Volume 172, Issue 3 pp. 195-214
MINI REVIEW
Free Access

Efficacy of plant products in controlling disease vector mosquitoes, a review

V. Edwin Hillary

V. Edwin Hillary

Department of Biosciences, Rajagiri College of Social Sciences, Cochin, Kerala, India

Contribution: Conceptualization (lead), Writing - original draft (lead), Writing - review & editing (equal)

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S. Antony Ceasar

Corresponding Author

S. Antony Ceasar

Department of Biosciences, Rajagiri College of Social Sciences, Cochin, Kerala, India

Correspondence

S. Antony Ceasar, Department of Biosciences, Rajagiri College of Social Sciences, Cochin – 683 104, Kerala, India.

Email: [email protected]

Contribution: Conceptualization (equal), Visualization (lead), Writing - original draft (equal), Writing - review & editing (equal)

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S. Ignacimuthu

S. Ignacimuthu

Xavier Research Foundation, St. Xavier's College, Palayamkottai, Tirunelveli, Tamil Nadu, India

The Anna and Donald Endowed Waite Chair, Creighton University, Omaha, NE, USA

Contribution: Conceptualization (supporting), Writing - original draft (supporting), Writing - review & editing (supporting)

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First published: 05 January 2024
Citations: 1

Abstract

Mosquitoes pose a severe threat to the environment as vectors of numerous harmful diseases affecting humans and animals. Mosquitoes transmit pathogens that cause dengue, chikungunya, malaria, zika, yellow fever, Japanese encephalitis, and filariasis. Today's mosquito control strategies heavily rely on the use of chemical insecticides such as N,N-diethyl-meta-toluamide (DEET), N,N-diethyl mandelic acid amide (DEM), and dimethyl phthalate (DMP). However, the widespread use of chemical insecticides has resulted in pollution, bio-magnification, and other health and environmental issues. It has also become ineffective because of the mosquitoes' aptitude to develop resistance, emphasizing the urgent need for safe, effective, and long-lasting strategies. An alternative and promising approach to circumventing these obstacles involves the implementation of insecticides made from natural compounds found in plants. Therefore, the scientific community has shifted its focus towards plant-based phytochemicals, oils, and extracts, as these are eco-friendly, safe, and cost-effective alternatives to conventional chemical insecticides. This review aims to provide details on current advances in plant-based products (plant compounds, extracts, and essential oils), which are used to control all the life cycle stages (egg, larva, pupa, and adult) of the mosquito genera Aedes, Anopheles, and Culex. Hopefully, this review will pave the way to devise control strategies against these challenging pests.

INTRODUCTION

Mosquitoes (Diptera: Culicidae) are major arthropod groups that existed over 180–220 million years ago (Gabrieli et al., 2014; Bird & McElroy, 2016; Benelli & Duggan, 2018; Hillary & Ceasar, 2021). Mosquitoes belong to two subfamilies (Gabrieli et al., 2014): Anophelinae (Anopheles) and Culicinae (Aedes, Culex, and Mansonia). These two subfamilies pose a severe threat to humans and animals because of their widespread occurrence. They transmit diseases like dengue, chikungunya, zika, yellow fever, malaria, Japanese encephalitis, and filariasis (Gabrieli et al., 2014; Bird & McElroy, 2016; Benelli & Duggan, 2018; Hillary & Ceasar, 2021). These mosquito-borne diseases endanger people in tropical and subtropical areas, confirming that half of the world's population is at higher risk (WHO, 2017, 2022a). The disease-transmitting female mosquitoes are attracted to host animals through moisture, lactic acid, carbon dioxide, and body heat. They depend on proteins and iron found in the blood to mature their eggs (Glenn et al., 2010). They suck blood using piercing mouthparts (proboscis) comprised of six different stylets (Glenn et al., 2010). The first two stylets act like teeth, and the second pair functions as clamp. The last two stylets inject chemicals into blood vessels to stimulate blood flow, which causes itching and bumps, while the other stylets suck the blood from the body (Kosova, 2003). Pathogen transmission occurs during this process (Kosova, 2003; Hillary et al., 2020).

Every year, the World Health Organization (WHO) reports nearly a billion cases and millions of deaths caused by mosquito-borne diseases (WHO, 2017, 2022a). For example, malaria alone killed >627 000 people in 2020 (WHO, 2022b). Dengue cases also increased eightfold in the last 2 decades (505 430 cases in 2000, >2.4 million in 2010, and 5.2 million in 2019) (WHO, 2017, 2022c). More than 60 countries were prone to chikungunya, which causes severe arthralgia and kills 95% of infected people (the first outbreak of chikungunya started in Kenya in 2004 and moved to India; subsequently, there were reports of local transmissions in Southeast Asia, France, and Italy). Recently, zika virus cases have also significantly increased, and an outbreak of the virus was detected in India in 2021. Climate change has become a severe issue with the rise of these mosquito-borne diseases (Reiter, 2001; Mordecai et al., 2020), due to various factors such as: (1) higher temperatures increase the geographic spread of mosquitoes by helping them to survive and breed (increased rainfall increases the amount of standing water, and droughts support breeding by forming stagnant pools), (2) warmer climates alter the season of disease transmission, and (3) temperature changes the behaviour of mosquitoes, including biting behaviour (WHO, 2022a).

Recent research from the London School of Hygiene and Tropical Medicine also found that malaria and dengue could affect about 8 million people worldwide in 2080. In brief, they found that rising global temperatures may contribute to increased dengue and malaria cases (Colón-González et al., 2021). These results were based on the projected population growth of roughly 4.5 billion and a rise in temperature of 3.7 °C by 2100 (Colón-González et al., 2021). Another study also revealed that global changes had increased the risk of mosquito-borne diseases (Baker et al., 2022). However, the history of mosquito-borne diseases such as malaria, yellow fever, and dengue shows that human activity and their effects on local ecology have typically been much more significant than the climate in determining their incidence or spread. It is thus inappropriate to estimate future prevalence using climate-based models alone (Reiter, 2001, 2008). The search for effective vaccines against these vector-borne diseases is still in progress (Mordecai et al., 2020). Therefore, we need to find safe and eco-friendly methods to reduce the burden of mosquito-borne diseases.

In order to lower the danger of mosquito-borne diseases, several mosquito life stages, such as eggs, larvae, and pupae, are typically targeted using various chemical insecticides such as carbamates, pyrethroids, organophosphates, N,N-diethyl-meta-toluamide (DEET), dimethyl phthalate (DMP), and N,N-diethyl mandelic acid amide (DEM) (Benelli, 2015; Ranson & Lissenden, 2016). But use of these chemical insecticides has caused harm on human health, such as vomiting, tremors, shakiness, and seizures (Ranson & Lissenden, 2016). In addition, it has increased mosquito populations globally (WHO, 2018, 2020; Richards et al., 2020). Due to these concerns and problems, researchers diverted their focus on finding novel and ecofriendly alternative mosquito control strategies (Benelli, 2015).

Many plants with bioactive properties have been recognized as important components in traditional medicine and food for humans. These naturally occurring medicinal plants contain various active ingredients (phytochemicals, extracts, and oils), potentially interfering in mosquitoes' life stages (egg, larva, pupa, and adult). In addition, researchers prefer plant-based products against vector mosquitoes as an alternative to harmful chemical insecticides. Several articles have been published on the insecticidal properties of botanicals against vector mosquitoes (Pålsson & Jaenson, 1999; Ghosh et al., 2012; Kalita et al., 2013; Lupi et al., 2013; Rehman et al., 2014; Shaalan & Canyon, 2015; Tehri & Singh, 2015; Naseem et al., 2016; Remia et al., 2017; Hikal et al., 2017; Bekele, 2018; Gharsan, 2019; Noronha et al., 2020; Ganesan et al., 2023). Sukumar et al. (1991) listed and discussed 344 botanical agents with potential mosquitocidal activities. A recent review by Senthil-Nathan et al. (2020) discusses the details of botanicals used for larvicidal activities. The current article is the first comprehensive review focusing on plant-based extracts, phytochemicals, secondary metabolites, and essential oils with their modes of action for controlling Aedes, Anopheles, and Culex species. This review will enable researchers to learn about plant-based products and the potential for improving state-of-the-art mosquito control strategies.

AEDES, ANOPHELES, AND CULEX MOSQUITOES

Aedes species spread significant infectious diseases, notably chikungunya, dengue, and zika. Over half of the world's population has been affected by deadly diseases transmitted by Aedes species (Hales et al., 2002; Yang et al., 2009). Aedes species can be found throughout the world, such as Aedes aegypti L. (primary vector), Aedes albopictus (Skuse) (secondary vector), Aedes mediovittatus (Coquillett) (Caribbean), Aedes polynesiensis Marks (western Pacific region), and Aedes scutellaris (Walker) (western Pacific region) (Honório et al., 2003; Harrington et al., 2005). These mosquitoes are day biters and live in dark places at the corners of houses, under curtains, clothes, umbrellas, etc. They breed in any rainwater or freshwater stored in artificial or natural containers. Eggs from Aedes sp. can stay alive for up to 2 years without water.

The genus Anopheles comprises the second most universally known mosquitoes, involved in transmitting malarial infection to humans. In India, more than 59 Anopheles species are distinguished, and among them, six species – Anopheles stephensi Liston, Anopheles gambiae Giles, Anopheles baimaii Sallum & Peyton, Anopheles culicifacies Giles, Anopheles fluviatilis James, and Anopheles coluzzii Coetzee & Wilkerson – are identified as vectors transmitting malarial disease globally. They live for a few weeks to months and produce thousands of eggs.

Culex is another important genus of mosquitoes found throughout the world. The Culex mosquitoes transmit filariasis, Japanese encephalitis, and avian West Nile fever. They also carry the nematode Wuchereria bancrofti Cobbold, which causes lymphatic filariasis (Holder et al., 1999). They live for a few weeks or months and lay thousands of eggs. Overall, Aedes, Anopheles, and Culex mosquitoes transmit diseases that cause 700 000 deaths annually (WHO, 2017). The burden of these diseases has also considerably increased over the last few years (Ferguson, 2018; Wilson et al., 2020). For example, dengue, chikungunya, zika, malaria, and yellow fever have increased more than 30-fold over the past 50 years. The fast spread of these vector-borne diseases can be explained by several complex factors, such as population growth, globalization, a lack of adequate vector control methods, poor access to high-quality health care, and climate change. Therefore, eco-friendly alternative control measures for managing mosquitoes are required to combat vector-borne diseases.

SYNTHETIC INSECTICIDES USED IN MOSQUITO MANAGEMENT

Many synthetic insecticides have been formulated against vector mosquitoes to reduce vector-borne diseases. But continued use of these synthetic insecticides has caused high toxicity to human skin and the nervous system (Landrigan, 2001; Maurya et al., 2007). Another major drawback with these insecticides is the increased incidence of resistance to these chemicals in mosquitoes, which has added additional concerns for humans. Apart from affecting humans, these chemicals also affect non-target organisms. For example, fish species like Colossoma macropomum (Cuvier), Hyphessobrycon erythrostigma (Fowler), Paracheirodon axelrodi (Schultz), Nannostomus unifasciatus Steindachner, and Otocinclus affinis (Steindachner) exposed to malathion insecticide showed high sensitivity with LC50 values of 111–1507 p.p.m. Nannostomus unifasciatus was the most sensitive (LC50 = 111 p.p.m.; Rico et al., 2011). These five fish species, when exposed to the benzimidazole fungicide carbendazim, also showed moderate toxic action (Rico et al., 2011). These results confirmed that synthetic insecticides are toxic to non-target organisms. In addition, several studies reveal that synthetic insecticides kill important beneficial organisms like birds, honeybees, ladybugs, ground beetles, spiders, hoverflies, lacewings, etc. Therefore, we need natural insecticides without side effects.

NATURAL METHODS FOR MOSQUITO CONTROL

Vector control strategies to prevent mosquito bites and to create a mosquito-free environment have been practiced since ancient times. In the olden days, people used smoke by burning cattle and goat dung to escape from mosquito bites (Kihampa, 2011). Later, they added herbs and the bark of trees to enhance the smoke effect to keep away mosquitoes successfully (Kihampa, 2011). These preliminary works encouraged researchers to investigate plant-based products to control vector mosquitoes (Sukumar et al., 1991). Several reports are available on the application of plant-based products for killing larvae and adult mosquitoes to protect humans from mosquito bites (Sukumar et al., 1991; Pålsson & Jaenson, 1999; Ghosh et al., 2012; Kalita et al., 2013; Lupi et al., 2013; Rehman et al., 2014; Shaalan & Canyon, 2015; Tehri & Singh, 2015; Remia et al., 2017; Hikal et al., 2017; Bekele, 2018; Bukar & Tukur, 2019; Gharsan, 2019; Senthil-Nathan, 2020; Haria, 2021) (Figure 1). These are discussed in detail below.

Details are in the caption following the image
An overview of plant-based products used against vector mosquitoes to control vector-borne diseases. (A) Vector mosquitoes such as Aedes aegypti, Anopheles stephensi, and Culex quinquefasciatus transmit deadly diseases to humans. (B) Integrated mosquito management (IMM), with the use of whole plants or specific parts of the plants such as flowers, leaves, stem, and roots with various formulations like isolating phytochemicals, plant extract, and oils for larvicidal, ovicidal, pupicidal, and adulticidal control of vector mosquitoes to protect humans from infectious diseases.

PLANT SECONDARY METABOLITES

Plants are well known to produce secondary metabolites, which help the plants from insect pests or diseases (de Souza Wuillda et al., 2019). The secondary metabolites may be divided into five groups (Figure 2): (1) alkaloids, (2) phenolic acids, (3) terpenoids, (4) carbohydrates, and (5) lipids (Bilal & Hassan, 2012). Plant secondary metabolites are vital as anti-nutritional components of food and animal feed. Other secondary metabolites, such as terpenoids, are involved in plant–plant communication, serve as attractants for pollinators, and protect the plants from insect attacks. These overall characteristics reveal that natural insecticides based on plant secondary metabolites may play a more pivotal role in vector control strategies than the existing control strategies (Khan et al., 2018). Therefore, researchers started utilizing plant secondary metabolites to control mosquitoes (de Souza Wuillda et al., 2019). So far, >2000 plant species are known to produce chemical factors and metabolites of value in mosquito control programs (Shaalan et al., 2005). Among the plant-based bioactive compounds, secondary metabolites showed good biological activity against insect pests, including vector mosquitoes (Bilal & Hassan, 2012). For instance, plant secondary metabolites, such as ocimenone, rotenone, capillin, quassin, and thymol, showed good ovicidal, larvicidal, and pupicidal activities against mosquitoes (Shaalan et al., 2005). Specifically, acetogenins from the Annonaceae family have drawn much attention due to their biological activities against mosquitoes (Domínguez-Martínez et al., 2003; Bobadilla et al., 2005). The secondary metabolites isolated from various plant extracts also affected mosquito nerve axons, synapsis, respiratory organs, and hormones (Varma & Dubey, 1999).

Details are in the caption following the image
Classification of plant chemicals. The plant chemicals are mostly divided into five classes such as carbohydrates, lipids, phenolic acids, terpenoids, and alkaloids based on their characterization and chemical structure.

However, only a few studies have been carried out on the utilization of secondary metabolites against vector mosquitoes, which required further investigations (Bilal & Hassan, 2012; Hazrat & Soaib, 2012; De Marinis, 2013; Giorgi et al., 2013; de Souza Wuillda et al., 2019). Hence, developing novel secondary metabolites from plants against vector mosquitoes would provide safe and harmless control measures in the future. In addition, there is also a need to establish an easy method to extract plant secondary metabolites that can be used at the home level.

Mode of action of secondary metabolites

Generally, the active ingredients of botanical extracts are secondary metabolites that interact with enzymes, receptors, signalling molecules, structural proteins, ion channels, nucleic acids, and other cellular components of target vector mosquitoes (Rattan, 2010). The secondary metabolites exhibited severe physiological disruptions, such as disruption of sodium and potassium ion exchange (caused by pyrethrin), inhibition of the acetylcholinesterase (AChE) enzyme, inhibition of cellular respiration (caused by rotenone), inhibition of chloride channels (caused by thymol), blockage of calcium channels (ryanodine), inhibition of octopamine receptors (thymol), and disruption of hormonal balance (Rattan, 2010). In these, the key enzyme, AChE (which enhances resistance in mosquitoes to digest organophosphorus and carbamate insecticides), is responsible for terminating nerve impulse transmission through the insect synaptic pathway. Hence, developing novel secondary metabolites to inhibit AChE and other key enzymes of vector mosquitoes may help control vector populations effectively.

PLANT EXTRACTS

The efficacy of plant extracts against vector mosquitoes may vary depending on the species selected, the solvents used for isolation, and their potential. Several studies have documented the effectiveness of plant extracts in controlling vector mosquitoes (Table S1).

Larvicidal activities of plant extracts

Natural extracts derived from plants become substitutes for synthetic insecticides to evaluate mosquito larvicidal activities (Komalamisra et al., 2005; Pavela et al., 2019; Falkowski et al., 2020). Aivazi & Vijayan (2009) assessed the larvicidal activity of Quercus infectoria Oliv. (Fagaceae) gall extracts against An. stephensi under laboratory conditions. Of the gallotannin, acetone, n-butanol, methanol, and ethanol acetate extracts tested, the ethyl acetate extract was found to be highly toxic (LC50 = 116.92 p.p.m.). Further, they used an ethyl acetate extract of Q. infectoria to testing the larvicidal activity against Anopheles subpictus Grassi and Culex tritaeniorhynchus Giles and observed the highest activities with LC50 = 67.24 and 88.50 p.p.m., respectively. From these results, they declared that Q. infectoria would effectively kill mosquitoes and control vector populations (Aivazi & Vijayan, 2009). Different plant parts of Ipomocea cairica (L.) Sweet were assessed for larvicidal efficacy against Aedes species. In those, acetone extract of I. cairica leaf was found to be the most potent against Ae. aegypti and Ae. albopictus (both LC50 = 450 p.p.m.; AhbiRami et al., 2014). A study on Glyosmis pentaphylla (Retz.) DC. leaf extract exhibited the highest larvicidal activities against three major vectors: Ae. aegypti, An. stephensi, and Culex quinquefasciatus (Say). The leaf extracts had larvicidal activity with LC50 values of 266.9, 0.4, and 58.5 p.p.m. against Ae. aegypti, An. stephensi, and Cx. quinquefasciatus, respectively (Ramkumar et al., 2016). The petroleum ether extract of Lantana camera L. and Hyptis suaveolens (L.) Kuntze was assessed against Ae. aegypti and Cx. quinquefasciatus larvae; it showed maximum larvicidal activity values of LC50 = 10.63 and 38.39 p.p.m. against Ae. aegypti and Cx. quinquefasciatus, respectively (Hari & Mathew, 2018). The acetone extracts of L. camara, Rhazyz stricta Decne., Acalypha fruticose Forssk., and Ruta chalepensis L. exhibited good larvicidal activities against Culex pipens L. larvae. Among these, the acetone extract of L. camara showed the highest mortality against Cx. pipens (LC50 = 168 p.p.m.), followed by those of R. stricta, A. fruticose, and R. chalepensis (LC50 = 248.7, 374.4, and 539.3, respectively; Al-Solami, 2021). These studies indicate that extracts from various medicinal plants have good larvicidal activities against Aedes, Anopheles, and Culex mosquitoes. Several other plant extracts have shown significant mosquito larvicidal activity (Table S1).

Pupicidal activities of plant extracts

Extracts from many plants have been tested for pupicidal activities against vector mosquitoes to combat infectious diseases (Table S1). The pupicidal activity investigated against An. stephensi using Gliricidia sepium (Jacq.) (Leguminosae) extract under laboratory conditions, recorded the highest mortality with an LC50 of 58.10 p.p.m. (Krishnappa et al., 2012). In another study, whole-plant extracts of Leucas aspera Willd. exhibited the highest mortality against pupae of An. stephensi, with an LC50 of 12.73 and an LC90 of 18.78 p.p.m. Extracts of L. aspera seem to potentially control the malarial vector An. stephensi, and may also act as a potentially safe insecticide against Ae. aegypti and Cx. quinquefasciatus (Kovendan et al., 2012). The petroleum ether, acetone, ethyl acetate, water, methanol, and ethanol extracts of Eichhornia crassipes (Mart.) Solms were tested for pupicidal activity against Cx. quinquefasciatus. Of all the extracts, ethanol extract had the highest mortality rate against Cx. quinquefasciatus, with an LC50 of 173.35 p.p.m. (Jayanthi et al., 2012). The seed kernel extract of soapnut (Sapindus emarginatus L.) showed good pupicidal activities against Ae. aegypti, An. stephensi, and Cx. quinquefasciatus. It exhibited 100% pupicidal mortality with an LC50 of 4.13 p.p.m. against Cx. quinquefasciatus, an LC50 of 3.64 p.p.m. against An. stephensi, and an LC50 of 3.52 p.p.m. against Ae. aegypti (Koodalingam et al., 2009). The hexane crude extracts of Solanum xanthocarpum L. exhibited the highest pupicidal activity with an LC50 of 448.41 p.p.m. and an LC90 of 1141.65 p.p.m. against Cx. quinquefasciatus. The hexane extract of Artemisia nilagirica L. revealed the highest pupicidal activity, with an LC50 of 542.11 p.p.m. and an LC90 of 991.29 p.p.m. against Cx. quinquefasciatus, and LC50 of 477.23 p.p.m. and an LC90 of 959.30 p.p.m. against Ae. aegypti (Panneerselvam et al., 2012). Hence, an hexane extract of A. nilagirica could be effective against Ae. aegypti and Cx. quinquefasciatus (Panneerselvam et al., 2012). These comprehensive studies indicate that extracts from medicinal plants have potent pupicidal activities against Ae. aegypti, An. stephensi, and Cx. quinquefasciatus. However, further studies are still needed to achieve a 100% mortality against vector mosquitoes to combat vector-borne diseases.

Ovicidal activities of plant extracts

Plant extracts have also been tested for ovicidal activities against vector mosquitoes (Benelli, 2015). For example, medicinal plants such as Andrographis paniculata (Burm.f.) Nees, Cassia accidentalis (L.) Link, and Euphrobia hirta L. have been utilized for ovicidal activities against An. stephensi mosquitoes. Using A. paniculata, C. accidentalis, and E. hirta plants, the authors isolated extracts of hexane, ethyl acetate, benzene, water, and methanol and performed ovicidal activity against An. stephensi. After 48 h of treatment, the methanolic extract of A. paniculata showed 100% ovicidal activity at 150 p.p.m., and the methanolic extracts of C. accidentalis and E. hirta revealed 100% ovicidal activity at 300 p.p.m. against An. stephensi (Panneerselvam & Murugan, 2013). The ovicidal activity against An. stephensi was performed with crude hexane, ethyl acetate, benzene, chloroform, and methanol extracts from the leaves of Eclipta alba (L.) L., Cardiospermum halicacabum L., and A. paniculata. After 48 h of treatment, 100% mortality was observed in the methanol and ethyl acetate extracts of A. paniculata and the methanol extract of E. alba at 200 p.p.m., whereas the benzene extract of C. halicacabum showed 100% mortality at 150 p.p.m. (Govindarajan, 2011). Elango et al. (2009) tested acetone, ethyl acetate, and methanol extracts from Aegle marmelos (L.) Corrêa, Andrographis lineata Nees, and Cocculus hirsutus (L.) W.Theob. against An. subpictus. The acetone, ethyl acetate, and methanol extracts gave the highest mortality of 88.3% (Ae. marmelos), 92.8% (An. lineata), and 94.0% (C. hirsutus), and the lowest mortality of 64.9% (Ae. marmelos), 71.1% (An. lineata), and 66.4% (C. hirsutus) against An. subpictus. Surprisingly, this is the first report on ovicidal activities against malarial vectors from southern India (Elango et al., 2009).

In another study, acetone, ethyl acetate, and methanol extracts of A. paniculata, Eclipta prostarta (L.) L., and Tagetes erecta L. were assessed for ovicidal activities against Ae. aegypti eggs. They recorded 100% mortality in ethyl acetate and methanol extracts (Baluselvakumar et al., 2012). Similarly, acetone, benzene, hexane, ethyl acetate, and methanol extracts of Melothria maderaspatana L. recorded 100% mortality against Ae. aegypti eggs at 240–120 p.p.m. (Baluselvakumar et al., 2012). Crude hexane, benzene, ethyl acetate, acetone, and methanol extracts of Acalypha alnifolia Klein ex Willd. were tested for ovicidal activities against Ae. aegypti, An. stephensi, and Cx. quinquefasciatus – the methanolic extract of A. alnifolia showed 100% mortality at 200 p.p.m. against all three mosquito species (Kovendan et al., 2013). Several other studies using various plants have also been checked for ovicidal activities to combat vector mosquitoes (Table S1). Intervention at the egg stage is generally the most effective and inexpensive way to control vector mosquitoes. However, these interventions may have some limitations against Ae. aegypti mosquitoes as they breed in different water sources.

Adulticidal activities of plant extracts

Plant extracts were also examined for adulticidal activities against vector mosquitoes (Table S1). Kamaraj et al. (2010) tested the adulticidal activity of eight plants – Aristolochia indica L., Senna angustifolia Mill., Diospyros melanoxylon Roxb., Dolichos biflorus L., Gymnema sylvestre R.Br., Justicia procumbens L., Mimosa pudica L., and Zingiber zerumbet (L.) Roscoe ex Sm. – in hexane, ethyl acetate, and methanol extracts against Culex gelidus Theobald and Cx. quinquefasciatus. The ethyl acetate extract of all eight plants exhibited the highest adulticidal activities against both mosquitoes (Kamaraj et al., 2010). The ethanol extracts of Eucalyptus globulus Labill., Cymbopogan citratus (DC.) Stapf, Artemisia gmelinii Weber ex Stechm, Justicia gendarussa Burm. fil., Myristica fragrans Houtt, Annona squamosal L., and Centella asiatica (L.) Urb. showed 100% adulticidal activity against An. stephensi (Senthilkumar et al., 2009). The highest adulticidal activity of Piper sarmentosum Roxb. with an LC50 of 0.14 p.p.m. against female Ae. aegypti was noted (Choochote et al., 2006). The ethanol extract of Camellia sinensis (L.) Kuntze showed adulticidal activity with LC50 = 272.1 and LC90 = 457.14 p.p.m. against An. stephensi, LC50 = 289.62 and LC90 = 457.14 p.p.m. against Ae. aegypti, and LC50 = 320.38 and LC90 = 524.57 p.p.m. against Cx. quinquefasciatus. In another study, methanol extract of A. alnifolia leaf showed 100% adulticidal activity (LC50 = 125 p.p.m.) against Ae. aegypti, An. stephensi, and Cx. quinquefasciatus (Kovendan et al., 2013).

Overall, plant extracts are promising for controlling vector mosquitoes at different life stages. However, prior to their use, these plant extracts must be verified for any acute or chronic toxicity against non-target organisms and humans (Ganesan et al., 2023).

Mode of action of plant extracts

The plant extracts generally target cellular mechanisms and potentially disturb the cellular functions of vector mosquitoes (Ganesan et al., 2023). The extracts from Piper nigrum L. exhibited larvicidal activity by reducing the enzyme levels of α- and β-carboxylesterase in Ae. aegypti (Lija-Escaline et al., 2015). The extracts of Alangium salvifolium (L.f.) Wangerin also reduced the levels of α- and β-carboxylesterase and superoxide dismutase enzymes in Ae. aegypti larvae (Thanigaivel et al., 2017). Water extract of Trigonella foenum-graecum L. (fenugreek) exhibited a larvicidal effect against Cx. quinquefasciatus by affecting various body tissues, including the midgut and nervous systems (Fallatah, 2010). Aristolochic acids, isolated from A. indica A. Juss., disturbed the epithelium layer (EL), muscles, and cells of An. stephensi larvae (Pradeepa et al., 2015). Similarly, the methanolic extract of A. catechu (L.f.) Willd. disturbed the midgut of Ae. aegypti larvae (Costa et al., 2012). Therefore, identifying novel plant extracts that disrupt cellular pathways is the best option for controlling mosquitoes.

PLANT COMPOUNDS

Mosquito control activities of plant compounds may vary due to numerous factors, such as plant species, plant parts, plant compounds, and the targeted vector mosquito species. Plant compounds from numerous species have exhibited ovicidal, larvicidal, pupicidal, and adulticidal activities against major vector mosquito species (Table S2). These plant compounds are recognized as carrying potential biocontrol properties and alternatives to synthetic chemical insecticides (Tripathi et al., 2009; Nyasembe & Torto, 2014; Tehri & Singh, 2015; Muema et al., 2017).

Plant compounds such as retrofractamide A, pipercide, guineensine, pellitorine, and piperine from Piper nigrum L. fruits showed good activity against Ae. aegypti larvae with LC50 of 0.039, 0.1, 0.89, 0.92, and 5.1 p.p.m., respectively. Spipnoohine and pipyahyine obtained from the petroleum ether extract of P. nigrum L. showed toxicity at 35.0 and 30.0 p.p.m. against Ae. aegypti larvae (Park et al., 2002). Nepseudin isolated from the roots of Cordia curassavica (Jacq.) Roem. & Schult., 4-methoxyneoduline isolated from Neorautanenia mitis (A.Rich.) Verdc., lapachol isolated from Cybistax antisyphilitica (Mart.) Mart., and ponscirin, naringin, and marmesin isolated from Poncirus trifoliata var. monstrosa (T.Itô) Swingle (Rajkumar & Jebanesan, 2008) revealed good larvicidal activity against Ae. aegypti, An. stephensi, and Cx. quinquefasciatus. 5-Methoxy psoralen and 8-methoxy psoralen isolated from Ficus carica L. showed LC50 of 9.4 and 56.3 p.p.m. and LC90 of 3.58 and 26.27 p.p.m. against Ae. aegypti larvae (Chung et al., 2011). Methyl-p-hydroxybenzoate derived from the methanol extract of Vitex trifolia L. showed robust larvicidal activity with LC50 values of 5.7 and 4.7 p.p.m. against Ae. aegypti and Cx. quinquefasciatus, respectively (Kannathasan et al., 2011). With their unique modes of action, some of these compounds may serve as scaffolds to develop safe insecticides shortly. Several other plant compounds have been reported as potential mosquitocidal agents against vector mosquitoes, tabulated in Table S2.

Mode of action of plant compounds

The plant compounds disturb proteins, ion channels, nucleic acids, and other cellular components in mosquitoes. Subsequently, these compounds disturb the central nervous system, receptor sites, and other cellular mechanisms. For example, the neem-based compound ‘NeemAzal’ was used against Ae. aegypti. The larvae treated with NeemAzal showed an increased level of protein (31%), carboxylesterase (121%), β-carboxylesterase (46%), alkaline phosphatase (37%), and acid phosphatase (62%). The pupae treated with NeemAzal had higher levels of AChE (116%) and acid phosphatase (43%), lower levels of β-carboxylesterase (12%) and α-carboxylesterase (46%), and no significant changes in alkaline phosphatase enzyme. These results confirmed that the plant compounds present in NeemAzal are actively involved in disturbing the enzymes of Ae. aegypti (Koodalingam et al., 2014). Another plant compound, plumbagin, isolated from the rhizome of Plumnago zeylanica L., disturbed the AChE enzyme of An. stephensi (Pradeepa et al., 2014). Therefore, if we can find plant compounds that will disrupt or deactivate other major enzymes such as Cytochrome P450's (enhance resistance in mosquitoes to digest pesticides), glutathione transferase (helps mosquitoes to survive exposure to pesticides), and vitellogenin (encodes yolk protein precursors for egg production), they will be beneficial to control vector mosquitoes effectively.

The midgut is an important organ involved in major processes such as digestion, ion transport, and osmoregulation. The gut region of mosquito larvae is commonly targeted using various plant compounds. For example, Senthilnathan et al. (2008) targeted the gut EL using a triterpene compound from Dysoxylum spp., which reduced the enzyme activity of the EL (Nathan et al., 2008). Catechin from L. aspera affected the midguts of Ae. aegypti, An. stephensi, and Cx. quinquefasciatus. The aristolochic acids derived from A. indica affected the midguts and larval muscles of Cx. quinquefasciatus larvae (Pradeepa et al., 2015). Similarly, oleic, linolic, and linolenic acids damage the Cx. quinquefasciatus larval midgut and fat body (de Melo et al., 2018). Therefore, identifying novel plant compounds to deactivate the enzymes or impair the midgut of vector mosquitoes may serve as effective control methods in mosquito management programs.

PLANT OILS

Several plant volatile oils derived from steam distillation are used against vector mosquitoes (Table S3). Plants such as Ocimum basilicium L., Hyptis suaveolens (L.) Poit., Salvia spp., Thymus spp., and many species from the grass family (Poaceae) contain oils such as lemongrass, citronella, and palmarosa. They possess good control activity (for 8 h) against vector mosquitoes. In Malaysia and Kuala Lumpur, people use essential oils derived from Litsea elliptica (Nees) Hook. fil., Polygonum minus Hudson, Piper aduncum L., Melaleuca cajuputi Powell, and Cymbopogan nardus (L.) Rendle as potential mosquito control agents (Rohani et al., 1997; Sulaiman et al., 2001). Cymbopogon excavatus (Hook. & Arn.) Stapf and C. nardus oils showed good activities for 2 h against vector mosquitoes (Govere et al., 2000). Lippia polystachya Griseb. and L. turbinate (Poleo) oils were tested for larvicidal effects against Cx. quinquefasciatus at concentrations of 0–160 p.p.m. in 100 mL distilled water for 24 h. After 24 h of treatment, the oils showed good larvicidal activities, with LC50 = 74.9 p.p.m. and LC90 = 124.9 p.p.m. (Gleiser & Zygadlo, 2007). The oil of Atalantia monophylla (L.) DC. had good larvicidal activities against Ae. aegypti, Cx. quinquefasciatus, and An. Stephensi, with LC50 ranging from 7 to 16 p.p.m. (Baskar et al., 2018). The volatile crude oil derived from Piper betle L. showed dose-dependent larvicidal activity against Ae. aegypti (Vasantha-Srinivasan et al., 2018). Using plant volatile oils as mosquito insecticides is well known and has many advantages (Soonwera, 2015a; De Souza et al., 2019; Luz et al., 2020; Marques et al., 2021). These oils are also effective and have a pleasant smell, making them acceptable for daily use and avoiding mosquito bites, which is also an excellent alternative to synthetic insecticides.

Mode of action of plant oils

Plant volatile oils are a good source for vector control as they possess diverse molecules with insect-control properties. The volatile oils derived from Commiphora molmol Engl. ex Tschirch affected the proteins of Cx. pipens. Similarly, the volatile oils from Allium sativum L. and Citrus limon (L.) Osbeck reduced the total protein level in Cx. pipens (Liu et al., 2014). Another volatile oil from the Alaskan yellow cider tree inhibited 50% of AChE enzyme activity in Ae. aegypti (Anderson & Coats, 2012). Piper betle L. oil also altered enzyme levels in Ae. aegypti (Vasantha-Srinivasan et al., 2018). Currently, only a few volatile oils are available to deter vector mosquitoes. Hence, identifying influential novel volatile oils from plants will provide new opportunities to control vector mosquitoes.

PLANT-BASED BIOINSECTICIDES FOR MOSQUITO CONTROL

Using chemical insecticides to get rid of mosquitoes is linked to severe health problems and, more importantly, the rise of mosquito resistance to the chemicals (Ohia & Ana, 2015; Dara, 2017). These situations have led scientists to look for safe and natural solutions, good for the environment and effective. Subsequently, scientists found that plant-based compounds have potential mosquitocidal activities. In addition to these phytocompounds, researchers found that bioinsecticides from plants are also good alternatives to chemical insecticides, which also affect numerous target locations in insects, including mosquitoes (Ohia & Ana, 2015; Dara, 2017; Şengül Demirak & Canpolat, 2022). For example, neem-based insecticides effectively controlled the mosquitoes and posed a lower risk of inducing resistance because of their multiple modes of action on mosquitoes (Pant et al., 2012). Another significant advantage of neem-based insecticides is that they require only a low concentration to control mosquitoes (Pant et al., 2012). The karanja oil-based insecticides also managed mosquitoes effectively (Pant et al., 2012). Similarly, acetogenin bioinsecticides have drawn much attention due to their biological activities against mosquitoes (Coria-Téllez et al., 2018; Louis et al., 2022).

Natural pyrethrins are also used as a substitute for synthetic dithiothreitol (DTT) (Schleier et al., 2011). However, they suffer from severe limitations, such as high instability and rapid disintegration in the presence of sunshine, which requires further investigation (Schleier et al., 2011). But, applying pyrethrin-based insecticides against Culex and Anopheles after sundown has decreased mosquito populations and provided protection (Veronesi et al., 2012). Despite our increasing knowledge of plant-based insecticides, only a few of them, such as neem-based insecticides, are commercially available on the market for mosquito control (Isman et al., 2011). One of the reasons for their limited use in the field is their formulation issues. Each commercially available plant-based insecticide composition should be formulated clearly in such a way that it should be bioactive to target insects and non-toxic to non-target organisms. Additionally, the formulation of these plant-based insecticides should make it possible to produce them in large amounts through plant biomass production and deliver them at prescribed dosages to reduce adverse effects and retain biological activity over a longer period. However, further studies are needed to develop bioinsecticides from various plants.

FUTURE OUTLOOK

Mosquitoes are a major cause of human arboviral diseases, such as dengue, the fastest-growing mosquito-borne disease, now endemic in more than 100 countries worldwide (WHO, 2017). As there are no commercial vaccines for diseases spread by mosquitoes, chemical insecticides are usually used to control mosquitoes. But the repeated use of chemical insecticides has caused alarm in the international scientific community via multiple study results (Margni et al., 2002; Desneux et al., 2007; Roghelia & Patel, 2017). These studies showed that chemical insecticides harm humans, animals, fish, and arthropods that are not their intended targets (Roghelia & Patel, 2017). This situation forced scientists to come up with new, effective, and environmentally acceptable ways to combat mosquitoes. Through continuous research, researchers have found plants that contain several compounds to be an excellent method to eliminate mosquitoes (Thiyagarajan et al., 2014; Siegwart et al., 2015). Several studies also have confirmed the effectiveness of plants against vector mosquitoes. But only a few biopesticides – such as citronella, pyrethrins, resmethrin, and neem (Neem Azal, Tre-san, MiteStop, Picksan LouseStop, and Wash Away Louse, all based on neem seed extracts) – are commercially produced and extensively used in mosquito control programs.

Besides these plant compounds, nanotechnology is another new technology with many potential uses in mosquito control (Borase et al., 2013; Hajra & Mondal, 2015; Zhang et al., 2016; Pilaquinga et al., 2019). This nanotechnology created novel nanoscale materials while remaining environmentally friendly. Therefore, recently, researchers synthesized nanoparticles (NPs) from botanical plants and started using them to eliminate mosquitoes. The phytochemical parts of plant extracts can break down the structure of an element into NPs more effectively than other natural sources (Borase et al., 2013; Hajra & Mondal, 2015; Zhang et al., 2016; Pilaquinga et al., 2019). Researchers also have found that organic compounds (polysaccharides, lipids, and proteins) and inorganic metals (such as silver nitrate, zinc, copper, gold, selenium, and sulfate) synthesized from plant extracts effectively control mosquitoes (Priya & Santhi, 2014; Soni & Prakash, 2014; Benelli, 2016; Benelli et al., 2017a; Foko et al., 2021). For example, synthesized silver, zinc, selenium, and gold NPs incorporating plant extracts displayed efficient larvicidal activity at deficient concentrations against mosquitoes (Borase et al., 2013; Hajra & Mondal, 2015; Zhang et al., 2016; Pilaquinga et al., 2019). Additionally, synthesized plant-derived NPs are considered cost-effective, ecologically benign, and time-efficient because they do not require high pressure, energy, temperature, or toxic chemicals (Jinu et al., 2018; Murugan et al., 2018). Overall, research suggests that the synthesis of plant-derived NPs can be used as an ideal eco-friendly approach for controlling mosquitoes in the future.

In light of these developments, it is important to also discuss their possible negative effect on the environment and non-target organisms (Calderón-Jiménez et al., 2017; Kalpana & Devi Rajeswari, 2018; Lekamge et al., 2018; Singh et al., 2018; Tunçsoy, 2018; Dikshit et al., 2021). For instance, research on the toxicity of Ag NPs on gram-negative bacteria showed severe damage to DNA when they interact with compounds that contain sulphur and phosphorus (Morones et al., 2005). Similarly, titanium dioxide NPs (TiO2 NPs) inhibited Bombyx mori (L.)'s development and moulting duration (Li et al., 2014). Studies on non-target organisms like zebrafish, earthworms, and bees also revealed toxicity of NPs (Zhu et al., 2008; Kim et al., 2014; Vicario-Parés et al., 2014; Kalpana & Devi Rajeswari, 2018). These results revealed that NPs have side effects on the animal kingdom, whether aquatic or terrestrial. A detailed toxicological survey of NPs is thus needed to help them be used in more ways and commercially.

AUTHOR CONTRIBUTIONS

Edwin Hillary: Conceptualization (lead); writing – original draft (lead); writing – review and editing (equal). S. Antony Ceasar: Conceptualization (equal); visualization (lead); writing – original draft (equal); writing – review and editing (equal). Ignacimuthu Savarimuthu: Conceptualization (supporting); writing – original draft (supporting); writing – review and editing (supporting).

ACKNOWLEDGEMENTS

Authors thank the Rajagiri College of Social Sciences for the research support.

    FUNDING INFORMATION

    This work was financially supported by Rajagiri College of Social Sciences (Autonomous), Kerala, India, under Seed Money for Faculty Minor Research.

    DECLARATION OF INTEREST

    None.

    ETHICS APPROVAL AND CONSENT TO PARTICIPATE

    Not applicable.

    CONSENT FOR PUBLICATION

    Not applicable.

    DATA AVAILABILITY STATEMENT

    Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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