- Silene otites (Caryophyllaceae): Attraction of nectar-seeking mosquitoes to inflorescence odours, and temporal variation of flower scent and flower visitors (2007)
- In this investigation the role of flower odours in attracting mosquitoes to Silene otites were evaluated. Although mosquitoes are known to visit diverse flowers for nectaring, S. otites is one of only two plant species world-wide in which mosquitoes have been described as pollinators. In several investigations flower odours were described as effective attractants for mosquitoes, however, in none of the studies the compounds responsible for attraction were identified. Therefore, in the present study the aim was to investigate the chemical components in Silene otites that play a role in the attraction of mosquitoes. The information obtained can be useful in developing odour-baited traps for monitoring and controlling mosquito populations. Inflorescence odour composition of 63 Silene otites samples of nine populations was determined. There was variability in scent composition among populations and between sexes (Chapter 1). Most populations and samples were dominated by phenyl acetaldehyde, but in some samples high relative amounts of lilac aldehyde or (Z)-3-hexen-1-ol and (Z)-3-hexenyl acetate were present. Populations geographically close to each other were not more similar in their scent than distant populations. Nevertheless, in wind tunnel bioassay experiments with inflorescence odours of S. otites of six populations and Culex pipiens molestus, no differences in attractivity of different populations were observed. Therefore, different scent spectra seem to have the same attractiveness for mosquitoes. In electroantennographic studies with the 13 most common and abundant odour compounds of S. otites, Culex pipiens molestus males and females responded similarly to dilution series but the strength of the responses differed among compounds. Linalool oxide (furanoid) and linalool evoked the strongest responses in male and female mosquitoes, and (Z)-3-hexenyl acetate was strongly active in females. Further, the differences in antennal responses between night- and crepuscular-active Culex pipiens molestus and day-active Aedes aegypti to headspace flower odour samples of S. otites were investigated using coupled gas chromatographic–electroantennographic detection (GC-EAD). No striking differences in antennal responses to the odour compounds have been found between the two mosquito species (Chapter 2). To identify the compounds responsible for attraction of mosquitoes to S. otites, wind tunnel bioassays with antennal sensitive compounds and C. p. molestus were conducted. Mosquitoes responded to 14 compounds in different magnitudes (65-20%) and acetophenone, linalool oxide (pyranoid), phenyl acetaldehyde and phenylethyl alcohol were found as more attractive in comparison to the least attractive compound, hexanol (Chapter 2). Further, in two-stimulus choice tests, mosquitoes were significantly more attracted to the mixture of the four most attractive compounds (all typical flower scent compounds) than the mixture of all 14 compounds (including flower and vegetative scents of S. otites). Therefore, it seems that mosquitoes prefer flower scent over a combination of flower and vegetative scents when searching for flowers. Wind tunnel bioassays were not only conducted with naïve mosquitoes but also with conditioned mosquitoes to test the hypothesis that mosquitoes can learn flower scent compounds (Chapter 3). Learning may help the mosquitoes to find the most suitable host-plant that they visit more frequently thereafter. Though Silene otites is described as night-pollinated, preliminary observations revealed that this plant species is also visited by several insects during day-time. Therefore, the temporal variations of flower scent and flower visitors were investigated in more detail (Chapter 4). Although S. otites emits highest amounts of floral volatiles at night, just after sunset, it also emits a smaller amount of volatiles during day-time. Interestingly, the temporal variations differed among scent compounds where five different emission patterns of compounds could be categorized. During day and night, a total of 60 flower-visiting insect species were recorded. During day beetles, brachyceran flies and hymenopteran species were found, whereas, at night the nocturnal Lepidoptera, nematoceran flies, and neuropteran individuals were found to visit the flowers. Insect species visiting the flowers during daytime may be attracted by the scent compounds emitted during day, while nocturnal insects may be attracted by compounds emitted during night. These differences in scent in the course of the day might be the results of pollinator mediated selection, and the ‘fine tuned’ odour emission of this plant seems to be an adaptation to the olfactory abilities and preferences of the day- and night-active pollinators, respectively.