Date Published: June 4, 2019
Publisher: Public Library of Science
Author(s): Daniel A. H. Peach, Elton Ko, Adam J. Blake, Gerhard Gries, Michel Renou.
Inflorescence patterns of ultraviolet (UV) absorption and UV-reflection are attractive to many insect pollinators. To understand whether UV inflorescence cues affect the attraction of nectar-foraging mosquitoes, we worked with the common house mosquito, Culex pipiens and with two plant species exhibiting floral UV cues: the tansy, Tanacetum vulgare, and the common hawkweed Hieraciumm lachenalii. Electroretinograms revealed that Cx. pipiens eyes can sense UV wavelengths, with peak sensitivity at 335 nm. Behavioural bioassays divulged that UV inflorescence cues enhance the attractiveness of inflorescence odour. In the presence of natural floral odour, female Cx. pipiens were attracted to floral patterns of UV-absorption and UV-reflection but preferred uniformly UV-dark inflorescences. Moreover, Cx. pipiens females preferred UV-dark and black inflorescence models to UV-dark and yellow inflorescence models. With feathers and pelts of many avian and mammalian hosts also being UV-dark and dark-coloured, foraging Cx. pipiens females may respond to analogous visual cues when they seek nectar and vertebrate blood resources.
Plant sugar, mainly in form of floral nectar, is the essential basic food for adult mosquitoes  that can serve as pollinators to the many plants they visit [2–6]. Floral semiochemicals are believed to attract mosquitoes to inflorescences [1,7,8], whereas visual floral cues were thought , and recently shown [10–12], to play a contributing role. Field observations suggest that mosquitoes most often visit light-coloured flowers [13–15] but preferential visitation to these types of flowers has yet to be rigorously tested . Exclusively visual cues of oxeye daisy inflorescences did not attract mosquitoes in laboratory experiments  but olfactory oxeye daisy cues alone or in combination with visual cues did . Both the yellow fever mosquito, Aedes aegypti (L.), and the northern house mosquito, Culex pipiens L., were more strongly attracted to a combination of olfactory and visual inflorescence cues than to olfactory inflorescence cues alone , revealing a contributing role of visual cues in mosquito attraction to inflorescences.
Our findings indicate that (1) compound eyes of Cx. pipiens can sense UV light; (2) visual inflorescence cues render inflorescence odour more attractive to Cx. pipiens; (3) the UV “bullseye” of H. lachenalii inflorescences (Fig 1B) attracts Cx. pipiens; (4) the UV-dark trait of inflorescences is a strong driver of Cx. pipiens attraction, and (5) stimuli dark in both human visible light and UV light are most attractive to Cx. pipiens. Below, we shall elaborate on these findings.
We have shown that nectar-foraging Cx. pipiens females respond to both olfactory and visual inflorescence cues. UV-sensitive eyes enable Cx. pipiens females to detect, and discern between, floral patterns of UV-absorption and UV-reflection, with preference for inflorescences with low reflection of both human-visible and UV light. With feathers and pelts of many avian and mammalian hosts being similarly dark, foraging mosquitoes may respond to analogous but contextually different visual cues when they seek nectar and vertebrate blood resources.