Functional development of carbon dioxide detection in the maxillary palp of Anopheles gambiae

Olfactory information drives several behaviours critical for the survival and persistence of insect pests and vectors. Insect behaviour is variable, linked to their biological needs, and regulated by physiological dynamics. For mosquitoes, CO(2) is an important cue that signifies the presence of a host, and which elicits activation and attraction. To investigate the genetic basis of olfactory modulation in mosquitoes, we assayed changes in CO(2) detection from receptor gene expression through physiological function to behaviour, associated with the onset of host seeking in the malaria vector, Anopheles gambiae. The gene encoding a subunit of the CO(2) receptor, AgGr22, was found to be significantly up-regulated in host-seeking females, consistent with a significant increase in sensitivity of CO(2)-responsive neurons (cpA) housed in capitate peg sensilla of the maxillary palp. In addition, the odorant receptor AgOr28, which is expressed in cpC neurons, was significantly up-regulated. In contrast, AgOr8, which is expressed in cpB neurons, was not affected by this change in physiological state, in agreement with results for the obligate co-receptor Orco. Moreover, the sensitivity of the cpB neuron to (R)-1-octen-3-ol, a well-known mammalian kairomone, did not change in response to the onset of host seeking. The concentration of CO(2) flux influenced both the propensity of A. gambiae to take off into the wind and the speed with which this activation occurred. Female A. gambiae mosquitoes responded to CO(2) whether mature for host seeking or not, but onset of host seeking enhanced sensitivity and speed of activation at relevant doses of CO(2)

Visual cues and host-plant preference of the bird cherry-oat aphid, Rhopalosiphum padi (Hemiptera: Aphididae)

Alate aphids respond to short (UV) and long (green-yellow) wavelength stimuli during host-plant searching behaviour. Although many aphids are attracted to yellow, the bird cherry-oat aphid, Rhopalosiphum padi, is attracted to green. As part of identifying suitable plant species for crop border plants for seed potatoes, the attraction of R. padi to different shades of green in relation to differences in spectral reflectance of three cultivars of a non-host, potato, and two host-plant species, maize and wheat, were determined. Choice experiments were carried out under laboratory conditions to evaluate aphid landing preference for stimuli of different colours. Rhopalosiphum padi alates preferred to land on the colour targets with the highest reflectance. Significantly more alates landed on yellow and lime colour targets with a maximum wavelength reflectance of 46 % from 600-610 nm and 26 % from 525-531 nm, respectively. The peak light reflectance of the crop plants ranged between 12 % (potato; 532-555 nm) and 20 % (wheat; 537-553 nm). The results on aphid landing preference for different colour targets suggest that R. padi will land in higher numbers on the wheat plants, compared to potato, due to their higher peak percentage reflectance. In addition, the study indicates that the wavelength reflectance curves of plants can be used as a characteristic to select possible crop border plants when making choices between seemingly equally suitable plant species/cultivars.Potatoes South Africa (PSA), the Technology and Human Resources for Industry Programme (THRIP), the National Research Foundation (NRF) and the University of Pretoria.http://www.journals.co.za/ej/ejour_ento.html2016-07-30am201

Age-dependent regulation of host seeking in Anopheles coluzzii

Behavioural attraction of the malaria vector Anopheles coluzzii to human host odour increases during adult maturation. We have previously demonstrated that the onset of host seeking in An. coluzzii coincides with an increased sensitivity of the CO2-sensitive neurons and abundance of chemosensory receptor gene transcripts in the maxillary palp. In this study, we extend our analysis to the antenna. Functional characterisation of the near-complete repertoire of odorant receptors (Ors) expressed in this tissue, to fractioned human odour, reveals a subset of salient human odorants to be detected by Ors at physiological relevant concentrations. When presented as a blend in their ratio of natural emission, these odorants elicit attraction by host-seeking mosquitoes, emphasising that Ors alone can mediate this behaviour. However, the same blend inhibits attraction in teneral mosquitoes. This switch in behavioural response indicates a change in valence during adult maturation. Quantitative analysis of Or transcript abundance and in vivo electrophysiological analysis reveal that the olfactory system of female An. coluzzii undergoes concerted changes that correlate with the onset of host seeking. We conclude that changes in Or abundance modulate peripheral olfactory coding, resulting in ecologically relevant behavioural effects

Olfactory responses of Rhopalosiphum padi to three maize, potato, and wheat cultivars and the selection of prospective crop border plants

Understanding host plant volatile – aphid interactions can facilitate the selection of crop border plants as a strategy to reduce plant virus incidence in crops. Crop border plant species with attractive odours could be used to attract aphids into the border crop and away from the main crop. As different cultivars of the same crop can vary in their olfactory attractiveness to aphids, selecting an attractive cultivar as a border crop is important to increase aphid landing rates. This study evaluated olfactory responses of the bird cherry-oat aphid, Rhopalosiphum padi (L.) (Hemiptera: Aphididae), to three cultivars each of maize [Zea mays L. (Poaceae)], potato [Solanum tuberosum L. (Solanaceae)], and wheat [Triticumaestivum L. (Poaceae)] with the aim of selecting an attractive crop border plant to reduce the incidence of the non-persistent Potato virus Y [PVY (Potyviridae)] in seed potatoes. Volatiles emitted by the crop cultivars were collected and identified using coupled gas chromatography/ mass spectrometry. Quantitative and qualitative differences were found among cultivars. Behavioural responses of alate R. padi to odours of the cultivars and synthetic compounds identified from the plants were determined with a four-arm olfactometer. Rhopalosiphum padi was attracted to odours emitted from maize cultivar 6Q-121, but did not respond to odours from the remaining eight crop cultivars. Volatile compounds from maize and wheat cultivars that elicited a behavioural response from R. padi and contributed to differences in plant volatile profiles included (Z)-3-hexenyl acetate (attractant) and a-farnesene, (E)-2-hexenal, indole, and (3E,7E)-4,8, 12-trimethyltrideca-1,3,7,11-tetraene (TMTT) (repellents). We conclude that maize cv. 6Q-121 is potentially suitable as a crop border plant based on the behavioural response of R. padi to the olfactory cues emitted by this cultivar. The findings provide insight into selecting crop cultivars capable of attracting R. padi to crop border plants.National Research Foundation (NRF), the University of Pretoria and the Swedish Foundation for Strategic Environmental Research (MISTRA) through the Plant-ComMistra programme. MISTRA and Carl Tryggers Stiftelse.http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1570-74582016-11-30hb201

Grass pollen affects survival and development of larval Anopheles arabiensis (Diptera: Culicidae)

Nutrients in breeding sites are critical for the survival and development of malaria mosquitoes, having a direct impact on vectorial capacity. Yet, there is a limited understanding about the natural larval diet and its impact on the individual fitness of mosquitoes. Recent studies have shown that gravid Anopheles arabiensis Patton (Diptera: Culicidae) are attracted by and oviposit in grass-associated habitats. The pollen provided by these grasses is a potential source of nutrients for the larvae. Here, we assess the effect of Typha latifolia L. (Poales: Typhaceae), Echinochloa pyramidalis Lamarck, Pennisetum setaceum Forsskål, and Zea mays L. pollen on larval survival and rate of development in An. arabiensis under laboratory conditions. In addition, we characterize the carbon to nitrogen ratio and the size of pollen grains as a measure of diet quality. Carbon-rich pollen with a small grain size (T. latifolia and P. setaceum; 9.7 ± 0.3 × 103 and 5.5 ± 0.2 × 104 µm3, respectively) resulted in enhanced rates of development of An. arabiensis. In contrast, the larva fed on the nitrogen-rich control diet (TetraMin) was slower to develop, but demonstrated the highest larval survival. Larvae fed on carbon-rich and large-grained Z. mays pollen (4.1 ± 0.2 × 105 µm3) survived at similar levels as those fed on the control diet and also took a longer time to develop compared with larvae fed on the other pollens. While males and females did not appear to develop differently on the different pollen diets, males consistently emerged faster than their female counterparts. These results are discussed in relation to integrated vector management

Mosquito host-seeking diel rhythm and chemosensory gene expression is affected by age and Plasmodium stages

Malaria parasites can affect vector-related behaviours, increasing transmission success. Using Anopheles gambiae and Plasmodium falciparum, we consider the effect of interaction between infection stage and vector age on diel locomotion in response to human odour and the expression of antennal chemosensory genes. We identified age-dependent behavioural diel compartmentalisation by uninfected females post-blood meal. Infection disrupts overall and diel activity patterns compared with age-matched controls. In this study, mosquitoes carrying transmissible sporozoites were more active, shifting activity periods which corresponded with human host availability, in response to human odour. Older, uninfected, blood-fed females displayed reduced activity during their peak host-seeking period in response to human odour. Age- and infection stage-specific changes in odour-mediated locomotion coincide with altered transcript abundance of select chemosensory genes suggesting a possible molecular mechanism regulating the behaviour. We hypothesize that vector-related behaviours of female mosquitoes are altered by infection stage and further modulated by the age post-blood meal of the vector. Findings may have important implications for malaria transmission and disease dynamics

Behavioural response of the house mosquitoes Culex quinquefasciatus and Culex pipiens molestus to avian odours and its reliance on carbon dioxide

How Culex (Diptera: Culicidae) mosquitoes select and discriminate between potential avian hosts is critical for understanding the epidemiology of West Nile virus. Therefore, the present authors studied the behavioural responses of Culex quinquefasciatus (Say) and Culex pipiens molestus (Forsskål) to headspace volatiles of three avian species [chicken and pigeon (sexes analysed separately), and magpie], presented either alone or in combination with 600 p.p.m. carbon dioxide (CO2). The attraction of Cx. quinquefasciatus to the headspace volatiles of both sexes of chicken, and of female pigeon, in combination with CO2 was significantly higher than that achieved by the CO2 and solvent control. Although Cx. p. molestus was attracted to headspace volatiles of chickens and magpies, it was repelled by those of female pigeons when combined with CO2. An increased effect between the avian volatiles and CO2 was observed for Cx. quinquefasciatus, whereas the addition of CO2 had no effect on the attraction of Cx. p. molestus females. The results of this study demonstrate that Cx. quinquefasciatus and Cx. p. molestus are attracted to the odour of potential avian hosts. Future studies aimed at identifying the bioactive volatile compounds in the headspace of chickens may contribute to the potential development of effective surveillance and control tools against Culex species