A material isolated from human hands that attracts female mosquitoes

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.

Impact of Consumption of Bananas on Attraction of Anopheles stephensi to Humans

Humans vary in attractiveness to mosquitoes, a phenomenon that is largely attributed to differences in physical cues such as heat and volatile odors emanating from breath and skin. Diet can change human odors but whether specific dietary components alter host attractiveness is largely unexplored. We identified bananas as a target for study following a survey of the internet for advice on avoiding mosquito bites. Human attractiveness to Anopheles stephensi Liston was measured using a glass vial bioassay where mosquito contacts were measured before and 1–3 h after ingestion of bananas or grapes. Consumption of grapes had no effect on the number of contacts but banana ingestion resulted in a significant increase in the overall number of contacts in spite of individual variation that included some subjects who showed no effect or decreases in contacts. Further tests with a single volunteer showed that the effect was repeatable and consistent across 15 trials. The magnitude of the increase was not affected by the number of bananas eaten. Increased contact counts after banana ingestion were also observed when A. gambiae Giles was tested. These results support the hypothesis that diet plays an important role in mediating host attractiveness to anopheline mosquitoes.

Olfaction in Asian tiger mosquito Aedes albopictus: flight orientation response to certain saturated carboxylic acids in human skin emanations

The flight orientation response of nonblood-fed and hungry Aedes albopictus females was studied in a Y-tube olfactometer at 10(-6) to 10(-2) g odor plumes of saturated carboxylic acids (C1-C20), in which C2-C18 were the main constituents of human skin emanations. Thirteen acids viz C1, C2, C3, C5, C6, C8 C9, C10, C12, C14, C16, C18, and C20 showed attractance at odor plumes ranging from 10(-5) to 10(-3) g doses, while five acids viz C4, C7, C11, C15, and C19 showed repellence at 10(-4) to 10(-2) g to test mosquitoes. Tridecanoic acid (C13) showed attractance only at 10(-4) g dose while higher doses caused repellence. Dose-dependent reversal of orientation behavior from attractance to repellence was observed at 10(-2) g plumes of C5, C9, C10, C13, C17, C19, and C20 acids. The outcome of the study will help in the identification of odoriferous acids as potential attractants, repellents, or attraction inhibitors, which may find their application in the repellent formulations and odor-baited traps for surveillance and control of mosquitoes.

Targeting a dual detector of skin and CO2 to modify mosquito host seeking

Female mosquitoes that transmit deadly diseases locate human hosts by detecting exhaled CO2 and skin odor. The identities of olfactory neurons and receptors required for attraction to skin odor remain a mystery. Here, we show that the CO2-sensitive olfactory neuron is also a sensitive detector of human skin odorants in both Aedes aegypti and Anopheles gambiae. We demonstrate that activity of this neuron is important for attraction to skin odor, establishing it as a key target for intervention. We screen ~0.5 million compounds in silico and identify several CO2 receptor ligands, including an antagonist that reduces attraction to skin and an agonist that lures mosquitoes to traps as effectively as CO2. Analysis of the CO2 receptor ligand space provides a foundation for understanding mosquito host-seeking behavior and identifies odors that are potentially safe, pleasant, and affordable for use in a new generation of mosquito control strategies worldwide.

Efficacy of three attractant blends tested in combination with carbon dioxide against natural populations of mosquitoes and biting flies at the Lower Suwannee Wildlife Refuge

Synthetic blends of chemicals identified previously from human skin emanations were evaluated against mosquito and biting fly populations at the Lower Suwannee Wildlife Refuge near Cedar Key, FL. Mosquito Magnet-Experimental traps were baited with the Red (400 ml acetone: 10 ml 1-hexen-3-ol:10 ml 1-octen-3-ol), Blue (400 ml acetone: 1 g/liter lactic acid:20 ml glycolic acid), or Green blend (400 ml acetone:1.5 g/liter lactic acid:20 ml dimethyl disulfide) plus CO2 or with CO2 alone (control). A relative index of efficacy was determined by dividing each mean blend trap catch by the mean control trap catch. Five mosquitoes (Aedes infirmatus, Ae. taeniorhynchus, Ae. triseriatus, Anopheles crucians, and Culex nigripalpus), 2 ceratopogonid (Culicoides floridensis and C. furens), and 1 tabanid (Diachlorus ferrugatus) and phlebotomine (Lutzomyia shannoni) species were trapped. The Red blend + CO2 treatment significantly increased collections of Ae. taeniorhynchus (3.4x), An. crucians (2.8X), total mosquitoes (2.7x), C. furens (17.6x), and L. shannoni (10.8x) compared with control traps. Traps baited with either the Blue or Green blends generally captured fewer insects than traps baited with the other 2 treatments. However, traps baited with the Green blend caught 7 x as many C. furens as the control traps. Responses clearly varied according to species; therefore, "one size does not fit all" when it comes to attractant blends.

Preliminary analysis of several attractants and spatial repellents for the mosquito, Aedes albopictus using an olfactometer

Mosquito attractants and spatial repellents hold great promise in controlling mosquito pests. In assessing the effectiveness of mosquito attractants and repellents, a good olfactometer system, and optimized testing conditions, are essential. In this research, we demonstrated the usefulness of an olfactometer system, and optimized testing conditions for Aedes albopictus (Diptera: Culicidae). We found no significant difference in the biting activity of the insect between 8:00 and 22:00. Furthermore, 5-10 day old mosquitoes were relatively strongly attracted, with bloodsucking rate 75.8%. The random capture rate (entered trap in absence of odor) was less than 20% for the 20-40 tested insects. Capture rates and systematic errors increased as the number of testing insects increased. Wind speed affected the capture rate significantly, whereas variations in temperature between 25-28°C did not result in significant difference. The wind speed of 0.2 m/s exhibited a higher capture rate, which was significantly different from those at either 0.1 m/s or 0.4 m/s (P < 0.05). At the wind speed of 0.2 m/s, time durations within the first 8 minutes correlated positively with capture rates (r(2) = 0.997), but further increase in time duration to 10 minutes did not result in further increase in capture rates. One percent of L-lactic acid in dichloromethane resulted in a consistently higher capture rate (43.8%) than that from a human odor (31.2%). Under our testing conditions, eugenol, anisaldehyde, geraniol, citronellal, citral, and linalool all exhibited some inhibition effect on mosquitoes to successfully trace human odor or 1% of L-lactic acid in dichloromethane. The results of these two tests indicate that the L-lactic acid/dichloromethane mixture may be used as an effective attractant to evaluate the effect of possible spatial repellents on Ae. albopictus.

Changes in host-seeking behavior of Puerto Rican Aedes aegypti after colonization

The effects of colonization on host-seeking behavior of mosquitoes was examined by comparing attraction responses of newly colonized Aedes aegypti (L.) from field-collected eggs in Puerto Rico to that of the Gainesville (Florida) strain, originally from Orlando (Florida) and in colony since 1952. Females from the Orlando and the F0 through F10 generations of the Puerto Rico strain were evaluated using attractant odors in a triple-cage dual-port olfactometer. Two attractant sources were used: odors from the hand of a volunteer and a standard blend of L-lactic acid, acetone, and dimethyl disulfide. Convergence of the percentage of attraction responses occurred around the F4-F6 generations of the Puerto Rico strain. Both the Orlando and Puerto Rico strains exhibited similar responses for tests with the remaining F7-F10 generations. A temporal effect on mosquito responses was observed for both strains regardless of the attractant blend used in tests. This study indicates that Ae. aegypti host-seeking behavior changes significantly over the first four to six generations after introduction into the laboratory, whereas the field-collected strain increases in attraction response until it stabilizes at a new level.

Effectiveness of synthetic versus natural human volatiles as attractants for Anopheles gambiae (Diptera: Culicidae) sensu stricto

Females of the African malaria vector, Anopheles gambiae Giles sensu stricto, use human volatiles to find their blood-host. Previous work has shown that ammonia, lactic acid, and aliphatic carboxylic acids significantly affect host orientation and attraction of this species. In the current study, these compounds were tested for their attractiveness relative to human emanations in vivo and in vitro. Emanations from a human hand, incubated sweat, and foot skin residues on a nylon sock were significantly attractive when tested against clean air. In a dual-choice test, foot skin residues were significantly more attractive than emanations from a human hand in vivo. Ammonia alone attracted more mosquitoes than fresh or incubated sweat. However, the odor of a human hand or of foot skin residues were more attractive than ammonia. A known attractive blend of ammonia with lactic acid and carboxylic acids was less effective than natural foot odorants. The results demonstrate that the synthetic blend based on skin odor is attractive for An. gambiae, but that in a choice situation in vitro natural skin odors are still preferred by the mosquito. Differences in volatile organic compound abundances between a worn sock and the synthetic blend may have resulted in stronger attraction to the sock. This suggests that candidate attractants should be evaluated with consideration of the strength of natural odorant sources. The data furthermore suggest that additional unidentified compounds from the human foot are involved in the host-seeking behavior of this mosquito species.

The effect of aliphatic carboxylic acids on olfaction-based host-seeking of the malaria mosquito Anopheles gambiae sensu stricto

The role of aliphatic carboxylic acids in host-seeking response of the malaria mosquito Anopheles gambiae sensu stricto was examined both in a dual-choice olfactometer and with indoor traps. A basic attractive blend of ammonia + lactic acid served as internal standard odor. Single carboxylic acids were tested in a tripartite blend with ammonia + lactic acid. Four different airflow stream rates (0.5, 5, 50, and 100 ml/min) carrying the compounds were tested for their effect on trap entry response in the olfactometer. In the olfactometer, propanoic acid, butanoic acid, 3-methylbutanoic acid, pentanoic acid, heptanoic acid, octanoic acid, and tetradecanoic acid increased attraction relative to the basic blend. While several carboxylic acids were attractive only at one or two flow rates, tetradecanoic acid was attractive at all flow rates tested. Heptanoic acid was attractive at the lowest flow rate (0.5 ml/min), but repellent at 5 and 50 ml/min. Mixing the air stream laden with these 7 carboxylic acids together with the headspace of the basic blend increased attraction in two quantitative compositions. Subtraction of single acids from the most attractive blend revealed that 3-methylbutanoic acid had a negative effect on trap entry response. In the absence of tetradecanoic acid, the blend was repellent. In assays with MM-X traps, both a blend of 7 carboxylic acids + ammonia + lactic acid (all applied from low density polyethylene-sachets) and a simple blend of ammonia + lactic acid + tetradecanoic acid were attractive. The results show that carboxylic acids play an essential role in the host-seeking behavior of An. gambiae, and that the contribution to blend attractiveness depends on the specific compound studied.

Identification of human-derived volatile chemicals that interfere with attraction of Aedes aegypti mosquitoes

It is known that human individuals show different levels of attractiveness to mosquitoes. In this study, we investigated the chemical basis for low attractiveness. We recorded behaviors of Aedes aegypti toward the hands of human volunteers and toward the volatile chemicals produced by their bodies. Some individuals, and their corresponding volatiles, elicited low upwind flight, relative attraction, and probing activity. Analyzing the components by gas chromatography coupled to electrophysiological recordings from the antennae of Aedes aegypti, enabled the location of 33 physiologically relevant compounds. The results indicated that higher levels of specific compounds may be responsible for decreased "attractiveness." In behavioral experiments, five of the compounds caused a significant reduction in upwind flight of Aedes aegypti to attractive human hands. Thus, unattractiveness of individuals may result from a repellent, or attractant "masking," mechanism.

A search for components in human body odour that attract females of Aedes aegypti

In a new type of wind tunnel, mosquitoes fly upwind towards host odour sources and towards human skin wash extracts obtained by rubbing the skin with a pad soaked in ethanol. We used this behavioural response as a bioassay to identify attractants in liquid chromatography fractions of such extracts. L-Lactic acid is a major constituent of skin wash extracts and it is a necessary component for the extract's effectiveness. As a single stimulus, however, L-lactic acid is only slightly effective. This indicates that the extract's high degree of effectiveness is based on a synergism of L-lactic acid and other odour components. The separation of the extract by liquid chromatography revealed three distinct regions of active fractions, only one of which contained L-lactic acid. The components of the other two regions have not yet been determined. A combination of fractions in these two regions together with L-lactic acid is as attractive as the complete extract.

Laboratory comparison of Aedes aegypti attraction to human odors and to synthetic human odor compounds and blends

Chemical lures can enhance the ability of traps to collect mosquitoes, selectively target species for capture, and provide a realistic assessment of the species and host-equivalent numbers of mosquitoes present in the local area. One approach to the development of chemical lures is to manufacture blends that comprise odors released in human emanations. These blends need to be safe for use in the environment, desirable from an economic standpoint, and transportable to the field for use in traps. In this report, we compared the attraction of mosquitoes to various chemicals, blends, and odors from humans. Noncompetitive (single-treatment) bioassays established that some blends are equivalent or more attractive to Aedes aegypti than human odors. Competitive bioassays were conducted; these involved simultaneous comparison of 2 treatments: single compounds to binary blends; binary blends to a trinary blend of L-lactic acid, acetone, and dimethyl disulfide; and the trinary blend to human odors from 3 volunteers. The overall trend was that the trinary blend was more attractive than binary blends, and binary blends were more attractive than single compounds. However, human odors were still significantly more attractive than the trinary blend. Therefore, further modifications and refinements to blends will be needed to better compete against human odors.

Electroantennogram and behavioral responses of Culex quinquefasciatus (Diptera: Culicidae) females to chemicals found in human skin emanations

The southern house mosquito, Culex quinquefasciatus Say (Diptera: Culicidae), is closely associated with humans and is a vector of filariasis. Use of semiochemicals for control of mosquitoes is a novel and potentially ecofriendly control approach. Human skin emanations are known to attract mosquitoes. In the current study, olfactory and behavioral responses of host-seeking female Cx. quinquefasciatus to various chemical components of human skin emanations (carboxylic acids, alcohols, and aldehydes) were evaluated separately at three doses (0.01, 0.1, and 1 microg) for electroantennogram (EAG) and three doses (0.1, 1, and 10 microg) for behavioral assay. Results of EAG studies indicated that all carboxylic acids elicited significant olfactory responses except tetradecanoic acid (C14) and octadecanoic acid (C18). In particular, hexanoic acid (C6) elicited a maximum, eight-fold olfactory response compared with the solvent control. Ethylene glycol and benzyl alcohol exhibited significant EAG and behavioral responses, whereas among aldehydes, nonanal elicited high EAG and behavioral responses, but only at all three doses tested compared with control. Some carboxylic acids elicited significant behavioral responses, attracting approximately 75% of females tested toward chemical stimuli in a Y-tube olfactometer.

The study of fingerprint characteristics of the emanations from human arm skin using the original sampling system by SPME-GC/MS

An efficient and noninvasive method consisting of an original sampling device, solid phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS) was developed to analyze volatile organic emanations from the skin of human arms. The emanations were sampled by SPME connected with the active sampling device for 30 min and transferred into GC-MS immediately for the consequent analysis. The sampling projects for 15 candidates were scheduled in both winter and spring with the same optimized conditions. Thirty-five compounds were finally identified according to various degrees of certainty. Different emission behaviors specified with principal component analysis (PCA) and similar fingerprint characteristics were observed clearly by comparisons of chromatograms of different seasons. Top ten emanations contributing to characteristics in different seasons were attempted to be described using comparisons based on common model strategy. The large amounts of experimental data were all handled by the corresponding chemometrics strategies with the homemade chromatographic data processing system. The results suggest that the analysis based on fingerprint characteristics of human skin emanations could provide useful and important clues to reveal biomarkers among the mixture of human skin emanations.

Limburger cheese as an attractant for the malaria mosquito Anopheles gambiae s.s

In the process of bloodfeeding, female Anopheles can transmit malaria parasites to humans. At night, while searching for blood, these insects respond to visual, physical and chemical properties of humans. Current research concentrates on the identification of kairomones, which guide mosquitoes to humans. Earlier observations on the biting behaviour of Anopheles gambiae s.s. on humans have now resulted in the discovery of a remarkable attractant for this important malaria vector, and it is thought that this will accelerate the development of odour-baited traps for malaria mosquito surveillance and control in sub-Saharan Africa, as discussed here by Bart Knols and Ruurd De Jong.

Attractiveness of pregnant women to the malaria vector, Anopheles arabiensis, in Sudan

The attractiveness of pregnant women for mosquitoes was investigated in a peri-urban site in New Halfa, eastern Sudan, in September-October 2003. For 20 nights, the mosquitoes feeding on nine pregnant and nine non-pregnant women sleeping under untreated bednets were collected. The women slept outdoors, in the yards of nine houses, each yard holding one pregnant and one non-pregnant woman. In general, each pregnant woman attracted significantly more Anopheles arabiensis (the main vector of Plasmodium falciparum in the area) than each non-pregnant women, with mean biting rates of 0.94 and 0.49 bites/woman-night, respectively (P = 0.005). In contrast, the two groups of women attracted similar numbers of the other mosquito species collected, which were all culicine. Impregnated bednets need to be used in the study area, at least by the pregnant women (who appear to be at particularly high risk of acquiring malaria).

Behavioural and electrophysiological responses of the malaria mosquito Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) to human skin emanations

Behavioural and electrophysiological responses of Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) to human skin emanations collected on glass beads were studied using a dual-port olfactometer and electroantannography. Glass beads to which skin emanations from human hands had been transferred elicited a level of attraction similar to a human hand. The attractiveness of these handled glass beads faded away 4 h after transfer onto the beads. Storage at -20 degrees C for up to 8 weeks showed a decreased but still attractive effect of the beads. In a choice test between one individual and four others, the emanations from the reference individual were significantly more attractive in three out of four cases. The headspace of handled glass beads elicited a dose-dependent EAG response. The substances causing EAG activity could be removed partially by dichloromethane, ethanol and pentane-ether. Glass beads provide a suitable neutral substrate for the transfer of human odour to enable chemical analysis of the human skin emanations for identification of kairomones of anthropophilic mosquitoes.

Synergistic attraction of Aedes aegypti (L.) to binary blends of L-lactic acid and acetone, dichloromethane, or dimethyl disulfide

Kairomones produced by humans provide female anthropophilic mosquitoes with vital cues used in host-seeking for a blood meal. These chemicals are emanated primarily by the skin and provide the mosquitoes a means to orient themselves to humans at a relatively close range. Chemical studies of these emanations have provided new ideas for the formulation of attractant blends. We report mosquito attraction responses for three binary blends and their separate components. The blends are comprised of L-lactic acid plus either acetone, dichloromethane, or dimethyl disulfide. At the emission rates used in our bioassays, these blends synergistically attract laboratory-reared female Aedes aegypti. Carbon dioxide is not a necessary component to yield high levels of attraction with these blends. It is postulated that at least one of these synergistic blends (L-lactic acid and acetone) produces mosquito attraction behavior similar to L-lactic acid and CO2.

An olfactometer for discriminating between attraction, inhibition, and repellency in mosquitoes (Diptera: Culicidae)

We constructed an olfactometer that differentiates and quantifies attraction, inhibition, and repellency to mosquitoes. Using this device, 22 formulations of various chemicals on gauze pads and 8 dilutions of DEET on skin were tested. Four novel formulations were discovered acting as true repellents, whereas DEET did not act as a true repellent but as an inhibitor.

Odor-mediated behavior of Afrotropical malaria mosquitoes

The African mosquito species Anopheles gambiae sensu lato s.l. and Anopheles funestus rank among the world's most efficient vectors of human malaria. Their unique bionomics, particularly their anthropophilic, endophagic and endophilic characters, guarantee a strong mosquito-host interaction, favorable to malaria transmission. Olfactory cues govern the various behaviors of female mosquitoes and here we review the role of semiochemicals in the life history of African malaria vectors. Recent evidence points towards the existence of human-specific kairomones affecting host-seeking A. gambiae s.l., and efforts are under way to identify the volatiles mediating this behavior. Based on examples from other Culicidae spp., it is argued that there is good reason to assume that mating, sugar feeding, and oviposition behavior in Afrotropical malaria vectors may also be mediated by semiochemicals. It is foreseen that increased knowledge of odor-mediated behaviors will be applied in the development of novel sampling techniques and possibly alternative methods of intervention to control malaria.

Differential attractiveness of isolated humans to mosquitoes in Tanzania

Individually occupied tents, modified for sampling mosquitoes, were used to assess the attractiveness of 3 humans for Anopheles gambiae Giles s.l., A. funestus Giles, Culex quinquefasciatus Say and Mansonia spp. in north-east Tanzania. Significant differences (P < 0.025) in attractiveness were observed for the first 3 species, whereas Mansonia did not show any preference. Because of the objective sampling method used, these results provide the first conclusive evidence that isolated people vary in attractiveness to mosquitoes.

Attraction of Lutzomyia longipalpis to human skin odours

Male and female Lutzomyia longipalpis sandfiles showed attraction to human skin emanations placed on warmed glass Petri dishes. Unfed virgin females were more strongly attracted than males, which also showed attraction. Four human subjects were tested and significant variation was found between the numbers of sandflies attracted to their skin emanations. This suggests that some individuals were more attractive than others. There was a significant difference between the response shown by sandflies from the Jacobina and Lapinha regions of Brazil, suggesting that sandflies from the Jacobina region were more anthropophilic. In addition, sandflies from Jacobina had a significantly higher level of activity than those from Lapinha. The role of sandfly attraction to humans as a risk factor in Leishmania transmission is considered.

The endogenous regulation of mosquito reproductive behavior

Most female mosquitoes require a meal of blood that provides protein for egg maturation. For reproduction to occur, two behavioral sequences are essential. One is concerned with finding a host for the blood meal and the other in finding a site on which to lay the eggs that result. Stimuli from both hosts and oviposition sites initiate the reproductive behaviors of host-seeking and pre-oviposition, respectively, that are discussed in this review. After sensory receptors perceive these stimuli, the central nervous system must integrate the information and associate it with a biologically appropriate response. Host-seeking appears to be the default behavior, expressed whenever host stimuli are present. However, if the female is successful in locating a host and ingesting blood, subsequent host-seeking is inhibited when the meal distends the abdomen above a certain threshold. Host-seeking inhibition continues during egg development as a result of a humoral mechanism even after the blood volume has been reduced by digestion. At the time when eggs are maturing and host-seeking is inhibited, pre-oviposition behavior predominates if the central nervous system receives oviposition site stimuli. This behavior is also initiated by a humoral factor. Several physiological states, including insemination, age, and nutrition, can modulate both host-seeking and pre-oviposition behaviors.