Controlling Mosquitoes with Semiochemicals: a Review Madelien Wooding1, Yvette Naudé1, Egmont Rohwer1 and Marc Bouwer2*
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Wooding et al. Parasites Vectors (2020) 13:80 https://doi.org/10.1186/s13071-020-3960-3 Parasites & Vectors REVIEW Open Access Controlling mosquitoes with semiochemicals: a review Madelien Wooding1, Yvette Naudé1, Egmont Rohwer1 and Marc Bouwer2* Abstract The use of semiochemicals in odour-based traps for surveillance and control of vector mosquitoes is deemed a new and viable component for integrated vector management programmes. Over 114 semiochemicals have been identifed, yet implementation of these for management of infectious diseases such as malaria, dengue, chikungunya and Rift Valley fever is still a major challenge. The difculties arise due to variation in how diferent mosquito spe- cies respond to not only single chemical compounds but also complex chemical blends. Additionally, mosquitoes respond to diferent volatile blends when they are looking for a mating partner, oviposition sites or a meal. Analyti- cally the challenge lies not only in correctly identifying these semiochemical signals and cues but also in develop- ing formulations that efectively mimic blend ratios that diferent mosquito species respond to. Only then can the formulations be used to enhance the selectivity and efcacy of odour-based traps. Understanding how mosquitoes use semiochemical cues and signals to survive may be key to unravelling these complex interactions. An overview of the current studies of these chemical messages and the chemical ecology involved in complex behavioural patterns is given. This includes an updated list of the semiochemicals which can be used for integrated vector control manage- ment programmes. A thorough understanding of these semiochemical cues is of importance for the development of new vector control methods that can be integrated into established control strategies. Keywords: Malaria, Vector mosquitoes, Anopheles, Aedes, Culex, Mosquito life-cycle, Semiochemicals, Chemical communication Background some minor species are also cause for concern. For Mosquitoes transmit serious infectious diseases that example, the Asian native mosquito An. stephensi is now include dengue, chikungunya, Rift Valley fever and established in Ethiopia [4] and An. arabiensis is deemed malaria. In Africa, malaria remains as one of the most a major cause of residual malaria transmission due to the serious vector-borne diseases [1]. Malaria is spread by species’ insecticide avoidance behaviours [2]. anopheline mosquitoes that transmit malaria parasites Vector control strategies such as indoor residual spray- to humans. Of the hundreds of Anopheles species found ing (IRS) and long-lasting insecticidal net (LLIN) pro- worldwide, only a few dozen mediate the transmission of grammes have played a crucial part in the reduction of the Plasmodium parasite to humans [2]. Africa has over malaria cases between 2002 and 2017 [5–7]. Tere are 128 native Anopheles mosquito species [3], with An. gam- two main problems afecting the future use of LLINs and biae (sensu stricto), An. coluzzii and An. funestus being IRS: frst, these strategies used alone or combined will the predominant malaria vector species [4]. However, not eradicate malaria incidences in high transmission areas and secondly, insecticide resistance of the major malaria vectors in Africa is widespread and increasing *Correspondence: [email protected] 2 Forestry and Agricultural Biotechnology Institute, University of Pretoria, [6, 7]. Another major concern with these vector control Hatfeld, Pretoria, South Africa strategies is that these tools mainly target Anopheles vec- Full list of author information is available at the end of the article tors that feed and rest indoors and have a preference to © The Author(s) 2020. 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The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/publi cdoma in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Wooding et al. Parasites Vectors (2020) 13:80 Page 2 of 20 feed at night. Changes in mosquito host preferences, time sensillum recordings (SSR) and gas chromatography cou- of feeding and an increase in outdoor feeding due to plas- pled to electroantennographic detection (GC-EAD), has ticity in mosquito behavioural responses have prompted greatly facilitated research in the feld of chemical ecol- the need for new and more environmentally friendly and ogy [14]. Chemical ecology of mosquitoes, specifcally robust vector control strategies that supplement current how mosquito behaviour is mediated by odour (Table 1), control strategies [5, 8]. Te development and incorpo- is a major feld of study that aims at improving control ration of novel vector control tools based on new scien- strategies that depend on synthetic odour lures which are tifc knowledge about mosquito behaviour and chemical popular for use in lure-and-kill strategies [9]. ecology into integrated vector management (IVM) pro- Tis literature review discusses chemical communica- grammes are needed in order to reduce the burden and tion in vector mosquitoes, with the focus on malaria vec- threat of mosquito-borne diseases [6]. tors. Key chemical cues and signals afecting diferent Studying the intricate strategies that malaria mosqui- behavioural patterns in the adult mosquito are discussed toes have evolved to survive in their environment may (Fig. 1). Te emphasis is on providing an updated list result in novel control methods for malaria. Mosquito of chemical attractants which can be used in IVM pro- survival depends not only on how successful they are at grammes. Known semiochemicals for eleven mosquito fnding suitable mating partners, oviposition sites and species are tabulated. blood or sugar meals [9], but also on how efective they are at avoiding predators by moving around undetected Semiochemicals as identifed during the diferent and fnding suitable shelter locations [10]. Mosquitoes mosquito life‑cycle stages can do this by detecting information from their environ- Chemical cues and signals mediate key behaviours during ment through a set of sensitive sensory organs [10]. Arti- the diferent stages in the life-cycle of an adult mosquito fcially interfering with these processes may result in the [15, 16]. Signals are produced for the specifc purpose ability to reduce mosquito populations and subsequently of changing the behaviour of an individual of a species, the incidence of malaria. whereas cues are not produced for any specifc pur- A range of environmental factors are detected by mos- pose, but they are detected by individuals and can lead quitoes and are used as cues that can afect their behav- to behavioural responses [17]. Host fnding by the adult iour. Changes that occur in the visual environment of the mosquito occurs through a series of behavioural steps mosquito obviously play an essential role, but other fac- that are infuenced by external chemical cues. A mos- tors such as fuctuations in the temperature or humidity quito can be primed for activity by a specifc chemical or levels also have an impact. Perhaps, the most important blend produced by a potential host. Tis is followed by cues are those that are present in the volatile chemical the initiation of fight to bring the insect within the vicin- environment that surrounds the mosquito [9]. Mosqui- ity of the source [18]. Te mosquito will then continue to toes use odours to locate their hosts and it is known that respond to external stimuli during fight for orientation diferent mosquito species depend on and detect difer- through optomotor anemotaxis (movement in response ent types of host odours [1, 11]. For example, An. arabi- to air currents and visual stimuli). Visual cues, such as ensis responds more strongly to carbon dioxide (CO 2) as contours against a background, that aid the mosquito a general cue to fnd a host, while An. gambiae relies on during upwind fight and physical cues, such as convec- other odours more specifc to humans together with CO2 tion heat and body moisture, play a role during host seek- acting as a long-range attractant [11, 12]. Understanding ing. Tese cues, together with chemical cues, infuence how these odours difer and how they are perceived by the orientation of a mosquito towards the host and can specifc mosquito species is an important feld that has induce a landing response [11, 12, 18]. Te individual been well documented but is still not understood entirely. importance of chemical, visual and physical cues that New knowledge on the chemical ecology of insects (the impact on the mosquito’s fight behaviour is well stud- study of chemical structures used by insects to mediate ied. However, the mechanism of how these cues work intra- and interspecifc interactions)