Yu Tong Qiu

Odor Coding in the Maxillary Palp of the Malaria Vector Mosquito Anopheles gambiae

BACKGROUND Many species of mosquitoes, including the major malaria vector Anopheles gambiae, utilize carbon dioxide (CO(2)) and 1-octen-3-ol as olfactory cues in host-seeking behaviors that underlie their vectorial capacity. However, the molecular and cellular basis of such olfactory responses remains largely unknown. RESULTS Here, we use molecular and physiological approaches coupled with systematic functional analyses to define the complete olfactory sensory map of the An. gambiae maxillary palp, an olfactory appendage that mediates the detection of these compounds. In doing so, we identify three olfactory receptor neurons (ORNs) that are organized in stereotyped triads within the maxillary-palp capitate-peg-sensillum population. One ORN is CO(2)-responsive and characterized by the coexpression of three receptors that confer CO(2) responses, whereas the other ORNs express characteristic odorant receptors (AgORs) that are responsible for their in vivo olfactory responses. CONCLUSIONS Our results describe a complete and highly concordant map of both the molecular and cellular olfactory components on the maxillary palp of the adult female An. gambiae mosquito. These results also facilitate the understanding of how An. gambiae mosquitoes sense olfactory cues that might be exploited to compromise their ability to transmit malaria.

Composition of Human Skin Microbiota Affects Attractiveness to Malaria Mosquitoes

The African malaria mosquito Anopheles gambiae sensu stricto continues to play an important role in malaria transmission, which is aggravated by its high degree of anthropophily, making it among the foremost vectors of this disease. In the current study we set out to unravel the strong association between this mosquito species and human beings, as it is determined by odorant cues derived from the human skin. Microbial communities on the skin play key roles in the production of human body odour. We demonstrate that the composition of the skin microbiota affects the degree of attractiveness of human beings to this mosquito species. Bacterial plate counts and 16S rRNA sequencing revealed that individuals that are highly attractive to An. gambiae s.s. have a significantly higher abundance, but lower diversity of bacteria on their skin than individuals that are poorly attractive. Bacterial genera that are correlated with the relative degree of attractiveness to mosquitoes were identified. The discovery of the connection between skin microbial populations and attractiveness to mosquitoes may lead to the development of new mosquito attractants and personalized methods for protection against vectors of malaria and other infectious diseases.

Interindividual variation in the attractiveness of human odours to the malaria mosquito Anopheles gambiae s.s.

Abstract Differences between human individuals in their attractiveness to female mosquitoes have been reported repeatedly, but the underlying mechanisms are not well understood. Skin emanations from 27 human individuals, collected on glass marbles, were tested against ammonia in a dual‐choice olfactometer to establish their degrees of attractiveness to anthropophilic Anopheles gambiae s.s. Giles (Diptera: Culicidae) mosquitoes. Ammonia was used as a standard odour source because of its proven attractiveness to An. gambiae s.s. Skin emanations from most volunteers attracted significantly more mosquitoes than ammonia. There were clear differences in the attractiveness of skin emanations from different volunteers relative to that of ammonia, as well as in the strength of the trap entry response. Consistent differences were observed when emanations from the three most and the three least attractive volunteers were tested pairwise. No gender or age effect was found for relative attractiveness or trap entry response. Emanations from volunteers with higher behavioural attractiveness elicited higher electroantennogram response amplitudes in two pairs, but in a third pair a higher electroantennogram response was found for the less attractive volunteer. These results confirm that odour contributes to the differences in attractiveness of humans to mosquitoes.

Flavonoids from cabbage are feeding stimulants for diamondback moth larvae additional to glucosinolates: Chemoreception and behaviour

In caterpillars two styloconic contact chemoreceptors on the maxillary galea are assumed to contain the main taste receptors involved in host plant selection. The diamondback moth, Plutella xylostella L. is a specialist feeder of plants in the Brassicaceae, a plant family characterized by the biosynthesis of glucosinolates. We used pea (Pisum sativum L., Leguminosae) as a neutral non‐host for a dual‐choice leaf disc assay to quantify feeding stimulation by glucosinolates and flavonoids. Increasing concentrations of sinigrin resulted in significant preferences for sinigrin‐treated leaf discs, with a threshold between 1 and 3 μM. Millimolar concentrations of four of the five flavonol triglucosides likewise elicited a significant preference for flavonoid‐treated leaf discs. A mixture of four flavonoids and sinigrin was significantly preferred over sinigrin‐treated leaf discs alone. Vigorous unicellular electrophysiological responses of medial maxillary styloconic taste sensilla were observed in response to five glucosinolates (glucocapparin, sinigrin, glucobrassicin, glucoiberin, and gluconasturtiin). This medial taste neuron responded in a dose‐dependent manner to a concentration series of sinigrin, with a threshold of response of ca. 1 μM. The lateral sensillum styloconicum contained a neuron sensitive to sucrose, glucose, and fructose. However, no responses in the two types of maxillary styloconic sensilla to the phagostimulatory flavonoids could be detected, suggesting that other taste organs mediate chemoreception of flavonoids. We conclude that diamondback moth larvae employ a combination of biosynthetically distinct categories of feeding stimulants which allows for a higher degree of discriminatory ability than when this would be based on glucosinolates alone.

Attractiveness of MM-X Traps Baited with Human or Synthetic Odor to Mosquitoes (Diptera: Culicidae) in The Gambia

Abstract Chemical cues play an important role in the host-seeking behavior of blood-feeding mosquitoes (Diptera: Culicidae). A field study was carried out in The Gambia to investigate the effects of human odor or synthetic odor blends on the attraction of mosquitoes. MM-X traps baited with 16 odor blends to which carbon dioxide (CO2) was added were tested in four sets of experiments. In a second series of experiments, MM-X traps with 14 odor blends without CO2 were tested. A blend of ammonia and L-lactic acid with or without CO2 was used as control odor in series 1 and 2, respectively. Centers for Disease Control and Prevention (CDC) traps were placed in a traditional house and an experimental house to monitor mosquito densities during the experiments. The MM-X traps caught a total number of 196,756 mosquitoes, with the most abundant species belonging to the genera Mansonia (70.6%), Anopheles (17.5%), and Culex (11.5%). The most abundant mosquito species caught by the CDC traps (56,290 in total) belonged to the genera Mansonia (59.4%), Anopheles (16.0% An. gambiae s.l. Giles, and 11.3% An. ziemanni Grünberg), and Culex (11.6%). MM-X traps baited with synthetic blends were in many cases more attractive than MM-X traps baited with human odors. Addition of CO2 to synthetic odors substantially increased the catch of all mosquito species in the MM-X traps. A blend of ammonia + L-lactic acid + CO2 + 3-methylbutanoic acid was the most attractive odor for most mosquito species. The candidate odor blend shows the potential to enhance trap collections so that traps will provide better surveillance and possible control.

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

Abstract.  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°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.

Chemoreception of oviposition inhibiting terpenoids in the diamondback moth Plutella xylostella

The effects of six terpenoids and two terpenoid containing extracts of the neem tree (Azadirachta indica A. Juss.) on oviposition by the diamondback moth (Plutella xylostella L., Yponomeutidae: Lepidoptera) were tested. Two drimane terpenoids, the sesquiterpenoid polygodial and the neem extract Margosan‐O exerted significant inhibitory effects at the dosages tested. Ablation experiments showed that both antennae and fore‐tarsi contributed to mediation of the inhibition by a drimane. Location of chemosensilla on prothoracic tarsi and ovipositor was examined by scanning electronmicroscopy. Electrophysiological recordings from ovipositor and tarsal taste sensilla showed that distilled water produced distinct responses from one neuron. In tarsal sensilla, ethanol and drimane solutions produced responses from two neurons, one of which might be the water cell that fired at a reduced rate. A drimane significantly decreased the responses of tarsal chemoreceptors to a cabbage leaf extract, which is a possible sensory mechanism leading to behavioural avoidance of this compound.

Behavioural responses of Anopheles gambiae sensu stricto to components of human breath, sweat and urine depend on mixture composition and concentration.

Host‐seeking behaviour of the anthropophilic malaria vector Anopheles gambiae sensu stricto (Diptera: Culicidae) is mediated predominantly by olfactory cues. Several hundreds of odour components have been identified from human emanations, but only a few have been proven to act as attractants or synergists in the host‐seeking behaviour of female An. gambiae. In previous work, aromatics, alcohols and ketones in human odours were found to elicit electrophysiological activity in antennal olfactory neurons of female An. gambiae. However, the behavioural effects of these compounds have not been investigated. In this study, behavioural responses of female An. gambiae to components of human breath, urine and sweat at a series of concentrations, or a single concentration in the case of acetone, were examined in combination with ammonia and L‐lactic acid in a dual‐choice olfactometer. The results showed that at specific concentrations 4‐ethylphenol, indole, 3‐methyl‐1‐butanol and two ketones inhibited the attractive effect of a mixture of ammonia and lactic acid. Acetone on its own was not attractive; however, when combined with lactic acid, the binary mixture was attractive. When combined with ammonia, acetone inhibited the attractiveness exerted by ammonia alone. Dodecanol and dimethyldisulphide did not affect the attraction exerted by ammonia and lactic acid at any of the concentrations tested. By contrast, a human‐specific armpit odour, 7‐octenoic acid, augmented the attraction exerted by the combination of ammonia and lactic acid at a specific dosage.

Comparison of searching strategies of five parasitoid species of Bemisia argentifolii Bellows and Perring (Hom., Aleyrodidae)

Amitus bennetti Viggiani and Evans, Encarsia formosa Gahan (two strains), Eretmocerus eremicus Rose and Zolnerowich, Eretmocerus mundus Mercet and Eretmocerus staufferi Rose and Zolnerowich, which are parasitoids of Bemisia argentifolii Bellows and Perring were compared with respect to their searching behaviour as part of a pre‐introduction evaluation programme. Within a 5 cm arena, host‐finding time was independent of the release distance from the host. Before oviposition, A. bennetti walked fastest, the E. formosa strains walked slowest and the Eretmocerus species intermediate. Leg length was not the most significant factor determining the differences in walking speed. After oviposition, A. bennetti and Er. eremicus had a lower walking speed and higher turn rate which is an indication of area‐restricted search. The effect was strongest for A. bennetti. All species showed preference for counter‐clockwise turns. Based on the walking speed alone, it is expected that A. bennetti will be the most efficient natural enemy of B. argentifolii, the Eretmocerus species intermediate and the E. formosa strains the least.

Life‐history and oviposition behaviour of Amitus bennetti, a parasitoid of Bemisia argentifolii

Amitus bennetti Viggiani & Evans (Hymenoptera: Platygasteridae) is a recently described parasitoid of the silverleaf whitefly, Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae). Behaviour and life history of the parasitoid are described. The first nymphal instar of B. argentifolii is preferred by the parasitoid, but the 1st through 4th instar may be parasitised. Females first investigate hosts with their antennae, then walk over the host, and eventually step with their front legs on the leaf and insert their ovipositor inside the host facing away from the host, while the hind legs are still on the host. The time from encounter to oviposition (=latency to oviposition) is shortest on the 1st instar. Oviposition duration (mean=39 s) comprises 50% of the handling time. Development time from egg to adult decreases from 72 days at 15 °C to 42 days at 20 °C to 28 days at 25 °C. We estimate that 400 degree days is required for development, with a development threshold of 10 °C. Adult longevity in the absence of hosts was 29, 26 and 19 days and with hosts present 8, 8 and 5 days at 15, 20 and 25 °C, respectively. Amitus bennetti is proovigenic and oviposits most eggs shortly after adult emergence. During the first day of their adult lives females laid 1, 31 and 49 eggs at 15, 20 and 25 °C, respectively. Compared with other parasitoid species, the development time of A. bennetti is very long, and the implications of this for management of B. argentifolii are discussed.