary-site spontaneous mutations and second-site suppressors resulting in heritable survivors from 1.2 million zygotes. Here we report that F₁ survivors due to primary-site deletions and indels occur at a 5.8 × 10⁻⁶ frequency, while survival due to second-site maternal-effect suppressors occur at a ~10⁻⁵ frequency. Survivors due to inherent lethal effector suppressors could result in a resistant field population, and we suggest that this risk may be mitigated by the use of dual redundant, albeit functionally unrelated, lethality systems.

Insect population control using conditional lethal systems could break down due to spontaneous mutations that render the system ineffective. Here the authors analyse the structure and frequency of such mutations in Drosophila and suggest the use of dual lethality systems to mitigate their survival.

Environmental influences on Aedes aegypti catches in Biogents Sentinel traps during a Californian "rear and release" program: Implications for designing surveillance programs

As Aedes aegypti continues to expand its global distribution, the diseases it vectors (dengue, Zika, chikungunya and yellow fever) are of increasing concern. Modern efforts to control this species include "rear and release" strategies where lab-reared mosquitoes are distributed throughout the landscape to replace or suppress invasive populations. These programs require intensive surveillance efforts to monitor their success, and the Biogents Sentinel (BGS) trap is one of the most effective tools for sampling adult Ae. aegypti. BGS trap catches can be highly variable throughout landscapes, so we investigated the potential impacts of environmental factors on adult Ae. aegypti capture rates during a "rear and release" program in California to better understand the relative contributions of true variability in population density across a landscape and trap context. We recorded male and female Ae. aegypti catches from BGS traps, with and without CO2, throughout control sites where no mosquitoes were released and in treatment sites where males infected with Wolbachia were released. BGS trap catches were positively influenced by higher proportions of shade or bushes in the front yard of the premises as well as the presence of potential larval habitats such as subterranean vaults. In contrast, an increase in residential habitat within a 100 m radius of trap locations negatively influenced BGS trap catches. For male Ae. aegypti, increased visual complexity of the trap location positively influenced capture rates, and the presence of yard drains negatively affected catch rates in control sites. Lastly, for BGS traps using CO2, higher catch rates were noted from traps placed greater than one meter from walls or fences for both male and female mosquitoes. These results have important implications for surveillance programs of Ae. aegypti throughout the Californian urban environment including adult monitoring during "rear and release" programs.

Attraction Versus Capture: Efficiency of BG-Sentinel Trap Under Semi-Field Conditions and Characterizing Response Behaviors for Female Aedes aegypti (Diptera: Culicidae)

Aedes aegypti (L.) is an important vector of viruses causing dengue, Zika, chikungunya, and yellow fever and as such is a threat to public health worldwide. Effective trapping methods are essential for surveillance of both the mosquito species and disease presence. The BG-Sentinel (BGS) is a widely used to trap Ae. aegypti but little is known of its efficiency, i.e., what proportion of the mosquitoes encountering the trap are captured. The first version of the BGS trap was predominantly white, and the current version is mostly navy blue. While this trap is often deployed without any olfactory lure, it can also be deployed with CO2 and/or a human skin odor mimic lure to increase capture rates. We tested the efficiency of capturing Ae. aegypti under semi-field conditions for the original white version without lures as well the blue version with and without various lure combinations. None of the configurations tested here captured 100% of the mosquitoes that encountered the trap. A navy-blue trap emitting CO2 and a skin odor mimic produced the highest capture (14% of the total insects in the semi-field cage), but its capture efficiency was just 5% (of mosquitoes encountering the trap). Mosquitoes often had multiple encounters with a trap that did not result in capture; they crossed over the trap entrance without being captured or landed on the sides of the trap. Understanding these behaviors and the factors that induce them has the potential to suggest improvement in trap design and therefore capture efficiency.

Urban Landscape Features Influence the Movement and Distribution of the Australian Container-Inhabiting Mosquito Vectors Aedes aegypti (Diptera: Culicidae) and Aedes notoscriptus (Diptera: Culicidae)

Urban landscape features play an important role in the distribution and population spread of mosquito vectors. Furthermore, current insecticide and novel rear-and-release strategies for urban mosquito management rarely consider the spatial structure of the landscape when applying control practices. Here, we undertake a mark-recapture experiment to examine how urban features influence the movement and distribution of Australian container-inhabiting Aedes vectors. We pay attention to the role of semipermanent water storage containers, called rainwater tanks, and the influence of movement barriers, such as roads, on the spread and distribution of vector populations. Results suggest that Aedes aegypti (Linnaeus) (Diptera: Culicidae) were more likely to be captured around rainwater tanks, and that released males travel throughout residential blocks but do not cross roads. Conversely, female Aedes notoscriptus (Skuse) (Diptera: Culicidae) movement was uninhibited by roads and rainwater tanks did not influence female distribution or oviposition behavior. Using an isotropic Gaussian kernel framework, we show that vector movement is likely to be greater when applying a temporal effect, than when estimated by traditional methods. We conclude that a greater understanding on the role of urban features on vector movement will be important in the new age of rear-and-release mosquito control strategies, particularly those where estimations of movement are important for ensuring efficacy of application.

First Field Release of a Genetically Engineered, Self-Limiting Agricultural Pest Insect: Evaluating Its Potential for Future Crop Protection

Alternative, biologically-based approaches for pest management are sorely needed and one approach is to use genetically engineered insects. Herein we describe a series of integrated field, laboratory and modeling studies with the diamondback moth, Plutella xylostella, a serious global pest of crucifers. A "self-limiting" strain of Plutella xylostella (OX4319L), genetically engineered to allow the production of male-only cohorts of moths for field releases, was developed as a novel approach to protect crucifer crops. Wild-type females that mate with these self-limiting males will not produce viable female progeny. Our previous greenhouse studies demonstrated that releases of OX4319L males lead to suppression of the target pest population and dilution of insecticide-resistance genes. We report results of the first open-field release of a non-irradiated, genetically engineered self-limiting strain of an agricultural pest insect. In a series of mark-release-recapture field studies with co-releases of adult OX4319L males and wild-type counterparts, the dispersal, persistence and field survival of each strain were measured in a 2.83 ha cabbage field. In most cases, no differences were detected in these parameters. Overall, 97.8% of the wild-type males and 95.4% of the OX4319L males recaptured dispersed <35 m from the release point. The predicted persistence did not differ between strains regardless of release rate. With 95% confidence, 75% of OX4319L males released at a rate of 1,500 could be expected to live between 3.5 and 5.4 days and 95% of these males could be expected to be detected within 25.8-34.9 m from the release point. Moth strain had no effect on field survival but release rate did. Collectively, these results suggest similar field behavior of OX4319L males compared to its wild-type counterpart. Laboratory studies revealed no differences in mating competitiveness or intrinsic growth rates between the strains and small differences in longevity. Using results from these studies, mathematical models were developed that indicate release of OX4319L males should offer efficacious pest management of P. xylostella. Further field studies are recommended to demonstrate the potential for this self-limiting P. xylostella to provide pest suppression and resistance management benefits, as was previously demonstrated in greenhouse studies.

Population structure and genetic diversity of Aedes aegypti and Aedes albopictus in Penang as revealed by mitochondrial DNA cytochrome oxidase I

The population genetics study is crucial as it helps in understanding the epidemiological aspects of dengue and help improving a vector control measures. This research aims to investigate the population genetics structure of two common species of Aedes mosquitoes in Penang; Aedes aegypti and Aedes albopictus using Cytochrome Oxidase I (COI) mitochondrial DNA (mtDNA) marker. Molecular investigations were derived from 440 bp and 418 bp mtDNA COI on 125 and 334 larvae of Aedes aegypti and Aedes albopictus respectively, from 32 locations in Penang. All samples were employed in the BLASTn for species identification. The haplotype diversity, nucleotide diversity, neutrality test and mismatch distribution analysis were conducted in DnaSP version 5.10.1. AMOVA analysis was conducted in ARLEQUIN version 3.5 and the phylogenetic reconstructions based on maximum likelihood (ML) and neighbor-joining (NJ) methods were implemented in MEGA X. The relationships among haplotypes were further tested by creating a minimum spanning tree using Network version 4.6.1. All samples were genetically identified and clustered into six distinct species. Among the species, Ae. albopictus was the most abundant (67.2%), followed by Ae. aegypti (25.2%) and the rest were counted for Culex sp. and Toxorhynchites sp. Both Ae. aegypti and Ae. albopictus show low nucleotide diversity (π) and high haplotype diversity (h), while the neutrality test shows a negative value in most of the population for both species. There are a total of 39 and 64 haplotypes recorded for Ae. aegypti and Ae. albopictus respectively. AMOVA analysis revealed that most of the variation occurred within population for both species. Mismatch distribution analysis showed bimodal characteristic of population differentiation for Ae. aegypti but Ae. albopictus showed unimodal characteristics of population differentiation. Genetic distance based on Tamura-Nei parameter showed low genetic divergent within population and high genetic divergent among population for both species. The maximum likelihood tree showed no obvious pattern of population genetic structure for both Ae. aegypti and Ae. albopictus from Penang and a moderate to high bootstrap values has supported this conclusion. The minimum spanning network for Ae. aegypti and Ae. albopictus showed five and three dominant haplotypes respectively, which indicates a mixture of haplotypes from the regions analysed. This study revealed that there is no population genetic structure exhibited by both Ae. aegypti and Ae. albopictus in Penang. Mutation has occurred rapidly in both species and this will be challenging in controlling the populations. However, further analysis needed to confirm this statement.

Broad dengue neutralization in mosquitoes expressing an engineered antibody

With dengue virus (DENV) becoming endemic in tropical and subtropical regions worldwide, there is a pressing global demand for effective strategies to control the mosquitoes that spread this disease. Recent advances in genetic engineering technologies have made it possible to create mosquitoes with reduced vector competence, limiting their ability to acquire and transmit pathogens. Here we describe the development of Aedes aegypti mosquitoes synthetically engineered to impede vector competence to DENV. These mosquitoes express a gene encoding an engineered single-chain variable fragment derived from a broadly neutralizing DENV human monoclonal antibody and have significantly reduced viral infection, dissemination, and transmission rates for all four major antigenically distinct DENV serotypes. Importantly, this is the first engineered approach that targets all DENV serotypes, which is crucial for effective disease suppression. These results provide a compelling route for developing effective genetic-based DENV control strategies, which could be extended to curtail other arboviruses.

Effects of larval diets and temperature regimes on life history traits, energy reserves and temperature tolerance of male Aedes aegypti (Diptera: Culicidae): optimizing rearing techniques for the sterile insect programmes

Background

Producing high quality sterile males is vital in Aedes aegypti rear-and-release birth control strategies. Larval diets, rearing temperatures, and their interactions determine the accumulation rates of essential nutrients in larvae, but these factors have been understudied in relation to mass-rearing techniques for producing eminent males.

Methods

We compared the effects of two larval diets, a cereal-legume-based diet (Khan’s diet) and a standard larval diet developed in the FAO/IAEA Insect Pest Control Laboratory (IAEA 2 diet). Diets were tested at selected temperatures for both larval and male adult life history traits, adult extreme temperature tolerance, and mating capacity relative to energy reserves of reared male adult Ae. aegypti.

Results

Khan’s diet resulted in shorter immature development time at each test temperature (except for 25 °C) than an IAEA 2 diet. Larvae reared at 28 °C and 32 °C with Khan’s diet demonstrated low pupation rates (c.80%). We accounted for these phenomena as secondary sex ratio manipulation, because a higher proportion of male adults emerged at 28 °C and 32 °C than that for the IAEA 2 diet. In general, the pupal development time shortened as temperature increased, resulting in higher teneral energy reserves in male mosquitoes. High energy reserves allowed male mosquitoes reared with Khan’s diet to have higher adult longevity (5–6 days longer when sugar-fed and 2–3 days longer when water-fed) and tolerance of heat stress than those fed on the IAEA 2 diet. The IAEA 2 diet produced larger male mosquitoes than Khan’s diet did: mosquitoes fed on Khan’s diet were 1.03–1.05 times smaller than those fed on the IAEA 2 diet at 28 °C and 32 °C. No evidence indicated reduced mating capacity for small mosquitoes fed on Khan’s diet.

Conclusions

Larvae reared at 28 °C and 32 °C with Khan’s diet were characterized by shorter immature development time compared with those fed on the IAEA 2 diet. Adult mosquitoes produced from that larval rearing condition exhibited a significant male bias, long lifespan, and better endurance against extreme temperatures relative to energy reserves. Thus, the larval diet at rearing temperature of 28 °C and 32 °C optimized rearing techniques for the sterile insect programmes. However, mating competitiveness and flight performance of adult males require further investigation.

A typology of community and stakeholder engagement based on documented examples in the field of novel vector control

Background

Despite broad consensus on the importance of community and stakeholder engagement (CSE) for guiding the development, regulation, field testing, and deployment of emerging vector control technologies (such as genetically engineered insects), the types of activities pursued have varied widely, as have the outcomes. We looked to previous CSE efforts for clarity about appropriate methods and goals. Our analysis yielded a typology of CSE, and related vocabulary, that describes distinctions that funders, organizers, and scholars should make when proposing or evaluating CSE.

Methods

We compiled available formal documentation of CSE projects, starting with projects mentioned in interviews with 17 key informants. Major features of these examples, including the initiators, target groups, timing, goals, and methods were identified using qualitative coding. Based on these examples, subcategories were developed for a subset of features and applied to the identified cases of CSE in the documents. Co-occurrence of subcategorized features was examined for patterns.

Results

We identified 14 documented examples CSE projects, which were comprised of 28 distinct CSE activities. We found no clear patterns with respect to timing. However, we found that grouping examples according to whether initiators or targets could enact the immediate desired outcome could help to clarify relationships between goals, methods, and targets.

Conclusion

Based on this analysis, we propose a typology that distinguishes three categories of CSE: engagement to inquire –where initiators are empowered to act on information collected through engagement with target groups; engagement to influence –where initiators engage to affect the actions of already-empowered target groups; and engagement to involve –where initiators engage to delegate authority to target groups. The proposed typology can serve as a guide for establishing the goals, identifying appropriate methods, and evaluating and reporting CSE projects by directing attention to important questions to be asked well before determining who to engage and how.

Mosquito borne diseases, such as malaria and dengue, are major causes of illness and death worldwide. Furthermore, it is getting harder to control mosquitoes and other disease-carrying pests because global climate change is facilitating their spread to new areas, and over time, mosquitoes develop resistance to pesticides. Scientists are therefore developing new methods for controlling mosquito vectors using new gene editing tools. However, releasing genetically engineered insects into the environment is controversial. Many experts recommend that communities and stakeholders be consulted about if or how to use these new methods, but there are few guidelines for the best way to do this. We examined published accounts of community and stakeholder engagement pertaining to novel vector control and looked for patterns across these cases. We found that many efforts were not described in published sources, but those that were could be grouped into three categories: engagement to inquire, engagement to influence, and engagement to involve.

Localized Control of Aedes aegypti (Diptera: Culicidae) in Miami, FL, via Inundative Releases of Wolbachia-Infected Male Mosquitoes

As part of the response to autochthonous Zika transmission in the United States, the City of South Miami implemented a 6-mo period in which Wolbachia-infected WB1 Aedes aegypti (L.) males were released into an ~170-acre area. Intracellular Wolbachia bacteria infections in Ae. aegypti cause early embryonic arrest (known as cytoplasmic incompatibility [CI]) and egg hatch failure, and inundative introductions have been suggested as a potential control tool. Throughout the release period, the Ae. aegypti population was monitored within both the release area and an equivalent area that did not receive WB1 male releases. The results show a significant reduction in egg hatch at the area receiving WB1 males, which is consistent with expectations for CI. Similarly, the number of Ae. aegypti was significantly reduced at the area receiving WB1 males, relative to the untreated area. The observed population reduction and results encourage additional work and replication of the Wolbachia biopesticide approach against Ae. aegypti, as an additional tool to be integrated with existing control tools for the control of this medically important vector and nuisance pest.