Mosquitoes Use Vision to Associate Odor Plumes with Thermal Targets

The complexities of blood-feeding patterns in mosquitoes and sandflies and the burden of disease: A minireview

Mosquitoes and sandflies exhibit a wide range of blood feeding patterns, targeting a wide range of vertebrate species, including birds, mammals, reptiles, and amphibians, for proteins vital for egg development. This broad host range increases the opportunity for them to acquire pathogens of numerous debilitating-and-fatal diseases from various animal reservoirs, playing a significant role in disease crossover between animals and humans, also known as zoonotic transmission. This review focuses on the intricate blood-feeding habits of these dipteran vectors, their sensory systems and the complex dance between host and pathogen during disease transmission. We delve into the influence of blood sources on pathogen spread by examining the insect immune response and its intricate interplay with pathogens. The remarkable sense of smell guiding them towards food sources and hosts is explored, highlighting the interplay of multiple sensory cues in their navigation. Finally, we examine the challenges in mosquito control strategies and explore innovations in this field, emphasizing the need for sustainable solutions to combat this global health threat. By understanding the biology and behaviour of these insects, we can develop more effective strategies to protect ourselves and mitigate the burden of vector-borne diseases.

Mosquitoes integrate visual and acoustic cues to mediate conspecific interactions in swarms

Male mosquitoes form aerial aggregations, known as swarms, to attract females and maximize their chances of finding a mate. Within these swarms, individuals must be able to recognize potential mates and navigate the social environment to successfully intercept a mating partner. Prior research has almost exclusively focused on the role of acoustic cues in mediating the male mosquito's ability to recognize and pursue females. However, the role of other sensory modalities in this behavior has not been explored. Moreover, how males avoid collisions with one another in the swarm while pursuing females remains poorly understood. In this study, we combined free-flight and tethered-flight simulator experiments to demonstrate that swarming Anopheles coluzzii mosquitoes integrate visual and acoustic information to track conspecifics and avoid collisions. Our tethered experiments revealed that acoustic stimuli gated mosquito steering responses to visual objects simulating nearby mosquitoes, especially in males that exhibited a strong response toward visual objects in the presence of female flight tones. Additionally, we observed that visual cues alone could trigger changes in mosquitoes' wingbeat amplitude and frequency. These findings were corroborated by our free-flight experiments, which revealed that Anopheles coluzzii modulate their thrust-based flight responses to nearby conspecifics in a similar manner to tethered animals, potentially allowing for collision avoidance within swarms. Together, these results demonstrate that both males and females integrate multiple sensory inputs to mediate swarming behavior, and for males, the change in flight kinematics in response to multimodal cues might allow them to simultaneously track females while avoiding collisions.

Thermal infrared directs host-seeking behaviour in Aedes aegypti mosquitoes

Mosquito-borne diseases affect hundreds of millions of people annually and disproportionately impact the developing world1,2. One mosquito species, Aedes aegypti, is a primary vector of viruses that cause dengue, yellow fever and Zika. The attraction of Ae. aegypti female mosquitos to humans requires integrating multiple cues, including CO2 from breath, organic odours from skin and visual cues, all sensed at mid and long ranges, and other cues sensed at very close range3-6. Here we identify a cue that Ae. aegypti use as part of their sensory arsenal to find humans. We demonstrate that Ae. aegypti sense the infrared (IR) radiation emanating from their targets and use this information in combination with other cues for highly effective mid-range navigation. Detection of thermal IR requires the heat-activated channel TRPA1, which is expressed in neurons at the tip of the antenna. Two opsins are co-expressed with TRPA1 in these neurons and promote the detection of lower IR intensities. We propose that radiant energy causes local heating at the end of the antenna, thereby activating temperature-sensitive receptors in thermosensory neurons. The realization that thermal IR radiation is an outstanding mid-range directional cue expands our understanding as to how mosquitoes are exquisitely effective in locating hosts.

Internal and external drivers interact to create highly dynamic mosquito blood-feeding behaviour

Blood-feeding, which is necessary for most female mosquitoes to reproduce, provides an opportunity for pathogen transmission. Blood-feeding is influenced by external factors such as light, temperature, humidity and intra- and inter-specific interactions. Physiologically, blood-feeding cycles are linked to nutritional conditions and governed by conserved hormonal signalling pathways that prepare mosquito sensory systems to locate and evaluate hosts. Human activities also alter mosquito blood-feeding behaviour through selection pressures such as insecticide usage, habitat and ecosystem alterations, and climate change. Notably, blood-feeding behaviour changes within a mosquito's lifespan, an underexplored phenomenon from an epidemiological standpoint. A review of the literature indicates that our understanding of mosquito biology and blood-feeding behaviour is predominantly based on studies of a handful of primarily tropical species. This focus likely skews our comprehension of the diversity of critical drivers of blood-feeding behaviour, especially under constraints imposed by harsh conditions. We found evidence of remarkable adaptability in blood-feeding and significant knowledge gaps regarding the determinants of host use. Specifically, epidemiological analyses assume host use is modified by external factors, while neglecting internal physiology. Integrating all significant factors is essential for developing effective models of mosquito-borne disease transmission in a rapidly changing world.

Wind Tunnels and Airflow-Driven Assays: Methods for Establishing the Cues and Orientation Mechanisms That Modulate Female Mosquito Attraction to Human Hosts

Understanding how female mosquitoes find a prospective host is crucial to developing means that can interfere with this process. Many methods are available to researchers studying cues and orientation mechanisms that modulate female mosquito attraction to hosts. Behaviors that can be monitored with these assays include activation, taking flight, upwind flight along an odor plume (optomotor anemotaxis), close approach to the stimulus (including hovering), and landing. Video recording can three-dimensionally document flight tracks and can correlate overall distribution patterns and moment-to-moment movements with odor contact and the presence of nearby cues such as a visual target. Here, we introduce mosquito host-seeking behaviors and methods to study them: wind tunnels (which allow orientation in free-flight), airflow-driven assays (using either tethered mosquitoes or small assay chambers that permit flight but also often dictate walking orientation), and still-air assays (wherein in odor concentration and spatial distribution are the orientation cues). We also describe factors that affect the assays and provide assay design considerations.

Mosquitoes as a model for understanding the neural basis of natural behaviors

Mosquito behaviors have been the subject of extensive research for over a century due to their role in the spread of human disease. However, these behaviors are also beginning to be appreciated as excellent models for neurobiological research in their own right. Many of the same behaviors and sensory abilities that help mosquitoes survive and reproduce alongside humans represent striking examples of generalizable phenomena of longstanding neurobiological interest. In this review, we highlight four prominent examples that promise new insight into (1) precise circadian tuning of sensory systems, (2) processing of complex natural odors, (3) multisensory integration, and (4) modulation of behavior by internal states.

Multimodal interactions in Stomoxys navigation reveal synergy between olfaction and vision

Stomoxys flies exhibit an attraction toward objects that offer no rewards, such as traps and targets devoid of blood or nectar incentives. This behavior provides an opportunity to develop effective tools for vector control and monitoring. However, for these systems to be sustainable and eco-friendly, the visual cues used must be specific to target vector(s). In this study, we modified the existing blue Vavoua trap, which was originally designed to attract biting flies, to create a deceptive host attraction system specifically biased toward attracting Stomoxys. Our research revealed that Stomoxys flies are attracted to various colors, with red proving to be the most attractive and selective color for Stomoxys compared to the other colors tested. Interestingly, our investigation of the cattle-Stomoxys interaction demonstrated that Stomoxys flies do not prefer a specific livestock fur color phenotype, despite variation in the spectrum. To create a realistic sensory impression of the trap in the Stomoxys nervous system, we incorporated olfactory cues from livestock host odors that significantly increased trap catches. The optimized novel polymer bead dispenser is capable of effectively releasing the attractive odor carvone + p-cresol, with strong plume strands and longevity. Overall, red trap baited with polymer bead dispenser is environmentally preferred.

Protocol for rearing and using mosquitoes for flight path tracking and behavioral characterization in wind tunnel bioassays

Mosquito behavioral assays are an important component in vector research and control tool development. Here, we present a protocol for rearing Anopheles mosquitoes, performing host-seeking behavioral bioassays, and collecting 3D flight tracks in a large wind tunnel. We describe steps for setting up host-seeking landing assays, both as a non-choice and as a dual-choice assay, and analyzing flight tracks. This protocol can be applied in the research of several behavioral traits, including nectar seeking, resting, mating, and oviposition behavior. For complete details on the use and execution of this protocol, please refer to Carnaghi et al.1.

À la carte: how mosquitoes choose their blood meals

Mosquitoes are important vectors for human diseases, transmitting pathogens that cause a range of parasitic and viral infections. Mosquito blood-feeding is heterogeneous, meaning that some human hosts are at higher risk of receiving bites than others, and this heterogeneity is multifactorial. Mosquitoes integrate specific cues to locate their hosts, and mosquito attraction differs considerably between individual human hosts. Heterogeneous mosquito biting results from variations in both host attractiveness and availability and can impact transmission of vector-borne diseases. However, the extent and drivers of this heterogeneity and its importance for pathogen transmission remain incompletely understood. Here, we review methods and recent data describing human characteristics that affect host-seeking behavior and host preferences of mosquito disease vectors, and the implications for vector-borne disease transmission.

Odour source distance is predictable from a time history of odour statistics for large scale outdoor plumes

Odour plumes in turbulent environments are intermittent and sparse. Laboratory-scaled experiments suggest that information about the source distance may be encoded in odour signal statistics, yet it is unclear whether useful and continuous distance estimates can be made under real-world flow conditions. Here, we analyse odour signals from outdoor experiments with a sensor moving across large spatial scales in desert and forest environments to show that odour signal statistics can yield useful estimates of distance. We show that achieving accurate estimates of distance requires integrating statistics from 5 to 10 s, with a high temporal encoding of the olfactory signal of at least 20 Hz. By combining distance estimates from a linear model with wind-relative motion dynamics, we achieved source distance estimates in a 60 × 60 m2 search area with median errors of 3-8 m, a distance at which point odour sources are often within visual range for animals such as mosquitoes.

Small dams drive Anopheles abundance during the dry season in a high malaria burden area of Malawi

This study explores the influence of small dams on the exposure to malaria vectors during the dry season in Kasungu district, Malawi, an area recently identified as high priority for malaria interventions by the National Malaria Control Programme. Small dam impoundments provide communities with a continuous supply of water for domestic and agricultural activities across sub-Saharan Africa and are considered vital to food security and climate change resilience. However, these permanent water bodies also create ideal breeding sites for mosquitoes in typically arid landscapes. The study focuses on a specific dam impoundment and its vicinity, aiming to assess its spatial and temporal influence on indoor vector densities. From May to August 2021, CDC light traps were used to measure indoor mosquito densities for two consecutive nights per month in three communities located at increasing distances from the dam (0, ~1 and ~2 km). Simultaneously, drone imagery was captured for each community, enabling the identification of additional standing water within approximately 400 m of selected households. Larval sampling was carried out within the impoundment periphery and in additional water bodies identified in the drone imagery. Generalised linear mixed models (GLMMs) were employed to analyse the indoor Anopheles abundance data, estimating the effects of household structure (open/closed eaves), month, temperature and water proximity on malaria vector exposure. Throughout 685 trapping nights, a total of 1256 mosquitoes were captured, with 33% (412) being female Anopheles. Among these, 91% were morphologically identified as Anopheles funestus s.l., and 5% as Anopheles gambiae s.l. Catches progressively decline in each consecutive trapping month as the environment became drier. This decline was much slower in Malangano, the community next to the dam, with abundance being notably higher in June and July. Further, the majority of An. gambiae s.l. were caught in May, with none identified in July and August. Anopheles larvae were found both in the impoundment and other smaller water bodies such as irrigation wells in each survey month; however, the presence of these smaller water bodies did not have a significant impact on adult female mosquito catches in the GLMM. The study concludes that proximity to the dam impoundment was the primary driver of differences between survey communities with the abundance in Chikhombwe (~1 km away) and Chiponde (~2 km away) being 0.35 (95% confidence interval [CI], 0.19-0.66) and 0.28 (95% CI, 0.16-0.47) lower than Malangano, respectively, after adjusting for other factors. These findings underscore the importance of targeted interventions, such as larval source management or housing improvements, near small dams to mitigate malaria transmission risks during the dry season. Further research is needed to develop cost-effective strategies for vector control within and around these impoundments.

C. elegans foraging as a model for understanding the neuronal basis of decision-making

Animals have evolved to seek, select, and exploit food sources in their environment. Collectively termed foraging, these ubiquitous behaviors are necessary for animal survival. As a foundation for understanding foraging, behavioral ecologists established early theoretical and mathematical frameworks which have been subsequently refined and supported by field and laboratory studies of foraging animals. These simple models sought to explain how animals decide which strategies to employ when locating food, what food items to consume, and when to explore the environment for new food sources. These foraging decisions involve integration of prior experience with multimodal sensory information about the animal's current environment and internal state. We suggest that the nematode Caenorhabditis elegans is well-suited for a high-resolution analysis of complex goal-oriented behaviors such as foraging. We focus our discussion on behavioral studies highlighting C. elegans foraging on bacteria and summarize what is known about the underlying neuronal and molecular pathways. Broadly, we suggest that this simple model system can provide a mechanistic understanding of decision-making and present additional avenues for advancing our understanding of complex behavioral processes.

Effect of textile colour on vector mosquito host selection: a simulated field study in Mali, West Africa

BACKGROUND

The effect of clothing colour on the biting rates of different vector mosquito species is not well understood. Studies under tropical field conditions are lacking. This study aimed to determine the influence of clothing colours on mosquito biting rates in rural and suburban settings in West Africa.

METHODS

We performed a simulated field study in a suburban and a rural site in Mali using Mosquito-Magnet traps utilizing CO2 and other attractants, which were covered with black, white, and black/white striped textile sheets covers. These targets operated continuously for 10 consecutive days with bright nights (around full moon) and 10 consecutive days with dark nights (around new moon). Trapped mosquitoes were collected and catch rates counted hourly. Mosquitoes were morphologically identified to the species complex level (Anopheles gambiae s.l. and Culex pipiens s.l.) or species level (Aedes aegypti). A subset of Anopheles specimens were further identified by molecular methods.

RESULTS

Under bright-night conditions, An. gambiae s.l. was significantly more attracted to black targets than to white and striped targets; during dark nights, no target preference was noted. During bright nights, Cx. pipiens s.l. was significantly more attracted to black and striped targets than to white targets; a similar trend was noted during dark nights (not significant). For day-active Ae. aegypti, striped targets were more attractive than the other targets and black were more attractive than white targets.

CONCLUSIONS

The study firstly demonstrated that under field conditions in Mali, West Africa, mosquito catch rates were influenced by different clothing colours, depending on mosquito species and light conditions. Overall, light colours were least attractive to host-seeking mosquitoes. Using white or other light-coloured clothing can potentially reduce bite exposure and risk of disease transmission in endemic tropical regions.

Conducting an Analysis of Mosquito Flight Behaviors in a Wind Tunnel

Prior to conducting wind tunnel experiments, mosquitoes must be prepared for testing. Important factors and state-dependent processes of the mosquito-like the sex, age, infection status, reproductive status, or nutritional status-should be evaluated and motivated by questions and hypotheses one seeks to address. Other critical external factors that can impact the mosquitoes' behavior and should be controlled for both in the colony and in the room where the wind tunnel experiments take place include the circadian rhythm, room temperature, light intensity, and relative humidity. Together, the internal and external factors, and wind tunnel design, ultimately control the behavior of the mosquito and, hence, the success of the experiments. In the present protocol, we describe methods using a standard wind tunnel design in which the fan pulls the air through the working section of the wind tunnel and the mosquito behavior is recorded by a multicamera system. Variations around the camera tracking system can be adapted according to the research questions being asked and include real-time tracking for both closed-loop and open-loop control of the stimulus environment or recording of the videos for off-line digitization and analysis. Within the working section, the sensory environment (odor, visual, wind) can be controlled to test the mosquito responses to different stimuli, and below we include different equipment and tools for modifying the stimuli the mosquito experiences during flight. Finally, the methods described here are applicable to multiple mosquito species, although the experiment parameters may need to be changed (e.g., ambient luminosity).

Experiments and Analysis of Mosquito Flight Behaviors in a Wind Tunnel: An Introduction

Mosquitoes detect and navigate to important resources, like a host, using combinations of different sensory stimuli. The relative importance of the sensory cues can change as the mosquito gets closer to their target. Other factors, both internal and external, can also influence the mosquito behavior. A mechanistic understanding of these sensory stimuli, and how they impact mosquito navigation, can now be readily studied using wind tunnels and associated computer vision systems. In this introduction, we present a behavioral paradigm using a wind tunnel for flight behavior analysis. The wind tunnel's large size with its associated cameras and software system for analysis of the mosquito flight tracks can be sophisticated and sometimes cost-prohibitive. Nevertheless, the wind tunnel's flexibility in allowing the testing of multimodal stimuli and scaling of environmental stimuli makes it possible to reproduce conditions from the field and test them in the laboratory, while also allowing the observation of natural flight kinematics.

Host preference patterns in domestic and wild settings: Insights into Anopheles feeding behavior

The adaptation of Anopheles malaria vectors to domestic settings is directly linked to their ability to feed on humans. The strength of this species-habitat association is unequal across the species within the genus, with the major vectors being particularly dependent on humans. However, our understanding of how blood-feeding behavior interacts with and adapts to environmental settings, including the presence of humans, remains limited. Using a field-based approach, we first investigated Anopheles community structure and feeding behavior patterns in domestic and sylvatic settings in La Lopé National Park in Gabon, Central Africa. We characterized the preference indices using a dual-host choice sampling approach across mosquito species, habitats, and seasons. We then quantified the plastic biting behavior of mosquito species in each habitat. We collected individuals from 16 Anopheles species that exhibited significant differences in species composition and abundance between sylvatic and domestic settings. The host-seeking behavior also varied among the seven most abundant species. The general attractiveness to each host, human or animal, remained relatively constant for each species, but with significant variations between habitats across species. These variations, to more generalist and to more anthropophilic behavior, were related to seasonal changes and distance from the village, respectively. Finally, we pointed out that the host choice of major malaria vectors changed in the absence of humans, revealing a plastic feeding behavior of these species. This study highlights the effect of humans on Anopheles distribution and feeding evolution. The characterization of feeding behavior in wild and domestic settings provides opportunities to better understand the interplay between genetic determinants of host preference and ecological factors. Our findings suggest that protected areas may offer alternative thriving conditions to major malaria vectors.

Carbon Dioxide, Methane, and Synthetic Cattle Breath Volatiles Attract Host-Seeking Stable Flies, Stomoxys calcitrans

Stable flies, Stomoxys calcitrans (L.), are blood-feeding ectoparasites of cattle. Host-seeking stable flies respond to various cattle host cues, but a potential role of cattle breath gases [carbon dioxide (CO2), methane (CH4)] and cattle breath volatiles (acetone, isoprene, 2-butanone, 2-propanol, propionic acid, 3-methyl butyric acid, phenol), alone or in combination, on host-seeking behavior of stable flies has not yet been comprehensively investigated. In laboratory and greenhouse experiments, we tested the hypotheses that (1) CO2 and CH4 interactively attract stable flies, (2) CO2 'gates' attraction of stable flies to CH4, and (3) breath volatiles on their own, or in combination with both CO2 and CH4, attract stable flies. In Y-tube olfactometer experiments, the blend of CH4 (0.5%) and CO2 (1%) in breathing air ('b-air') attracted significantly more female flies than CH4, or CO2, in b-air. The flies' responses to CH4 were contingent upon their prior or concurrent exposure to CO2. In two-choice experiments in a large greenhouse compartment, significantly more flies landed on the host-look-alike barrel that disseminated a blend of CO2 and CH4 in b-air (CO2/CH4/b-air) than on the barrel disseminating either b-air or CO2. Moreover, significantly more flies landed on the barrel that disseminated synthetic breath volatiles (SBVs) than on the barrel disseminating b-air. The blend of CO2/CH4/b-air and SBVs elicited more fly landings on barrels than CO2/CH4/b-air but not than SBVs. SBVs, possibly combined with both CH4 and CO2, could be developed as a lure to enhance trap captures of stable flies in livestock production facilities.

Multisensory integration in Anopheles mosquito swarms: The role of visual and acoustic information in mate tracking and collision avoidance

Male mosquitoes form aerial aggregations, known as swarms, to attract females and maximize their chances of finding a mate. Within these swarms, individuals must be able to recognize potential mates and navigate the dynamic social environment to successfully intercept a mating partner. Prior research has almost exclusively focused on the role of acoustic cues in mediating the male mosquito's ability to recognize and pursue flying females. However, the role of other sensory modalities in this behavior has not been explored. Moreover, how males avoid collisions with one another in the dense swarm while pursuing females remains poorly understood. In this study, we combined free-flight and tethered flight simulator experiments to demonstrate that swarming Anopheles coluzzii mosquitoes integrate visual and acoustic information to track conspecifics and avoid collisions. Our tethered experiments revealed that acoustic stimuli gated mosquito steering responses to visual objects simulating nearby mosquitoes, especially in males that exhibited attraction to visual objects in the presence of female flight tones. Additionally, we observed that visual cues alone could trigger changes in mosquitoes' wingbeat amplitude and frequency. These findings were corroborated by our free-flight experiments, which revealed that mosquitoes modulate their flight responses to nearby conspecifics in a similar manner to tethered animals, allowing for collision avoidance within swarms. Together, these results demonstrate that both males and females integrate multiple sensory inputs to mediate swarming behavior, and for males, the change in flight kinematics in response to multimodal cues allows them to simultaneously track females while avoiding collisions.

Genetic and behavioral differences between above and below ground Culex pipiens bioforms

Efficiency of mosquito-borne disease transmission is dependent upon both the preference and fidelity of mosquitoes as they seek the blood of vertebrate hosts. While mosquitoes select their blood hosts through multi-modal integration of sensory cues, host-seeking is primarily an odor-guided behavior. Differences in mosquito responses to hosts and their odors have been demonstrated to have a genetic component, but the underlying genomic architecture of these responses has yet to be fully resolved. Here, we provide the first characterization of the genomic architecture of host preference in the polymorphic mosquito species, Culex pipiens. The species exists as two morphologically identical bioforms, each with distinct avian and mammalian host preferences. Cx. pipiens females with empirically measured host responses were prepared into reduced representation DNA libraries and sequenced to identify genomic regions associated with host preference. Multiple genomic regions associated with host preference were identified on all 3 Culex chromosomes, and these genomic regions contained clusters of chemosensory genes, as expected based on work in Anopheles gambiae complex mosquitoes and in Aedes aegypti. One odorant receptor and one odorant binding protein gene showed one-to-one orthologous relationships to differentially expressed genes in A. gambiae complex members with divergent host preferences. Overall, our work identifies a distinct set of odorant receptors and odorant binding proteins that may enable Cx. pipiens females to distinguish between their vertebrate blood host species, and opens avenues for future functional studies that could measure the unique contributions of each gene to host preference phenotypes.

Animal behavior: Mosquitos ride the wave

Some insects have a frustrating knack for avoiding a swatter. A new study shows that mosquitos not only evade the visual image of the looming threat, they also surf the wave of air the swatter creates.

Robotic Odor Source Localization via Vision and Olfaction Fusion Navigation Algorithm

Robotic odor source localization (OSL) is a technology that enables mobile robots or autonomous vehicles to find an odor source in unknown environments. An effective navigation algorithm that guides the robot to approach the odor source is the key to successfully locating the odor source. While traditional OSL approaches primarily utilize an olfaction-only strategy, guiding robots to find the odor source by tracing emitted odor plumes, our work introduces a fusion navigation algorithm that combines both vision and olfaction-based techniques. This hybrid approach addresses challenges such as turbulent airflow, which disrupts olfaction sensing, and physical obstacles inside the search area, which may impede vision detection. In this work, we propose a hierarchical control mechanism that dynamically shifts the robot's search behavior among four strategies: crosswind maneuver, Obstacle-Avoid Navigation, Vision-Based Navigation, and Olfaction-Based Navigation. Our methodology includes a custom-trained deep-learning model for visual target detection and a moth-inspired algorithm for Olfaction-Based Navigation. To assess the effectiveness of our approach, we implemented the proposed algorithm on a mobile robot in a search environment with obstacles. Experimental results demonstrate that our Vision and Olfaction Fusion algorithm significantly outperforms vision-only and olfaction-only methods, reducing average search time by 54% and 30%, respectively.

Quantifying Mosquito Host Preference

The most dangerous mosquito species for human health are those that blood feed preferentially and frequently on humans (anthropophilic mosquitoes). These include prolific disease vectors such as the African malaria mosquito Anopheles gambiae and yellow fever mosquito Aedes aegypti The chemosensory basis for anthropophilic behavior exhibited by these disease vectors, as well as the factors that drive interindividual differences in human attractiveness to mosquitoes, remain largely uncharacterized. Here, we concisely review established methods to quantify mosquito interspecific and intraspecific host preference in the laboratory, as well as semi-field and field environments. Experimental variables for investigator consideration during assays of mosquito host preference across these settings are highlighted.

Mosquito Pollination and Sugar Detection Methods: An Overview

The interactions between mosquitoes and plants and, particularly, between mosquitoes and plant sugars from flowers and other structures are often overlooked and are vastly underresearched compared to mosquito-vertebrate or mosquito-pathogen interactions. Given the importance of mosquito nectar-feeding behavior, as well as its impact on vectorial capacity and its implications for vector suppression, a better understanding of mosquito-plant interactions is needed. Direct observation of mosquitoes visiting plants to obtain sugar and other nutrients can be difficult because females may leave flowers to seek a blood meal from the observer, but this can be overcome with the right experimental procedures. This article discusses methods for the detection of sugar in mosquitoes and for assessing mosquito pollination.

Mosquitoes escape looming threats by actively flying with the bow wave induced by the attacker

To detect and escape looming threats, night-flying insects must rely on other senses than vision alone. Nocturnal mosquitoes can evade looming objects in the dark, but how they achieve this is still unknown. Here, we show how night-active female malaria mosquitoes escape from rapidly looming objects that simulate defensive actions of blood-hosts. First, we quantified the escape performance of flying mosquitoes from an event-triggered mechanical swatter, showing that mosquitoes use swatter-induced airflow to increase their escape success. Secondly, we used high-speed videography and deep-learning-based tracking to analyze escape flights in detail, showing that mosquitoes use banked turns to evade the threat. By combining escape kinematics data with numerical simulations of attacker-induced airflow and a mechanistic movement model, we unraveled how mosquitoes control these banked evasive maneuvers: they actively steer away from the danger, and then passively travel with the bow wave produced by the attacker. Our results demonstrate that night-flying mosquitoes can detect looming objects when visual cues are minimal, suggesting that they use attacker-induced airflow both to detect the danger and as a fluid medium to move with away from the threat. This shows that escape strategies of flying insects are more complex than previous visually induced escape flight studies suggest. As most insects are of similar or smaller sizes than mosquitoes, comparable escape strategies are expected among millions of flying insect species. The here-observed escape maneuvers are distinct from those of mosquitoes escaping from odor-baited traps, thus providing new insights for the development of novel trapping techniques for integrative vector management.

An update and review of arthropod vector sensory systems: Potential targets for behavioural manipulation by parasites and other disease agents

For over a century, vector ecology has been a mainstay of vector-borne disease control. Much of this research has focused on the sensory ecology of blood-feeding arthropods (black flies, mosquitoes, ticks, etc.) with terrestrial vertebrate hosts. Of particular interest are the cues and sensory systems that drive host seeking and host feeding behaviours as they are critical for a vector to locate and feed from a host. An important yet overlooked component of arthropod vector ecology are the phenotypic changes observed in infected vectors that increase disease transmission. While our fundamental understanding of sensory mechanisms in disease vectors has drastically increased due to recent advances in genome engineering, for example, the advent of CRISPR-Cas9, and high-throughput "big data" approaches (genomics, proteomics, transcriptomics, etc.), we still do not know if and how parasites manipulate vector behaviour. Here, we review the latest research on arthropod vector sensory systems and propose key mechanisms that disease agents may alter to increase transmission.

Visual and thermal stimuli modulate mosquito-host contact with implications for improving malaria vector control tools

Malaria prevention relies on mosquito control interventions that use insecticides and exploit mosquito behavior. The rise of insecticide resistance and changing transmission dynamics urgently demand vector control innovation. To identify behavioral traits that could be incorporated into such tools, we investigated the flight and landing response of Anopheles coluzzii to human-like host cues. We show that landing rate is directly proportional to the surface area of thermal stimulus, whereas close-range orientation is modulated by both thermal and visual inputs. We modeled anopheline eye optics to theorize the distance at which visual targets can be detected under a range of conditions, and experimentally established mosquito preference for landing on larger targets, although landing density is greater on small targets. Target orientation does not affect landing rate; however, vertical targets can be resolved at greater distance than horizontal targets of the same size. Mosquito traps for vector control could be significantly enhanced by incorporating these features.

The Shockwè trap: a human-baited exposure-free device for surveillance and behaviour studies of anthropophilic vectors

Background: The human biting rate (MBR) and entomological inoculation rate (EIR) are common parameters routinely used to measure the risk of malaria transmission. Both parameters can be estimated using human landing catches (HLC). Although it is considered the gold-standard, HLC puts collectors at higher risk of infection with mosquito-transmitted pathogens. Methods: A novel exposure-free host-seeking mosquito electrocution trap, the Shockwè trap (SHK), was developed and its efficiency for monitoring mosquito community composition and abundance was compared with human landing catches (HLC) as the gold-standard. Field experiments were performed in Massavasse village, southern Mozambique. Simultaneous indoor and outdoor collections of nocturnal host-seeking mosquitoes were carried out using the SHK and HLC methods. The relative sampling efficiency of SHK was estimated as the ratio of the numbers of mosquitoes caught in SHK compared HLC. Proportionality and density-dependence between SHK and HLC catches were estimated by mean of Bayesian regression approaches. Results: A total of 69,758 and 27,359 host-seeking mosquitoes comprising nineteen species and four genera, were collected by HLC and SHK respectively. In general, SHK and HLC sampled similar numbers of mosquito species, with the exceptions of the least common species Aedes sudanensis, Ae. subargenteus, and Coquillettidia versicolor that were caught only by HLC. The relative sampling efficiency and proportionality between SHK and matched HLC catches varied greatly between species and collection site. However, all mosquitoes collected by SHK were unfed, confirming the Shockwè trap design's performance and reliability as a successful mosquito exposure free sampling approach. Conclusions: Results demonstrate that SHK is a safe and reliable human-exposure free device for monitoring the occurrence of a wide range of mosquito, including major malaria and arboviruses vector species. However, improvements are needed to increase its sampling efficiency for less abundant mosquito species.

Quantifying Mosquito Attraction Using a Uniport Olfactometer

Female mosquitoes respond to the world around them by using chemosensory organs, such as their antennae, to detect volatile compounds emitted from a vertebrate host. These chemosensory systems facilitate the interpretation of external stimuli from the periphery by connecting to the central nervous system and eliciting behaviors necessary for survival, such as obtaining a blood meal. This innate behavior leads to the transmission of pathogens, including dengue virus, chikungunya virus, and Zika virus. Olfaction is a primary sense mosquitoes use to differentiate between vertebrate hosts, and studying it can lead to novel strategies to reduce the risk of disease. In this protocol, we present an olfactory-driven behavioral assay using a uniport olfactometer that measures mosquito attraction rate to a specific stimulus. We include details of the behavioral assay and data analysis as well as how to prepare the mosquitoes before their introduction into the olfactometer. This uniport olfactometer behavioral assay is currently one of the most reliable methods to study mosquito attraction to a single stimulus.

Quantifying Mosquito Attraction Behavior Using Olfactometry

When blood feeding from human hosts, female mosquitoes can transmit life-threatening pathogens to humans, including dengue virus, chikungunya virus, and Zika virus. Olfaction is the primary sense mosquitoes use to locate and differentiate hosts and studying it can lead to new strategies to reduce the risk of disease. To effectively study host-seeking behavior in mosquitoes, a repeatable, quantitative assay that isolates olfaction from other cues is critical for interpreting mosquito behavior. Here, we contribute an overview of methods and best practices for the study of mosquito attraction (or lack thereof) by using olfactometry to quantify behavior. In the accompanying protocols, we present an olfactory-based behavioral assay using a uniport olfactometer that measures mosquito attraction rate to specific stimuli. We include construction details, setup of the uniport olfactometer, details of the behavioral assay, and data analysis guidelines, as well as how to prepare the mosquitoes before their introduction into the olfactometer. This uniport olfactometer behavioral assay is currently one of the most reliable methods to study mosquito attraction to a single olfactory stimulus.

Mosquito (Diptera: Culicidae) Vision and Associated Electrophysiological Techniques

Mosquitoes are considered the world's deadliest animal because of the pathogens they spread. Additionally, they are an unbearable nuisance in many areas. Visual stimuli play an important role in the mosquito life cycle, helping them find vertebrate hosts, floral nectar, and oviposition sites. Here, we review mosquito vision, including its influences on mosquito behavior, the photoreceptors involved, and mosquito spectral sensitivity, as well as provide an overview of techniques used for the analysis of mosquito vision, including electroretinograms, single-cell recordings, and the use of opsin-deficient mutants. We anticipate that this information will be useful for researchers studying mosquito physiology, evolution, ecology, and management.

Mosquito Repellent Efficacy of Australian Blue Cypress Callitris intratropica Essential Oil and a Topical Formulation under Laboratory and Field Conditions

Mosquito repellents are important for personal protection against nuisance and potentially infectious mosquito bites. Repellent activity of Australian blue cypress essential oil (EO) and a commercially formulated skin lotion containing blue cypress EO (topical formulation) were compared with 20% DEET (N, N-diethyl-3 toluamide) against mosquitoes under laboratory and field conditions in North Queensland, Australia. On a volunteer's forearm, 1 mL of candidate material was applied to approximately 600 cm2 of exposed skin. When blue cypress EO was applied at various concentrations (0.5%-10.5%), it did not fully prevent mosquito landing or biting. However, a dose-dependent increase, approaching 80% protection, was observed at high EO concentrations. On the basis of these results, three concentrations (5%, 10%, and 20%) of blue cypress EO were selected for complete protection time (CPT) experiments. Topical formulation (undiluted) was also included in CPT experiments. Although some protection was afforded, mosquito landing/probing were still recorded immediately after application for both blue cypress EO and its topical formulation. Specifically, protection declined for blue cypress EO from 80% to 70% (laboratory) and from 93% to 50% (field) within 1 hour. For topical formulation, protection declined from 85% to 75% in the laboratory and from 63% to 50% in the field. In comparison, DEET maintained a 100% protection throughout the testing period of up to 1 h, and there was no landing/probing observed in volunteers who had applied DEET. To conclude, both blue cypress products provided some protection against mosquito bites, which decreased soon after application.

Tethered Preparation for the Analysis of Mosquito Visual-Motor Responses Using Modular Visual Displays

In the present protocol, we describe methods to assess mosquito visual-motor responses using the Reiser-Dickinson light-emitting diode (LED) panels arranged in a cylindrical arena and fixed-tethered preparations where the insect cannot adjust its orientation relative to the visual display. Variations around this approach might be better adapted for the specific requirements of each research project and must be considered by the investigators. Other types of displays may provide other stimulation possibilities (e.g., color range, refresh rate, field of view). Also, other types of preparations, such as rotating (magneto-tethered) preparations where the insect can rotate around a vertical axis and reorient relative to the visual display, may reveal other aspects of mosquito optomotor responses. Finally, the methods described here are applicable to multiple species and were used to produce data published previously using 6-d-old Aedes aegypti females.

Retrospective Review of and Introduction to the Analysis of Mosquito Optomotor Responses

Adult hematophagous female mosquitoes require nutrients and proteins from vertebrate blood to produce progeny. To find these hosts, mosquitoes rely on olfactory, thermal, and visual cues. Among these sensory modalities, vision has received far less attention than olfaction, in part because of a lack of experimental tools providing sufficient control on the delivery of visual stimuli and the recording of mosquito responses. Although free-flight experiments (e.g., wind tunnel and cage) ensure higher ecological relevance and allow the observation of more natural flight dynamics, tethered flight assays offer a greater level of control on the suite of sensory stimuli experienced by mosquitoes. In addition, these tethered assays provide a stepping stone toward understanding the neural underpinnings of mosquito optomotor behavior. Advances in computer vision tracking systems and programmable light-emitting diode displays have permitted significant discoveries in models such as the fly Drosophila melanogaster Here, we introduce the use of these methods with mosquitoes.

Ingested histamine and serotonin interact to alter Anopheles stephensi feeding and flight behavior and infection with Plasmodium parasites

Blood levels of histamine and serotonin (5-HT) are altered in human malaria, and, at these levels, we have shown they have broad, independent effects on Anopheles stephensi following ingestion by this invasive mosquito. Given that histamine and 5-HT are ingested together under natural conditions and that histaminergic and serotonergic signaling are networked in other organisms, we examined effects of combinations of these biogenic amines provisioned to A. stephensi at healthy human levels (high 5-HT, low histamine) or levels associated with severe malaria (low 5-HT, high histamine). Treatments were delivered in water (priming) before feeding A. stephensi on Plasmodium yoelii-infected mice or via artificial blood meal. Relative to effects of histamine and 5-HT alone, effects of biogenic amine combinations were complex. Biogenic amine treatments had the greatest impact on the first oviposition cycle, with high histamine moderating low 5-HT effects in combination. In contrast, clutch sizes were similar across combination and individual treatments. While high histamine alone increased uninfected A. stephensi weekly lifetime blood feeding, neither combination altered this tendency relative to controls. The tendency to re-feed 2 weeks after the first blood meal was altered by combination treatments, but this depended on mode of delivery. For blood delivery, malaria-associated treatments yielded higher percentages of fed females relative to healthy-associated treatments, but the converse was true for priming. Female mosquitoes treated with the malaria-associated combination exhibited enhanced flight behavior and object inspection relative to controls and healthy combination treatment. Mosquitoes primed with the malaria-associated combination exhibited higher mean oocysts and sporozoite infection prevalence relative to the healthy combination, with high histamine having a dominant effect on these patterns. Compared with uninfected A. stephensi, the tendency of infected mosquitoes to take a second blood meal revealed an interaction of biogenic amines with infection. We used a mathematical model to project the impacts of different levels of biogenic amines and associated changes on outbreaks in human populations. While not all outbreak parameters were impacted the same, the sum of effects suggests that histamine and 5-HT alter the likelihood of transmission by mosquitoes that feed on hosts with symptomatic malaria versus a healthy host.

Hunting with heat: thermosensory-driven foraging in mosquitoes, snakes and beetles

Animals commonly use thermosensation, the detection of temperature and its variation, for defensive purposes: to maintain appropriate body temperature and to avoid tissue damage. However, some animals also use thermosensation to go on the offensive: to hunt for food. The emergence of heat-dependent foraging behavior has been accompanied by the evolution of diverse thermosensory organs of often exquisite thermosensitivity. These organs detect the heat energy emitted from food sources that range from nearby humans to trees burning in a forest kilometers away. Here, we examine the biophysical considerations, anatomical specializations and molecular mechanisms that underlie heat-driven foraging. We focus on three groups of animals that each meet the challenge of detecting heat from potential food sources in different ways: (1) disease-spreading vector mosquitoes, which seek blood meals from warm-bodied hosts at close range, using warming-inhibited thermosensory neurons responsive to conductive and convective heat flow; (2) snakes (vipers, pythons and boas), which seek warm-blooded prey from ten or more centimeters away, using warmth-activated thermosensory neurons housed in an organ specialized to harvest infrared radiation; and (3) fire beetles, which maximize their offspring's feeding opportunities by seeking forest fires from kilometers away, using mechanosensory neurons housed in an organ specialized to convert infrared radiation into mechanosensory stimuli. These examples highlight the diverse ways in which animals exploit the heat emanating from potential food sources, whether this heat reflects ongoing metabolic activity or a recent lightning strike, to secure a nutritious meal for themselves or for their offspring.

Mosquito biology: Scents and selectability

Malaria-transmitting mosquitoes are skilled human hunters, selectively choosing their prey based on a complex array of sensory cues. A new study unveils a distinct pattern of preference for human-associated olfactory cues that underlies the selective behavior of these mosquitoes.

Human scent guides mosquito thermotaxis and host selection under naturalistic conditions

The African malaria mosquito Anopheles gambiae exhibits a strong innate drive to seek out humans in its sensory environment, classically entering homes to land on human skin in the hours flanking midnight. To gain insight into the role that olfactory cues emanating from the human body play in generating this epidemiologically important behavior, we developed a large-scale multi-choice preference assay in Zambia with infrared motion vision under semi-field conditions. We determined that An. gambiae prefers to land on arrayed visual targets warmed to human skin temperature during the nighttime when they are baited with carbon dioxide (CO2) emissions reflective of a large human over background air, body odor from one human over CO2, and the scent of one sleeping human over another. Applying integrative whole body volatilomics to multiple humans tested simultaneously in competition in a six-choice assay, we reveal high attractiveness is associated with whole body odor profiles from humans with increased relative abundances of the volatile carboxylic acids butyric acid, isobutryic acid, and isovaleric acid, and the skin microbe-generated methyl ketone acetoin. Conversely, those least preferred had whole body odor that was depleted of carboxylic acids among other compounds and enriched with the monoterpenoid eucalyptol. Across expansive spatial scales, heated targets without CO2 or whole body odor were minimally or not attractive at all to An. gambiae. These results indicate that human scent acts critically to guide thermotaxis and host selection by this prolific malaria vector as it navigates towards humans, yielding intrinsic heterogeneity in human biting risk.

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.

Neurogenetic identification of mosquito sensory neurons

Anopheles mosquitoes, as vectors for the malaria parasite, are a global threat to human health. To find and bite a human, they utilize neurons within their sensory appendages. However, the identity and quantification of sensory appendage neurons are lacking. Here we use a neurogenetic approach to label all neurons in Anopheles coluzzii mosquitoes. We utilize the homology assisted CRISPR knock-in (HACK) approach to generate a T2A-QF2^w knock-in of the synaptic gene bruchpilot. We use a membrane-targeted GFP reporter to visualize the neurons in the brain and to quantify neurons in all major chemosensory appendages (antenna, maxillary palp, labella, tarsi, and ovipositor). By comparing labeling of brp>GFP and Orco>GFP mosquitoes, we predict the extent of neurons expressing ionotropic receptors (IRs) or other chemosensory receptors. This work introduces a valuable genetic tool for the functional analysis of Anopheles mosquito neurobiology and initiates characterization of the sensory neurons that guide mosquito behavior.

Quantifying Aedes aegypti Host Odor Preference Using a Two-Port Olfactometer

Blood-feeding mosquitoes are a leading threat to global public health-vectoring dangerous infections including Zika, dengue, and malaria. Mosquitoes identify and target hosts for blood meals by using visual, thermal, and chemical cues. Here we describe an assay for measuring odor-based host-preference behavior-that is, the preferential approach toward one host over another based on differences in the volatile compounds they emit. The assay can be adapted for use with diverse odor sources, from live animals and their breath to odor-scented sleeves with controlled amounts of CO₂ Mosquitoes in this assay fly upwind to within 30 cm of the odor source and then enter a small trap. We therefore believe this assay best replicates medium- to short-range host-seeking, when females approach and are preparing to land on a host animal. We also find that relative response in a two-choice test shows less trial-to-trial variation than the absolute number of responsive mosquitoes, which appears more sensitive to exogenous factors such as rearing conditions. This assay has been used to better understand mosquito host-seeking decisions, which can provide fundamental insight into the brain and behavior as well as information useful for the design of novel vector control strategies.

Synergistic attraction of Western black-legged ticks, Ixodes pacificus, to CO2 and odorant emissions from deer-associated microbes

Foraging ticks reportedly exploit diverse cues to locate their hosts. Here, we tested the hypothesis that host-seeking Western black-legged ticks, Ixodes pacificus, and black-legged ticks, I. scapularis, respond to microbes dwelling in sebaceous gland secretions of white-tailed deer, Odocoileus virginianus, the ticks' preferred host. Using sterile wet cotton swabs, microbes were collected from the pelage of a sedated deer near forehead, preorbital, tarsal, metatarsal and interdigital glands. Swabs were plated on agar, and isolated microbes were identified by 16S rRNA amplicon sequencing. Of 31 microbial isolates tested in still-air olfactometers, 10 microbes induced positive arrestment responses by ticks, whereas 10 others were deterrent. Of the 10 microbes prompting arrestment by ticks, four microbes-including Bacillus aryabhattai (isolates A4)-also attracted ticks in moving-air Y-tube olfactometers. All four of these microbes emitted carbon dioxide and ammonia as well as volatile blends with overlapping blend constituents. The headspace volatile extract (HVE) of B. aryabhattai (HVE-A4) synergistically enhanced the attraction of I. pacificus to CO₂. A synthetic blend of HVE-A4 headspace volatiles in combination with CO₂ synergistically attracted more ticks than CO₂ alone. Future research should aim to develop a least complex host volatile blend that is attractive to diverse tick taxa.

Optimal policies for Bayesian olfactory search in turbulent flows

In many practical scenarios, a flying insect must search for the source of an emitted cue which is advected by the atmospheric wind. On the macroscopic scales of interest, turbulence tends to mix the cue into patches of relatively high concentration over a background of very low concentration, so that the insect will detect the cue only intermittently and cannot rely on chemotactic strategies which simply climb the concentration gradient. In this work we cast this search problem in the language of a partially observable Markov decision process and use the Perseus algorithm to compute strategies that are near-optimal with respect to the arrival time. We test the computed strategies on a large two-dimensional grid, present the resulting trajectories and arrival time statistics, and compare these to the corresponding results for several heuristic strategies, including (space-aware) infotaxis, Thompson sampling, and QMDP. We find that the near-optimal policy found by our implementation of Perseus outperforms all heuristics we test by several measures. We use the near-optimal policy to study how the search difficulty depends on the starting location. We also discuss the choice of initial belief and the robustness of the policies to changes in the environment. Finally, we present a detailed and pedagogical discussion about the implementation of the Perseus algorithm, including the benefits-and pitfalls-of employing a reward-shaping function.

Biology and Behaviour of Aedes aegypti in the Human Environment: Opportunities for Vector Control of Arbovirus Transmission

Aedes aegypti is a ubiquitous vector of arboviruses mostly in urbanised areas throughout the tropics and subtropics and a growing threat beyond. Control of Ae. aegypti is difficult and costly, and no vaccines are available for most of the viruses it transmits. With practical control solutions our goal, ideally suitable for delivery by householders in affected communities, we reviewed the literature on adult Ae. aegypti biology and behaviour, within and close to the human home, the arena where such interventions must impact. We found that knowledge was vague or important details were missing for multiple events or activities in the mosquito life cycle, such as the duration or location of the many periods when females rest between blood feeding and oviposition. The existing body of literature, though substantial, is not wholly reliable, and evidence for commonly held "facts" range from untraceable to extensive. Source references of some basic information are poor or date back more than 60 years, while other information that today is accepted widely as "fact" is not supported by evidence in the literature. Many topics, e.g., sugar feeding, resting preferences (location and duration), and blood feeding, merit being revisited in new geographical regions and ecological contexts to identify vulnerabilities for exploitation in control.

Development of an automated biomaterial platform to study mosquito feeding behavior

Mosquitoes carry a number of deadly pathogens that are transmitted while feeding on blood through the skin, and studying mosquito feeding behavior could elucidate countermeasures to mitigate biting. Although this type of research has existed for decades, there has yet to be a compelling example of a controlled environment to test the impact of multiple variables on mosquito feeding behavior. In this study, we leveraged uniformly bioprinted vascularized skin mimics to create a mosquito feeding platform with independently tunable feeding sites. Our platform allows us to observe mosquito feeding behavior and collect video data for 30-45 min. We maximized throughput by developing a highly accurate computer vision model (mean average precision: 92.5%) that automatically processes videos and increases measurement objectivity. This model enables assessment of critical factors such as feeding and activity around feeding sites, and we used it to evaluate the repellent effect of DEET and oil of lemon eucalyptus-based repellents. We validated that both repellents effectively repel mosquitoes in laboratory settings (0% feeding in experimental groups, 13.8% feeding in control group, p < 0.0001), suggesting our platform's use as a repellent screening assay in the future. The platform is scalable, compact, and reduces dependence on vertebrate hosts in mosquito research.

Behavioral algorithms and neural mechanisms underlying odor-modulated locomotion in insects

Odors released from mates and resources such as a host and food are often the first sensory signals that an animal can detect. Changes in locomotion in response to odors are an important mechanism by which animals access resources important to their survival. Odor-modulated changes in locomotion in insects constitute a whole suite of flexible behaviors that allow insects to close in on these resources from long distances and perform local searches to locate and subsequently assess them. Here, we review changes in odor-mediated locomotion across many insect species. We emphasize that changes in locomotion induced by odors are diverse. In particular, the olfactory stimulus is sporadic at long distances and becomes more continuous at short distances. This distance-dependent change in temporal profile produces a corresponding change in an insect's locomotory strategy. We also discuss the neural circuits underlying odor modulation of locomotion.

Visual threats reduce blood-feeding and trigger escape responses in Aedes aegypti mosquitoes

The diurnal mosquitoes Aedes aegypti are vectors of several arboviruses, including dengue, yellow fever, and Zika viruses. To find a host to feed on, they rely on the sophisticated integration of olfactory, visual, thermal, and gustatory cues emitted by the hosts. If detected by their target, this latter may display defensive behaviors that mosquitoes need to be able to detect and escape in order to survive. In humans, a typical response is a swat of the hand, which generates both mechanical and visual perturbations aimed at a mosquito. Here, we used programmable visual displays to generate expanding objects sharing characteristics with the visual component of an approaching hand and quantified the behavioral response of female mosquitoes. Results show that Ae. aegypti is capable of using visual information to decide whether to feed on an artificial host mimic. Stimulations delivered in a LED flight arena further reveal that landed Ae. aegypti females display a stereotypical escape strategy by taking off at an angle that is a function of the direction of stimulus introduction. Altogether, this study demonstrates that mosquitoes landed on a host mimic can use isolated visual cues to detect and avoid a potential threat.

Carbon dioxide and blood-feeding shift visual cue tracking during navigation in Aedes aegypti mosquitoes

Haematophagous mosquitoes need a blood meal to complete their reproductive cycle. To accomplish this, female mosquitoes seek vertebrate hosts, land on them and bite. As their eggs mature, they shift attention away from hosts and towards finding sites to lay eggs. We asked whether females were more tuned to visual cues when a host-related signal, carbon dioxide, was present, and further examined the effect of a blood meal, which shifts behaviour to ovipositing. Using a custom, tethered-flight arena that records wing stroke changes while displaying visual cues, we found the presence of carbon dioxide enhances visual attention towards discrete stimuli and improves contrast sensitivity for host-seeking Aedes aegypti mosquitoes. Conversely, intake of a blood meal reverses vertical bar tracking, a stimulus that non-fed females readily follow. This switch in behaviour suggests that having a blood meal modulates visual attention in mosquitoes, a phenomenon that has been described before in olfaction but not in visually driven behaviours.

Comparisons of chemosensory gene repertoires in human and non-human feeding Anopheles mosquitoes link olfactory genes to anthropophily

We investigate the genetic basis of anthropophily (human host use) in a non-model mosquito species group, the Anopheles farauti complex from the southwest Pacific. This complex has experienced multiple transitions from anthropophily to zoophily, contrasting with well-studied systems (the global species Aedes aegypti and the African Anopheles gambiae complex) that have evolved to be specialist anthropophiles. By performing tests of selection and assessing evolutionary patterns for >200 olfactory genes from nine genomes, we identify several candidate genes associated with differences in anthropophily in this complex. Based on evolutionary patterns (phylogenetic relationships, fixed amino acid differences, and structural differences) as well as results from selection analyses, we identify numerous genes that are likely to play an important role in mosquitoes’ ability to detect humans as hosts. Our findings contribute to the understanding of the evolution of insect olfactory gene families and mosquito host preference as well as having potential applied outcomes.

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Genomes of Anopheles mosquitoes with differing host preferences were sequenced

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Evolutionary comparisons were performed on >200 insect chemosensory genes

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These comparisons revealed candidate genes involved in human feeding

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Two of the main candidates identified were co-receptor Ir8a and Or75

Discrete roles of Ir76b ionotropic coreceptor impact olfaction, blood feeding, and mating in the malaria vector mosquito Anopheles coluzzii

Anopheline mosquitoes rely on their highly sensitive chemosensory apparatus to detect diverse chemical stimuli that drive the host-seeking and blood-feeding behaviors required to vector pathogens for malaria and other diseases. This process incorporates a variety of chemosensory receptors and transduction pathways. We used advanced in vivo gene-editing and -labeling approaches to localize and functionally characterize the ionotropic coreceptor AcIr76b in the malaria mosquito Anopheles coluzzii, where it impacts both olfactory and gustatory systems. AcIr76b has a broad expression pattern in female adult antennal grooved pegs, coeloconic sensilla, and T1 and T2 sensilla on the labellum, stylets, and tarsi, as well as the larval sensory peg. AcIr76b is colocalized with the Orco odorant receptor (OR) coreceptor in a subset of cells across the female antennae and labella. In contrast to Orco and Ir8a, chemosensory coreceptors that appear essential for the activity of their respective sets of chemosensory neurons in mosquitoes, AcIr76b−/− mutants maintain wild-type peripheral responses to volatile amines on the adult palps, labellum, and larval sensory cone. Interestingly, AcIr76b−/− mutants display significantly increased responses to amines in antennal grooved peg sensilla, while coeloconic sensilla reveal significant deficits in responses to several acids and amines. Behaviorally, AcIr76b mutants manifest significantly female-specific insemination deficits, and although AcIr76b−/− mutant females can locate, alight on, and probe artificial blood hosts, they are incapable of blood feeding successfully. Taken together, our findings reveal a multidimensional functionality of Ir76b in anopheline olfactory and gustatory pathways that directly impacts the vectorial capacity of these mosquitoes.