Odour-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 odours, methods that harness a mosquito sense of smell hold great promise in providing novel tools. A broad array of sensory neurons ‘hardwire’ mosquito preference and tune its nose to resources important for survival and reproduction. A mosquito’s needs, however, frequently change between mating, nectar feeding, blood feeding and oviposition, and with that the odours to which it orients. Mosquitoes thus have to ‘toggle’ between sensory modes, which involves peripheral [44, 45] and/or central modulatory factors . In this study, we evaluated whether a combination of odours from spatio-temporally different origins would synergize attraction, or alternatively, constitute olfactory nonsense to a mosquito nose and potentially mask attractiveness. Parous mosquitoes switch in behaviour from nectar feeding to a combination of blood host and nectar feeding . Comparison between parous and non-parous female mosquitoes may thus highlight differences that are due to this switch in preference. This study showed that combining synthetic mimics of floral and human odour attract nulliparous and parous mosquitoes. As field populations are comprised of mosquitoes whose odour preferences vary with, for example, 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 60 years, attraction of female mosquitoes to blood-host mimicking odours and plant-based attractants has rarely combined odours from different origin [46-53]. In those studies, where odours of presumed floral and vertebrate origin were combined, mixed results were obtained, by and large not indicating synergy [36-39]. In this study, although mosquitoes were attracted to a combination of floral and human odour, 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 [36-39]. In a recent field studies in Kenya, a combination of plant- and human-derived odours was observed to elicit a masking effect in trapping Aedes aegypti .
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 this study 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 odour 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 odours [54, 55].
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, 56]. 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 blood meal were equally attracted to either lure, as they are known to alternate between sugar meals and blood meals .
Combining the floral and skin odour blends is also of interest as they induce sensory activity in entirely different classes of sensory neurons, with floral odours being detected by olfactory receptors (ORs), whereas the detection of the human odour blend, consisting of amines and acids, is entirely restricted to ionotropic receptors (IRs) expressed in grooved peg sensilla . Accordingly, the input from floral and human odour is complementary and induces responses in separate olfactory sub-circuits . Combination of input from these sub-circuits often leads to synergistic trap catches in other insect taxa example in Drosophila flies . In mosquitoes, however, the relative importance of the OR and IR sub-circuitry may differ between distinct behaviours, such as orientation to nectar versus blood host resources . How a combination of input from these classes of sensory neurons influences capture rates in mosquitoes, and for example, 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 odour-coding difference between the taxa, it may also simply be due to that the combination, release rates and ratios require further adjustment. Inconsistency between laboratory and semi-field results observed is likely to be due to the fact that in the laboratory there was closeness of the mosquitoes, hence relatively sensing high concentration while in the semi-field the large space and weather had dilution effect.
The results further suggest that a previous blood meal experience modulates olfactory preference. Shifts in blood host preference have been reported for mosquitoes [61, 62]. Similarly, shifts in preference have been found depending on internal state, such as age, mating status, physiological status, and blood feeding status [63-65]. The modulation observed here, from floral to skin odour, also implies that following a blood meal mosquito may increasingly ‘weigh’ input from the IR circuitry, tuned to vertebrate hosts, in behavioural preference. Further research is needed to more in-depth evaluate the protracted effects of a blood meal on nutritional status and preference modulation, as implied by results in this study.
From an applied perspective the results offer interesting angles. Although the study did not find any augmentation of trap catches by combining floral and skin odours, 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 odours) [19, 66, 67] or searching for hosts to support reproduction (skin lure, a human skin-mimicking blend of volatiles) [28, 68, 69]. Although there was significant attraction to either floral and skin odour 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 at different concentrations of the materials and new odour blends compared to that of humans in natural field settings.