Behavioural and Electrophysiological Responses of Female Anopheles gambiae Mosquitoes to Volatiles from a Mango Bait

Olfactory preferences and chemical differences of fruit scents for Aedes aegypti mosquitoes

Feeding on the sugar of fruits and flowers is vital for mosquitoes and increases their lifespan, reproduction, and flight activity. Olfaction is a key sensory modality in mediating mosquito responses to sugar sources. Previous studies have demonstrated that natural nectar sources from fruits and flowers can vary in attractiveness to mosquitoes, with some sources preferred over others. However, how the attractiveness of different fruits relates to the chemical composition of their scent and the responses they evoke from the mosquito's peripheral olfactory system, is still not understood. In this study, we use closely related fruit species and their varieties to examine how changes in scent chemistry can influence the fruit's attractiveness to Aedes aegypti mosquitoes and examine how the mosquito's olfactory responses (via electroantennogram recordings, or EAG) correlate with those differences. Our results show that mosquitoes are attracted to the scents of certain fruits (Mangifera indica, Prunus perspica, Psidium guajava, Musa acuminata), whereas others (Pyrus communis, Citrus limon) elicited responses not significantly different from the negative control. Chemical analyses of the scents showed that attractive fruits have distinct chemical profiles, and amongst closely related fruits, minor changes in the relative proportions of scent compounds can modify the attractiveness. These minor differences in the fruit scent were not reflected in the EAG responses, which showed similar responses to scents from different fruit species and closely related varieties. Experimentally altering the chemical proportion of a single compound in attractive scents caused a significant decrease in attraction to levels similar to the less attractive cultivars. Our results demonstrate that mosquitoes are sensitive to compound proportions in attractive odors, which have implications for the olfactory processing of complex odor sources, like those from plants or blood hosts.

Bioassays for the evaluation of the attractiveness of attractive targeted sugar bait (ATSB) against Anopheles mosquitoes in controlled semi-field systems

BACKGROUND

Sugar feeding is an essential aspect of mosquito biology that may be exploited for mosquito control by adding insecticides to sugar attractants, so-called 'attractive targeted sugar baits' (ATSBs). To optimize their effectiveness, ATSB products need to be maximally attractive at both short and long range and induce high levels of feeding. This study aimed to assess the attractiveness and feeding success of Anopheles mosquitoes exposed to attractive sugar baits (ASBs).

METHOD

Experiments were conducted in 2 × 5 × 2-m cages constructed within the semi-field systems (SFS) at Ifakara Health Institute, Bagamoyo, Tanzania. Male and female Anopheles gambiae s.s. and An. funestus s.s. mosquitoes were exposed to either 20% sucrose or different ASB station prototypes produced by Westham Co. in either (1) no-choice experiments or (2) choice experiments. Mosquitoes were exposed overnight and assessed for intrinsic or relative olfactory attraction using fluorescent powder markers dusted over the ASB stations and 20% sucrose and for feeding using uranine incorporated within the bait station and food dye in 20% sucrose controls.

RESULTS

Both male and female An. gambiae and An. funestus mosquitoes were attracted to the ASBs, with no significant difference between the sexes for each of the experiments conducted. Older mosquitoes (3-5 days) were more attracted to the ASBs (OR = 8.3, [95% CI 6.6-10.5] P < 0.001) than younger mosquitoes (0-1 day). Similarly, older mosquitoes responded more to 20% sucrose (OR = 4.6, [3.7-5.8], P < 0.001) than newly emerged Anopheles. Of the four prototypes tested, the latest iteration, ASB prototype v1.2.1, showed the highest intrinsic attraction of both Anopheles species, attracting 91.2% [95% CI 87.9-94.5%]. Relative to ATSB v1.1.1, the latest prototype, v.1.2.1, had higher attraction (OR = 1.19 [95% CI 1.07-1.33], P < 0.001) and higher feeding success (OR = 1.71 [95% CI 1.33-2.18], P < 0.001).

CONCLUSIONS

Data from these experiments support using ASBs v1.2.1, deployed in large-scale epidemiological trials, as it is the most attractive and shows the highest feeding success of the Westham prototypes tested. The findings indicate that future bioassays to evaluate ATSBs should use mosquitoes of both sexes, aged 3-5 days, include multiple species in the same cage or chamber, and utilize both non-choice and choice tests with a standard comparator.

Mosquito floral visitation and pollination

We often consider mosquitoes through an 'anthropocentric lens' that disregards their interactions with nonhuman and nonpathogenic organisms, even though these interactions can be harnessed for mosquito control. Mosquitoes have been recognized as floral visitors, and pollinators, for more than a century. However, we know relatively little about mosquito-plant interactions, excepting some nutrition and chemical ecology-related topics, compared with mosquito-host interactions, and frequently use flawed methodology when investigating them. Recent work demonstrates mosquitoes use multimodal sensory cues to locate flowers, including ultraviolet visual cues, and we may underestimate mosquito pollination. This review focuses on current knowledge of how mosquitoes locate flowers, floral visitation assay methodology, mosquito pollination, and implications for technologies such as sterile male mosquito release through genetic control programs or Wolbachia infection.

Chemical ecology of nectar-mosquito interactions: recent advances and future directions

Mosquitoes, males and females, rely on sugar-rich resources, including floral nectar as a primary source of sugar to meet their energy and nutritional needs. Despite advancements in understanding mosquito host-seeking and blood-feeding preferences, significant gaps in our knowledge of the chemical ecology mediating mosquito-nectar associations remain. The influence of such association with nectar on mosquito behavior and the resulting effects on their fitness are also not totally understood. It is significant that floral nectar frequently acts as a natural habitat for various microbes (e.g. bacteria and yeast), which substantially alter nectar characteristics, influencing the nutritional ecology of flower-visiting insects, such as mosquitoes. The role of nectar-inhabiting microbes in shaping the nectar-mosquito interactions remains, however, under-researched. This review explores recent advances in understanding the role of such multitrophic interactions on the fitness and life history traits of mosquitoes and outlines future directions for research toward their control as disease vectors.

Sugar feeding by invasive mosquito species on ornamental and wild plants

Feeding on plant-derived sugars is an essential component of mosquito biology, affecting key aspects of their lives such as survival, metabolism, and reproduction. Among mosquitoes, Aedes aegypti and Aedes albopictus are two invasive mosquito species in the US, and are vectors of diseases such as dengue fever, chikungunya, and Zika. These species live in heavily populated, urban areas, where they have high accessibility to human hosts as well as to plants in backyards and public landscapes. However, the range of plants that are suitable sugar hosts for these species remains to be described, despite the importance of understanding what plants may attract or repel mosquitoes to inform citizens and municipal authorities accordingly. Here, we tested whether Ae. aegypti and Ae. albopictus would sugar-feed on eleven commonly planted ornamental plant species. We confirmed feeding activity using the anthrone method and identified the volatile composition of plant headspace using gas-chromatography mass-spectroscopy. These chemical analyses revealed that a broad range of olfactory cues are associated with plants that mosquitoes feed on. This prompted us to use plant DNA barcoding to identify plants that field-caught mosquitoes feed on. Altogether, results show that native and invasive mosquito species can exploit a broader range of plants than originally suspected, including wild and ornamental plants from different phyla throughout the Spring, Summer and Fall seasons.

Mosquito Electroantennography

Mosquitoes use multiple cues to locate food sources (animal and plants), mates, and oviposition sites. The sense of smell plays an important role in these behaviors, and olfactory cues are detected primarily by the appendages on the head-in particular, the antennae. Thus, mosquito olfaction can be studied by conducting electroantennogram (EAG) recordings. EAGs have emerged as a reliable technique to screen for bioactive compounds and are useful in the development of attractants and repellents for mosquito population control. Here, we focus on comparing the two main experimental approaches used in the literature (whole-body and whole-head mounting) and highlight key considerations for conducting EAGs in multiple species and for obtaining reliable and reproducible data.

Electrophysiological and Behavioral Responses of Plutella xylostella (Lepidoptera: Plutellidae) to Volatiles from a Non-host Plant, Geranium, Pelargonium × hortorum (Geraniaceae)

The diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae), is a notorious cruciferous vegetable pest globally. Mechanically damaged geranium (Pelargonium × hortorum) can strongly repel DBM, but specific plant bioactive compounds responsible for such effects have not been identified. The headspace volatiles from wounded geranium were analyzed using gas chromatography-electroantennographic detection and gas chromatography-mass spectrometry. The electrophysiological and behavioral responses of DBM females to these chemicals and two blends were investigated. The results showed that five components myrcene, γ-terpinene, linalool, camphor, and terpinen-4-ol in geranium plants mediated the repellence of DBM mated females. These substances significantly repelled the oviposition of females, and blend-2 (a mixture of the five components with a ratio 1:5:3:4:3) was most effective. The slow-release blend-2 had a repellent range of 0.9 m and a repellent longevity of 26 days. These five substances are promising behavioral regulators of the destructive moths and could be potential candidates for "push" components in plant-based "push-pull" strategies.

Bioactive Volatiles From Push-Pull Companion Crops Repel Fall Armyworm and Attract Its Parasitoids

Fall armyworm, Spodoptera frugiperda, is a serious invasive pest in Africa but “Push-Pull” companion cropping can substantially reduce infestation. Here, we elucidate the underpinning chemical ecology mechanisms. We hypothesized that companion crop volatiles repel herbivores (push) while attracting natural enemies (pull). Headspace volatiles collected from companion plants (Desmodium intortum, Desmodium uncinatum, Brachiaria Mulato II) were used in bioassays and electrophysiological recordings with S. frugiperda and parasitoid wasps. Insect populations, plant damage and herbivore parasitism were assessed in field plots. Coupled GC-electroantennogram (GC-EAG) recordings showed robust responses to certain aromatic and terpenoid volatile compounds. In wind tunnel bioassays, maize volatiles mixed with Desmodium volatiles were less attractive to moths than maize alone. In oviposition bioassays, S. frugiperda laid significantly fewer eggs on maize when Desmodium volatiles were present. Conversely, in an olfactometer bioassay, parasitoid wasps were attracted to the scent of both Desmodium spp. (intercrop) and the Brachiaria border crop. Our data provide evidence of the mechanisms underpinning reduced S. frugiperda infestation in the Push-Pull companion cropping system, i.e., volatiles from companion crops repel S. frugiperda while attracting its parasitoid natural enemies. These findings explain why Push-Pull field plots had fewer S. frugiperda larvae and lower crop damage than monocropped maize.

Attraction of the Mosquitoes Aedes aegypti and Aedes albopictus (Diptera: Culicidae) to a 3-Part Phytochemical Blend in a Mesocosm

Aedes aegypti (L.) and Aedes albopictus (Skuse) mosquitoes of both sexes were attracted to a 3-part volatile synthetic phytochemical blend but differed according to their component ratios, 7:3:2 or 1:1:1, and their initial concentrations. These arbovirus vectors were presented with the blends as baits in paired baited and blank CFG traps in a large greenhouse mesocosm. Ae. aegypti attraction was highest at a 7:3:2 blend ratio, but at a concentration half that found most effective for an anopheline mosquito species in outdoor screenhouses. Both lower and higher concentrations yielded substantially lower attraction scores for Ae. aegypti. By contrast, the few tests conducted on Ae. albopictus showed that it was not as sensitive to concentration, but again it was more responsive to the 7:3:2 ratio of components than to the 1:1:1 ratio. The two sexes of both species were represented equally in the trap catches, indicating the potential value of this and similar attractive blends for population surveillance and control of Aedes mosquitoes.

Characterizations of botanical attractant of Halyomorpha halys and selection of relevant deorphanization candidates via computational approach

Halyomorpha halys has been recognized as a global cross-border pest species. Along with well-established pheromone trapping approaches, there have been many attempts to utilize botanical odorant baits for field monitoring. Due to sensitivity, ecological friendliness, and cost-effectiveness for large-scale implementation, the selection of botanical volatiles as luring ingredients and/or synergists for H. halys is needed. In the current work, botanical volatiles were tested by olfactometer and electrophysiological tests. Results showed that linalool oxide was a potential candidate for application as a behavioral modifying chemical. It drove remarkable attractiveness toward H. halys adults in Y-tube assays, as well as eliciting robust electroantennographic responsiveness towards antennae. A computational pipeline was carried out to screen olfactory proteins related to the reception of linalool oxide. Simulated docking activities of four H. halys odorant receptors and two odorant binding proteins to linalool oxide and nerolidol were performed. Results showed that all tested olfactory genes were likely to be involved in plant volatile-sensing pathways, and they tuned broadly to tested components. The current work provides insights into the later development of field demonstration strategies using linalool oxide and its molecular targets.

Development of an Attractive Toxic Sugar Bait for the Control of Aedes j. japonicus (Diptera: Culicidae)

Both female and male mosquitoes consume sugar meals to obtain carbohydrates used for energy. This behavior has recently been identified as a possible mosquito control target, as the World Health Organization has urged for the development of integrated vector management. This is critical as many medically important mosquito species are developing insecticide resistance, resulting in current control strategies becoming less effective. Additionally, the traditional use of insecticides is detrimental to many beneficial insects such as pollinators. The main goal of this study was to develop an attractive toxic sugar bait (ATSB) to limit the populations of a local invasive mosquito, Aedes j. japonicus (Theobald) (Diptera: Culicidae). An ATSB is a lure bait composed of an attractant odorant, a toxic component, and sugar that the mosquitoes can feed on. ATSBs are cost-effective, sustainable, environmentally friendly, and can be species-specific. Mosquitoes were isolated into cages or cups and each group had access to either a toxic sugar solution (containing boric acid), a control solution or a choice between the two. We tested multiple fruits, including mango, peach, blueberries, and blackberries, as well as a soda and grape juice and monitored their survival for 96 h. We found that this species fed on all tested fruit solutions and that the groups that imbibed toxic solutions died within 48 h, indicating that boric acid is an effective oral toxin against Ae. j. japonicus. Further experiments will be conducted in the field to determine the ATSBs efficacy and to monitor potential effects on off-target species.

Grass-like plants release general volatile cues attractive for gravid Anopheles gambiae sensu stricto mosquitoes

Background

Understanding the ecology and behaviour of disease vectors, including the olfactory cues used to orient and select hosts and egg-laying sites, are essential for the development of novel, insecticide-free control tools. Selected graminoid plants have been shown to release volatile chemicals attracting malaria vectors; however, whether the attraction is selective to individual plants or more general across genera and families is still unclear.

Methods

To contribute to the current evidence, we implemented bioassays in two-port airflow olfactometers and in large field cages with four live graminoid plant species commonly found associated with malaria vector breeding sites in western Kenya: Cyperus rotundus and C. exaltatus of the Cyperaceae family, and Panicum repens and Cynodon dactylon of the Poaceae family. Additionally, we tested one Poaceae species, Cenchrus setaceus, not usually associated with water. The volatile compounds released in the headspace of the plants were identified using gas chromatography/mass spectrometry.

Results

All five plants attracted gravid vectors, with the odds of a mosquito orienting towards the choice-chamber with the plant in an olfactometer being 2–5 times higher than when no plant was present. This attraction was maintained when tested with free-flying mosquitoes over a longer distance in large field cages, though at lower strength, with the odds of attracting a female 1.5–2.5 times higher when live plants were present than when only water was present in the trap. Cyperus rotundus, previously implicated in connection with an oviposition attractant, consistently elicited the strongest response from gravid vectors. Volatiles regularly detected were limonene, β-pinene, β-elemene and β-caryophyllene, among other common plant compounds previously described in association with odour-orientation of gravid and unfed malaria vectors.

Conclusions

The present study confirms that gravid Anopheles gambiae sensu stricto use chemical cues released from graminoid plants to orientate. These cues are released from a variety of graminoid plant species in both the Cyperaceae and Poaceae family. Given the general nature of these cues, it appears unlikely that they are exclusively used for the location of suitable oviposition sites. The utilization of these chemical cues for attract-and-kill trapping strategies must be explored under natural conditions to investigate their efficiency when in competition with complex interacting natural cues.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04939-4.

Electrophysiological, behavioural and biochemical effect of Ocimum basilicum oil and its constituents methyl chavicol and linalool on Musca domestica L

Ocimum basilicum essential oil (EO) was evaluated for its biological effects on M. domestica. Characterization of O. basilicum EO revealed the presence of methyl chavicol (70.93%), linalool (9.34%), epi-α-cadinol (3.69 %), methyl eugenol (2.48%), γ-cadinene (1.67%), 1,8-cineole (1.30%) and (E)-β-ocimene (1.11%). The basil EO and its constituents methyl chavicol and linalool elicited a neuronal response in female adults of M. domestica. Adult female flies showed reduced preference to food source laced with basil EO and methyl chavicol. Substrates treated with EO and methyl chavicol at 0.25% resulted in an oviposition deterrence of over 80%. A large ovicidal effect was found for O. basilicum EO (EC₅₀ 9.74 mg/dm³) followed by methyl chavicol (EC₅₀ 10.67 mg/dm³) and linalool (EC₅₀ 13.57 mg/dm³). Adults exposed to EO (LD₅₀ 10.01 μg/adult) were more susceptible to contact toxicity than to methyl chavicol and linalool (LD₅₀ 13.62 μg/adult and LD₅₀ 43.12 μg/adult respectively). EO and its constituents methyl chavicol and linalool also induced the detoxifying enzymes Carboxyl esterase (Car E) and Glutathione S - transferases (GST).

Evaluation of Mosquito Attractant Candidates Using a High-Throughput Screening System for Aedes aegypti (L.), Culex quinquefasciatus Say. and Anopheles minimus Theobald (Diptera: Culicidae)

Several types of olfactometers have been used to evaluate mosquito responses to agents that mimic natural volatiles that repel or attract. The Y-tube olfactometer has been widely used to study repellents and attractants, while the high-throughput screening system assay has only been used to study repellents. Whether the high-throughput screening system assay is suitable for evaluating attractants is unknown. We evaluated the responses to four lactic-acid-based mixtures and two non-lactic-acid-based chemical lure candidates using the high-throughput screening system (HITSS) for three mosquito species (laboratory strains and field populations of both Aedes aegypti (L.) and Culex quinquefasciatus Say.; laboratory strain of Anopheles minimus Theobald) under laboratory-controlled conditions. HITSS assay results showed that KU-lure #1 elicited the greatest percent attraction for pyrethroid-resistant and -susceptible Ae. aegypti. KU-lure #6 elicited the strongest attractive response for pyrethroid-susceptible and -resistant Cx. quinquefasciatus and pyrethroid-susceptible An. minimus. The response to the lures from each species was independent of the pyrethroid susceptibility status (Ae. aegypti, p = 0.825; Cx. quinquefasciatus, p = 0.056). However, a significant difference in attraction to KU-lure #6 was observed between diurnal and nocturnal mosquitoes (Cx. quinquefasciatus vs. Ae. aegypti, p = 0.014; An. minimus vs. Ae. aegypti, p = 0.001). The laboratory-level HITSS assay effectively selects potential lure candidates. Because the host-seeking behavior differs between mosquito species, further studies are needed to develop species-specific attractants. Additional studies in semi-field screen houses using commercial traps are necessary to evaluate the accuracy of these laboratory assay results.

Dose-Response Assay for Synthetic Mosquito (Diptera: Culicidae) Attractant Using a High-Throughput Screening System

Natural volatile host cues play a critical role for mosquito orientation and locating a blood source for egg production. Similar olfactory activation responses have allowed the use and development of artificial chemical attractants to lure mosquitoes to trapping devices. Using a pre-formulated commercial product mixture of different attractant chemicals, a high-throughput screening system (HITSS) is used to screen varying doses of chemical required to activate behavioral responses. Two strains of Aedes aegypti (L.): permethrin-susceptible (USDA) and -resistant (Pu Teuy) phenotypes and one Culex quinquefasciatus Say. (NIH) laboratory strain were tested. Overall, mosquitoes showed repellency between 1.0 g and to 10.0 g dose of each compound. However, by progressively reducing the dose, Cx. quinquefasciatus showed a greater positive percent attraction (88.9%) at 0.025 g, whereas the USDA and Pu Teuy Ae. aegypti produced optimum attractant activation at 0.005 g (72.6% and 58.9%, respectively) without significant difference within species (p > 0.05). In parallel control assays, Cx. quinquefasciatus was significantly attracted to 1 g of dry ice (carbon dioxide) (76%) more than Ae. aegypti (USDA) (12.2%). The HITSS was originally designed to measure three chemical actions to sublethal concentrations of chemicals by mosquitoes: toxicity and the two primary behavior avoidance responses (contact excitation and spatial repellency). These findings demonstrate that the HITSS assay, with only minor modifications, allows comparison screening of candidate compounds as potential attractants for anemotactic responses under laboratory-controlled conditions. Further investigations will be required to equate measurements obtained from controlled laboratory assays to more varied field conditions for attracting natural mosquito populations.

Mosquito Attractants

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

Essential Oil Headspace Volatiles Prevent Invasive Box Tree Moth (Cydalima perspectalis) Oviposition-Insights from Electrophysiology and Behaviour

The box tree moth (Cydalima perspectalis Walker) is an invasive species in Europe causing severe damage both in natural and ornamental boxwood (Buxus spp.) vegetation. Pest management tactics are often based on the use of chemical insecticides, whereas environmentally-friendly control solutions are not available against this insect. The application of essential oils may provide effective protection against oviposition and subsequent larval damage. Oviposition deterrence of cinnamon, eucalyptus and lavender essential oils was tested on female C. perspectalis in behavioural bioassays. Our results indicate that all the studied essential oils may be adequate deterrents; however, cinnamon oil exhibited the strongest effect. To determine the physiologically active compounds in the headspace of the essential oils, gas chromatography coupled with electroantennography recordings were performed in parallel with gas chromatography-mass spectrometry to identify the volatile constituents. In addition, the release rates of various components from vial-wick dispensers were measured during the oviposition bioassay. These results may serve as a basis for the development of a practical and insecticide-free plant protection method against this invasive moth species.