Odor-based mosquito control tools slowly find their way into application, thereby diversifying the toolbox available to local vector control schemes. With much of mosquito life revolving around odors, methods that harness a mosquito sense of smell hold great promise in providing novel tools. A broad array of sensory neurons hardwires mosquito preference and tunes its nose to resources important for survival and reproduction. A mosquito’s needs, however, frequently changes between mating, nectar feeding, blood feeding and oviposition, and with that the odors to which it orients. Mosquitoes thus have to ‘toggle’ between sensory modes, which involves peripheral [43,44] and/or central modulatory factors [44]. Here we evaluated whether a combination of odors from spatio-temporally different origins would synergize attraction, or, alternatively, constitute olfactory nonsense to a mosquito nose and potentially mask attractiveness. We show that combining synthetic mimics of floral and human odor attract nulliparous and parous mosquitoes. As field populations are comprised of mosquitoes whose odor preferences vary with e.g. age, nutritional and gonotrophic state, such complex, multiplexed blends may be more effective and take a broader sweep of the mosquito population.
Over the past sixty years, attraction of female mosquitoes to blood-host mimicking odours and plant-based attractants has rarely combined odors from different origin [45- 52]. In those studies where odors of presumed floral and vertebrate origin were combined, mixed results were obtained, by and large not indicating synergy [37-40]. In our study, although mosquitoes were attracted by a combination of floral and human odor, the combination did not augment or synergize capture rates, in spite of each blend individually being attractive. This is largely in line with earlier reports [37-40]. Somehow the added sensory input does not translate in an enhanced ‘attractiveness’ of the signal. This could in part be because the nutritional status of the mosquitoes in our cohorts was similar, whereas ‘synergy’ or ‘augmentation’ of trap capture for a multiplexed lure would more readily emerge in field populations with mosquitoes in diverse physiological states. Further, it may also be that that odor sources, although placed in very close proximity of each other, do not create fully merged plumes, which mosquitoes may perceive as two separate sources instead of an augmented single source. Indeed, insects are exquisitely capable of neurologically parse incompletely mixed strands of odors [53,54].
Of further interest is the observation that in semi-field experiments nulliparous females preferred floral volatiles to other blends, whereas this preference disappeared in parous females. This demonstrated a well-known mosquito food proclivity: female mosquitoes generally take sugar meals before they seek a blood meal, and some species strongly prefer sugar over blood or rarely bite until after a sugar meal or even not until after several weeks of sugar feeding [19,55]. The relatively young (4-5 days old) and nulliparous females in this study may thus follow such pattern and first cater to their low energy reserves before seeking blood. In contrast, parous females, which likely have increased their energy levels through a previous bloodmeal were equally attracted to either lure, as they are known to alternate between sugar meals and blood meals [19].
Combining the floral and skin odor blends is also of interest as they induce sensory activity in entirely different classes of sensory neurons, with floral odors being detected by olfactory receptors (ORs), whereas the detection of the human odor blend, consisting of amines and acids, is entirely restricted to ionotropic receptors (IRs) expressed in grooved peg sensilla[56]. Accordingly, the input from floral and human odor is complementary and induces responses in separate olfactory subcircuits [57]. Combination of input from these subcircuits often lead to synergistic trap catches in other insect taxa example in Drosophila flies [58]. In mosquitoes, however, the relative importance of the OR and IR subcircuitry may differ between distinct behaviors, such as orientation to nectar versus blood host resources [59]. How a combination of input from these classes of sensory neurons influences capture rates in mosquitoes, and e.g. synergize capture rates of each blend separately, has not been systematically analysed. The tests performed here indicate that different from some other insect taxa, IR and OR input does, perhaps surprisingly, not necessarily synergize. Whereas this may indicate a fundamental odor-coding difference between the taxa, it may also simply be due to that the combination, release rates and ratios require further adjustment.
The results further suggest that a previous blood meal experience modulates olfactory preference. Shifts in blood host preference has been reported for mosquitoes [60,61]. Similarly, shifts in preference have been found depending on internal state, such as age, mating status, physiological status, and blood feeding status [62-64]. The modulation observed here, from floral to skin odor, also implies that following a bloodmeal mosquitoes may increasingly ‘weigh’ input from the IR circuitry, tuned to vertebrate hosts, in behavioural preference. Further research is needed to more in depth evaluate protracted effects of a blood meal on nutritional status and preference modulation, as implied by our results.
From an applied perspective the results offer interesting angles. Although we did not find any augmentation of trap catches by combining floral and skin odors, the combination did catch both nulliparous and parous mosquitoes (which differs slightly from earlier reports, 37-40), and would therefore attract mosquitoes relatively independently of physiological status, these being either searching to replenish carbohydrate energy reserves for flight and maintenance (floral odors) [19, 65,66] or searching for hosts to support reproduction (skin lure, a human skin-mimicking blend of volatiles) [28,67,68]. Although there was significant attraction to either floral and skin odor blends, alone and in combination, to both parous and non-parous females, this study did not compare the attraction of the blends to a living human. Further research is needed to assess the attractiveness of the new odour blends compared to that of humans in natural field settings.