Visual-Olfactory Integration in the Human Disease Vector Mosquito Aedes aegypti

A conserved odorant receptor underpins borneol-mediated repellency in culicine mosquitoes

The use of essential oils derived from the camphor tree to repel mosquitoes is an ancient practice that originated in Southeast Asia and gradually spread to China and across Europe via the Maritime Silk Road. The olfactory mechanisms by which these oils elicit avoidance behavior are unclear. Here we show that plant bicyclic monoterpenoids and borneol specifically activate a neural pathway that originates in the orphan olfactory receptor neuron of the capitate peg sensillum in the maxillary palp, and projects to the mediodorsal glomerulus 3 in the antennal lobe. This neuron co-locates with two olfactory receptor neurons tuned to carbon dioxide and octenol that mediate human-host detection. We also confirm that borneol elicits repellency against human-seeking female mosquitoes. Understanding the functional role of the mosquito maxillary palp is essential to investigating olfactory signal integration and host-selection behavior.

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.

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.

The black-legged tick Ixodes scapularis detects CO2 without the Haller's organ

Both male and female ticks have a strong innate drive to find and blood-feed on hosts. Carbon dioxide (CO2) is considered a critical behavioral activator and attractant for ticks and an essential sensory cue to find hosts. Yet, how CO2 activates and promotes host seeking in ticks is poorly understood. CO2 responses were studied in the black-legged tick Ixodes scapularis, the primary vector for Lyme disease in North America. Adult males and females were exposed to 1%, 2%, 4% or 8% CO2, and changes in walking behavior and foreleg movement were analyzed. CO2 is a potent stimulant for adult I. scapularis, even at lower concentrations (1%). Behavioral reactions depended on the animal's state: walking ticks increased their walking speed, while stationary ticks started to wave their forelegs and began to quest - both behaviors resembling aspects of host seeking. Only in sporadic cases did stationary animals start to walk when exposed to CO2, supporting the hypothesis that CO2 acts as an activator rather than an attractant. Furthermore, I. scapularis did not show a clear concentration preference and was not tuned more robustly to breath-like CO2 concentrations (∼4%) than to the other concentrations tested. Moreover, convincing evidence is provided showing that the foreleg Haller's organ is not necessary for CO2 detection. Even with a disabled or amputated Haller's organ, I. scapularis responded robustly to CO2, signifying that there must be CO2-sensitive structures important for tick host seeking that have not yet been identified.

Aedes albopictus is not an arbovirus aficionado - Impacts of sylvatic flavivirus infection in vectors and hosts on mosquito engorgement on non-human primates

The contact structure between vertebrate hosts and arthropod vectors plays a key role in the spread of arthropod-borne viruses (arboviruses); thus, it is important to determine whether arbovirus infection of either host or vector alters vector feeding behavior. Here we leveraged a study of the replication dynamics of two arboviruses isolated from their ancestral cycles in paleotropical forests, sylvatic dengue-2 (DENV-2) and Zika (ZIKV), in one non-human primate (NHP) species from the paleotropics (cynomolgus macaques, Macaca fascicularis) and one from the neotropics (squirrel monkeys, Saimiri boliviensis) to test the effect of both vector and host infection with each virus on completion of blood feeding (engorgement) of the mosquito Aedes albopictus. Although mosquitoes were starved and given no choice of hosts, engorgement rates varied dramatically, from 0% to 100%. While neither vector nor host infection systematically affected engorgement, NHP species and body temperature at the time of feeding did. We also interrogated the effect of repeated mosquito bites on cytokine expression and found that epidermal growth factor (EGF) and macrophage migration inhibitory factor (MIF) concentrations were dynamically associated with exposure to mosquito bites. This study highlights the importance of incorporating individual-level heterogeneity of vector biting in arbovirus transmission models.

New classifications for quantum bioinformatics: Q-bioinformatics, QCt-bioinformatics, QCg-bioinformatics, and QCr-bioinformatics

Bioinformatics has revolutionized biology and medicine by using computational methods to analyze and interpret biological data. Quantum mechanics has recently emerged as a promising tool for the analysis of biological systems, leading to the development of quantum bioinformatics. This new field employs the principles of quantum mechanics, quantum algorithms, and quantum computing to solve complex problems in molecular biology, drug design, and protein folding. However, the intersection of bioinformatics, biology, and quantum mechanics presents unique challenges. One significant challenge is the possibility of confusion among scientists between quantum bioinformatics and quantum biology, which have similar goals and concepts. Additionally, the diverse calculations in each field make it difficult to establish boundaries and identify purely quantum effects from other factors that may affect biological processes. This review provides an overview of the concepts of quantum biology and quantum mechanics and their intersection in quantum bioinformatics. We examine the challenges and unique features of this field and propose a classification of quantum bioinformatics to promote interdisciplinary collaboration and accelerate progress. By unlocking the full potential of quantum bioinformatics, this review aims to contribute to our understanding of quantum mechanics in biological systems.

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.

Effect of Temperature on Mosquito Olfaction

Mosquitoes use a wide range of cues to find a host to feed on, eventually leading to the transmission of pathogens. Among them, olfactory cues (e.g., host-emitted odors, including CO2, and skin volatiles) play a central role in mediating host-seeking behaviors. While mosquito olfaction can be impacted by many factors, such as the physiological state of the insect (e.g., age, reproductive state), the impact of environmental temperature on the olfactory system remains unknown. In this study, we quantified the behavioral responses of Aedes aegypti mosquitoes, vectors of dengue, yellow fever, and Zika viruses, among other pathogens, to host and plant-related odors under different environmental temperatures.

Context and the functional use of information in insect sensory ecology

Context-specific behaviors emerge from the interaction between an animal's internal state and its external environment. Although the importance of context is acknowledged in the field of insect sensory ecology, there is a lack of synthesis on this topic stemming from challenges in conceptualizing 'context'. We address this challenge by gleaning over the recent findings on the sensory ecology of mosquitoes and other insect pollinators. We discuss internal states and their temporal dynamics, from those lasting minutes to hours (host-seeking) to those lasting days to weeks (diapause, migration). Of the many patterns reviewed, at least three were common to all taxa studied. First, different sensory cues gain prominence depending on the insect's internal state. Second, similar sensory circuits between related species can result in different behavioral outcomes. And third, ambient conditions can dramatically alter internal states and behaviors.

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.

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.

Effect of temperature on mosquito olfaction

Mosquitoes use a wide range of cues to find a host to feed on, eventually leading to the transmission of pathogens. Among them, olfactory cues ( e.g. , host emitted odors, including CO 2 , and skin volatiles) play a central role in mediating host seeking behaviors. While mosquito olfaction can be impacted by many factors, such as the physiological state of the insect ( e.g. , age, reproductive state), the impact of environmental temperature on the olfactory system remains unknown. In this study, we quantified the behavioral responses of Aedes aegypti mosquitoes, vectors of dengue, yellow fever and Zika viruses, to host and plant related odors under different environmental temperatures.

Multisite imaging of neural activity using a genetically encoded calcium sensor in the honey bee

Understanding of the neural bases for complex behaviors in Hymenoptera insect species has been limited by a lack of tools that allow measuring neuronal activity simultaneously in different brain regions. Here, we developed the first pan-neuronal genetic driver in a Hymenopteran model organism, the honey bee, and expressed the calcium indicator GCaMP6f under the control of the honey bee synapsin promoter. We show that GCaMP6f is widely expressed in the honey bee brain, allowing to record neural activity from multiple brain regions. To assess the power of this tool, we focused on the olfactory system, recording simultaneous responses from the antennal lobe, and from the more poorly investigated lateral horn (LH) and mushroom body (MB) calyces. Neural responses to 16 distinct odorants demonstrate that odorant quality (chemical structure) and quantity are faithfully encoded in the honey bee antennal lobe. In contrast, odor coding in the LH departs from this simple physico-chemical coding, supporting the role of this structure in coding the biological value of odorants. We further demonstrate robust neural responses to several bee pheromone odorants, key drivers of social behavior, in the LH. Combined, these brain recordings represent the first use of a neurogenetic tool for recording large-scale neural activity in a eusocial insect and will be of utility in assessing the neural underpinnings of olfactory and other sensory modalities and of social behaviors and cognitive abilities.

Neuromodulation and differential learning across mosquito species

Mosquitoes can change their feeding behaviours based on past experiences, such as shifting from biting animals to biting humans or avoiding defensive hosts (Wolff & Riffell 2018 J. Exp. Biol. 221, jeb157131. (doi:10.1242/jeb.157131)). Dopamine is a critical neuromodulator for insects, allowing flexibility in their feeding preferences, but its role in the primary olfactory centre, the antennal lobe (AL), remains unclear (Vinauger et al. 2018 Curr. Biol. 28, 333-344.e8. (doi:10.1016/j.cub.2017.12.015)). It is also unknown whether mosquitoes can learn some odours and not others, or whether different species learn the same odour cues. We assayed aversive olfactory learning in four mosquito species with different host preferences, and found that they differentially learn odours salient to their preferred host. Mosquitoes that prefer humans learned odours found in mammalian skin, but not a flower odour, and a nectar-feeding species only learned a floral odour. Comparing the brains of these four species revealed significantly different innervation patterns in the AL by dopaminergic neurons. Calcium imaging in the Aedes aegypti AL and three-dimensional image analyses of dopaminergic innervation show that glomeruli tuned to learnable odours have significantly higher dopaminergic innervation. Changes in dopamine expression in the insect AL may be an evolutionary mechanism to adapt olfactory learning circuitry without changing brain structure and confer to mosquitoes an ability to adapt to new hosts.

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.

Opsin1 regulates light-evoked avoidance behavior in Aedes albopictus

BACKGROUND

Mosquitoes locate a human host by integrating various sensory cues including odor, thermo, and vision. However, their innate light preference and its genetic basis that may predict the spatial distribution of mosquitoes, a prerequisite to encounter a potential host and initiate host-seeking behaviors, remains elusive.

RESULTS

Here, we first studied mosquito visual features and surprisingly uncovered that both diurnal (Aedes aegypti and Aedes albopictus) and nocturnal (Culex quinquefasciatus) mosquitoes significantly avoided stronger light when given choices. With consistent results from multiple assays, we found that such negative phototaxis maintained throughout development to adult stages. Notably, female mosquitoes significantly preferred to bite hosts in a shaded versus illuminated area. Furthermore, silencing Opsin1, a G protein-coupled receptor that is most enriched in compound eyes, abolished light-evoked avoidance behavior of Aedes albopictus and attenuated photonegative behavior in Aedes aegypti. Finally, we found that field-collected Aedes albopictus also prefers darker area in an Opsin1-dependent manner.

CONCLUSIONS

This study reveals that mosquitoes consistently prefer darker environment and identifies the first example of a visual molecule that modulates mosquito photobehavior.

The neural basis of interspecies interactions in insects

As insects move through the world, they continuously engage in behavioral interactions with other species. These interactions take on a spectrum of forms, from inconsequential encounters to predation, defense, and specialized symbiotic partnerships. All such interactions rely on sensorimotor pathways that carry out efficient categorization of different organisms and enact behaviors that cross species boundaries. Despite the universality of interspecies interactions, how insect brains perceive and process salient features of other species remains unexplored. Here, we present an overview of major questions concerning the neurobiology and evolution of behavioral interactions between species, providing a framework for future research on this critical role of the insect nervous system.

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

Human attractive cues and mosquito host-seeking behavior

Female mosquitoes use chemical and physical cues, including vision, smell, heat, and humidity, to orient toward hosts. Body odors are produced by skin resident bacteria that convert metabolites secreted in sweat into odorants that confer the characteristic body scent. Mosquitoes detect these compounds using olfactory receptors in their antennal olfactory receptor neurons. Such information is further integrated with the senses of temperature and humidity, as well as vision, processed in the brain into a behavioral output, leading to host finding. Knowledge of human scent components unveils a variety of odorants that are attractive to mosquitoes, but also odor-triggering repellency. Finding ways to divert human-seeking behavior by female mosquitoes using odorants can possibly mitigate mosquito-borne pathogen transmission.

Studies on the Volatiles Composition of Stored Sheep Wool, and Attractancy toward Aedes aegypti Mosquitoes

To discover new natural materials for insect management, commercially available stored sheep wool was investigated for attractancy to female adult Aedes aegypti mosquitoes. The volatiles from sheep wool were collected by various techniques of headspace (HS) extractions and hydrodistillation. These extracts were analyzed using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-flame ionization detector (GC-FID) coupled with GC-MS. Fifty-two volatile compounds were detected, many of them known for their mosquito attractant activity. Seven compounds were not previously reported in sheep products. The volatile composition of the extracts varied significantly across collections, depending on the extraction techniques or types of fibers applied. Two types of bioassay were conducted to study attractancy of the sheep wool volatiles to mosquitoes: laboratory bioassays using glass tubes, and semi-field bioassays using large, screened outdoor cages. In bioassays with glass tubes, the sheep wool hydrodistillate and its main component, thialdine, did not show any significant attractant activity against female adult Ae. aegypti mosquitoes. Semi-field bioassays in two large screened outdoor cages, each equipped with a U.S. Centers for Disease Control (CDC) trap and the various bait setups with Vortex apparatus, revealed that vibrating wool improved mosquito catches compared to the setups without wool or with wool but not vibrating. Sheep wool, when vibrated, may release intensively volatile compounds, which could serve as olfactory cues, and play significant role in making the bait attractive to mosquitoes. Sheep wool is a readily available, affordable, and environment-friendly material. It should have the potential to be used as a mosquito management and surveillance component in dynamic bait setups.

The olfactory gating of visual preferences to human skin and visible spectra in mosquitoes

Mosquitoes track odors, locate hosts, and find mates visually. The color of a food resource, such as a flower or warm-blooded host, can be dominated by long wavelengths of the visible light spectrum (green to red for humans) and is likely important for object recognition and localization. However, little is known about the hues that attract mosquitoes or how odor affects mosquito visual search behaviors. We use a real-time 3D tracking system and wind tunnel that allows careful control of the olfactory and visual environment to quantify the behavior of more than 1.3 million mosquito trajectories. We find that CO₂ induces a strong attraction to specific spectral bands, including those that humans perceive as cyan, orange, and red. Sensitivity to orange and red correlates with mosquitoes’ strong attraction to the color spectrum of human skin, which is dominated by these wavelengths. The attraction is eliminated by filtering the orange and red bands from the skin color spectrum and by introducing mutations targeting specific long-wavelength opsins or CO₂ detection. Collectively, our results show that odor is critical for mosquitoes’ wavelength preferences and that the mosquito visual system is a promising target for inhibiting their attraction to human hosts.

Vision in mosquitoes plays a critical but understudied role in their attraction to hosts. Here, the authors show that encounter with an attractive odor gates the mosquito attraction to specific colors, especially the long wavelengths reflected from human skin. Filtering the long wavelengths reflected from the human skin or knocking-out the ability for the mosquito to detect the wavelengths, suppressed their attraction. This work transforms our understanding of mosquito vision from the conventional view that vision does little in mediating mosquito-host interactions, to the recognition that vision plays a critical role.

Elimination of vision-guided target attraction in Aedes aegypti using CRISPR

Blood-feeding insects, such as the mosquito, Aedes (Ae.) aegypti, use multiple senses to seek out and bite humans.^1,2 Upon exposure to the odor of CO₂, the attention of female mosquitoes to potential targets is greatly increased. Female mosquitoes are attracted to high-contrast visual cues and use skin olfactory cues to assist them in homing in on targets several meters away.^3-9 Within close range, convective heat from skin and additional skin odors further assist the mosquitoes' evaluation as to whether the object of interest might be a host.^10,11 Here, using CRISPR-Cas9, we mutated the gene encoding Op1, which is the most abundant of the five rhodopsins expressed in the eyes of Ae. aegypti. Using cage and wind-tunnel assays, we found that elimination of op1 did not impair CO₂-induced target seeking. We then mutated op2, which encodes the rhodopsin most similar to Op1, and also found that there was no impact on this behavior. Rather, mutation of both op1 and op2 was required for abolishing vision-guided target attraction. In contrast, the double mutants exhibited normal phototaxis and odor-tracking responses. By measuring the walking optomotor response, we found that the double mutants still perceived optic flow. In further support of the conclusion that the double mutant is not blind, the animals retained an electrophysiological response to light, although it was diminished. This represents the first genetic perturbation of vision in mosquitoes and indicates that vision-guided target attraction by Ae. aegypti depends on two highly related rhodopsins.

Understanding and interpreting mosquito blood feeding studies: the case of Aedes albopictus

Blood feeding is a fundamental mosquito behavior with consequences for pathogen transmission and control. Feeding behavior can be studied through two lenses - patterns and preference. Feeding patterns are assessed via blood meal analyses, reflecting mosquito-host associations influenced by environmental and biological parameters. Bias can profoundly impact results, and we provide recommendations for mitigating these effects. We also outline design choices for host preference research, which can take many forms, and highlight their respective (dis)advantages for preference measurement. Finally, Aedes albopictus serves as a case study for how to apply these lessons to interpret data and understand feeding biology. We illustrate how assumptions and incomplete evidence can lead to inconsistent interpretations by reviewing Ae. albopictus feeding studies alongside prevalent narratives about perceived behavior.

Multimodal integration across spatiotemporal scales to guide invertebrate locomotion

Locomotion is a hallmark of organisms that has enabled adaptive radiation to an extraordinarily diverse class of ecological niches, and allows animals to move across vast distances. Sampling from multiple sensory modalities enables animals to acquire rich information to guide locomotion. Locomotion without sensory feedback is haphazard, therefore sensory and motor systems have evolved complex interactions to generate adaptive behavior. Notably, sensory-guided locomotion acts over broad spatial and temporal scales to permit goal-seeking behavior, whether to localize food by tracking an attractive odor plume or to search for a potential mate. How does the brain integrate multimodal stimuli over different temporal and spatial scales to effectively control behavior? In this review, we classify locomotion into three ordinally ranked hierarchical layers that act over distinct spatiotemporal scales: stabilization, motor primitives, and higher-order tasks, respectively. We discuss how these layers present unique challenges and opportunities for sensorimotor integration. We focus on recent advances in invertebrate locomotion due to their accessible neural and mechanical signals from the whole brain, limbs and sensors. Throughout, we emphasize neural-level description of computations for multimodal integration in genetic model systems, including the fruit fly, Drosophila melanogaster, and the yellow fever mosquito, Aedes aegypti. We identify that summation (e.g. gating) and weighting-which are inherent computations of spiking neurons-underlie multimodal integration across spatial and temporal scales, therefore suggesting collective strategies to guide locomotion.

Contributions of the Conserved Insect Carbon Dioxide Receptor Subunits to Odor Detection

The CO₂ receptor in mosquitoes is broadly tuned to detect many diverse odorants. The receptor consists of three subunits (Gr1, Gr2, and Gr3) in mosquitoes but only two subunits in Drosophila: Gr21a (Gr1 ortholog) and Gr63a (Gr3 ortholog). We demonstrate that Gr21a is required for CO₂ responses in Drosophila, as has been shown for Gr63a. Next, we generate a Drosophila double mutant for Gr21a and Gr63a, and in this background, we functionally express combinations of Aedes Gr1, Gr2, and Gr3 genes in the CO₂ empty neuron. Only two subunits, Gr2 and Gr3, suffice for response to CO₂. Addition of Gr1 increases sensitivity to CO₂, whereas it decreases the response to pyridine. The inhibitory effect of the antagonist isobutyric acid is observed upon addition of Gr1. Gr1 therefore increases the diversity of ligands of the receptor and modulates the response of the receptor complex.

Multimodal synergisms in host stimuli drive landing response in malaria mosquitoes

Anopheles mosquitoes transmit malaria, which affects one-fifth of the world population. A comprehensive understanding of mosquito behaviour is essential for the development of novel tools for vector control and surveillance. Despite abundant research on mosquito behaviour, little is known on the stimuli that drive malaria vectors during the landing phase of host-seeking. Using behavioural assays with a multimodal step approach we quantified both the individual and the combined effect of three host-associated stimuli in eliciting landing in Anopheles coluzzii females. We demonstrated that visual, olfactory and thermal sensory stimuli interact synergistically to increase the landing response. Furthermore, if considering only the final outcome (i.e. landing response), our insect model can bypass the absence of either a thermal or a visual stimulus, provided that at least one of these is presented simultaneously with the olfactory stimuli, suggesting that landing is the result of a flexible but accurate stimuli integration. These results have important implications for the development of mosquito control and surveillance tools.

Understanding host utilization by mosquitoes: determinants, challenges and future directions

Mosquito host utilization is a key factor in the transmission of vector-borne pathogens given that it greatly influences host-vector contact rates. Blood-feeding patterns of mosquitoes are not random, as some mosquitoes feed on particular species and/or individuals more than expected by chance. Mosquitoes use a number of cues including visual, olfactory, acoustic, and thermal stimuli emitted by vertebrate hosts to locate and identify their blood meal sources. Thus, differences in the quality/intensity of the released cues may drive host selection by mosquitoes at both inter- and intra-specific levels. Such patterns of host selection by mosquitoes in space and time can be structured by factors related to mosquitoes (e.g. innate host preference, behavioural plasticity), to hosts (e.g. emission of host-seeking cues, host availability) or to both (e.g. pathogen infection). In this study, we review current evidence, from phenomena to mechanisms, of how these factors influence host utilization by mosquitoes. We also review the methodologies commonly used in this research field and identify the major challenges for future studies. To bridge the knowledge gaps, we propose improvements to strengthen traditional approaches and the use of a functional trait-based approach to infer mosquito host utilization in natural communities.

What gaze direction can tell us about cognitive processes in invertebrates

Most visually guided animals shift their gaze using body movements, eye movements, or both to gather information selectively from their environments. Psychological studies of eye movements have advanced our understanding of perceptual and cognitive processes that mediate visual attention in humans and other vertebrates. However, much less is known about how these processes operate in other organisms, particularly invertebrates. We here make the case that studies of invertebrate cognition can benefit by adding precise measures of gaze direction. To accomplish this, we briefly review the human visual attention literature and outline four research themes and several experimental paradigms that could be extended to invertebrates. We briefly review selected studies where the measurement of gaze direction in invertebrates has provided new insights, and we suggest future areas of exploration.

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.

The mosquito taste system and disease control

Mosquitoes are a widely diverse group of organisms, comprising ∼3,500 species that live in an enormous range of habitats. Some species are vectors of diseases that afflict hundreds of millions of people each year. Although understanding of mosquito olfaction has progressed dramatically in recent years, mosquito taste remains greatly understudied. Since taste is essential to feeding, egg laying, and mating decisions in insects, improved understanding of taste in mosquitoes could provide new mechanistic insight into many aspects of their behavior. We provide a guide to current knowledge in the field, and we suggest a wealth of opportunities for research that are now enabled by recent scientific and technological advances. We also propose means by which taste might be exploited in new strategies for mosquito control, which may be urgently needed as the geographical ranges of vector species increase with climate change.

The neuroecology of insect-plant interactions: the importance of physiological state and sensory integration

Natural behaviorally important stimuli are combinations of cues that are integrated by the nervous system to elicit behavior. Nonetheless, these cues dynamically change in time and space. In turn, the animal's internal state can cause changes in the encoding and representation of these stimuli. Despite abundant behavioral examples, links between the neural bases of sensory integration and the internal state-dependency of these responses remains an active study area. Recent studies in different insect models have provided new insights into how plasticity and the insect's internal state may influence odor representation. These studies show that complex stimuli are represented in unique percepts that are different from their sensory channels and that the representations may be modulated by physiological state.

Using virtual worlds to understand insect navigation for bio-inspired systems

Insects perform a wide array of intricate behaviors over large spatial and temporal scales in complex natural environments. A mechanistic understanding of insect cognition has direct implications on how brains integrate multimodal information and can inspire bio-based solutions for autonomous robots. Virtual Reality (VR) offers an opportunity assess insect neuroethology while presenting complex, yet controlled, stimuli. Here, we discuss the use of insects as inspiration for artificial systems, recent advances in different VR technologies, current knowledge gaps, and the potential for application of insect VR research to bio-inspired robots. Finally, we advocate the need to diversify our model organisms, behavioral paradigms, and embrace the complexity of the natural world. This will help us to uncover the proximate and ultimate basis of brain and behavior and extract general principles for common challenging problems.

An updated antennal lobe atlas for the yellow fever mosquito Aedes aegypti

The yellow fever mosquito Aedes aegypti is a prolific vector of arboviral and filarial diseases that largely relies on its sense of smell to find humans. To facilitate in-depth analysis of the neural circuitry underlying Ae. aegypti olfactory-driven behaviors, we generated an updated in vitro atlas for the antennal lobe olfactory brain region of this disease vector using two independent neuronal staining methods. We performed morphological reconstructions with replicate fixed, dissected and stained brain samples from adult male and female Ae. aegypti of the LVPib12 genome reference strain and determined that the antennal lobe in both sexes is comprised of approximately 80 discrete glomeruli. Guided by landmark features in the antennal lobe, we found 63 of these glomeruli are stereotypically located in spatially invariant positions within these in vitro preparations. A posteriorly positioned, mediodorsal glomerulus denoted MD1 was identified as the largest spatially invariant glomerulus in the antennal lobe. Spatial organization of glomeruli in a recently field-derived strain of Ae. aegypti from Puerto Rico was conserved, despite differences in antennal lobe shape relative to the inbred LVPib12 strain. This model in vitro atlas will serve as a useful community resource to improve antennal lobe annotation and anatomically map projection patterns of neurons expressing target genes in this olfactory center. It will also facilitate the development of chemotopic maps of odor representation in the mosquito antennal lobe to decode the molecular and cellular basis of Ae. aegypti attraction to human scent and other chemosensory cues.

The olfactory system of the yellow fever mosquito Aedes aegypti is highly tuned for the detection of human odorants, as well as other chemical cues influencing host and food-search behavior, egg-laying and mating. To provide insights into the neuroanatomical organization of the olfactory system of this globally important disease vector, we have generated an updated in vitro atlas for the primary smell processing center of the Ae. aegypti brain, called the antennal lobe. These new guide maps facilitate systematic interrogation of antennal lobe morphology and naming of associated substructures in dissected brain samples of this species labeled with two common neural staining methods. We report that landmark features of the Ae. aegypti antennal lobe morphology and spatial organization appear conserved between mosquito sexes and across geographically divergent strains of this mosquito species. An improved understanding of Ae. aegypti antennal lobe neuroanatomy and how attractive or repellent odorant stimuli are encoded in this brain center has the potential to rapidly accelerate reverse engineering of synthetic chemical blends that effectively lure, confuse or repel this major disease vector.

Lure, retain, and catch malaria mosquitoes. How heat and humidity improve odour-baited trap performance

BACKGROUND

When seeking a human for a blood meal, mosquitoes use several cues to detect and find their hosts. From this knowledge, counter-flow odour-baited traps have been developed that use a combination of CO₂, human-mimicking odour, visual cues and circulating airflow to attract and capture mosquitoes. Initially developed for monitoring, these traps are now also being considered as promising vector control tools. The traps are attractive to host-seeking mosquitoes, but their capture efficiency is low. It has been hypothesized that the lack of short-range host cues, such as heat and increased local humidity, often prevent mosquitoes from getting close enough to get caught; this lack might even trigger avoidance manoeuvres near the capture region.

METHODS

This study investigated how close-range host cues affect the flight behaviour of Anopheles female malaria mosquitoes around odour-baited traps, and how this affects trap capture performance. For this, a novel counter-flow odour-baited trap was developed, the M-Tego. In addition to the usual CO₂ and odour-blend, this trap can provide the short-range host cues, heat and humidity. Systematically adding or removing these two cues tested how this affected the trap capture percentages and flight behaviour. First, capture percentages of the M-Tego with and without short-range host cues to the BG-Suna trap were compared, in both laboratory and semi-field testing. Then, machine-vision techniques were used to track the three-dimensional flight movements of mosquitoes around the M-Tego.

RESULTS

With heat and humidity present, the M-Tego captured significantly more mosquitoes as capture percentages almost doubled. Comparing the flight behaviour around the M-Tego with variable close-range host cues showed that when these cues were present, flying mosquitoes were more attracted to the trap and spent more time there. In addition, the M-Tego was found to have a better capture mechanism than the BG-Suna, most likely because it does not elicit previously observed upward avoiding manoeuvres.

CONCLUSIONS

Results suggest that adding heat and humidity to an odour-baited trap lures more mosquitoes close to the trap and retains them there longer, resulting in higher capture performance. These findings support the development of control tools for fighting mosquito-borne diseases such as malaria.

Mechanism and plasticity of vectors' host-seeking behavior

The host-seeking behavior of disease vector insects is central to the transmission of pathogens. In this context, an improved understanding of the mechanisms that allow vectors to detect, identify and locate a potential host will be crucial to refine existing control strategies and invent new ones. Host-seeking is mediated by the integration of cues that are processed by multiple sensory modalities, and provide robust information about host location and quality. Responses to these cues are plastic and vary as a function of the vector's internal state, age, and previous experience. Vectors also integrate other factors such as time of day, or even the level of defensiveness of the host. Here, we review the most recent advances on the molecular basis of host-seeking behavior, with a particular emphasis on disease vector mosquitoes.

Multimodal interactions in insect navigation

Animals travelling through the world receive input from multiple sensory modalities that could be important for the guidance of their journeys. Given the availability of a rich array of cues, from idiothetic information to input from sky compasses and visual information through to olfactory and other cues (e.g. gustatory, magnetic, anemotactic or thermal) it is no surprise to see multimodality in most aspects of navigation. In this review, we present the current knowledge of multimodal cue use during orientation and navigation in insects. Multimodal cue use is adapted to a species’ sensory ecology and shapes navigation behaviour both during the learning of environmental cues and when performing complex foraging journeys. The simultaneous use of multiple cues is beneficial because it provides redundant navigational information, and in general, multimodality increases robustness, accuracy and overall foraging success. We use examples from sensorimotor behaviours in mosquitoes and flies as well as from large scale navigation in ants, bees and insects that migrate seasonally over large distances, asking at each stage how multiple cues are combined behaviourally and what insects gain from using different modalities.

Linking Mosquito Ecology, Traits, Behavior, and Disease Transmission

Mosquitoes are considered to be the deadliest animals on Earth because the diseases they transmit claim at least a million human lives every year globally. Here, we discuss the scales at which the effects of ecological factors cascade to influence epidemiologically relevant behaviors of adult mosquitoes. In particular, we focused our review on the environmental conditions (coarse-scale variables) that shape the life-history traits of larvae and adult mosquitoes (fine-scale traits), and how these factors and their association, in turn, modulate adult behaviors to influence mosquito-borne disease transmission. Finally, we explore the integration of physical, physiological, and behavioral information into predictive models with epidemiological applications.

The olfactory basis of orchid pollination by mosquitoes

Nectar feeding by mosquitoes is important for survival and reproduction, and hence disease transmission. However, we know little about the sensory mechanisms that mediate mosquito attraction to sources of nectar, like those of flowers, or how this information is processed in the mosquito brain. Using a unique mutualism between Aedes mosquitoes and Platanthera obtusata orchids, we reveal that the orchid’s scent mediates this mutualism. Furthermore, lateral inhibition in the mosquito’s antennal (olfactory) lobe—via the neurotransmitter GABA—is critical for the representation of the scent. These results have implications for understanding the olfactory basis of mosquito nectar-seeking behaviors.

Mosquitoes are important vectors of disease and require sources of carbohydrates for reproduction and survival. Unlike host-related behaviors of mosquitoes, comparatively less is understood about the mechanisms involved in nectar-feeding decisions, or how this sensory information is processed in the mosquito brain. Here we show that Aedes spp. mosquitoes, including Aedes aegypti, are effective pollinators of the Platanthera obtusata orchid, and demonstrate this mutualism is mediated by the orchid’s scent and the balance of excitation and inhibition in the mosquito’s antennal lobe (AL). The P. obtusata orchid emits an attractive, nonanal-rich scent, whereas related Platanthera species—not visited by mosquitoes—emit scents dominated by lilac aldehyde. Calcium imaging experiments in the mosquito AL revealed that nonanal and lilac aldehyde each respectively activate the LC2 and AM2 glomerulus, and remarkably, the AM2 glomerulus is also sensitive to N,N-diethyl-meta-toluamide (DEET), a mosquito repellent. Lateral inhibition between these 2 glomeruli reflects the level of attraction to the orchid scents. Whereas the enriched nonanal scent of P. obtusata activates the LC2 and suppresses AM2, the high level of lilac aldehyde in the other orchid scents inverts this pattern of glomerular activity, and behavioral attraction is lost. These results demonstrate the ecological importance of mosquitoes beyond operating as disease vectors and open the door toward understanding the neural basis of mosquito nectar-seeking behaviors.

Progress in the use of genetic methods to study insect behavior outside Drosophila

In the span of a decade we have seen a rapid progress in the application of genetic tools and genome editing approaches in 'non-model' insects. It is now possible to target sensory receptor genes and neurons, explore their functional roles and manipulate behavioral responses in these insects. In this review, we focus on the latest examples from Diptera, Lepidoptera and Hymenoptera of how applications of genetic tools advanced our understanding of diverse behavioral phenomena. We further discuss genetic methods that could be applied to study insect behavior in the future.

Does membrane feeding compromise the quality of Aedes aegypti mosquitoes?

Modified Aedes aegypti mosquitoes are being mass-reared for release in disease control programs around the world. Releases involving female mosquitoes rely on them being able to seek and feed on human hosts. To facilitate the mass-production of mosquitoes for releases, females are often provided blood through artificial membrane feeders. When reared across generations there is a risk that mosquitoes will adapt to feeding on membranes and lose their ability to feed on human hosts. To test adaptation to membrane feeding, we selected replicate populations of Ae. aegypti for feeding on either human arms or membrane feeders for at least 8 generations. Membrane-selected populations suffered fitness costs, likely due to inbreeding depression arising from bottlenecks. Membrane-selected females had higher feeding rates on membranes than human-selected ones, suggesting adaptation to membrane feeding, but they maintained their attraction to host cues and feeding ability on humans despite a lack of selection for these traits. Host-seeking ability in small laboratory cages did not differ between populations selected on the two blood sources, but membrane-selected females were compromised in a semi-field enclosure where host-seeking was tested over a longer distance. Our findings suggest that Ae. aegypti may adapt to feeding on blood provided artificially, but this will not substantially compromise field performance or affect experimental assessments of mosquito fitness. However, large population sizes (thousands of individuals) during mass rearing with membrane feeders should be maintained to avoid bottlenecks which lead to inbreeding depression.