Male swarming aggregation pheromones increase female attraction and mating success among multiple African malaria vector mosquito species

An overview of technologies available to monitor behaviours of mosquitoes

Mosquito-borne diseases such as malaria, dengue, Zika, and chikungunya cause significant morbidity and mortality globally, resulting in over 600,000 deaths from malaria and around 36,000 deaths from dengue each year, with millions of people infected annually, leading to substantial economic losses. The existing mosquito control measures, such as long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS), helped to reduce the infections. However, mosquito-borne diseases are still among the deadliest diseases, forcing us to improve the existing control methods and look for alternative methods simultaneously. Advanced monitoring techniques, including remote sensing, and geographic information systems (GIS) have significantly enhanced the efficiency and effectiveness of mosquito control measures. Mosquitoes' behavioural traits, such as locomotion, blood-feeding, and fertility are the key determinants of disease transmission and epidemiology. Technological advancements, such as high-resolution cameras, infrared imaging, and artificial intelligence (AI) driven object detection models, including groundbreaking convolutional neural networks, have provided efficient and precise options to monitor various mosquito behaviours, including locomotion, oviposition, fertility, and host-seeking. However, they are not commonly employed in mosquito-based research. This review highlights the novel and significant advancements in behaviour-monitoring tools, mostly from the last decade, due to cutting-edge video monitoring technology and artificial intelligence. These advancements can offer enhanced accuracy, efficiency, and the ability to quickly process large volumes of data, enabling detailed behavioural analysis over extended periods and large sample sizes, unlike traditional manual methods prone to human error and labour-intensive. The use of behaviour-assaying techniques can support or replace existing monitoring techniques and directly contribute to improving control measures by providing more accurate and real-time data on mosquito activity patterns and responses to interventions. This enhanced understanding can help establish the role of behavioural changes in improving epidemiological models, making them more precise and dynamic. As a result, mosquito management strategies can become more adaptive and responsive, leading to more effective and targeted interventions. Ultimately, this will reduce disease transmission and significantly improve public health outcomes.

No sexual pheromones in Anopheles mosquitoes?

Swarming behavior is the cornerstone of the anopheline mating system. At dusk, males congregate in monospecific swarms in which females come to find a mate once in their lives. Although many Anopheles species coexist in sympatry, hybrids are infrequent, suggesting the existence of strong premating reproductive barriers. Chemical cues, particularly pheromones, often play a crucial role in bringing sexes together in a species-specific manner among insects. While the existence of sexual pheromones in Anopheles species has been postulated, only a few studies developed experimental designs to investigate their presence. Here, we discuss the contrasting and debatable findings regarding both long-range and contact sex pheromones in the context of swarm ecology in Anopheles species.

Lipid Metabolism in Insect Vectors of Diseases

According to the World Health Organization vector-borne diseases account for more than 17% of all infectious diseases, causing more than 700,000 deaths annually. Vectors are organisms that are able to transmit infectious pathogens between humans, or from animals to humans. Many of these vectors are hematophagous insects, which ingest the pathogen from an infected host during a blood meal, and later transmit it into a new host. Malaria, dengue, African trypanosomiasis, yellow fever, leishmaniasis, Chagas disease, and many others are examples of diseases transmitted by insects.Both the diet and the infection with pathogens trigger changes in many metabolic pathways, including lipid metabolism, compared to other insects. Blood contains mostly proteins and is very poor in lipids and carbohydrates. Thus, hematophagous insects attempt to efficiently digest and absorb diet lipids and also rely on a large de novo lipid biosynthesis based on utilization of proteins and carbohydrates as carbon source. Blood meal triggers essential physiological processes as molting, excretion, and oogenesis; therefore, lipid metabolism and utilization of lipid storage should be finely synchronized and regulated regarding that, in order to provide the necessary energy source for these events. Also, pathogens have evolved mechanisms to hijack essential lipids from the insect host by interfering in the biosynthesis, catabolism, and transport of lipids, which pose challenges to reproduction, survival, fitness, and other insect traits.In this chapter, we have tried to collect and highlight the current knowledge and recent discoveries on the metabolism of lipids in insect vectors of diseases related to the hematophagous diet and pathogen infection.

Microcapsules of mesoporous silica and cyclodextrin modified loaded with nonanal and decanal for effective control of Sitotroga cerealella in grain storage environments

BACKGROUND

The Angoumois grain moth, Sitotroga cerealella, is a destructive pest of maize, wheat, and rice, causing economic losses and threatening food security. This study aimed to develop and characterize microcapsules of mesoporous silica nanospheres (MSN) and cyclodextrin-modified mesoporous silica nanospheres (CDMSN) containing two aldehydes, nonanal and decanal, found in plant essential oils, to assess their attractiveness to S. cerealella populations.

RESULTS

Microcapsules with 2:1 ratio of nonanal and decanal exhibited an average encapsulation efficiency of 39.82% for MSN loaded with nonanal and decanal (MSN-ND) and 46.10% for CDMSN loaded with nonanal and decanal (CDMSN-ND). They have an elliptical shape with particle sizes of 115 nm for MSN and 175 nm for CDMSN. Gas chromatography-mass spectrometry analysis revealed in vitro release of nonanal in MSN at 96.24% and decanal at 96.42% by the 36th day. CDMSN showed releases of 93.83% for nonanal and 93.74% for decanal by the 50th day. CDMSN-ND attracted adult S. cerealella for 43 days, while MSN-ND remained effective for 29 days. In mass trapping assays in simulated grain warehouse, both MSN-ND and CDMSN-ND trapped over 50% of the adult population within 7 days, significantly reducing grain infestation rates below 10% by inhibiting F1 adult emergence. At temperatures ranging from 20 °C to 35 °C, both microcapsules exhibited significant and effective attraction rates for S. cerealella. Stored wheat seeds treated with CDMSN and CDMSN-ND over 1 year showed no significant differences in key germination parameters.

CONCLUSION

Microencapsulated nonanal and decanal offer a promising, sustainable approach for controlling S. cerealella infestation in stored grains, contributing to global food security. © 2024 Society of Chemical Industry.

An expanded neurogenetic toolkit to decode olfaction in the African malaria mosquito Anopheles gambiae

Anopheles gambiae uses its sense of smell to hunt humans. We report a two-step method yielding cell-type-specific driver lines for enhanced neuroanatomical and functional studies of its olfactory system. We first integrated a driver-responder-marker (DRM) system cassette consisting of a linked T2A-QF2 driver, QUAS-GFP responder, and a gut-specific transgenesis marker into four chemoreceptor genes (Ir25a, Ir76b, Gr22, and orco) using CRISPR-Cas9-mediated homology-directed repair. The DRM system facilitated rapid selection of in-frame integrations via screening for GFP+ olfactory sensory neurons (OSNs) in G1 larval progeny, even at genomic loci such as orco where we found the transgenesis marker was not visible. Next, we converted these DRM integrations into T2A-QF2 driver-marker lines by Cre-loxP excision of the GFP responder, making them suitable for binary use in transcuticular calcium imaging. These cell-type-specific driver lines tiling key OSN subsets will support systematic efforts to decode olfaction in this prolific malaria vector.

Karyotypic stasis and swarming influenced the evolution of viral tolerance in a species-rich bat radiation

The emergence of COVID-19 and severe acute respiratory syndrome (SARS) has prioritized understanding bats' viral tolerance. Myotis bats are exceptionally species rich and have evolved viral tolerance. They also exhibit swarming, a cryptic behavior where large, multi-species assemblages gather for mating, which has been hypothesized to promote interspecific hybridization. To resolve the coevolution of genome architecture and their unusual antiviral tolerance, we undertook a phylogenomic analysis of 60 Old World Myotis genomes. We demonstrate an extensive history of introgressive hybridization that has replaced the species phylogeny across 17%-93% of the genome except for pericentromeric regions of macrochromosomes. Introgression tracts were enriched on microchromosome regions containing key antiviral pathway genes overexpressed during viral challenge experiments. Together, these results suggest that the unusual Myotis karyotype may have evolved to selectively position immune-related genes in high recombining genomic regions prone to introgression of divergent alleles, including a diversity of interleukin loci responsible for the release of pro-inflammatory cytokines.

Female Aedes aegypti mosquitoes use communal cues to manage population density at breeding sites

Where a female mosquito lays her eggs creates the conditions for reproductive success. Here we identify a communal behavior among ovipositing female mosquitoes. When choosing equal breeding sites, gravid Aedes aegypti aggregate more often than expected. This aggregation occurs when water contact is restricted and does not require the presence of eggs. Instead, the aggregation is regulated by the number of females present at the breeding site. Using assays with both occupied and empty oviposition sites, we show that the Orco olfactory co-receptor and a carbon dioxide receptor, Gr3, detect the presence of mosquitoes. orco mutants aggregate more often in empty sites, suggesting attractive olfactory cues influence females to associate with one another. Gr3 mutant females do not prefer either site, suggesting that the CO2 receptor is necessary to evaluate mosquito population density at breeding sites. Further, raising CO2 levels is sufficient to cause wild-type mosquitoes to avoid empty oviposition sites. Our results demonstrate that female mosquitoes can regulate their own population density at breeding sites using attractive and repellent communal cues.

Impact of the microbiome on mosquito-borne diseases

Mosquito-borne diseases present a significant threat to human health, with the possibility of outbreaks of new mosquito-borne diseases always looming. Unfortunately, current measures to combat these diseases such as vaccines and drugs are often either unavailable or ineffective. However, recent studies on microbiomes may reveal promising strategies to fight these diseases. In this review, we examine recent advances in our understanding of the effects of both the mosquito and vertebrate microbiomes on mosquito-borne diseases. We argue that the mosquito microbiome can have direct and indirect impacts on the transmission of these diseases, with mosquito symbiotic microorganisms, particularly Wolbachia bacteria, showing potential for controlling mosquito-borne diseases. Moreover, the skin microbiome of vertebrates plays a significant role in mosquito preferences, while the gut microbiome has an impact on the progression of mosquito-borne diseases in humans. As researchers continue to explore the role of microbiomes in mosquito-borne diseases, we highlight some promising future directions for this field. Ultimately, a better understanding of the interplay between mosquitoes, their hosts, pathogens, and the microbiomes of mosquitoes and hosts may hold the key to preventing and controlling mosquito-borne diseases.

Data-driven networking of global transcriptomics and male sexual development in the main malaria vector, Anopheles funestus

Deaths from malaria remain staggering despite global support that drives research into new territories. One major gap is our understanding of the sexual biological aspects of the male mosquito, which maintain the vector population solidity. Although Anopheles funestus s.s. is an extremely efficient African vector, little is known about the network between its sexual physiology and gene expression. The Culicidae male's sexual maturity involves a suite of physiological changes, such as genitalia rotation that is necessary for successful mating to occur. We show that mating success is guided by genes and physiological plasticity. Transcriptome analysis between newly emerged males (immature) versus males with rotating genitalia (maturing) provides insight into possible molecular mechanisms regulating male sexual behaviour. Putative transcripts that were associated with male sexual maturation were identified and validated. The discovery of the functions of these transcripts could lead to identifying potential targets for innovative vector control interventions, and mosquito population suppression.

Swarming and mate selection in Anopheles gambiae mosquitoes (Diptera: Culicidae)

Treating both male and female Anopheles gambiae as if they are "boids" (a computer program that mimics flocking in birds) explains much of the swarming and mating behavior in this important group of malaria vectors. It is suggested that species specific swarm sites act as the mate recognition system in anophelines and it is proposed that virgin females respond to the swarm site per se rather than the swarm itself. Given the high operational sex ratio and the inability of any male to dominate all females within the swarm, it is considered that chance, rather than sexual selection, is the most important determinant of mating. The male being in the swarm may be a sufficiently strong signal to the female of his fitness, so that more elaborate sexual selection is unnecessary. The possibility of alternative mechanisms for mating may also exist but need to be investigated further. Given the importance of swarms as the isolating mechanism between species, emphasis should be placed on determining the characteristics of swarm sites and markers between them.

An angiotensin converting enzyme homolog is required for volatile pheromone detection, odorant binding protein secretion and normal courtship behavior in Drosophila melanogaster

In many arthropods, including insects responsible for transmission of human diseases, behaviors that include mating, aggregation, and aggression are triggered by detection of pheromones. Extracellular odorant binding proteins are critical for pheromone detection in many insects and are secreted into the fluid bathing the olfactory neuron dendrites. In Drosophila melanogaster, the odorant binding protein LUSH is essential for normal sensitivity to the volatile sex pheromone, 11-cis vaccenyl acetate (cVA). Using a genetic screen for cVA pheromone insensitivity, we identified ANCE-3, a homolog of human angiotensin converting enzyme that is required for detection of cVA pheromone. The mutants have normal dose-response curves for food odors, although olfactory neuron amplitudes are reduced in all olfactory neurons examined. ance-3 mutants have profound delays in mating, and the courtship defects are primarily but not exclusively due to loss of ance-3 function in males. We demonstrate that ANCE-3 is required in the sensillae support cells for normal reproductive behavior, and that localization of odorant binding proteins to the sensillum lymph is blocked in the mutants. Expression of an ance-3 cDNA in sensillae support cells completely rescues the cVA responses, LUSH localization, and courtship defects. We show the courtship latency defects are not due to effects on olfactory neurons in the antenna nor mediated through ORCO receptors, but instead stem from ANCE-3-dependent effects on chemosensory sensillae in other body parts. These findings reveal an unexpected factor critical for pheromone detection with profound influence on reproductive behaviors.

Combinatorial encoding of odors in the mosquito antennal lobe

Among the cues that a mosquito uses to find a host for blood-feeding, the smell of the host plays an important role. Previous studies have shown that host odors contain hundreds of chemical odorants, which are detected by different receptors on the peripheral sensory organs of mosquitoes. But how individual odorants are encoded by downstream neurons in the mosquito brain is not known. We developed an in vivo preparation for patch-clamp electrophysiology to record from projection neurons and local neurons in the antennal lobe of Aedes aegypti. Combining intracellular recordings with dye-fills, morphological reconstructions, and immunohistochemistry, we identify different sub-classes of antennal lobe neurons and their putative interactions. Our recordings show that an odorant can activate multiple neurons innervating different glomeruli, and that the stimulus identity and its behavioral preference are represented in the population activity of the projection neurons. Our results provide a detailed description of the second-order olfactory neurons in the central nervous system of mosquitoes and lay a foundation for understanding the neural basis of their olfactory behaviors.

The sex pheromone heptacosane enhances the mating competitiveness of sterile Aedes aegypti males

BACKGROUND

Aedes aegypti is a vector that transmits various viral diseases, including dengue and Zika. The radiation-based sterile insect technique (SIT) has a limited effect on mosquito control because of the difficulty in irradiating males without reducing their mating competitiveness. In this study, the insect sex pheromone heptacosane was applied to Ae. aegypti males to investigate whether it could enhance the mating competitiveness of irradiated males.

METHODS

Heptacosane was smeared on the abdomens of Ae. aegypti males that were allowed to mate with untreated virgin females. The insemination rate was used to assess the attractiveness of heptacosane-treated males to females. The pupae were irradiated with different doses of X-rays and γ-rays, and the emergence, survival time, egg number, and hatch rate were detected to find the optimal dose of X-ray and γ-ray radiation. The males irradiated at the optimal dose were smeared with heptacosane, released in different ratios with untreated males, and mated with females. The effect of heptacosane on the mating competitiveness of irradiated mosquitoes was then evaluated by the hatch rate, induced sterility, and mating competitiveness index.

RESULTS

Applying heptacosane to Ae. aegypti males significantly increased the insemination rate of females by 20%. Pupal radiation did not affect egg number but significantly reduced survival time and hatch rate. The emergence of the pupae was not affected by X-ray radiation but was affected by γ-ray radiation. Pupae exposed to 60 Gy X-rays and 40 Gy γ-rays were selected for subsequent experiments. After 60 Gy X-ray irradiation or 40 Gy γ-ray irradiation, the average hatch rate was less than 0.1%, and the average survival time was more than 15 days. Moreover, at the same release ratio, the hatch rate of the irradiated group perfumed with heptacosane was lower than that of the group without heptacosane. Conversely, the male sterility and male mating competitiveness index were significantly increased due to the use of heptacosane.

CONCLUSIONS

The sex pheromone heptacosane enhanced the interaction between Ae. aegypti males and females. Perfuming males irradiated by X-rays or γ-rays with heptacosane led to a significant increase in mating competitiveness. This study provided a new idea for improving the application effect of SIT.

Traveling across Life Sciences with Acetophenone-A Simple Ketone That Has Special Multipurpose Missions

Each metabolite, regardless of its molecular simplicity or complexity, has a mission or function in the organism biosynthesizing it. In this review, the biological, allelochemical, and chemical properties of acetophenone, as a metabolite involved in multiple interactions with various (mi-cro)organisms, are discussed. Further, the details of its biogenesis and chemical synthesis are provided, and the possibility of its application in different areas of life sciences, i.e., the status quo of acetophenone and its simple substituted analogs, is examined. In particular, natural and synthetic simple acetophenone derivatives are analyzed as promising agrochemicals and useful scaffolds for drug research and development.

The chemical code for attracting Culex mosquitoes

Mosquitoes use chemical codes to locate and discriminate among vertebrate hosts to obtain a blood meal. Recent advances have allowed for the identification of the chemical codes used by mosquitoes to locate and discriminate humans from other vertebrate hosts. Humans are incidental “dead-end” hosts for the West Nile virus, which is maintained in an enzootic cycle, primarily through its transmission between infected birds by Culex mosquitoes. Host-seeking Culex mosquitoes are attracted to the odor of chicken, which are used in sentinel traps to monitor West Nile virus transmission. Using combined gas chromatography and electroantennography and mass spectrometry we identify a blend of volatile organic compounds present in chicken emanates, including mostly salient bioactive compounds previously identified in human emanates. When released at their identified ratios, this blend elicits behavioral responses of Culex pipiens molestus and Culex quinquefasciatus similar to that to the natural chicken odor. Tested under field conditions, this blend attract Culex spp. and other species of mosquitoes using birds among their hosts. This study provides evidence for conserved chemical codes for resource location by mosquitoes, and highlights the intricate role of CO2 for host-seeking mosquitoes. The identification of conserved chemical codes, which drive innate preference behaviors that are fundamental for survival and reproduction, provides important substrates for future control interventions targeting disease vector mosquitoes.

Kinetics of Odorant Recognition with a Graphene-Based Olfactory Receptor Mimicry

Malaria vector mosquito species rely on a handful of specific pheromones for mating; one of them, sulcatone (6-methyl-5-hepten-2-one), is also found in human exudation. Therefore, a complete understanding of the insect’s olfaction, and rapid real-time methods for odorant detection, are required. Here, we mimic the odorant recognition of the nerve cells of an insect’s antenna with a synthetic graphene-based bio-electro-interfacial odorant receptor. By this means, we obtain the kinetics of the genuine odorant recognition reaction and compare them to electro-antennogram data that represent the more complex scenario of a living insect. The odorant-binding proteins OBP 9A and 9B only associate with their ligands weakly, showing KDs of between 2.1 mM and 3 mM, while the binding kinetics of OBP proteins depend on the structural feature of a cystine knot and are modulated by the local milieu within a protein-aided enhancement zone.

Mosquito Olfactory Response Ensemble enables pattern discovery by curating a behavioral and electrophysiological response database

Many experimental studies have examined behavioral and electrophysiological responses of mosquitoes to odors. However, the differences across studies in data collection, processing, and reporting make it difficult to perform large-scale analyses combining data from multiple studies. Here we extract and standardize data for 12 mosquito species, along with Drosophila melanogaster for comparison, from over 170 studies and curate the Mosquito Olfactory Response Ensemble (MORE), publicly available at https://neuralsystems.github.io/MORE. We demonstrate the ability of MORE in generating biological insights by finding patterns across studies. Our analyses reveal that ORs are tuned to specific ranges of several physicochemical properties of odorants; the empty-neuron recording technique for measuring OR responses is more sensitive than the Xenopus oocyte technique; there are systematic differences in the behavioral preferences reported by different types of assays; and odorants tend to become less attractive or more aversive at higher concentrations.

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MORE is a database of behavioral and electrophysiological responses to odors

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MORE includes data from 170 studies covering 12 species of mosquitoes along with flies

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MORE shows differences in odor preferences measured with different assays

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Empty-neuron technique measures responses more sensitively than the oocyte technique

Reverse chemical ecology guides the screening for Grapholita molesta pheromone synergists

BACKGROUND

Pheromone-based management is a leading nonpesticidal strategy among integrated pest management options. Improving the potency of pheromone products by adding synergists would be a practical way to popularize pheromone-based management as well as to reduce pesticide use.

RESULTS

Using reverse chemical ecology, synergists for Grapholita molesta sex pheromone were screened. Combined molecular docking and in vitro binding assay led to the determination of four potentially active odorants showing high affinity to G. molesta pheromone binding protein 2 (GmolPBP2). Thereafter, the high affinity between Codlemone and GmolPBP2 was further verified by exploration of GmolPBP2-Codlemone interactions. As the only sex pheromone synergist validated by both laboratory behavioral tests and field trapping, Codlemone was used to optimize commercial sex attractants currently used in G. molesta control. The recommended formulation [(Z)-8-dodecenyl acetate:(E)-8-dodecenyl acetate:Codlemone = 95:4:10] was found to trap about five to six times more G. molesta adults than the commercial sex attractant [(Z)-8-dodecenyl acetate:(E)-8-dodecenyl acetate: (Z)-8-dodecenol = 95:4:1].

CONCLUSION

Codlemone is an excellent pheromone synergist that can be potentially sensed by GmolPBP2, which can remarkably improve the potency of G. molesta sex attractants. It is believed that the introduction of reverse chemical ecology would increase the chance of discovering pheromone synergists, promoting the development of more efficacious pheromone products that can be used in controlling G. molesta and beyond. © 2021 Society of Chemical Industry.

Artificial Light as a Modulator of Mosquito-Borne Disease Risk

Light is a fundamental cue regulating a host of biological responses. The artificial modification thereof demonstrably impacts a wide range of organisms. The use of artificial light is changing in type, extent and intensity. Insect vector-borne diseases remain a global scourge, but surprisingly few studies have directly investigated the interactions between artificial light and disease vectors, such as mosquitoes. Here we briefly overview the progress to date, which highlights that artificial light must be considered as a modulator of mosquito-borne disease risk. We discuss where the mechanisms may lie, and where future research could usefully be directed, particularly in advancing understanding of the biological effects of the light environment. Further understanding of how artificial light may modulate mosquito-borne disease risk may assist in employing and redesigning light regimes that do not increase, and may even mitigate, already significant disease burdens, especially in the developing world.

Swarming Behavior in Anopheles gambiae (sensu lato): Current Knowledge and Future Outlook

Effective management of insect disease vectors requires a detailed understanding of their ecology and behavior. In Anopheles gambiae sensu lato (s.l.) (Diptera: Culicidae) mating occurs during swarming, but knowledge of their mating behavior under natural conditions is limited. Mosquitoes mate in flight over specific landmarks, known as swarm markers, at particular locations. Swarms consist of males; the females usually approach the swarm and depart following copulation. The number of mating pairs per swarm is closely associated with swarm size. The shape and height of swarm markers vary and may depend on the environmental conditions at the swarm's location. Male-male interactions in mosquito swarms with similar levels of attractive flight activity can offer a mating advantage to some individuals. Flight tone is used by mosquitoes to recognize the other sex and choose a desirable mate. Clarifying these and other aspects of mosquito reproductive behavior can facilitate the development of population control measures that target swarming sites. This review describes what is currently known about swarming behavior in Anopheles gambiae s.l., including swarm characteristics; mating within and outside of swarms, insemination in females, and factors affecting and stimulating swarming.

Wild populations of malaria vectors can mate both inside and outside human dwellings

BACKGROUND

Wild populations of Anopheles mosquitoes are generally thought to mate outdoors in swarms, although once colonized, they also mate readily inside laboratory cages. This study investigated whether the malaria vectors Anopheles funestus and Anopheles arabiensis can also naturally mate inside human dwellings.

METHOD

Mosquitoes were sampled from three volunteer-occupied experimental huts in a rural Tanzanian village at 6:00 p.m. each evening, after which the huts were completely sealed and sampling was repeated at 11:00 p.m and 6 a.m. the next morning to compare the proportions of inseminated females. Similarly timed collections were done inside local unsealed village houses. Lastly, wild-caught larvae and pupae were introduced inside or outside experimental huts constructed inside two semi-field screened chambers. The huts were then sealed and fitted with exit traps, allowing mosquito egress but not entry. Mating was assessed in subsequent days by sampling and dissecting emergent adults caught indoors, outdoors and in exit traps.

RESULTS

Proportions of inseminated females inside the experimental huts in the village increased from approximately  60% at 6 p.m. to approximately 90% the following morning despite no new mosquitoes entering the huts after 6 p.m. Insemination in the local homes increased from approximately 78% to approximately 93% over the same time points. In the semi-field observations of wild-caught captive mosquitoes, the proportions of inseminated An. funestus were 20.9% (95% confidence interval [CI]: ± 2.8) outdoors, 25.2% (95% CI: ± 3.4) indoors and 16.8% (± 8.3) in exit traps, while the proportions of inseminated An. arabiensis were 42.3% (95% CI: ± 5.5) outdoors, 47.4% (95% CI: ± 4.7) indoors and 37.1% (CI: ± 6.8) in exit traps.

CONCLUSION

Wild populations of An. funestus and An. arabiensis in these study villages can mate both inside and outside human dwellings. Most of the mating clearly happens before the mosquitoes enter houses, but additional mating happens indoors. The ecological significance of such indoor mating remains to be determined. The observed insemination inside the experimental huts fitted with exit traps and in the unsealed village houses suggests that the indoor mating happens voluntarily even under unrestricted egress. These findings may inspire improved vector control, such as by targeting males indoors, and potentially inform alternative methods for colonizing strongly eurygamic Anopheles species (e.g. An. funestus) inside laboratories or semi-field chambers.

Cuticular hydrocarbons are associated with mating success and insecticide resistance in malaria vectors

Anopheles coluzzii females, important malaria vectors in Africa, mate only once in their lifetime. Mating occurs in aerial swarms with a high male-to-female ratio, where traits underlying male mating success are largely unknown. Here, we investigated whether cuticular hydrocarbons (CHCs) influence mating success in natural mating swarms in Burkina Faso. As insecticides are widely used in this area for malaria control, we also determined whether CHCs affect insecticide resistance levels. We find that mated males have higher CHC abundance than unmated controls, suggesting CHCs could be determinants of mating success. Additionally, mated males have higher insecticide resistance under pyrethroid challenge, and we show a link between resistance intensity and CHC abundance. Taken together, our results suggest that CHC abundance may be subject to sexual selection in addition to selection by insecticide pressure. This has implications for insecticide resistance management, as these traits may be sustained in the population due to their benefits in mating even in the absence of insecticides.

In this study, Adams et al. investigate the effect of cuticular hydrocarbons on mating success in natural mosquito mating swarms. These hydrocarbons confer both higher mating success and increased resistance to pyrethroid, suggesting sexual selection for insecticide resistance in this population secondary to mating success.

The Expression of Chemosensory Genes in Male Maxillary Palps of Anopheles coluzzii (Diptera: Culicidae) and An. quadriannulatus

Because of its importance as a malaria vector, Anopheles coluzzii's Coetzee & Wilkerson olfactory system has been studied extensively. Among this work is a series of studies comparing the expression of chemosensory genes in olfactory organs in females and/or males of these species. These have identified species- and female-biased chemosensory gene expression patterns. However, many questions remain about the role of chemosensation in male anopheline biology. To pave the way for future work we used RNAseq to compare chemosensory gene expression in the male maxillary palps of An. coluzzii and its sibling species An. quadriannulatus Theobald. As expected, the chemosensory gene repertoire is small in the male maxillary palps. Both species express the tuning receptors Or8 and Or28 at relatively high levels. The CO2 receptor genes Gr22-Gr24 are present in both species as well, although at much lower level than in females. Additionally, several chemoreceptors are species-specific. Gr37 and Gr52 are exclusive to An. coluzzii, whereas Or9 and Gr60 were detected only in An. quadriannulatus. Furthermore, several chemosensory genes show differential expression between the two species. Finally, several Irs, Grs, and Obps that show strong differential expression in the female palps, are absent or lowly expressed in the male palps. While many questions remain about the role of chemosensation in anopheline male biology, these results suggest that the male maxillary palps could have both a sex- and species-specific role in the perception of chemical stimuli. This work may guide future studies on the role of the male maxillary palp in these species.

Olfaction in Anopheles mosquitoes

As vectors of disease, mosquitoes are a global threat to human health. The Anopheles mosquito is the deadliest mosquito species as the insect vector of the malaria-causing parasite, which kills hundreds of thousands every year. These mosquitoes are reliant on their sense of smell (olfaction) to guide most of their behaviors, and a better understanding of Anopheles olfaction identifies opportunities for reducing the spread of malaria. This review takes a detailed look at Anopheles olfaction. We explore a range of topics from chemosensory receptors, olfactory neurons, and sensory appendages to behaviors guided by olfaction (including host-seeking, foraging, oviposition, and mating), to vector management strategies that target mosquito olfaction. We identify many research areas that remain to be addressed.

Mosquito sound communication: are male swarms loud enough to attract females?

Given the unsurpassed sound sensitivity of mosquitoes among arthropods and the sound source power required for long-range hearing, we investigated the distance over which female mosquitoes detect species-specific cues in the sound of station-keeping mating swarms. A common misunderstanding, that mosquitoes cannot hear at long range because their hearing organs are 'particle-velocity' receptors, has clouded the fact that particle velocity is an intrinsic component of sound whatever the distance to the sound source. We exposed free-flying Anopheles coluzzii females to pre-recorded sounds of male An. coluzzii and An. gambiae s.s. swarms over a range of natural sound levels. Sound levels tested were related to equivalent distances between the female and the swarm for a given number of males, enabling us to infer distances over which females might hear large male swarms. We show that females do not respond to swarm sound up to 48 dB sound pressure level (SPL) and that louder SPLs are not ecologically relevant for a swarm. Considering that swarms are the only mosquito sound source that would be loud enough to be heard at long range, we conclude that inter-mosquito acoustic communication is restricted to close-range pair interactions. We also showed that the sensitivity to sound in free-flying males is much enhanced compared to that of tethered ones.

Mosquito Attractants

Vector control and personal protection against anthropophilic mosquitoes mainly rely on the use of insecticides and repellents. The search for mosquito-attractive semiochemicals has been the subject of intense studies for decades, and new compounds or odor blends are regularly proposed as lures for odor-baited traps. We present a comprehensive and up-to-date review of all the studies that have evaluated the attractiveness of volatiles to mosquitoes, including individual chemical compounds, synthetic blends of compounds, or natural host or plant odors. A total of 388 studies were analysed, and our survey highlights the existence of 105 attractants (77 volatile compounds, 17 organism odors, and 11 synthetic blends) that have been proved effective in attracting one or several mosquito species. The exhaustive list of these attractants is presented in various tables, while the most common mosquito attractants - for which effective attractiveness has been demonstrated in numerous studies - are discussed throughout the text. The increasing knowledge on compounds attractive to mosquitoes may now serve as the basis for complementary vector control strategies, such as those involving lure-and-kill traps, or the development of mass trapping. This review also points out the necessity of further improving the search for new volatile attractants, such as new compound blends in specific ratios, considering that mosquito attraction to odors may vary over the life of the mosquito or among species. Finally, the use of mosquito attractants will undoubtedly have an increasingly important role to play in future integrated vector management programs.

Olfactory systems across mosquito species

There are 3559 species of mosquitoes in the world (Harbach 2018) but, so far, only a handful of them have been a focus of olfactory neuroscience and neurobiology research. Here we discuss mosquito olfactory anatomy and function and connect these to mosquito ecology. We highlight the least well-known and thus most interesting aspects of mosquito olfactory systems and discuss promising future directions. We hope this review will encourage the insect neuroscience community to work more broadly across mosquito species instead of focusing narrowly on the main disease vectors.

Modulation of odour-guided behaviour in mosquitoes

Mosquitoes are emerging as model systems with which to study innate behaviours through neuroethology and functional genomics. Decades of work on these disease vectors have provided a solid behavioural framework describing the distinct repertoire of predominantly odour-mediated behaviours of female mosquitoes, and their dependence on life stage (intrinsic factors) and environmental cues (extrinsic factors). The purpose of this review is to provide an overview of how intrinsic factors, including adult maturation, age, nutritional status, and infection, affect the attraction to plants and feeding on plant fluids, host seeking, blood feeding, supplemental feeding behaviours, pre-oviposition behaviour, and oviposition in female mosquitoes. With the technological advancements in the recent two decades, we have gained a better understanding of which volatile organic compounds are used by mosquitoes to recognise and discriminate among various fitness-enhancing resources, and characterised their neural and molecular correlates. In this review, we present the state of the art of the peripheral olfactory system as described by the neural physiology, functional genomics, and genetics underlying the demonstrated changes in the behavioural repertoire in female mosquitoes. The review is meant as a summary introduction to the current conceptual thinking in the field.