Monoterpenes:
linalool, cuminaldehyde,
1,8-cineole, limonene, fenchone,
eugenol, γ-terpineol, cinnamic alcohol,
geraniol, β-citronellol,
-menthane-3,8 diol,
α-pinene, β-pinene, -cymene,
thymol, terpinolene, camphor,
citronellal, sabinene, carvacrol
Sesquiterpenes:
guaiol, α-bisabolol, α-cadinol,
germacrene D, β-caryophyllene,
nootkatone
Diterpenoids:
diterpene alcohol, phytol
Aromatic phenol
Coumarin
Essential oils have been efficiently used against a variety of pests and for crop protection in the world and they are potential alternatives to synthetic insecticides used against mosquitoes. Essential oils are very complex natural mixtures that consist of a variety of volatile molecules, which are hydrocarbons (terpenes and sesquiterpenes), oxygenated hydrocarbons and phenylpropenes ( Table 1 ). Essential oils are synthesized in the cytoplasm and plastids of plant cells through mevalonic acid and 2- C -methyl-erythritol 4-phosphate (MEP) pathways, respectively [ 113 ]. Essential oils target the insect nervous system and cause neurotoxic effects through several mechanisms by inhibiting the activity of AChE, and blocking octopamine receptors and GABA-gated chloride channels [ 114 , 115 ]. About 90% of essential oils are composed of monoterpenes, which are determined to be active ingredients for potential plant-based larvicides and cause inhibition of AChE activity in insects [ 116 ]. Monoterpenes, such as linalool, cuminaldehyde, 1,8-cineole, limonene and fenchone, cause inhibition of AChE and accumulation of acetylcholine in synapses and state of permanent stimulation, which results in ataxia [ 117 , 118 ]. According to Hideyukiu and Mitsuo [ 119 ], a mixture of monoterpenoids is a more potent inhibitor of AChE than single monoterpenoid application and acts synergistically.
The octopaminergic system of insects is another target for essential oils that block octopamine receptors and cause acute and sub-lethal behavioral effects on insects. The increase in cyclic AMP levels, induced upon binding of octopamine to octopamine-receptors, can be inhibited by a mixture of essential oils (eugenol, γ-terpineol and cinnamic alcohol). Moreover, octopamine receptor binding is significantly reduced with low doses of eugenol alone [ 120 , 121 ]. Another possible target for essential oils is ligand-gated chloride channels. Essential oils consist of monoterpenes, such as linalool, methyl eugenol, estragole, citronellal, inhibit GABA-gated chloride channels by binding at the receptor site and increase the chloride anion influx into the neurons, which lead to hyper-excitation of the central nervous system, convulsions, and finally death of insects [ 122 , 123 ].
Many plant oils possess ovicidal, larvicidal, pupaecidal and repellent activities against various mosquito species, some of which will be discussed below. Essential oils of plants from the Lamiaceae, Poaceae, Rutaceae and Myrtaceae families are well-known for repellent activity [ 103 ]. Essential oils obtained from citronella, lemon and eucalyptus are commercially available and recommended by the U.S. Environmental Protection Agency (US EPA) as repellent ingredients for application on the skin because of their low toxicity. For example, P -menthane-3,8 diol (PMD) is an active component of the lemon eucalyptus plant and responsible for the repellency in mosquitoes [ 124 ].
Most of the monoterpenes and sesquiterpenes of essential oils are known with repellent activities [ 125 ]. Among monoterpenes, α-pinene, γ-pinene, p -cymene, eugenol, limonene, thymol, terpinolene, citronellol, camphor and citronellal are responsible for mosquito repellency [ 126 , 127 ]. Representative molecules of sesquiterpenes are guaiol, α-bisabolol, α-cadinol, germacrene D, β-caryophyllene and nootkatone. β-caryophyllene is known to exhibit strong repellent activity against Aedes mosquitoes [ 126 ]. Repellent and larvicidal activities of monoterpenes from the essential oils of Thymus plant against Cx. pipiens pallens , Cx. quinquefasciatus , and Cx. pipiens biotype molestus have been determined [ 128 , 129 , 130 ]. Larvicidal activities of phenolic terpenes, such as thymol and carvacrol, of Satureja species were observed against Cx. pipiens biotype molestus [ 131 ]. Moreover, repellent and larvicidal activities of carvacrol were determined in the field trials against Ae. albopictus mosquitoes in Bologna (Italy) [ 132 ]. Cinnamomum osmophloeum and Carum copticum essential oils had larvicidal activity against Cx. quinquefasciatus and Cx. pipiens , respectively [ 107 , 133 ]. Toxicity of β-citronellol, geraniol and linalool from Pelargonium roseum essential oil was also detected in Cx. pipiens [ 134 ]. High larvicidal and pupaecidal activities of essential oils from Cinnamomum verum , Citrus aurantifolia , Cuminum cyminum , Syzygium aromaticum , Laurus nobilis , Lippia berlandieri and Pimpinella anisum were reported from Cx. quinquefasciatus [ 135 ]. Artemisia absinthium essential oils also showed toxic effects against larval populations of Aedes , Anopheles , and Culex mosquitoes [ 136 ]. Essential oils isolated from Tagetes lucida , Lippia alba , Lippia origanoides , Eucalyptus citriodora , Cymbopogon citratus , Cymbopogon flexuosus , Citrus sinensis , Swinglea glutinosa , and Cananga odorata plants showed larvicidal activities on Ae. aegypti larvae [ 137 ]. Oviposition deterrence and ovicidal activity of some of essential oils, peppermint oil, basil oil, rosemary oil, and citronella oil from Mentha piperita , Ocimum basilicum , Rosmarinus officinalis , Cymbopogon nardus and Apium graveolens were also reported in Ae. aegypti [ 138 ]. Manh et al. [ 139 ] also showed toxicity of essential oils from Eucalyptus and Cymbopogon aromatic plants to the larvae of Ae. aegypti . Essential oils also cause toxicity at different developmental stages and have repellent activities against adult Anopheles mosquitoes [ 140 ]. Essential oils extracted from Cymbopogon proximus , Lippia multiflora and Ocimum canum had larvicidal and ovicidal activities against An. gambiae and Ae. aegypti mosquitoes [ 141 ]. Besides monoterpenes and sesquiterpenes, phytol (a diterpene alcohol) and coumarin (an aromatic phenol) were both determined to be responsible for the biting deterrence effect in Ae. aegypti [ 142 ].
Repellent activity of essential oils is generally attributed to individual chemical compounds, but synergistic effects of plant metabolites have been observed when the effect of an active compound is enhanced by other major compounds or modulated by minor compounds. The efficacy of the major compounds is enhanced by minor compounds through different mechanisms, which may cause higher bioreactivity compared to isolated compounds of essential oils. The synergistic effect is also observed with mixture of oils. The synergistic action of the major compounds in essential oils results in higher repellent and larvicidal activity and toxicity to insects [ 140 , 143 , 144 , 145 ]. A combination of blends assayed on An. gambiae mosquitoes indicated that blends of oils showed higher repellency compared to the individual oil used [ 146 ]. It has been also reported that essential oils composed of a mixture of active components might reduce resistance in mosquito population by acting at different target sites or with a different mode of action [ 139 ].
Neem-based insecticides are extensively used for protection against various pests all over the world. Neem trees, Azadirachta indica , is a member of the Meliaceae family and are originated from India and distributed throughout all South- and Southeast-Asian countries, including Pakistan, Sri Lanka, Thailand, Malaysia, and Indonesia [ 147 ]. The main product of the neem is the oil extracted from the seeds and contains at least 100 active compounds, including azadirachtin, meliantriol, salannin, desacetyl salannin, nimbin, desacetyl nimbin, nimbidin and nimbolides [ 148 ]. Limonoids are the major active compound of the neem oil and act as an insect growth inhibitor. Azadirachtin is a triterpenoid and highly oxidized limonoid, one of the most potent active compounds of the neem extract and found in higher concentrations (0.2–0.6%) in the seeds of the neem compared to other parts of the neem tree [ 149 , 150 ]. Various isomers of azadirachtin (azadirachtin A to G) were identified and azadirachtin A and B isomers are the most abundant isomers in the plant tissues. In addition, azadirachtin A is the most active biological ingredient which shows insecticidal activity compared to the other analogs [ 151 , 152 , 153 ].
Generally, neem-based products are effective in the juvenile stages of insects. Azadirachtin is structurally similar to insect hormones known as ecdysones that are involved in the process of metamorphosis. The main mechanism of action of azadirachtin is to impair the homeostasis of insect hormones by interfering with the endocrine system. Azadirachtin acts as ecdysone blocker and causes severe growth and molting aberrations by affecting ecdysteroid and juvenile hormone titers [ 154 ]. The feeding deterrent activity of azadirachtin is mediated through azadirachtin’s interference with phagostimulants that are important in normal feeding behavior of mosquitos [ 155 ].
Neem-based biopesticides have a wide range of effects against insects, such as re-pellency, feeding deterrence, ovicidal activity, fecundity suppression, toxicity, insect growth regulation, deterrence of egg-laying, disruption of growth and reproduction, and inhibition of metamorphosis [ 156 , 157 , 158 , 159 , 160 ]. Larvicidal activity of the neem oil has been reported in controlling mosquito larvae in different breeding sites under natural field conditions [ 161 ]. Ayinde et al. [ 162 ] reported the repellent and larvicidal potential of the emulsified neem seed oil formulation as a suitable alternative for commercially available insecticides against An. gambiae in Nigeria. Oils of neem and karanj were also found to have larvicidal, ovicidal and oviposition deterrent activities against Ae. aegypti and Ae. albopictus mosquitoes [ 163 ]. The effects of the neem limonoids azadirachtin, salannin, deacetylgedunin, gedunin, 17-hydroxyazadiradione and deacetylnimbin were analyzed, and azadirachtin, salannin and deacetylgedunin showed the highest larvicidal activity against An. stephensi [ 164 ]. Larval mortality and repellent activity were also achieved from neem essential oils against An. gambiae [ 162 ]. A neem extract, neemarin, also showed significant mortality rates at larvae, pupae, and adult stages of Cx. quinquefasciatus and An. stephensi , where the former showed lower mortality rates [ 165 ].
Pyrethrum is a plant-based insecticide obtained from flower heads of Tanacetum cinerariifolium . Pyrethrum extract is composed of six active ingredients derived from esters of chrysanthemic acid: pyrethrin I, cinerin I, and jasmolin I, and esters of pyrethric acid: pyrethrin II, cinerin II, and jasmolin II [ 166 ]. They target the nervous system of insects and cause neurotoxic effects through blocking the voltage-gated sodium channels in nerve axons, thereby cause hyperactivity and convulsions by a rapid knockdown effect [ 167 ]. The mode of action of pyrethrins is similar to that of DDT and many synthetic organochlorine insecticides. Thus, pyrethrins can be alternatively used instead of organophosphates and organochlorides. While it is less toxic to mammals, it has higher toxicity to fish and aquatic invertebrates. When used together with a conventional synergist, such as piperonyl butoxide (PBO), their activity is increased and harmful effects to non-target organisms are reduced [ 168 ]. The usage of natural pyrethrins in mosquito control is supported with the finding that pyrethrum had knock-down effect, repellency, and blood-feeding inhibition in pyrethroid-resistant An. gambiae strains [ 169 ]. Electroantennogram responses of pyrethrum in Ae. aegypti and An. gambiae mosquitoes were detected while no response is observed in maxillary palps, indicating that the repellency effect of pyrethrum is mediated by the olfactory systems of mosquitoes [ 170 ]. Moreover, the molecular mechanism of pyrethrum repellency was investigated and a synergistic mechanism involving dual activation of olfactory repellency pathways and voltage-gated sodium channels has been determined [ 170 ].
Alkaloids are nitrogen-containing natural products found in bacteria, fungi, animals, and plants. They are commonly isolated from plants and found in large quantities in many members of the Berberidaceae, Fabaceae, Solanaceae, and Ranunculaceae families. The alkaloids obtained from these plants are used extensively in conventional insect repellents [ 171 , 172 , 173 ]. The mode of action of alkaloids varies depending on the type of alkaloids and interferes with major cellular and physiological functions by affecting AChE receptors in the nervous system, regulating hormonal activity, and causing toxicity [ 174 ]. Alkaloids are not volatile like essential oils. However, they could be used as repellents against mosquitoes by burning plants to generate an insecticidal smoke that repels insects and directly causes toxicity [ 124 ]. In Ae. aegypti , the inhibitory effect of natural alkaloids on AChE activity was determined by using molecular docking studies. Among the 25 different alkaloids tested, alpha-solanine has been found to fit into the AChE1 binding pocket and potentially be the best inhibitor of AChE1 [ 175 ].
Extracts of the castor bean ( Ricinus communis , Euphorbiaceae) contain the alkaloid ricinine and have a strong insecticidal effect. It showed strong larvicidal activity against larvae of An. arabiensis [ 176 ]. Additionally, pyridine alkaloid from R. communis showed bioactivity against An. gambiae larvae and adults [ 177 ]. The larvicidal activity of alkaloids against Ae. albopictus , Cx. pipiens pallens and Ae. aegypti has also been determined [ 178 , 179 ]. Alkaloid from Arachis hypogaea plant also had larvicidal toxicity against An. stephensi and Ae. aegypti mosquitoes [ 180 ].
Nicotine is an alkaloid derived from tobacco plant ( Nicotiana tobacco ) that mostly consists of phenolic compounds, such as nicotine and diterpene. Nicotine, nornicotine and anabasine mimic the neurotransmitter acetylcholine, which causes symptoms similar to organophosphate or carbamate insecticides [ 160 ]. Extracts of tobacco leaves were mixed with bio-oil and high repellent activity was observed against Ae. aegypti [ 181 ]. Furthermore, nicotine has been found to be the most dominant compound among the other active compounds of the repellent mixture, including nicotine, d-limonene, indole, and pyridine. In addition, the repellent compound was harmless to human skin as confirmed by sensitivity tests on volunteers.
Besides the most common plant-based bioinsecticides mentioned above, there are other natural plant metabolites that show insecticidal properties. Among them, flavonoids elicit larvicidal activity by inhibiting AChE in mosquito larvae [ 182 ]. They could also act as respiratory inhibitors and result in the disturbance of the larval respiratory system. Alkaloids have multiple effects including inhibition of the AChE enzyme, degradation of cell membranes, and they may act as stomach poisons [ 182 ]. It has been shown that flavonoid and alkaloid components of bangle rhizome extract from Zingiber montanum act differently against Ae. aegypti [ 183 ]. Flavonoids from Derris trifoliata extract also exhibited larvicidal activity against Ae. aegypti [ 184 ]. Rotenone is an isoflavonoid extracted from roots and stems of Derris ( Derris elliptica , Derris involute ), Lonchocarpus ( Lonchocarpus utilis , Lonchocarpus urucu ) and Tephrosia virginiana [ 160 ]. It has long been used as a biopesticide due to less harmful effects to the environment. Rotenone has the potential to be used as a larvicide to control mosquitoes and interferes with the cellular respiration system of insects and prevents energy production [ 185 ].
It is important that inherent activity of candidate bioinsecticides should be assessed before they can be effectively used against mosquito populations. The World Health Organization has established methods to screen the efficacy and field application acceptability of new compounds as potential mosquito larvicides and adulticides (for IRS and ITNs); they are laboratory studies, small-scale and large-scale field trials [ 186 , 187 , 188 ]. Laboratory studies focus on determination of biopotency, efficacy, residual activity, irritant or repellent properties, diagnostic concentration, and possible cross-resistance of candidate larvicides or adulticides. In laboratory bioassays, mosquito larvae are exposed to various concentrations of larvicides, and a mortality rate based on lethal concentration (LC) of the larvicide for 50% and 90% mortality (LC50 and LC90) or for 50% and 90% inhibition of adult emergence (IE50 and IE90) is recorded. LC values are determined and can then be compared with the LC50 or LC90 values of other insecticides to assess the activity of the compound as “sufficiently effective”. For adulticides, LC is determined by tarsal contact to treated papers. The “time to first take-off” (FT) for the 50% and 90% of the mosquitoes to take off (FT50 and FT90) after exposure to treated substrates are measured to determine the irritant or repellent activity of an adulticide. Insecticide-treated nets are used for bioassays of adult mosquitoes to determine the efficacy and residual activity of different dosages of the candidate compounds. Moreover, efficacy and wash-resistance of ITNs against susceptible mosquito species should be determined using standard WHO cone bioassays or tunnel tests [ 188 ]. The efficacy criteria for cone bioassays are ≥80% mortality or ≥95% knock-down, and for the tunnel test, it is ≥80% mortality or ≥90% blood-feeding inhibition. Candidate larvicides and adulticides are also tested against multi-resistant mosquito strains and a susceptible reference strain to assess the cross-resistance and, if detected, biochemical, immunological, and molecular methods are used to determine the mechanism of resistance [ 189 ].
Once candidate compounds are selected from laboratory tests, they are subjected to small-scale field testing in natural breeding sites (such as drains sewage water tanks, ponds, rice plots, etc.) or under simulated field conditions (artificial containers filled with water, experimental huts). Larvicidal efficacy is determined by the level of inhibition of emergence of adults and the percentage reduction in larval and pupal densities, while adulticidal efficacy can be assessed in terms of mortality, residual effect, deterrence, blood-feeding inhibition and induced exophily. These trials elucidate efficacy of candidate compounds against different mosquito species in different breeding sites, determine optimum field application dosage of the compound and possible impact on the mosquito behavior. Abiotic parameters that may influence the efficacy of the product and effect on non-target organisms can also be observed. Those larvicides and adulticides that show promise in small-scale field trials should be validated in larger-scale field trials against natural mosquito populations in natural breeding habitats using optimum field dosages. At this stage, the storage, handling, and application of the insecticide formulation should be considered for proper functioning of application and dispersal of the bioinsecticide in natural ecosystems.
There are also potential limitations to the efficacy of bioinsecticides, such as environmental conditions, mosquito fitness, mosquito resistance as well as the parts of the plants used, solvents used in extraction steps, insecticide dose and exposure time [ 190 , 191 ]. These effects should be considered for successful assessment of novel bioinsecticides in mosquito control. While efficacy tests provide promising information on possible mosquitocidal effects, new compounds from plant origin, the identification of actual active ingredient for efficacy and their mode of action are still waiting to be resolved.
Most of the bioinsecticides are now effective alternatives to chemical insecticides and have become an integral part of the integrated mosquito management (IMM) programs because the development of resistance to bioinsecticides is low due to their multiple mode of actions [ 192 , 193 ]. The synergic mixture of the active compounds in plant extracts also minimizes resistance development [ 167 ]. However, resistance already developed to extensively used chemical insecticides is a major problem that limits the success rate of novel bioinsecticides against mosquito populations. Insecticide resistance should be reduced or reverted (which takes time) in order to apply new and effective bioinsecticides in resistant populations. Surveillance of mosquito resistance and effective resistance management strategies should be routinely conducted to determine the levels, mechanisms, and geographic distribution of resistance in field populations of mosquitoes for increasing efficacy of bioinsecticides [ 44 ]. Moreover, proper application technologies should be considered as they greatly influence the bioinsecticide efficacy.
Surveillance of resistance development to many different insecticides are determined by dose-mortality bioassays, the World Health Organization tube testing, and Centers for Disease Control and Prevention (CDC) bottle bioassay for mosquitoes [ 11 , 44 , 194 , 195 ]. In the dose-mortality assay, the resistance ratio (RR) is determined in a susceptible population to monitor changes in resistance over time. The RR is calculated from LC50 values of the field and susceptible populations, in which an RR lower than five indicates susceptibility or low resistance and an RR value higher than ten indicates high resistance. In the WHO tube testing, the insecticide susceptibility status of the selected mosquitoes is evaluated through susceptibility tests measuring the mortality rate twenty-four hour after exposure [ 44 ]. A mortality rate lower than 98% indicates occurrence of resistance and should be confirmed with biochemical and molecular analysis. A mortality rate less than 90% confirms the existence of resistant genes in the tested mosquito populations. The CDC bottle bioassay is a measure of insecticide effectiveness, where diagnostic doses (DDs) and diagnostic times (DTs) are determined for candidate compounds using susceptible mosquitoes prior to testing in field mosquito populations. The DD is a measure of insecticide dose that kills 100% of susceptible mosquitoes within a certain period of time (DT). A mortality rate lower than 97% is an indication of resistance that needs to be confirmed, and below 80% suggests strong resistance at the recommended DT. The DD and DT values for some active ingredients are available for Anopheles and Aedes mosquito populations and these parameters should be defined for a particular insecticide and mosquito population [ 195 ].
It is evident that no single strategy is effective enough to solve insecticide resistance of mosquitoes. According to the WHO [ 44 ], one strategy to prevent the resistance problem is rotational usage of different classes of bioinsecticides with different modes of action. There are several new plant-based larvicides with different modes of action (discussed in Section 5 ) and they could be good alternatives for mosquito control in larval stages. Additionally, multiple interventions that affect different stages of mosquitoes (such as larvae and adults) can be used together to manage insecticide resistance. It is also suggested that different classes of insecticides with different modes of action can be used in neighboring geographic locations. To successfully implement these strategies, knowledge of the mode of action of the novel bioinsecticide is essential. The resistance mechanism developed by the local population of mosquitoes should also be determined to reduce cross-resistance effects.
RNA interference (RNAi) mediated loss-of-function technique has been proposed for pest management programs [ 196 , 197 ] and to study insecticide resistance [ 198 ]. Genes responsible for resistance development in insects (e.g., genes for DDT or pyrethroid resistance) can be identified and used as a target for the development of novel RNAi based insecticides. Several delivery methods including nonmicrobial and microbial are used routinely to induce RNAi in mosquito larvae [ 199 ]. Nonmicrobial delivery methods consist of soaking, injection, nanoparticles and dehydration and rehydration. Although soaking and injection methods are used as excellent tools in RNAi research, they have no application in the field. Soaking, injection methods and nanoparticles have been effectively used to introduce dsRNA into first-instar Ae. aegypti larvae [ 200 ] and fourth instars of Ae. aegypti [ 201 ]. In mosquitoes, a chitosan/dsRNA-based nanoparticle has also been used in feeding the larvae of An. gambiae mosquitoes which led to successful gene silencing of two chitin synthase genes and increased susceptibilities to DTT [ 202 ]. Such an RNAi-based bioinsecticide can be potentially used as an effective strategy to enhance the efficacy of new bioinsecticides for mosquito control.
Another technology used for the manipulation of insect behavior is “Specialized Pheromone and Lure Application Technology (SPLAT)”. SPLAT is a chemical controlled-release emulsion technology, and it has been used as an alternative management strategy to target the aquatic life stages of mosquitoes [ 203 ]. SPLAT emulsions can be formulated by using a variety of compounds, such as sex pheromones, attractants, repellents, phagostimulants and insecticides. SPLAT consists of both aqueous and non-aqueous components. The aqueous component of the SPLAT emulsion is involved in the liquid property of the product and evaporates within 3 h upon application. The non-aqueous component of the emulsion is the controlled-release device that releases active ingredients (e.g., semio-chemical or pesticides) at a controlled rate for 2 weeks to 6 months by protecting the active ingredients from environmental, chemical, and biological degradation. It has been reported that combination of attractant and larvicidal agents in a single formulation and biodegradable matrices causes significant increase in larval mosquito mortality, specifically Cx. quinquefasciatus , compared to formulations consisting of larvicidal agents alone in semi-field trials (e.g., large-screened greenhouse and emulating field conditions) [ 204 ]. The major benefits of this technology are a timely-manner release of both pheromone and insecticide, reduced insecticide resistance, and persistence in the field [ 203 ].
Synthetic chemicals used to control mosquitoes are now causing serious health problems and, more importantly, resistant mosquitoes that lead to search for more effective, healthier, safer, and eco-friendly natural solutions. Phytochemicals derived from plant resources are excellent targets to search for bioactive compounds because plants synthesize these chemicals naturally in response to their environment (such as against insect predators and microbial attacks), thus, plants are indeed natural insecticide sources. While searching the literature for plant-based compounds, we have encountered a tremendous number of efforts to identify and evaluate compounds that could have potential mosquitocidal activity with negative impact on mosquito physiology at different development stages. Since phytochemicals have multiple modes of action and exert their effects on multiple target sites in insects, their efficacy can be enhanced when used as a blend (e.g., mixture of oils) against mosquitoes. In addition, insects are more likely to develop resistance to a single chemical compound rather than a mixture of compounds. Thus, a combinatorial usage of phytochemicals would limit development of resistance in mosquitoes. Phytochemicals have short residual half-life which could be advantageous when synergistically used together with other biological control agents [ 205 ]. It is encouraging that these features of phytochemicals make them alternative natural solutions for the development of suitable products to interfere with the mosquito–host interaction and reduce disease transmission.
Among the phytochemicals, essential oils are extensively studied and their repellent activities against mosquitoes makes them favorable natural chemicals. However, they are volatile compounds, and this causes issues in their long-term applications in mosquito control. In recent years, new technologies, such as microencapsulation and nanoemulsion, have been used to overcome this problem by enhancing the duration and efficacy of essential oils [ 140 ]. Since ITNs are one of the major intervention methods to control mosquitoes, the incorporation of plant-based insect repellents in fabrics seems a prompt and alternative way to provide safer protection against mosquito bites. Fabrics treated with microencapsulated citronella essential oil have been reported to provide higher repellent activity and longer lasting protection, up to three weeks, against insects compared to the fabrics sprayed with ethanol solution of the essential oil [ 206 ]. Grancaric et al. [ 207 ] also reported that microencapsulated immortelle oil had the highest repellent efficacy against Ae. aegypti compared to immortelle oil alone on cotton samples. In another study, microcapsules composed of two biopesticides, namely citronella essential oil and citriodiol, were prepared and applied to cotton textiles using a variety of techniques. As a result, citriodiol-treated cotton fabrics had a prolonged durability, and 100% repellent activity for more than 30 days after its application [ 208 ]. Additionally, encapsulation of citronella oil into microcapsules of poly ε-caprolactone has been considered as an effective and sustained release system with potential application in protection against mosquitoes [ 209 ]. Encapsulated citronella oil nanoemulsions prepared by high pressure homogenization at varying amounts of surfactant and glycerol were tested for mosquito repellency. It has been shown that increasing concentration of glycerol and surfactant improved the stability of the emulsion causing prolonged mosquito protection [ 210 ]. These results clearly indicate that through microencapsulation and nanoemulsion formulation technologies, effective and longer usage of essential oils on cotton fabrics or ITNs can be achieved.
Neem-based insecticides can also be effectively used for the control of mosquitoes. They are considered more eco-friendly than synthetic insecticides and are less prone to induce resistance because of their multiple modes of action on insects. Another advantage of neem oil formulations is that it causes mortality at relatively low concentrations making them potential alternatives to synthetic insecticides in the control of malaria vectors. Microencapsulation of neem seed oil and karanja oil has been used for the control of larvae of Ae. aegypti [ 211 ]. The major drawback of using neem oil is that its dosage should be considered when applied in the field because neem can cause risks to non-target organisms at higher doses.
Natural pyrethrins are now considered as a potential alternative to synthetic DTT and can overcome hazardous effects of pyrethroids. However, they have major drawbacks which include their high instability and quick degradation in the presence of sunlight. Stability concerns and short duration of their knockdown effect cause inadequate field applications against mosquito populations during the day [ 212 ]. However, the application of pyrethrin-based insecticides after sunset against Culex and Anopheles has shown a decrease in mosquito populations and protection against non-target insects [ 213 ]. Pyrethrins are also more effective when used with a synergist. They can be easily degraded before having an impact on mosquitoes, thus should be applied with a synergist of non-synthetic origin. Since pyrethrin-based chemicals are detected via mosquito olfactory organs and processed through olfactory signal transduction mechanisms, pyrethrin-based repellent molecules should be developed and implemented in order to interfere with the host-seeking behavior of mosquitoes for an effective reduction in disease transmission.
Despite our increasing knowledge on plant-based bioactive compounds and their multiple mode of actions on insects, a few of them, such as essential oil-based and neem-based insecticides, have been commercially available for pest management [ 205 ]. One of the reasons that causes their limited usage in the field is the formulation problem to overcome phytotoxic effects. The chemical composition of each compound should be formulated in such a way that it should be bioactive to target insects and non-toxic to non-target organisms. In addition, formulation of plant-based bioinsecticides should ensure that it can be produced in large quantities through biomass production of plants and administered in recommended dosages to minimize toxic effects, and biological activity can be maintained for longer shelf-life. As discussed above, microencapsulation and nanoemulsion technologies have benefits in solving formulation issues of phytochemicals. A new formulation in the form of tablets containing a lectin preparation showed mosquitocidal activity against different developmental stages of Ae. aegypti mosquitoes, and this formulation method is proposed as a new control strategy for Ae. aegypti populations [ 214 ]. Phytochemicals break down rapidly and this causes a need for continuous and more frequent applications in the field for a satisfactory impact on mosquito control. Further studies are needed with the implementation of new methods for the development of effective bioinsecticides from other plant-based bioactive compounds.
Mosquitoes are important vectors of devastating diseases, and their hazardous effects are far beyond eradication. The occurrence/reoccurrence of mosquitoes in endemic, non-endemic, and new regions of the world has led to extensive use of synthetic chemicals to control transmission of mosquito-borne diseases. With the increase of resistant mosquitoes and toxicity issues to target and non-target organisms, safer, biodegradable, target-specific alternatives have been considered to replace conventional mosquito control strategies. Phytochemicals have gained importance to overcome mosquito control problems as being considered natural, environmentally safe, less toxic, inexpensive, and, more importantly, less prone to mosquito resistance. Variety of plant extracts have been reported to have mosquitocidal or repellent activity against mosquito vectors, mostly depending on laboratory assays, but there are limitations for their efficacy and applicability in the field. Problems associated with their formulation and commercialization, non-standardization in evaluation of their bioactivities, and their persistence for longer durations should be resolved for development of effective and sustainable methods for their usage. There is no doubt that there are more bioactive compounds that require exploring, and future research should focus on searching for plant-based products with the ultimate goal of deploying them as a reliable remedy to control mosquito populations and mosquito-borne diseases.
The authors thank the editorial board of Insects and the reviewers for comments and suggestions.
Conceptualization, M.Ş.Ş.D.; original draft preparation, M.Ş.Ş.D. and E.C.; review and editing, M.Ş.Ş.D. and E.C. All authors have read and agreed to the published version of the manuscript.
This research received no external funding.
Data availability statement, conflicts of interest.
The authors declare no conflict of interest.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
IMAGES
VIDEO
COMMENTS
Vector-borne diseases account for more than 17% of all infectious diseases, causing more than 700,000 deaths annually . More than 80% of the global population is at risk of vector-borne diseases, with mosquito-borne diseases (MBDs) being the largest contributor . MBDs such as dengue, Zika, chikungunya, West Nile virus (WNV), eastern equine ...
The issue will highlight mosquito control technologies at varying stages of development and includes both opinion pieces and research articles with laboratory and field-based data on control strategy development. This article is part of the theme issue 'Novel control strategies for mosquito-borne diseases'.
1. Introduction. Vector-borne diseases (VBDs) cause significant morbidity and mortality worldwide, accounting for as much as 17% of the global infectious disease burden (Organization, W.H, 2017a).Over one billion people are infected with VBDs annually and more than one million die from those infections each year (Organization, W.H, 2014).Of all the known VBDs, mosquito-borne infectious ...
The issue will highlight mosquito control technologies at varying stages of development and includes both opinion pieces and research articles with laboratory and field-based data on control strategy development. This article is part of the theme issue 'Novel control strategies for mosquito-borne diseases'. Keywords: mosquito; novel approach ...
Combating mosquito-borne diseases using genetic control ...
Mosquitoes act as vectors of pathogens that cause most life-threatening diseases, such as malaria, Dengue, Chikungunya, Yellow fever, Zika, West Nile, Lymphatic filariasis, etc. To reduce the transmission of these mosquito-borne diseases in humans, several chemical, biological, mechanical, and pharmaceutical methods of control are used. However, these different strategies are facing important ...
Knowledge Gaps in Mosquito Biology and the Need for Multiscale, Multidisciplinary Perspectives. Mosquito-borne diseases claim millions of human lives annually, and their prevention and eradication have been a major priority in the global public health agenda over the last two decades [1].In a bid to identify solutions for the health risks posed by mosquitoes, research across scales ranging ...
More than 80% of the global population is at risk of a vector-borne disease, with mosquito-borne diseases being the largest contributor to human vector-borne disease burden. Although many global processes, such as land-use and socioeconomic change, are thought to affect mosquito-borne disease dynamics, research to date has strongly focused on the role of climate change. Here, we show, through ...
This Research Topic, " Emerging Mosquito-Borne Diseases and Novel Biocontrol Strategies ", focuses on current and sound research addressing one or more of the abovementioned biocontrol strategies, related genomic surveillance, evolutionary genomics of mosquito species, and insecticide resistance. The Research Topic brings a collection of ...
Projecting the risk of mosquito-borne diseases in a warmer ...
Mosquito-borne diseases have become more common as previously geographically isolated diseases have spread globally. Chikungunya, dengue, Japanese encephalitis, malaria, West Nile, yellow fever, and Zika are a few of the common and emerging viral diseases spread by mosquitoes. A thorough patient history, physical, and knowledge of diagnostic ...
Author summary Mosquito control interventions are widely used to reduce mosquito-borne diseases, but it is unclear what combination of interventions are most effective in reducing human disease. Given the wide range of mosquito species and the diseases they transmit, different interventions strategies have been implemented across many regions globally, with varying degrees of success. This ...
Patterns of human movement and employment, water management, and buildings and local-area infrastructure are all parts of this complex urban system that contribute to mosquito-borne disease as an ...
Mosquito control interventions are widely used to reduce mosquito-borne diseases, but it is unclear what combination of interventions are most effective in reducing human disease. Given the wide range of mosquito species and the diseases they transmit, different interventions strategies have been implemented across many regions globally, with ...
The expansion of mosquito-borne diseases such as dengue, yellow fever, and chikungunya in the past 15 years has ignited the need for active surveillance of common and neglected mosquito-borne infectious diseases. The surveillance should be designed to detect diseases and to provide relevant field-based data for developing and implementing effective control measures to prevent outbreaks before ...
For more than two decades, extensive research has been conducted into the use of EO data as a tool to inform responses to mosquito-borne diseases (Hay et al., 1998a; Kalluri et al., 2007; Kotchi et al., 2019).Main objectives include identifying risk areas at various spatial scales (Rogers et al., 2002), identifying seasonality in risk in different locations (Hay et al., 1998b), and forecasting ...
Although. Mosquito-borne diseases are distributed by different mosquito types that cause malaria, dengue, chikungunya and Zika virus. Viruses and protozoan parasites mainly cause essential vector ...
Trends in mosquito species distribution modeling: insights for ...
2. Resurgence of Diseases Transmitted by Mosquitoes. The three main mosquito genera, Anopheles, Aedes, and Culex, transmit the causative agents of numerous important diseases to humans as well as animals [11,12,13,14].In this chapter, we briefly describe the resurgence of essential disease agents transmitted by mosquitoes and their impact on humans and animals.
Culicidae) and Mosquito-Borne Diseases in North Africa
Each year, diseases resulting from a mosquito bite cause at least 1 million deaths worldwide, according to estimates. Fortunately, not all types of mosquito-borne illnesses are endemic in the U.S ...
Author summary Dengue is a rapidly spreading mosquito-borne disease transmitted primarily by Aedes aegypti mosquitoes. As climate change leads to extremes in rainfall and temperature, the abundance and populations of these vectors will be affected, thus influencing transmission of dengue. Using satellite-derived climate data for Kenya, we classified months that experienced highly abnormal ...
Experts say that infection rates in Canada for mosquito-borne diseases are extremely low, but evolving environmental factors — including climate change — are changing the way that mosquitoes ...
This Research Topic, " Emerging Mosquito-Borne Diseases and Novel Biocontrol Strategies ", focuses on current and sound research addressing one or more of the abovementioned biocontrol strategies, related genomic surveillance, evolutionary genomics of mosquito species, and insecticide resistance. The Research Topic brings a collection of ...
The World Mosquito Program estimates that mosquito-borne diseases kill one million people and infect up to 700 million people annually. As the environment continues to deteriorate, climate change and altered weather patterns have lengthened mosquito season and expanded the geographical range of these infectious creatures. According to the CDC, the primary diseases spread to humans by mosquitos ...
There's an outbreak of the rare mosquito-borne disease Eastern Equine Encephalitis in the Northeast. ... How climate change is affecting the spread of mosquito-borne illnesses There's an outbreak ...
Plant-Based Bioinsecticides for Mosquito Control