This is a more direct method of identifying plants with a potential use than general screening of all plants in an area. Questions are asked about plant usage, abundance and source. It is common practice to conduct ethnobotanical surveys using structured interviews, combined with the collection of plant voucher Specimens (Figure 2), to evaluate plant use by indigenous ethnic groups. This is the targeted search for medicinal plants through in-depth interviews with key informants knowledgeable in folk-lore and traditional medicine. The discovery of new plant-based repellents is heavily reliant on ethnobotany. Plant-based repellents are still extensively used in this traditional way throughout rural communities in the tropics because for many of the poorest communities the only means of protection from mosquito bites that are available, and indeed for some of these communities, as in the Europe and North America “natural” smelling repellents are preferred because plants are perceived as a safe and trusted means of mosquito bite prevention. Plants have also been used for centuries in the form of crude fumigants where plants were burnt to drive away nuisance mosquitoes and later as oil formulations applied to the skin or clothes which was first recorded in writings by ancient Greek, Roman and Indian scholars (Figure 1). This repellency of plant material has been exploited for thousands of years by man, most simply by hanging bruised plants in houses, a practice that is still in wide use throughout the developing countries. However, it is most likely that many plant volatiles are deterrent or repellent because they have high vapour toxicity to insects. In Anopheles gambiae, the DEET receptor OR83b is stimulated by citronellal, but is also modulated by the TRPA1 cation channel. Interestingly, the same odour receptors that respond to DEET also respond to thujone eucalyptol and linalool in Culex quinquefasciatus. Plants commonly produce volatile “green leaf volatiles” when leaves are damaged in order to deter herbivores, and several authors have shown strong responses of mosquito odour receptors to this class of volatiles including geranyl acetate and citronellal, 6-methyl-5- hepten-2-one and geranylacetone. Insects detect odours when that volatile odour binds to odorant receptor (OR) proteins displayed on ciliated dendrites of specialized odour receptor neurons (ORNs) that are exposed to the external environment, often on the antennae and maxillary palps of the insect, and some ORNs, such as OR83b that is important in olfaction and blocked by the gold-standard synthetic repellent DEET (N, N-diethyl-3-methylbenzamide), are highly conserved across insect species. The fact that several of these compounds are repellent to haematophagous insects could be an evolutionary relict from a plant-feeding ancestor, as many of these compounds evolved as repellents to phytophagous insects, and this repellent response to potentially toxic compounds is well conserved in the lineage of Diptera (True Flies). Although the primary functions of these compounds is defence against phytophagous insects, many are also effective against mosquitoes and other biting Diptera, especially those volatile components released as a consequence of herbivory. Most can be grouped into five major chemical categories: (1) nitrogen compounds (primarily alkaloids), (2) terpenoids, (3) phenolics, (4) proteinase inhibitors, and (5) growth regulators. These chemicals fall into several categories, including repellents, feeding deterrents, toxins, and growth regulators. Most plants contain compounds that they use in preventing attack from phytophagous (plant eating) insects. This paper presents a summary of recent information on testing, efficacy and safety of plant-based repellents as well as promising new developments in the field. There is a need for further standardized studies in order to better evaluate repellent compounds and develop new products that offer high repellency as well as good consumer safety. To date insufficient studies have followed standard WHO Pesticide Evaluation Scheme guidelines for repellent testing. Recently, commercial repellent products containing plant-based ingredients have gained increasing popularity among consumers, as these are commonly perceived as “safe” in comparison to long-established synthetic repellents although this is sometimes a misconception. Knowledge on traditional repellent plants obtained through ethnobotanical studies is a valuable resource for the development of new natural products. Plant-based repellents have been used for generations in traditional practice as a personal protection measure against host-seeking mosquitoes.
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