| Volatiles emanating from hosts play an important role in modulating host-seeking bebavior of mosquitoes. The domestic mosquito species Culex pipiens pallens Coquillet, the most extensive in distribution, had a marked preference for birds, such as chicks, ducks and pigeons. The main aim of the present study was to investigate the chemical components in odors of chick, pigeon and mouse that play a role in the host-seeking behaviour of this mosquito species. In addition, Cx. pipiens molestus and Cx. pipiens pallens belong to Cx. pipiens complex, but the former is autogenous and the latter is anautogenous. So, the secondary aim of the present thesis was to investigate the behavioral response of Cx. pipiens molestus to animals and certain odor chemicals. The knowledge obtained could be applied in developing new attractants and repellents that can be used to control mosquitoes and monitor mosquito populations in the field. Additional, fundamental knowledge on host-seeking bebavior can be helpful for understanding the mechamisms of mosquito-host interaction.1. Establishment of pigeon preference Cx. pipiens pallens strain and mouse preference Cx.pipiens pallens strainHost preference differences in interspecies for Cx. pipiens pallens mosquitoes have been demonstrated by both field and laboratory experiments. Preference for pigeon over mouse has been recorded. So field population of Cx. pipiens pallens could be separated into pigeon preference strain and mouse preference strain through host selection experiment. Based on such a presumption, selective experiments were carried out using a selective cage in the laboratory. Pigeon and Kuming mouse were used as selective hosts. In the first selection, the proportion choosing pigeon and mouse was 27% and 16% respectively. The proportion choosing pigeon and mouse could be increased through several times selections. To the sixth selection, for the population prefered for pigeon, the proportion choosing pigeon and mouse was 55% and 10% respectively, as well as for the population prefered for mouse, the proportion choosing pigeon and mouse was 31% and 34% respectively(P>0.05). In addition, the proportion changed little during following six selections, and it indicated that the field Cx.pipiens pallens population has been separated into pigeon preference strain and mouse preference strain. The study would provide excellent stocks for genetic studies on host selection.2. Analysis of volatiles from pigeon, chick and Kunming mouse by Gas Chromatography/Mass SpectrometryIn order to identify the chemicals in host odors that play a role in the host-seeking behaviour of this mosquito species, the chemical components of the three host odors were analyzed. Chemical components were collected using GDX-101 adsorptive process and solvent extraction of feather or fur and analyzed by gas-chromatography-mass spectrometry (GC/MS). Results as follows: (1) Chemical components in pigeon odors. Forty-two components from GDX-101 absorbed odors and 59 components from methylene chloride etracts of feather were detected. The compounds found prominently in GDX-101 absorbed odors included hydrocarbon and aromatics (81%), ketones, aldehydes and carboxylic acids (13%). The compounds found prominently in feather extracts included hydrocarbons and aromatics (60%), ketones, aldehydes, carboxylic acids, alcohols and esters (32%). (2) Chemical components in chick odors. In GDX-101 absorbed odors, 69 components were detected belonging to five main classes: n-alkanes, ketones and aldehydes, carboxylic acids, n-heterocyclic aromatics and terpenes. Aldehydes were the most compounds in our analysis, compositing of 17.4% (12 aldehydes in a total of 69 compounds). Prominent components were indole and 4-methyphenol (10.98% and 10.69% respectively, peak area percentage). In feather extracts, 61 components were detected belonging to six main classes: n-alkanes, ketones, alkenes, carboxylic acid esters, n-heterocyclic aromatics and cholests. Alkanes and carboxylic acid esters were the most compounds in this analysis, compositing of 53% and 13% respectively. Prominent components were cholests (41.26%, peak area percentage). (3) Chemical components in Kunming mouse odors. In GDX-101 absorbed odors, 30 components including n-alkanes and alkenes (compositing of 33.3%), n-heterocyclic aromatics (found 8 in 30 compounds), ketones and aldehydes (found 7 in 30 compounds), carboxylic acids (found 3 in 30compounds) were detected. Forty-five components from fur extracts including n-alkanes and alkenes (compositing of 46.7%), ketones and aldehydes (found 7 in 45 compounds), carboxylic acids and esters (found 6 in 45 compounds), n- heterocyclic aromatics (found 3 in 45 compounds) and cholests (found 4 in 45 compounds) were detected. Prominent components were cholests (41.99%, peak area percentage). In this thesis, only compounds with a≥90% match, however, have been given a tentative identification.3. Evaluation of odor chemicals influence Cx. pipiesn pallens mosquito host-seeking behaviorEAG and behavioral bioassays were conducted in laboratory. Based on GC/MS and other results in human odors, the electrophysiological responses of Cx. pipiens pallens females to L-lactic acid, ammonia, benzaldehyde andα-pinene at four doses (0.01, 0.1, 1 and 10μL/mL) were tested. Results indicated that lactic acid, benzaldehyde and ammonia elicited significant olfactory responses exceptα-pinene. Lactic acid showed a maximum EAG response at 1μL/mL, but a decrease in response was observed with an increase in the dose. Ammonia elicited a maximum response of 4.907±1.357 mV at 10μL/mL. Ammonia and benzaldehyde elicited dose-dependent responses.α-pinene did not elicit significant reoponses at all four doses. The studies further demonstrate that chemical communication plays a crucial role for female Cx. pipiens pallens in host-seeking.A dual-choice Y-tube olfactomer was used to investigate the attractiveness of 19 chemicals mainly identified tentatively from chick, pigeon and mouse odors to Cx.pipiens pallens. These chemicals were from the following five classes: carboxylic acids, aldehydes, n-alkanes and alkenes, monoterpenes, and n-heterocyclic aromatics. Octanoic acid and tetradecane acid elicited significant attraction that averaged of 84.1 and 64.6% at 10μL/mL respectively. L-lactic acid elicited attraction that averaged of 57.27% (P=0.042) at 10μL/mL, but L-lactic acid did not elicit attraction at 0.1 and 1μL/mL. Nonanoic acid and benzoic acid did not attract Cx.pipiens pallens with no differences between responses to treatment and control at any doses (P=1.133, P=0.675). All aldehydes did not elicit attraction of Cx.pipiens pallens. In addition, responses significantly declined as the dose was increased. The least response to nonanal, benzaldehyde and hexanal averaged less than 15% at the higest dose 10μL/mL. The results indicated that these aldehydes may be potent mosquito repellents, but this needs to be further studied by using repellent experiments. Five alkanes and alkenes did not elicit significant attraction of Cx.pipiens pallens at levels not significantly higher than the control(P>0.05). In n-heterocyclic aromatics group, indole and 4-mythtlphenol exhibited attraction averaged more than 80% at 0.1μL/mL(P=0.0047, P=0,0096), however, the responses declined as the doses increasing. Naphthalene did not elicit attraction of Cx.pipiens pallens at three doses (P>0.05). Ammonia showed attraction of 73% at the highest dose (10μL/mL), but the response was only about 57% at the lowest dose (0.1μL/mL).α-pinene did not elicit significant attractant responses at any dose(P>0.05).Octanoic acid, ammonia, 4-mythtlphenol, nonanal andβ-myrcene were selected for further examination in bioassays with pigeon preference Cx. pipiens pallens and mouse preference Cx. pipiens pallens. The results showed that there were no differences between behavioral responses of pigeon preference Cx.pipiens pallens and mouse preference Cx.pipiens pallens to five chemicals with exception of octanoic acid and 4-mythtlphenol which elicited greater attraction of pigeon preference Cx.pipiens pallens than that of mouse preference Cx.pipiens pallens at 10μL/mL and 0.1μL/mL respectively (P=0.007, P=0.024). The chemicals which attracted pigeon preference Cx. pipiens pallens were also attractive to mouse preference Cx.pipiens pallens with only differences in attractive percentage. Ammonia identified only in mouse odors elicited attraction of two host preference population4. Behavioral responses of Cx. pipiens molestus to hosts and chemicalsAutogenous mosquitoes do not require blood to mature an initial egg batch. We studied the expression of host-seeking bebaviour for Cx.pipiens molestus during its first gonotrophic cycle. A selected cage and a Y-tube olfactometer have been used to study host location behaviour. Pigeon, Kunming mouse and 5 chemicals were tested for their attraction for autogenous Cx.pipiens molestus. The proportion choosing pigeon and mouse was 7.7% and 8.3% respectively. Similarly, five chemicals elicited behavioral responses of autogenous Cx.pipiens molestus females. Octanoic acid and ammonia elicited a maximum attraction that averaged of 72.02% and 70.57% at 10μL/mL respectively. 4-methyphenol elicited a maximum attraction that averaged of 76.45% at 0.1μL/ml.β-myrcene did not elicit significant attraction of Cx. pipiens molestus at any dose (P=0.92). Nonanal did not elicited attraction that averaged of 17.97% and 12.58% at 1μL/mL and 10μL/mL respectively. The studies showed that most autogenous females can seek host (or chemichals) and fly into host traps during first gonotrophic cycle, if the host is available. The results implied that olfactory receptors have been matured when the adults emerged. Comparative studies on host-seeking behavior of autogenous and anautogenous mosquitoes would facilitate the study of the mechanism of host-seeking behavior expression or inhibition which would be useful for development of new mosquito control methods such as controlling the mosquito biting behaviors.
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