Evidence of multiple inseminations in the field in Aedes albopictus

A bacterium against the tiger: further evidence of the potential of noninundative releases of males with manipulated Wolbachia infection in reducing fertility of Aedes albopictus field populations in Italy

BACKGROUND

Incompatible insect technique (IIT) is a population suppression approach based on the release of males with manipulated Wolbachia infection inducing egg inviability in wild females. We here present results of multiple field releases of incompatible ARwP males carried out in 2019 in a 2.7-ha green area within urban Rome (Italy) to assess the effect on Aedes albopictus egg viability. Data are compared with results obtained in 2018, when the approach was tested for the first time in Europe.

RESULTS

An average of 4674 ARwP males were released weekly for 7 weeks, resulting in a mean ARwP:wild male ratio of 1.1:1 (versus 0.7:1 in 2018). Egg-viability dynamics in ovitraps significantly varied between treated and control sites, with an estimated overall reduction of 35% (versus 15% in 2018). The estimated proportion of females classified as mated with ARwP males was 41.8% and the viability rate of eggs laid by these females (9.5%) was on average significantly lower than that of females only mated with wild males (87.8%); however, high variability in fertility was observed. Values of ARwP male competitiveness were 0.36 and 0.73 based on the overall viability rate of eggs in ovitraps and on female fertility, respectively; thus, well above the conventional 0.2 threshold for an effective suppressive impact in the field.

CONCLUSIONS

Results further support the potential of IIT as a tool to contribute to Ae. albopictus control in the urban context, stressing the need for larger field trials to evaluate the cost-efficacy of the approach in temperate regions. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

Insemination in Aedes aegypti and Aedes albopictus

Aedes mosquitoes are the vectors of several arboviruses that cause human disease. A better understanding of their reproduction helps to improve their management and contributes insights into the fundamental biology of mosquitoes. During mating, inseminated mosquito females receive seminal fluids and sperm from males that they then store in the spermathecae. In Aedes aegypti and Aedes albopictus, most mated females become resistant to further insemination within 2 h of initial insemination. Although the male seminal fluids are known to be involved in initiating the resistance of inseminated females to further insemination, the mechanism underlying this resistance is not well-understood. The determination of insemination status is a key step in investigating the behavioral and molecular interactions between males and females and for exploring the proximate influences and evolutionary implications of interspecific copulations. Several methods exist for determining insemination status, as discussed here. The choice of method depends on the research question and the availability of resources.

Interspecific mating bias may drive Aedes albopictus displacement of Aedes aegypti during its range expansion

Aedes albopictus is the most invasive mosquito in the world and often displaces Ae. aegypti in regions where their populations overlap. Interspecific mating has been proposed as a possible cause for this displacement, but whether this applies across the range of their sympatry remains unclear. Aedes albopictus and Ae. aegypti collected from allopatric and sympatric areas in China were allowed to interact in cage experiments with different crosses and sex-choices. The results confirm that asymmetric interspecific mating occurs in these populations with matings between allopatric Ae. albopictus males and Ae. aegypti females being significantly higher (55.2%) than those between Ae. aegypti males and Ae. albopictus females (27.0%), and sympatric mosquitoes showed a similar but lower frequency bias, 25.7% versus 6.2%, respectively. The cross-mated females can mate second time (remate) with the respective conspecific males and the 66.7% remating success of female Ae. albopictus was significantly higher than the 9.3% of Ae. aegypti females. Furthermore, 17.8% of the matings of Ae. albopictus males exposed to mixed pools of Ae. albopictus and Ae. aegypti females and 9.3% of the matings of Ae. aegypti males with mixed Ae. aegypti and Ae. albopictus females were interspecific. The difference in the length of clasper between male Ae. albopictus (0.524 mm) and Ae. aegypti (0.409 mm) may be correlated with corresponding mates. We conclude that stronger Ae. albopictus male interspecific mating and more avid female intraspecific remating result in a satyr effect and contribute to competitive displacement of Ae. aegypti as allopatric Ae. albopictus invade during range expansion.

Strategic Approach, Advances, and Challenges in the Development and Application of the SIT for Area-Wide Control of Aedes albopictus Mosquitoes in Reunion Island

Simple Summary

Aedes albopictus is a well-established competent arbovirus vector in Reunion Island, a French overseas territory in the Indian Ocean, occurring in a range of natural to urban environments where it represents a major threat to public health. Following the 2006 Chikungunya outbreak and periodic occurrence of dengue epidemics, the sterile insect technique (SIT) emerged as the most environment-friendly option for integration with the current vector control strategy that relies mainly on the elimination of breeding sites and insecticide applications. This paper describes the trajectory that has been followed in assessing the feasibility of SIT against Ae. albopictus in Reunion Island, and reviews some of the main achievements since 2009. These include essential scientific information so far obtained on the biology and ecology of Ae. albopictus, and the development of the requisite technological capabilities for the production and release of sexually competitive sterile males. Furthermore, it also draws attention to the strategies established to streamline the decision-making process, including an awareness campaign to enhance public understanding, efforts to secure public acceptance and regulatory validation of SIT pilot testing for small-scale suppression of wild Ae. albopictus in selected urban sites on the island.

Abstract

The global expansion of Aedes albopictus, together with the absence of specific treatment and vaccines for most of the arboviruses it transmits, has stimulated the development of more sustainable and ecologically acceptable methods for control of disease transmission through the suppression of natural vector populations. The sterile insect technique (SIT) is rapidly evolving as an additional tool for mosquito control, offering an efficient and more environment-friendly alternative to the use of insecticides. Following the devastating chikungunya outbreak, which affected 38% of the population on Reunion Island (a French overseas territory in the southwest of the Indian Ocean), there has been strong interest and political will to develop effective alternatives to the existing vector control strategies. Over the past 10 years, the French Research and Development Institute (IRD) has established an SIT feasibility program against Ae. albopictus on Reunion Island in collaboration with national and international partners. This program aimed to determine whether the SIT based on the release of radiation-sterilized males is scientifically and technically feasible, and socially acceptable as part of a control strategy targeting the local Ae. albopictus population. This paper provides a review of a multi-year and a particularly broad scoping process of establishing the scientific and technological feasibility of the SIT against Ae. albopictus on Reunion Island. It also draws attention to some prerequisites of the decision-making process, through awareness campaigns to enhance public understanding and support, social adoption, and regulatory validation of the SIT pilot tests.

A bacterium against the tiger: preliminary evidence of fertility reduction after release of Aedes albopictus males with manipulated Wolbachia infection in an Italian urban area

BACKGROUND

Novel tools are needed to reduce the nuisance and risk of exotic arbovirus transmission associated with the colonization of temperate regions by Aedes albopictus. The incompatible insect technique (IIT) is a population suppression approach based on cytoplasmic incompatibility between males with manipulated endosymbionts and wild females. Here, we present the results of the first field experiment in Europe to assess the capacity of an Ae. albopictus line (ARwP) deprived of its natural endosymbiont Wolbachia and transinfected with a Wolbachia strain from the mosquito Culex pipiens, to sterilize wild females.

RESULTS

We released ∼ 4500 ARwP males weekly for 6 weeks in a green area within urban Rome (Italy) and carried out egg (N = 13 442), female (N = 128) and male (N = 352) collections. Egg (N = 13 783) and female (N = 48) collections were also carried out at two untreated control sites. The percentage of viable eggs during release was, on average, significantly lower in treated sites than in control sites, with the greatest difference (16%) seen after the fourth release. The ARwP to wild male ratio in the release spots between day 3 after the first ARwP male release and day 7 after the last release was, on average, 7:10. Released males survived up to 2 weeks. Approximately 30% of females collected in the release spots showed 100% sterility and 20% showed strongly reduced fertility compared with control sites.

CONCLUSIONS

Results support the potential of IIT as a tool contributing to Ae. albopictus control in the urban context, and stress the need for larger field trials to evaluate the cost-efficacy of the approach in suppressing wild populations. © 2019 Society of Chemical Industry.

Spermathecal Filling in Aedes aegypti and Aedes albopictus: Effects of Female and Male Body Sizes and Species

The mosquitoes Aedes aegypti (L.) (Diptera: Culicidae) and Aedes albopictus (Skuse) (Diptera: Culicidae) are ecologically similar species that have evolved independently in their native ranges, Ae. aegypti in Africa and Ae. albopictus in Asia. Where their invasive ranges overlap, interspecific mating is facilitated by both species swarming to mate around bloodmeal hosts during daylight. Here, we test hypotheses to account for variation in spermathecal filling in females of Ae. aegypti and Ae. albopictus, particularly, the effect of mosquito size on the number of spermathecae containing sperm after intraspecific and interspecifc matings of these two species. Results show that significantly more spermathecae contained sperm in large than in small Ae. albopictus females, but there was no effect of Ae. aegypti female body size on the average number of spermathecae filled. Among inseminated females, there was no effect of male size or cross-type on number of spermathecae with sperm. Differences in the effect of female size on the number of spermathecae that store sperm in Ae. aegypti and Ae. albopictus imply differences in the mating biology of these two species.

Cytoplasmic incompatibility management to support Incompatible Insect Technique against Aedes albopictus

Background

The transinfection of the endosymbiotic bacterium Wolbachia provides a method to produce functionally sterile males to be used to suppress mosquito vectors. ARwP is a wPip Wolbachia infected Aedes albopictus which exhibits a bidirectional incompatibility pattern with wild-types. We coupled a modelistic approach with laboratory experiments to test ARwP as a control tool and evaluate the possible occurrence of population replacement following the release of ARwP females in a wild-type (S_ANG) population. Repeated male-only releases were simulated and tested in the laboratory in comparison with releases contaminated with 1% ARwP females. Model simulations also investigated how migration affects the outcome of contaminated releases. Finally, the mean level of egg fertility and the long-term evolution of populations constituted by two Wolbachia infection types were studied by testing S_ANG and ARwP Ae. albopictus and performing more general model simulations.

Results

The model was parametrized with laboratory data and simulations were compared with results of biological trials. Small populations of ARwP males and females were theoretically and experimentally demonstrated to rapidly become extinct when released in larger S_ANG populations. Male-only releases at a 5:1 ratio with respect to the wild-type males led to a complete suppression of the S_ANG population in a few generations. Contaminated releases were efficient as well but led to population replacement in the long term, when the wild-type population approached eradication. Migration significantly contrasted this trend as a 5% population turnover was sufficient to avoid any risk of population replacement. At equal frequencies between ARwP and S_ANG individuals, the mean egg fertility of the overall population was more than halved and suppression was self-sustaining until one of the two infection types extinguished.

Conclusions

In the case of bidirectional incompatibility patterns, the repeated release of incompatible males with small percentages of contaminant females could lead to population replacement in confined environments while it could be managed to target high efficiency and sustainability in wild-type suppression when systems are open to migration. This possibility is discussed based on various contexts and taking into consideration the possibility of integration with other control methods such as SIT and the use of larvicides.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3208-7) contains supplementary material, which is available to authorized users.

Age and Body Size Influence Sperm Quantity in Male Aedes albopictus (Diptera: Culicidae) Mosquitoes

Aedes albopictus (Skuse) (Diptera: Culicidae) is a vector of several arboviruses impacting human health, including dengue, chikungunya, and potentially Zika. Vector control strategies that deploy modified males into the field are in use or under development and require a solid understanding of male biology; unfortunately, there has been limited effort to understand male Ae. albopictus reproductive biology, including sperm production and capacity. We tested whether body size and age affect spermatogenesis in Ae. albopictus. In general, older and larger males produced more sperm than their younger or smaller counterparts. Large males continued spermatogenesis well after 10-d post-eclosion (dpe), augmenting their reserves by 39%. By contrast, small males stopped producing sperm at 10 dpe. These results contribute to a deeper understanding of Ae. albopictus reproductive physiology. We discuss the usefulness of these findings in the context of Ae. albopictus life history and their utility in optimizing male mosquito release strategies.

Shared larval rearing environment, sex, female size and genetic diversity shape Ae. albopictus bacterial microbiota

The Asian tiger mosquito Aedes albopictus became of public health concern as it can replicate and transmit viral and filarial pathogens with a strong invasive success over the world. Various strategies have been proposed to reduce mosquito population's vectorial capacity. Among them, symbiotic control of mosquito borne disease offers promising perspectives. Such method is likely to be affected by the dynamics of mosquito-associated symbiotic communities, which might in turn be affected by host genotype and environment. Our previous study suggested a correlation between mosquitoes’ origin, genetic diversity and midgut bacterial diversity. To distinguish the impact of those factors, we have been studying the midgut bacterial microbiota of two Ae. albopictus populations from tropical (La Réunion) and temperate (Montpellier) origins under controlled laboratory conditions. the two populations experienced random mating or genetic bottleneck. Microbiota composition did not highlight any variation of the α and β-diversities in bacterial communities related to host’s populations. However, sizes of the mosquitoes were negatively correlated with the bacterial α-diversity of females. Variations in mosquito sex were associated with a shift in the composition of bacterial microbiota. The females’ mosquitoes also exhibited changes in the microbiota composition according to their size and after experiencing a reduction of their genetic diversity. These results provide a framework to investigate the impact of population dynamics on the symbiotic communities associated with the tiger mosquito.

Mosquito pornoscopy: Observation and interruption of Aedes aegypti copulation to determine female polyandric event and mixed progeny

Ades aegypti is the most important arbovirus vector in the world, and new strategies are under evaluation. Biological studies mentioning the occurrence of a second mate in Aedes aegypti can interfere with vector control program planning, which involves male mosquito release technique. This study presents different experiments to show the occurrence of mixed progeny. Mixed male crosses (using a combination of different type of males in confinement with virgin females) showed no polyandric female. Individual crosses with male substitution in every gonotrophic cycle also did not show any polyandric female. Individual crosses with a 20 minutes interval, with subsequent male change, showed that only a few females presented mixed offspring. The copulation breach in three different moments, group A with full coitus length, group B the coitus was interrupted in 5-7 seconds after the start; and group C, which the copulation was interrupted 3 seconds after started. In summary, group A showed a majority of unique progeny from the first male; group B showed the higher frequency of mixed offspring and group C with the majority of the crosses belonging to the second male. To conclude, the occurrence of a viable second mate and mixed offspring is only possible when the copulation is interrupted; otherwise, the first mate is responsible for mixed progeny.

A mosquito sperm's journey from male ejaculate to egg: Mechanisms, molecules, and methods for exploration

The fate of mosquito sperm in the female reproductive tract has been addressed sporadically and incompletely, resulting in significant gaps in our understanding of sperm-female interactions that ultimately lead to fertilization. As with other Diptera, mosquito sperm have a complex journey to their ultimate destination, the egg. After copulation, sperm spend a short time at the site of insemination where they are hyperactivated and quickly congregate near the entrance of the spermathecal ducts. Within minutes, they travel up the narrow ducts to the spermathecae, likely through the combined efforts of female transport and sperm locomotion. The female nourishes sperm and maintains them in these permanent storage organs for her entire life. When she is ready, the female coordinates the release of sperm with ovulation, and the descending egg is fertilized. Although this process has been well studied via microscopy, many questions remain regarding the molecular processes that coordinate sperm motility, movement through the reproductive tract, maintenance, and usage. In this review, we describe the current understanding of a mosquito sperm's journey to the egg, highlighting gaps in our knowledge of mosquito reproductive biology. Where insufficient information is available in mosquitoes, we describe analogous processes in other organisms, such as Drosophila melanogaster, as a basis for comparison, and we suggest future areas of research that will illuminate how sperm successfully traverse the female reproductive tract. Such studies may yield molecular targets that could be manipulated to control populations of vector species. Mol. Reprod. Dev. 83: 897-911, 2016 © 2016 Wiley Periodicals, Inc.

Mission Accomplished? We Need a Guide to the 'Post Release' World of Wolbachia for Aedes-borne Disease Control

Historically, sustained control of Aedes aegypti, the vector of dengue, chikungunya, yellow fever, and Zika viruses, has been largely ineffective. Subsequently, two novel 'rear and release' control strategies utilizing mosquitoes infected with Wolbachia are currently being developed and deployed widely. In the incompatible insect technique, male Aedes mosquitoes, infected with Wolbachia, suppress populations through unproductive mating. In the transinfection strategy, both male and female Wolbachia-infected Ae. aegypti mosquitoes rapidly infect the wild population with Wolbachia, blocking virus transmission. It is critical to monitor the long-term stability of Wolbachia in host populations, and also the ability of this bacterium to continually inhibit virus transmission. Ongoing release and monitoring programs must be future-proofed should political support weaken when these vectors are successfully controlled.

Rapid evolution of female-biased genes among four species of Anopheles malaria mosquitoes

Understanding how phenotypic differences between males and females arise from the sex-biased expression of nearly identical genomes can reveal important insights into the biology and evolution of a species. Among Anopheles mosquito species, these phenotypic differences include vectorial capacity, as it is only females that blood feed and thus transmit human malaria. Here, we use RNA-seq data from multiple tissues of four vector species spanning the Anopheles phylogeny to explore the genomic and evolutionary properties of sex-biased genes. We find that, in these mosquitoes, in contrast to what has been found in many other organisms, female-biased genes are more rapidly evolving in sequence, expression, and genic turnover than male-biased genes. Our results suggest that this atypical pattern may be due to the combination of sex-specific life history challenges encountered by females, such as blood feeding. Furthermore, female propensity to mate only once in nature in male swarms likely diminishes sexual selection of post-reproductive traits related to sperm competition among males. We also develop a comparative framework to systematically explore tissue- and sex-specific splicing to document its conservation throughout the genus and identify a set of candidate genes for future functional analyses of sex-specific isoform usage. Finally, our data reveal that the deficit of male-biased genes on the X Chromosomes in Anopheles is a conserved feature in this genus and can be directly attributed to chromosome-wide transcriptional regulation that de-masculinizes the X in male reproductive tissues.

The importance of male body size on sperm uptake and usage, and female fecundity in Aedes aegypti and Aedes albopictus

Background

Adult mosquito density is a critical factor in the transmission of arboviruses by container Aedes spp. mosquitoes. Female fecundity drives population growth, and therefore contributes to adult mosquito density. Previous studies have focused on female body size as the major determinant of fecundity, paying little attention to male condition. In this study, we examined the effects of male body size on the abundance of sperm in spermatheca, depletion of sperm over time, and female fecundity.

Methods

We generated males in two size classes using different larval densities, and allowed them to mate with females generated from a moderately dense larval environment. We counted sperm in female spermatheca in a sample of females immediately after mating, then every week for four weeks post-mating. We provided weekly blood meals to females and determined their fecundity over four weeks after the initial blood meal.

Results

We found significantly more sperm in Aedes albopictus females than in Aedes aegypti, and detected depletion of sperm in Ae. aegypti, but not in Ae. albopictus. We did not see significant differences in number of sperm in spermathecae in relation to male body size in either species over subsequent gonotrophic cycles. We found a significant effect of male body size on fecundity in Ae. albopictus, but not Ae. aegypti, with a 46 % increase in fecundity for female Ae. albopictus offered four blood meals.

Conclusions

Our results suggest substantial differences in the mating biology of these ecologically similar species and the importance of considering males in understanding female fecundity. The substantial increase in fecundity in Ae. albopictus has implications for population growth, estimating vector density, and modeling the transmission of pathogens.

Combining the Sterile Insect Technique with the Incompatible Insect Technique: III-Robust Mating Competitiveness of Irradiated Triple Wolbachia-Infected Aedes albopictus Males under Semi-Field Conditions

Combination of the sterile insect technique with the incompatible insect technique is considered to be a safe approach to control Aedes albopictus populations in the absence of an accurate and scalable sex separation system or genetic sexing strain. Our previous study has shown that the triple Wolbachia-infected Ae. albopictus strain (wAlbA, wAlbB and wPip) was suitable for mass rearing and females could be completely sterilized as pupae with a radiation dose of at least 28 Gy. However, whether this radiation dose can influence the mating competitiveness of the triple infected males was still unknown. In this study we aimed to evaluate the effects of irradiation on the male mating competitiveness of the triple infected strain under laboratory and semi-field conditions. The results herein indicate that irradiation with a lower, female-sterilizing dose has no negative impact on the longevity of triple infected males while a reduced lifespan was observed in the wild type males (wAlbA and wAlbB) irradiated with a higher male-sterilizing dose, in small cages. At different sterile: fertile release ratios in small cages, triple-infected males induced 39.8, 81.6 and 87.8% sterility in a wild type female population at 1:1, 5:1 and 10:1 release ratios, respectively, relative to a fertile control population. Similarly, irradiated triple infected males induced 31.3, 70.5 and 89.3% sterility at 1:1, 5:1 and 10:1 release ratios, respectively, again relative to the fertile control. Under semi-field conditions at a 5:1 release ratio, relative to wild type males, the mean male mating competitiveness index of 28 Gy irradiated triple-infected males was significantly higher than 35 Gy irradiated wild type males, while triple infected males showed no difference in mean mating competitiveness to either irradiated triple-infected or irradiated wild type males. An unexpected difference was also observed in the relative male mating competitiveness of the triple infected strain after irradiation at 28 Gy dose in small vs large cages, with a higher male mating competitiveness index calculated from results of experiments in the large cages. Based on these results, we consider that the male mating performance of the triple infected strain after irradiation at 28 Gy, a dose required for complete female sterility and the avoidance of population replacement, is approximately equal to that of the wild type males under semi-field conditions. Though field evaluation is required, this suggests that the triple infected strain is suitable for irradiation and release as part of a combined SIT-IIT approach to Ae. albopictus control.

Historical inability to control Aedes aegypti as a main contributor of fast dispersal of chikungunya outbreaks in Latin America

The arrival of chikungunya fever (CHIKF) in Latin American countries has been expected to trigger epidemics and challenge health systems. Historically considered as dengue-endemic countries, abundant Aedes aegypti populations make this region highly vulnerable to chikungunya virus (CHIKV) circulation. This review describes the current dengue and CHIKF epidemiological situations, as well as the role of uncontrolled Ae. aegypti and Aedes albopictus vectors in spreading the emerging CHIKV. Comments are included relating to the vector competence of both species and failures of surveillance and vector control measures. Dengue endemicity is a reflection of these abundant and persistent Aedes populations that are now spreading CHIKV in the Americas. This article forms part of a symposium in Antiviral Research on "Chikungunya discovers the New World."

Back to the future: the sterile insect technique against mosquito disease vectors

With the global burden of mosquito-borne diseases increasing, and some conventional vector control tools losing effectiveness, the sterile insect technique (SIT) is a potential new tool in the arsenal. Equipment and protocols have been developed and validated for efficient mass-rearing, irradiation and release of Aedines and Anophelines that could be useful for several control approaches. Assessment of male quality is becoming more sophisticated, and several groups are well advanced in pilot site selection and population surveillance. It will not be long before SIT feasibility has been evaluated in various settings. Until perfect sexing mechanisms exist, combination of Wolbachia-induced phenotypes, such as cytoplasmic incompatibility and pathogen interference, and irradiation may prove to be the safest solution for population suppression.

Asymmetric Mating Interference between Two Related Mosquito Species: Aedes (Stegomyia) albopictus and Aedes (Stegomyia) cretinus

Aedes (Stegomyia) albopictus (Skuse) and Aedes (Stegomyia) cretinus Edwards are closely related mosquito species with common morphological features and bio-ecological similarities. Recent mosquito surveillance in Athens, Greece, showed that they are sympatric mosquito species, with Ae. albopictus [corrected] developing quite higher population densities than Ae. cretinus [corrected] . The potential of mating interference between these species was investigated by reciprocal and homologous mating experiments in cages under laboratory conditions. In non-choice interspecific crosses (groups of males and females) females of both species produced sterile eggs. Insemination rate was 58% for Ae. cretinus females and only 1% for Ae. albopictus [corrected] females. Aedes albopictus males were sexually aggressive and inseminated Ae. cretinus females (31%) in choice experiments, where males of one species had access to mate with females of both species. Whereas, interspecific mating of Ae. albopictus [corrected] females with Ae. cretinus males in the co-occurrence of Ae. cretinus females was weaker (4%). Aedes cretinus females from non-choice crossing with Ae. albopictus [corrected] or Ae. cretinus males were paired individually with conspecific males. The percentage of fertile Ae. cretinus females was 17.5% when had encaged before with Ae. albopictus [corrected] males, compared to 100% when Ae. cretinus [corrected] females were encaged with conspecific males only. Probable ecological consequences of asymmetric mating between these ecologically homologous species in nature are discussed.

Long-term Effects of Carbohydrate Availability on Mating Success of Newly Eclosed Aedes albopictus (Diptera: Culicidae) Males

Sugar availability varies greatly in nature, and determining how this affects male mosquito fitness is essential for understanding population dynamics. We allowed male Aedes albopictus (Skuse) carbohydrate access for increasing intervals of time immediately after eclosion and we evaluated their fitness by comparing mortality, mating success, and sperm transfer. We compared individual male Ae. albopictus, which were offered water or 20% sucrose solution for 24, 48, or 72 h. As predicted, there were significant increases in fitness for each additional day of sucrose access. Following sugar exposure, we allowed males daily access to three virgin females. We assessed mating success through observation of spermatozoa in the female spermathecae. When individuals of the same age were compared, males with sugar access exhibited significantly greater mating success than water-treated males in all treatments. The total number of spermathecae filled by males with sugar access in the 48- and 72-h treatments was also significantly greater on some days; these were 3-5 d posteclosion in the 48-h treatment and 5-6 d posteclosion in the 72-h treatment. We conclude that extended sugar access at eclosion is important for maximizing fitness in male Ae. albopictus and should be applicable to sterile male release efforts, especially when laboratory-reared males suffered from other disadvantages. We recommend retaining adult males for 3 d posteclosion prior to release to improve their mating success in male release initiatives.

Evidence of limited polyandry in a natural population of Aedes aegypti

The mosquito Aedes aegypti is a vector of yellow fever, dengue, and chikungunya. Control of the insect is crucial to stop the spread of dengue and chikungunya, so it is critically important to understand its mating behavior. Primarily, based on laboratory behavior, it has long been assumed that Ae. aegypti females mate once in their lifetime. However, multiple inseminations have been observed in semi-field and laboratory settings, and in closely related species. Here, we report the first evidence of polyandry in a natural population of Ae. aegypti. Female Ae. aegypti were captured around the New Orleans, LA, metropolitan area. They were offered a blood meal and allowed to lay eggs, which were reared to the third-instar larval stage. A parentage analysis using four microsatellite loci was performed. Out of 48 families, 3 showed evidence of multiple paternity. An expanded analysis of these three families found that one family group included offspring contributed by three fathers, and the other two included offspring from two fathers. This result establishes that polyandry can occur in a small proportion of Ae. aegypti females in a natural setting. This could complicate future genetic control efforts and has implications for sampling for population genetics.

Seasonal variation in size estimates of Aedes albopictus population based on standard mark-release-recapture experiments in an urban area on Reunion Island

The implementation of the sterile insect technique for area-wide vector control requires that natural population density be accurately estimated to determine both the appropriate time to treat and the adequate number of sterile males for release. Herein, we used mark-release-recapture (MRR) to derive seasonal abundance estimates of Aedes albopictus population sizes within a delimited geographical area in Reunion Island. Population size of Ae. albopictus was estimated through four mark-release-recapture experiments carried out separately in different seasons. Marked males and females were released each time, and recaptured using BG sentinel traps for six consecutive days. Data were used to estimate the population size using a conceptual model that incorporates the variation in daily mortality rates. The likely influence of environmental factors on the magnitude of catches and on population fluctuation was analyzed. A total of 2827 mosquitoes (1914 males and 913 females) were marked and released on four occasions during dry and wet seasons. After release, 138 males (7.21%) and 86 females (9.41%) of the marked specimens were recaptured in subsequent samplings. The effectiveness of the daily captures of wild and released mosquitoes was significantly influenced by meteorological conditions such as temperature, rainfall, wind speed and light intensity. The estimates of Ae. albopictus population size obtained with our model estimator ranged from 298 to 1238 males and 604 to 2208 females per ha, with seasonal variability - higher population size in the humid season. The presented results will be essential in designing more effective sterile male release strategies for long-term suppression of wild Ae. albopictus populations.

The effect of virus-blocking Wolbachia on male competitiveness of the dengue vector mosquito, Aedes aegypti

Background

The bacterial endosymbiont Wolbachia blocks the transmission of dengue virus by its vector mosquito Aedes aegypti, and is currently being evaluated for control of dengue outbreaks. Wolbachia induces cytoplasmic incompatibility (CI) that results in the developmental failure of offspring in the cross between Wolbachia-infected males and uninfected females. This increases the relative success of infected females in the population, thereby enhancing the spread of the beneficial bacterium. However, Wolbachia spread via CI will only be feasible if infected males are sufficiently competitive in obtaining a mate under field conditions. We tested the effect of Wolbachia on the competitiveness of A. aegypti males under semi-field conditions.

Methodology/Principal Findings

In a series of experiments we exposed uninfected females to Wolbachia-infected and uninfected males simultaneously. We scored the competitiveness of infected males according to the proportion of females producing non-viable eggs due to incompatibility. We found that infected males were equally successful to uninfected males in securing a mate within experimental tents and semi-field cages. This was true for males infected by the benign wMel Wolbachia strain, but also for males infected by the virulent wMelPop (popcorn) strain. By manipulating male size we found that larger males had a higher success than smaller underfed males in the semi-field cages, regardless of their infection status.

Conclusions/Significance

The results indicate that Wolbachia infection does not reduce the competitiveness of A. aegypti males. Moreover, the body size effect suggests a potential advantage for lab-reared Wolbachia-males during a field release episode, due to their better nutrition and larger size. This may promote Wolbachia spread via CI in wild mosquito populations and underscores its potential use for disease control.

Dengue is a tropical, potentially lethal disease transmitted by mosquitoes. A new control method involves the release of mosquitoes infected by the bacterium Wolbachia that blocks the transmission of the dengue virus to humans. However, possible negative effects of Wolbachia on mosquito reproductive success could substantially slow the spread of this bacterium in mosquito populations, reducing the feasibility of this method. We found that male mosquitoes infected by Wolbachia are equally successful in finding and mating with females within experimental tents and semi-field cages that mimic mosquito natural habitat. Moreover, larger, well-fed mosquitoes were more successful in semi-field cages, suggesting that Wolbachia mosquitoes that are reared in the lab, and are generally larger than wild mosquitoes, might have an advantage during the time they are being released in the field. Hence, in contrast to other control methods (e.g., the use of sterile males or genetically modified mosquitoes), the use of Wolbachia does not seem to compromise male performance, making it a candidate for disease control.

The smart aerial release machine, a universal system for applying the sterile insect technique

Background

Beyond insecticides, alternative methods to control insect pests for agriculture and vectors of diseases are needed. Management strategies involving the mass-release of living control agents have been developed, including genetic control with sterile insects and biological control with parasitoids, for which aerial release of insects is often required. Aerial release in genetic control programmes often involves the use of chilled sterile insects, which can improve dispersal, survival and competitiveness of sterile males. Currently available means of aerially releasing chilled fruit flies are however insufficiently precise to ensure homogeneous distribution at low release rates and no device is available for tsetse.

Methodology/Principal Findings

Here we present the smart aerial release machine, a new design by the Mubarqui Company, based on the use of vibrating conveyors. The machine is controlled through Bluetooth by a tablet with Android Operating System including a completely automatic guidance and navigation system (MaxNav software). The tablet is also connected to an online relational database facilitating the preparation of flight schedules and automatic storage of flight reports. The new machine was compared with a conveyor release machine in Mexico using two fruit flies species (Anastrepha ludens and Ceratitis capitata) and we obtained better dispersal homogeneity (% of positive traps, p<0.001) for both species and better recapture rates for Anastrepha ludens (p<0.001), especially at low release densities (<1500 per ha). We also demonstrated that the machine can replace paper boxes for aerial release of tsetse in Senegal.

Conclusions/Significance

This technology limits damages to insects and allows a large range of release rates from 10 flies/km² for tsetse flies up to 600 000 flies/km² for fruit flies. The potential of this machine to release other species like mosquitoes is discussed. Plans and operating of the machine are provided to allow its use worldwide.

Boosting the sterile insect technique to control mosquitoes

Mosquitoes are vectors of major diseases. Auto-dissemination recently proved very efficient to control Aedes species, using adult females contaminated with dissemination stations of juvenile hormone to treat breeding habitats, but cannot be used at large scales. Here we propose to combine it to the Sterile Insect Technique (SIT) to create a new control concept, named 'boosted SIT' that might enable the area-wide eradication of mosquitoes and many other vectors and insect pests.

Male reproductive biology of Aedes mosquitoes

Among Aedes mosquitoes are species responsible for transmission of serious pathogens to humans. To cope with the current threats to long-term effectiveness of the traditional vector control methods, non-conventional control strategies are being developed. These include autocidal control such as the release of sterile males (sterile insect technique) and the release of Wolbachia-infected males to induce sexual sterility (incompatible insect technique) and pathogen-refractory strain replacement variations using Wolbachia. Sterile male types of techniques particularly depend on released males' ability to successfully mate with wild females. For that reason, a good understanding of male mating biology, including a thorough understanding of the reproductive system and mating capacity, increases the likelihood of success of such genetic vector control programmes. Here we review the literature concerning the reproduction of Aedes mosquitoes with an emphasis on the male biology. We consider sexual maturation, mate finding, insemination, male reproductive capacity, and the occurrence of multiple matings. We also discuss which parameters are of greatest importance for the successful implementation of autocidal control methods and propose questions for future research.

Review: Improving our knowledge of male mosquito biology in relation to genetic control programmes

The enormous burden placed on populations worldwide by mosquito-borne diseases, most notably malaria and dengue, is currently being tackled by the use of insecticides sprayed in residences or applied to bednets, and in the case of dengue vectors through reduction of larval breeding sites or larviciding with insecticides thereof. However, these methods are under threat from, amongst other issues, the development of insecticide resistance and the practical difficulty of maintaining long-term community-wide efforts. The sterile insect technique (SIT), whose success hinges on having a good understanding of the biology and behaviour of the male mosquito, is an additional weapon in the limited arsenal against mosquito vectors. The successful production and release of sterile males, which is the mechanism of population suppression by SIT, relies on the release of mass-reared sterile males able to confer sterility in the target population by mating with wild females. A five year Joint FAO/IAEA Coordinated Research Project brought together researchers from around the world to investigate the pre-mating conditions of male mosquitoes (physiology and behaviour, resource acquisition and allocation, and dispersal), the mosquito mating systems and the contribution of molecular or chemical approaches to the understanding of male mosquito mating behaviour. A summary of the existing knowledge and the main novel findings of this group is reviewed here, and further presented in the reviews and research articles that form this Acta Tropica special issue.

Sexually dimorphic body size and development time plasticity in Aedes mosquitoes (Diptera: Culicidae)

BACKGROUND

Sexual size dimorphism (SSD) in insects often accompanies a sexual difference in development time, sexual bimaturism (SBM).

GOAL

To determine whether three Aedes mosquito species have similar plasticity in SSD, attain sexual dimorphism through similar strategies, and whether SSD and SBM are associated.

ORGANISMS

Aedes albopictus, Aedes aegypti, and Aedes triseriatus (Diptera: Culicidae).

METHODS

In four different food availability environments, we quantified plastic responses of relative growth rate (RGR), development time, and adult body size in individually reared males and females.

RESULTS

Food availability affected RGR differently for the sexes for all three species. The RGR of males and females differed significantly in the 0.1 g/L food treatment. This difference did not account for observed SSD. Food levels over which the largest changes in RGR were observed differed among the species. Male and female adult mass and development time were jointly affected by food availability in a pattern that differed among the three species, so that degree of SSD and SBM changed differentially with food availability for all three species. Development time was generally less sexually dimorphic than mass, particularly in A. albopictus. At lower food levels, A. aegypti and A. triseriatus had accentuated dimorphism in development time. These results, combined with our knowledge of mosquito life history, suggest that a direct benefit of SBM is improbable for mosquitoes and that the observed intersexual differences in development time are more likely byproducts of selection for SSD.

Male mating performance and cytoplasmic incompatibility in a wPip Wolbachia trans-infected line of Aedes albopictus (Stegomyia albopicta)

Wolbachia pipientis Hertig (Rickettsiales: Rickettsiaceae) is a maternally inherited endosymbiont of a large number of insects and other arthropods that induces various effects on host reproductive biology. Among these, cytoplasmic incompatibility (CI) is a form of sterility induced in eggs produced by mating between infected males and females uninfected or infected by an incompatible Wolbachia strain. This phenomenon has been proposed as a potential way to produce functionally sterile males to be used in genetic control programmes. In this paper, we report on experiments carried out to evaluate the mating performances of males of an Aedes albopictus (Stegomyia albopicta) (Diptera: Culicidae) line (ARwP), harbouring a new Wolbachia infection [the wPip strain from Culex pipiens Linnaeus (Diptera: Culicidae)], in comparison with naturally infected males (SR line). ARwP males did not differ from SR males with regard to insemination capacity. Mating competitiveness did not differ significantly between lines in either laboratory or greenhouse conditions. Moreover, crosses with SR females were characterized by a 100% CI regardless of ARwP male age. All of these findings suggest that ARwP males may represent a very efficient tool for control programmes against Ae. albopictus based on the release of functionally sterile males.

Reproductive strategies of Aedes albopictus (Diptera: Culicidae) and implications for the sterile insect technique

Male insects are expected to optimize their reproductive strategy according to the availability of sperm or other ejaculatory materials, and to the availability and reproductive status of females. Here, we investigated the reproductive strategy and sperm management of male and virgin female Aedes albopictus, a mosquito vector of chikungunya and dengue viruses. The dynamics of semen transfer to the female bursa inseminalis and spermathecae were observed. Double-mating experiments were conducted to study the effect of time lapsed or an oviposition event between two copulations on the likelihood of a female double-insemination and the use of sperm for egg fertilization; untreated fertile males and radio-sterilised males were used for this purpose. Multiple inseminations and therefore the possibility of sperm competition were limited to matings closely spaced in time. When two males consecutively mated the same female within a 40 min interval, in ca. 15% of the cases did both males sire progeny. When the intervals between the copulations were longer, all progeny over several gonotrophic cycles were offspring of the first male. The mating behavior of males was examined during a rapid sequence of copulations. Male Ae. albopictus were parceling sperm allocation over several matings; however they would also attempt to copulate with females irrespective of the available sperm supply or accessory gland secretion material. During each mating, they transferred large quantities of sperm that was not stored for egg fertilization, and they attempted to copulate with mated females with a low probability of transferring their genes to the next generation. The outcomes of this study provided in addition some essential insights with respect to the sterile insect technique (SIT) as a vector control method.

Transient population dynamics of mosquitoes during sterile male releases: modelling mating behaviour and perturbations of life history parameters

The release of genetically-modified or sterile male mosquitoes offers a promising form of mosquito-transmitted pathogen control, but the insights derived from our understanding of male mosquito behaviour have not fully been incorporated into the design of such genetic control or sterile-male release methods. The importance of aspects of male life history and mating behaviour for sterile-male release programmes were investigated by projecting a stage-structured matrix model over time. An elasticity analysis of transient dynamics during sterile-male releases was performed to provide insight on which vector control methods are likely to be most synergistic. The results suggest that high mating competitiveness and mortality costs of released males are required before the sterile-release method becomes ineffective. Additionally, if released males suffer a mortality cost, older males should be released due to their increased mating capacity. If released males are of a homogenous size and size-assortative mating occurs in nature, this can lead to an increase in the abundance of large females and reduce the efficacy of the population-suppression effort. At a high level of size-assortative mating, the disease transmission potential of the vector population increases due to male releases, arguing for the release of a heterogeneously-sized male population. The female population was most sensitive to perturbations of density-dependent components of larval mortality and female survivorship and fecundity. These findings suggest source reduction might be a particularly effective complement to mosquito control based on the sterile insect technique (SIT). In order for SIT to realize its potential as a key component of an integrated vector-management strategy to control mosquito-transmitted pathogens, programme design of sterile-male release programmes must account for the ecology, behaviour and life history of mosquitoes. The model used here takes a step in this direction and can easily be modified to investigate additional aspects of mosquito behaviour or species-specific ecology.

Molecular evidence of polyandry in the citrus mealybug, Planococcus citri (Hemiptera: Pseudococcidae)

The occurrence of polyandry in Planococcuscitri, presumed by earlier observations of mating behavior, was confirmed using microsatellite genotyping of pools of over 400 eggs resulting from controlled crosses of one female with two males. The genetic contribution of both mated males was confirmed in 13 out of 43 crosses. In three crosses it was possible to determine that only the first male fertilized the eggs, which may be due to sperm competition or unviable sperm supply. The microsatellite analysis also allowed the confirmation of aspects of the chromosomal inheritance detected previously in cytogenetic studies in Planococcuscitri, namely that only one of the alleles is transmitted by the male, indicating that the males are functionally haploid, supporting the observation of Paternal Genome Elimination (PGE) in these insects.

The sterile insect technique for controlling populations of Aedes albopictus (Diptera: Culicidae) on Reunion Island: mating vigour of sterilized males

Reunion Island suffers from high densities of the chikungunya and dengue vector Aedes albopictus. The sterile insect technique (SIT) offers a promising strategy for mosquito-borne diseases prevention and control. For such a strategy to be effective, sterile males need to be competitive enough to fulfil their intended function by reducing wild mosquito populations in natura. We studied the effect of irradiation on sexual maturation and mating success of males, and compared the sexual competitiveness of sterile versus wild males in the presence of wild females in semi-field conditions. For all untreated or sterile males, sexual maturation was completed within 13 to 20 h post-emergence and some males were able to inseminate females when 15 h old. In the absence of competition, untreated and sterile males were able to inseminate the same number of virgin females during 48 h, in small laboratory cages: an average of 93% of females was inseminated no matter the treatment, the age of males, and the sex ratio. Daily mating success of single sterile males followed the same pattern as for untreated ones, although they inseminated significantly fewer females after the ninth day. The competitiveness index of sterile males in semi-field conditions was only 0.14 when they were released at 1-day old, but improved to 0.53 when the release occurred after a 5-day period in laboratory conditions. In SIT simulation experiments, a 5∶1 sterile to wild male ratio allowed a two-fold reduction of the wild population’s fertility. This suggests that sterile males could be sufficiently competitive to mate with wild females within the framework of an SIT component as part of an AW-IPM programme for suppressing a wild population of Ae. albopictus in Reunion Island. It will be of interest to minimise the pre-release period in controlled conditions to ensure a good competitiveness without increasing mass rearing costs.