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.

Molecular basis for odorant receptor tuning: a short C-terminal sequence is necessary and sufficient for selectivity of mosquito Or8

A birth-and-death evolutionary model for odorant receptor gene repertoires presumes the creation of repertoires with the capacity for high-level diversity and rapid ligand specificity change. This changes the recognised odour space, directly affecting fitness-related behaviours and ultimately affecting adaptation to new environments and resources. The proximate molecular mechanisms underlying the tuning of odorant receptor repertoires, and thus peripheral olfaction, are unclear. In the present study, we report a concrete example of this model of odorant receptor evolution leading to rapid changes in receptor tuning that leave the peripheral neuronal circuitry intact. We identified a conserved odorant receptor gene in mosquitoes, Or8, which in Culex quinquefasciatus underwent a duplication and inversion event. The paralogues differ in only minor structural changes manifesting at the C-terminus. We assessed the specificity of the paralogous odorant receptors and receptor neurones. We found that the functional tuning of the receptor was indeed reflected in minor differences in amino acid structure. Specifically, we found that enantiomeric specificity of these mosquito Or8 paralogues relies on eight C-terminal amino acids encoded in the final exon of the gene; thus, the birth of a paralogous odorant receptor can change the tuning of the peripheral olfactory system.

Molecular identification of bloodmeals and species composition in Culicoides biting midges

Investigations of host preferences in haematophagous insects, including Culicoides biting midges (Diptera: Ceratopogonidae), are critical in order to assess transmission routes of vector-borne diseases. In this study, we collected and morphologically identified 164 blood-engorged Culicoides females caught in both light traps and permanent 12-m high suction traps during 2008-2010 in Sweden. Molecular analysis of the mitochondrial cytochrome c oxidase subunit I (COI) gene in the biting midges was performed to verify species classification, discern phylogenetic relationships and uncover possible cryptic species. Bloodmeal analysis using universal vertebrate cytochrome b primers revealed a clear distinction in host selection between mammalophilic and ornithophilic Culicoides species. Host sequences found matches in horse (n = 59), sheep (n = 39), cattle (n = 26), Eurasian elk (n = 1) and 10 different bird species (n = 18). We identified 15 Culicoides species previously recorded in Scandinavia and four additional species haplotypes that were distinctly different from the described species. All ornithophilic individuals (n = 23) were caught exclusively in the suction traps, as were, interestingly, almost all mammalophilic species (n = 41), indicating that many biting midge species may be able to cover long distances after completing a bloodmeal. These results add new information on the composition of Culicoides species and their host preferences and their potential long-distance dispersal while blood-engorged.

Smelling your way to food: can bed bugs use our odour?

The resurgence in developed countries of the common bed bug, Cimex lectularius, has led to a search for new sustainable methods to monitor and control this human ectoparasite. Because of increased resistance to insecticides, traps baited with attractive cues are considered a promising method to be developed into efficient monitoring tools for bed bugs. Despite their potential as attractants, only a few studies have investigated the odorant cues implicated in the attraction of bed bugs to human hosts. In this study, we used aeration extracts from human volunteers to assess the role of olfaction in host searching by bed bugs. By coupled gas chromatography and single sensillum recordings on all the antennal sensilla, we measured the electrophysiological response elicited by the compounds present in our human odour extracts. Only five compounds were clearly detected by the olfactory receptor neurons housed in the smooth-peg sensilla of the bed bugs. We tested the behavioural effect of these extracts in a still-air arena and showed a gradient of repellence linked to the dose, as well as a higher propensity of local search behaviour associated with human odours containing a lower ratio of 6-methyl-5-hepten-2-one to C7–C10 aldehydes. We conclude that human odour alone has a weak influence on the behaviour of C. lectularius and we propose that human kairomones may have a significant impact on bed bug behaviour in combination with heat and carbon dioxide, the only two currently known attractive vertebrate cues used by bed bugs for host seeking.

Enantiomeric selectivity in behavioural and electrophysiological responses of Aedes aegypti and Culex quinquefasciatus mosquitoes

1-Octen-3-ol is a kairomone for many haematophagous insects including mosquitoes. Numerous studies have examined the effects of racemic 1-octen-3-ol; however, few studies have investigated the role of individual enantiomers in relation to mosquito attraction. In the present study, we investigated the behavioural and electrophysiological responses of two mosquito species, Aedes aegypti and Culex quinquefasciatus, to individual enantiomers and mixtures of 1-octen-3-ol, employing a laboratory Y-tube olfactometer and single sensillum recordings. The olfactory receptor neurons of both Ae. aegypti and Cx. quinquefasciatus had a significantly higher response to the (R)-1-octen-3-ol enantiomer compared to the (S)-1-octen-3-ol enantiomer at 10-9 g μl-1 to 10-6 g μl-1. Behaviourally, Ae. aegypti was more responsive to the (R)-1-octen-3-ol enantiomer, showing an increase in flight activity and relative attraction compared to Cx. quinquefasciatus. The (R)-1-octen-3-ol enantiomer caused an increase in activation for Cx. quinquefasciatus. However, the most notable effect was from an (R:S)-1-octen-3-ol mixture (84:16) that caused significantly more mosquitoes to sustain their flight and reach the capture chambers (demonstrated by a reduced non-sustained flight activity), suggesting that it may have a behaviourally excitatory effect. For Cx. quinquefasciatus, a reduced relative attraction response was also observed for all treatments containing the (R)-1-octen-3-ol enantiomer, either on its own or as part of a mixture, but not with the (S)-1-octen-3-ol enantiomer. This is the first time enantiomeric selectivity has been shown for Ae. aegypti using electrophysiology in vivo. The implications of these results for exploitation in mosquito traps are discussed.

Monoamines and neuropeptides in antennal lobe interneurons of the desert locust, Schistocerca gregana: an immunocytochemical study

As a first step towards unravelling some of the complexity of the signalling and modulatory mechanisms in the antennal lobe (AL) of the desert locust Schistocerca gregaria, I analysed the immunocytochemical identity of AL interneurons. Antibodies against serotonin, histamine, locustatachykinin, leucokinin and FMRFamide were used to reveal the morphology of interneurons ramifying in the AL. In addition, double-labelling experiments were performed in order to demonstrate colocalisation of GABA and locustatachykinin and to investigate the ramification patterns of immunolabelled interneurons and physiologically characterised olfactory projection neurons (PNs) injected with Lucifer yellow. Immunoreactivity to these antibodies revealed six different types of interneurons with different patterns of ramification within the glomerular neuropil: (1, 2) Centrifugal interneurons displaying serotonin immunoreactivity, which arborised extensively within the AL and extended varicose fibres into the microglomerular core where close associations with dendrites of AL PNs could be distinguished. (3) Histamine-immunoreactive centrifugal interneurons with arborisations in the protocerebrum and the dorsal non-glomerular regions of the AL and the lobus glomerulatus (LG). (4) Locustatachykinin-immunoreactive local interneurons, colocalising GABA, arborising throughout the AL and extending varicose fibres throughout the glomerular neuropil where close associations with dendrites of AL PNs could be distinguished. (5) Leucokinin-immunoreactive descending neurons connecting the protocerebrum, the AL, the LG and all ganglia of the ventral nerve cord. These neurons displayed sparse innervation of the AL and extended varicose fibres into the interglomerular space. (6) FMRF-amide-immunoreactive centrifugal interneurons, connecting the lateral protocerebrum with the AL and the LG, which arborised sparsely within these neuropils and displayed similar innervation of the microglomeruli as (1) and (2).

The antennal lobe of orthoptera - anatomy and evolution

The first odor-processing neuropils of insects comprise glomeruli, islets of neuropil, that are supplied by olfactory receptor neurons and give rise to efferent axons to higher brain centers. Glomeruli size and organization varies in a taxon-specific manner across the Insecta, suggesting possible correlates between their organization and chemosensory behaviors in different insect groups. Comparative studies of antennal lobe glomeruli within the Orthoptera have been used to infer how the various taxon-specific arrangements of odorant-processing structures (glomeruli) might have evolved. The cellular arrangements in glomeruli have been surveyed using anterograde filling and Golgi impregnation of antennal receptor neurons projecting to the antennal lobe in Stenopelmatidae, Tettigoniidae, Gryllidae, Tetrigidae and Acrididae. These taxa, which represent the two sub-orders of Orthoptera, reveal a high correlation between the neural architecture of the glomeruli and structures within the glomeruli. Using a recent molecular phylogeny of the Orthoptera we have mapped the occurrence of glomerular characteristics to infer the evolution of antennal lobe structures in orthopterans. The functional implications of these results are discussed.

Juvenile-hormone-mediated plasticity of aggregation behaviour and olfactory processing in adult desert locusts

In desert locusts Schistocerca gregaria, aggregation behaviour is elicited by aggregation pheromones. In this study, we show that the behavioural response to the major and most potent adult aggregation pheromone component, phenylacetonitrile, is age- and juvenile-hormone-dependent. Furthermore, we show that juvenile hormone influences the responsiveness of olfactory interneurons in the antennal lobe to aggregation pheromone, whereas the responsiveness of antennal receptor neurons is not changed. Old locusts and locusts injected with juvenile hormone, in contrast to young locusts and locusts deprived of juvenile hormone through allatectomy, i.e. after surgical removal of the gland producing this hormone, do not display any aggregation behaviour, as indicated by long-term behavioural observations. The lack of positive olfactory-guided behaviour coincides with an impairment of the central olfactory system, which displays a lower number of neurons responding to aggregation pheromone. Indirect and direct actions of juvenile hormone at different levels of the central nervous system may thus contribute to the regulation and modulation of behavioural responsiveness in the locust.

The maxillary palp sensory pathway of Orthoptera

Primary sensory projections and arborisations of higher-order neurons associated with the maxillary palps were examined in Tettigoniidae, Gryllidae, Tetrigidae and Acrididae representing the two sub-orders of Orthoptera, Ensifera and Caelifera. Anterograde filling and Golgi impregnation of maxillary receptor neurons revealed two patterns of innervation, the ensiferous and the caeliferous type. In both ensiferans and caeliferans, receptor neurons arborised within the tritocerebrum, the antennal motor- and mechano-sensory centre and the lobus glomerulatus. In ensiferans, additional areas of innervation were found in the lobus glomerulatus and in a previously undescribed neuropil, here referred to as the accessory lobus glomerulatus. In relation to the anatomical data a putative functional segregation of the neuropil into gustatory-, olfactory- and mechano-sensory centres is implied.

Integration of behaviourally relevant odours at the central nervous level in solitary and gregarious third instar locusts, Schistocerca gregaria

Integration of behaviourally relevant odours at the central nervous level of 3rd instar nymphal desert locusts, Schistocerca gregaria, showed phase- and (developmental) stage-dependent characteristics which correlated with differences in the number of olfactory sensilla. Antennal lobe (AL) neurons of gregarious locusts generally responded more frequently and showed a higher sensitivity to the tested stimuli. However, AL neurons of solitary locusts responded significantly more frequently to phenylacetonitril, the major component of the adult aggregation pheromone. Pheromone-specific, plant-specific and pheromone-plant generalist neurons were found in both phases. The response spectra of pheromone-specific neurons correlated with the potential behavioural significance of attractant chemical cues in the environment. Neurons of both phases responded specifically to stage-specific aggregation-pheromone components and to other chemical cues that may be involved in the location of suitable roosting and foraging sites.