Impact of Environmental Changes on the Chemical Composition of Mosquitocidal Plants

Received 07 Dec, 2024

Accepted 28 Jan, 2025

Published 29 Jan, 2025

Mosquitocidal plants known for their ability to kill and repel mosquitoes depend on their chemical composition to be effective. However, the production and abundance of these chemical compounds are increasingly threatened by major environmental changes. Warmer temperatures, increased UV-B radiation, elevated CO₂ levels and altered precipitation patterns are major factors that can influence the balance of these phytochemicals. In addition, pollution and other anthropogenic activities such as dust pollution can cause physiological stress for plants and consequently affect phytochemicals production. These changes have a huge impact on the use of traditional remedies available to locals, reliance on chemical insecticides, disease transmission, and environmental degradation among others. Addressing these issues requires a multifaceted approach involving research, adaptation strategies, community involvement, and sustainable cultivation practices. This review highlights the importance of understanding how environmental factors are affecting mosquitocidal plants and stresses the need for adaptive strategies to ensure their continued efficacy. Conclusively, preserving the efficacy of mosquitocidal plants necessitates an integrative effort involving scientific advancements and community-based initiatives to combat the dynamic challenges posed by environmental changes.

INTRODUCTION

Mosquitoes are without a doubt the vectors of disease that are arthropods and are of significant public health importance. The endemicity of diseases like malaria, lymphatic filariasis, and a variety of viral infections in a region is dependent on the presence of vectors that can transmit these diseases. Plasmodium falciparum and Plasmodium vivax which affect nearly 500 million people each year and kill nearly 3 million people cause malaria in human beings¹. Several people have been infected with mosquito-borne arboviruses, with the most common being yellow fever and dengue fever². In 2019, there were 229 million reported cases of malaria globally, resulting in 409,000 deaths. The African regional disease accounts for 90% of all cases and deaths each year³.

The reduction in the transmission of deadly diseases like malaria, dengue fever, and the Zika virus is largely dependent on the control of mosquitoes. Mosquito control with insecticides is now the most widely used disease control strategy. Insecticide resistance has emerged, and synthetic insecticides have been discovered to be harmful to the environment, non-target species, and human health. Bio-control strategies for mosquito vectors based on plant-based materials are encouraged to avoid these issues. For centuries, humans have used plant materials by bruising or burning them, which causes the phytochemicals secretion from the plants that help to ward off biting insects particularly mosquitoes⁴. These plants secrete very volatile compounds that repel the hematophagous insects away from the hosts⁵. Secondary compounds from plants, such as alkanes, alkenes, alkynes, simple aromatics, lactones, essential oils, fatty acids, terpenes, alkaloids, steroids, flavonoids, and pterocarpus, have shown potential in controlling mosquito larvae. Saponin in M. koenigii causes death to adult insects by reducing the insect’s ability to uptake food, which consequently leads to instability while hindering development and reproduction⁶. Flavonoids are important in using plants for insect control because they have been found to attract insects to toxic plants or repel insects. They have also been found to suppress egg-laying in insects. The geographical distribution and the chemical composition of the world’s vegetation, especially plants that are used for medicine in most of the world is changing rapidly. Temperature and wind pattern changes affect rainfall which then affects the development and growth of plants. Research has identified the presence of different secondary metabolites in medicinal and aromatic plants. Due to growing limitations on the use of synthetic insecticides, there is greater attention on sustainable methods of controlling hematophagous insects, such as the use of plants. Environment and agronomic practices both influence phytochemical content in plants⁷. Therefore, a thorough understanding of the impacts of the environmental and agronomic factors is required to improve the level of predictability of desired compounds, whether for human health or pest control. The objective of this study is to investigate the effect of environmental changes on the chemical composition of mosquitocidal plants. The study aims to identify how these changes influence the plant’s effectiveness in mosquito control.

Chemical composition of mosquitocidal plants: Plants are composed of secondary metabolites which they use as a way to protect themselves from the herbivores who feed on them and other abiotic pressures. Several categories of phytochemicals, primarily secondary metabolites derived from various plant species-including alkaloids, steroids, terpenoids, essential oils, and phenolics have been documented for their insecticidal properties. More than 2,000 plant species have been recognized for producing chemical compounds and metabolites that are valuable in pest management strategies. Members of the plant families Solanaceae, Asteraceae, Cladophoraceae, Labiatae, Miliaceae, Oocystaceae, and Rutaceae have varying degrees of larval, adulticidal or repellent activity against various mosquito species⁸.

Phytochemicals in essential oil: Essential oils have proven to be effective in combating various pests and safeguarding crops globally, offering a promising alternative to synthetic insecticides for mosquito control. These oils consist of several volatile compounds, such as hydrocarbons (terpenes and sesquiterpenes), oxygenated hydrocarbons, and phenylpropenes. High larvicidal and pupicidal activities have been reported for plant oils gotten from Cinnamomum verum, Cuminum cyminum, Syzygium aromaticum, Laurus nobilis, Citrus aurantifolia, Lippia berlandieri, and Pimpinella anisum against Cx. quinquefasciatus⁹. Essential oils extracted from Tagetes lucida, Lippia alba, Lippia origanoides, Eucalyptus citriodora, Cymbopogon citratus, Cymbopogon flexuosus, Citrus sinensis, Swinglea glutinosa, and Cananga odorata have shown mortality against Ae. aegypti larvae¹⁰. Additionally, oils like peppermint, basil, rosemary, and citronella from Mentha piperita, Ocimum basilicum, Rosmarinus officinalis, Cymbopogon nardus, and Apium graveolens suppress fecundity and demonstrate effectiveness against the development of eggs into larvae in Aedes aegypti¹¹. Cymbopogon proximus, Ocimum canum and Lippia multiflora plant oils have shown effective mortality activity against the larvae and eggs of An. gambiae and Ae. aegypti mosquitoes¹². Furthermore, oils from Cinnamomum osmophloeum and Carum copticum demonstrated mortality against Cx. quinquefasciatus and Cx. Pipiens larvae¹³. Essential oils disrupt the insect nervous system, inducing neurotoxicity through multiple mechanisms, such as inhibiting acetylcholinesterase (AChE) activity, blocking octopamine receptors, and interfering with GABA-gated chloride channels¹⁴. Monoterpenes which is the functional compound responsible for AChE interference in insects is the main composition of about 90% of essential oils and generally potential plant-based larvicides¹⁵. The Thymus plant produces Monoterpenes which have been proven to be effective in deterring Culex mosquitoes and have shown significant mortality activity against larvae of Culex mosquitoes including Pipiens, Pallens and Quinquefasciatus¹⁶. Mortality activities of β-citronellol, geraniol and linalool which are compounds from Pelargonium roseum essential oil were also observed against Cx. Pipiens¹⁷. The composition of an essential oil obtained from the flowers of Cannabis sativa L., with significant mortality activity against mosquito larvae and adults was found to be 25.0% myrcene, 45.4% beta-caryophyllene,17.9% α-pinene, 8.3% humulene, 5.2% β-pinene, 5.1% ocimene, and 3.0% of farnesene¹⁸.

Terpenes: This compound is plentiful in citrus, eucalyptus and conifers which are found in their leaves, flowers, stems, and roots. Terpenes contain over 25,000 volatile organic compounds of different concentrations and levels of toxicity that are useful for insect and pathogenic microbe defense. Cyclopentanoid (iridoid), a compound derived from monoterpene, is extremely bitter and an effective insect repellent. Iridoids can inhibit the production of prostaglandins, thereby suppressing insect growth and development. Iridoid increases the larval stage of insects thereby reducing insect growth and survival rates in vitro¹⁹. Pyrethroids are neurotoxin-modulating monoterpene esters elicited by flowers and leaves. Because studies have shown that pyrethroid is environmentally safe, it is used as an active ingredient in the majority of commercial insecticides. Mosquito repellents include terpene-rich Chenopodium ambrosioides and neem extracts. Callicarpenal, a naturally occurring type of terpenoid from Callicarpa Americana has been studied for its significant repellent activities against biting from mosquitoes²⁰.

Alkaloids: Alkaloids are naturally occurring nitrogen-containing compounds which are frequently synthesized in plants. This compound from these plants’ alkaloids is widely used in conventional insect repellents^21,22. Unlike essential oils, alkaloids are non-volatile; however, they prove effective as mosquito repellents when plants are burned to release smoke, which deters insects²³. Ricinine is found in castor bean extracts (Ricinus communis) which have a strong insecticidal effect. High mortality rates were recorded for its activity against Anopheles arabiensis larvae²⁴. Furthermore, Ricinus communis produces pyridine alkaloids which showed larvicidal and adulticidal activity against An. gambiae²⁵. Aedes albopictus, Culex pipiens pallens, and Aedes aegypti have also demonstrated susceptibility to the larvicidal activity of alkaloids²⁶. Alkaloids derived from the Arachis hypogaea plant have also been shown to cause mortality against Anopheles stephensi and Aedes aegypti mosquito larvae²⁷. Additionally, Nicotiana tobacco produces Nicotine, which is an alkaloid. Nicotine is mostly made up of phenols that mimic neurotransmitters that cause symptoms resembling those caused by organophosphate or carbamate insecticides, same with nornicotine, and anabasine²⁸.

Steroids: They fulfill diverse tasks such as hormones, pheromones or insect deterrents. A previous study has isolated a steroid compound from the foliage of Cestrum diurnum effective against An. subpictus²⁹. Similarly, the activity of the methanol extract of mature foliage of Solanum villosum has been reported against Culex quinquefasciatus fourth instar larvae³⁰. Also, β-sitosterol a new phytosteroid isolated from Abutilon indicum, has been reported for its mortality activity against Aedes, Anopheles and Culex mosquito larvae³¹. Banerjee et al.³² observed the presence of steroidal compounds in the methanol extracts of Limonia acidissima leaves which showed significant larvicidal activity with LC₅₀ values of 1.73 ppm against Culex mosquitoes after exposure for three days.

FACTORS THAT DETERMINE THE PRODUCTION OF PHYTOCHEMICALS

Natural insect management agents are increasingly being used to manage different arthropod pests and vectors because they provide a safer alternative to hazardous chemical insecticides. A holistic understanding of soil biological and ecological factors can improve efficacy and bio-control success. Factors such as the variety of the cultivar, the specific plant species, the age of the plant, the particular organ being examined, as well as environmental conditions like weather and soil type, along with cultivation techniques and agricultural practices, all contribute to the variability of phytochemical levels and profiles in plants, such as alkaloids³³. Environmental conditions such as drought, sulfate and nitrogen nutrients, season, and the diurnal cycle have a significant impact on these phytochemicals and their associated enzymes⁷. Overall, the level of phytochemical production is influenced by the interplay between environmental and genetic factors.

Formation of the plant organ: The developmental stage of the organ of a plant can influence the phytochemical composition of that plant³⁴. The phytochemical composition of a plant organ can change substantially, with about 10% traces of the phytochemicals in the early stages to approximately 50-70% phytochemical content when the plant flowers, while in some cases, the phytochemicals accumulate in abundant before the full development of the organ. A decrease in the amount of eugenol and methyl eugenol was observed as the leaves of Ocimum sanctum begin to develop. It is also worthy noting that yield and composition variability with organ formation might be due to the secretion structure. Plants with their secretory structures outside the plants may release secretions with organ maturation as a result of trichome cuticle disruption. In contrast, those with their secretions internally typically demonstrate consistency in the composition of their phytochemicals and their yield³⁵.

Plant part: Differences in composition can also be seen in flowers that are adapted to be pollinated by insects (entomophilous flowers), where secretion of phytochemicals can serve as orientation cues, and as such phytochemicals released from floral parts are different from those secreted by other parts of the plants³⁶. According to Kuropka et al.³⁷, a very low amount of monoterpenes, almost unavailable was produced in Achillea ptarmica leaves, stems and roots essential oil, while the monoterpenes in floral parts were abundant. Similarly, the floral volatiles of Lavandula pinnata differed from the phytochemicals synthesized in the green parts (stems and leaves) during the flowering and vegetative phases³⁵.

Seasonal variation: The chemical constituent of phytochemicals changes with the season in some plant species, so harvesting at the right time is critical. Crithmum maritimum produced abundant sabinene compound when it was flowering, while γ-terpinene was the most abundantly produced phytochemical in other vegetative phase³⁸. In addition, abundant Sesquiterpene was discovered in the essential oil of Achillea millefolium harvested during the vegetative phase while in plants harvested during the flowering phase, monoterpene was the most abundant constituent of the essential oil. A pure essential oil obtained from Mentha piperita contains a significant amount of menthol and ethanol, less than 4% menthofuran, and pulegone of less than 2%. When floral structures development reaches 50% (mid-to late August), the amount of the menthofuran increases with the yield of the essential oil while in September and October during over-maturation, the amount of oil yield tends to decrease with the amount of menthofuran and an increases, up to 60% in the amount of menthol³⁹.

Mechanical or chemical injuries: Under normal conditions, the plants produce a large amount of phytochemicals, which is considered constitutive production. When a plant experiences a traumatic injury, it may trigger de novo production, leading to the formation of new compounds that were not previously present. This process is referred to as induced production. The type of induced response stimulated is dependent on the species, its stage of development, water and sunlight availability. In Picea plant, the stem resin is produced in the canals of the axial resin in the cortex but the effect of injuries from mechanical activities, feeding from insects or fungal infection causes the accumulation of traumatic resin ducts within the developing xylem. Martin et al.⁴⁰ noticed very different morphological changes six to nine days after a spray treatment with methyl jasmonate (MeJA). The changes include denser cytoplasm and thin cell walls in xylem cells adjacent to the cambium, causing the development of more traumatic resin ducts. Visibility of the lumen of the traumatic canals was also observed fifteen days after treatment, which lead to the formation of a ring within the youngest xylem portion. Accumulation of resin in the lumen was also observed. The MeJA treatment increased the total accumulation of mono- and diterpenes in addition to inducing new canal development, but the absolute amount of sesquiterpenes remained unchanged.

Pollution: Although it is known that air pollution adversely impacts the production of secondary metabolite production, especially essential oil composition, it is difficult to assess because the observed results may be confused with responses to other stressors. Ozone is one of the most serious plant pollutants. Limited research has investigated the effects of air pollutants on the chemical composition of essential oils, with reported findings often being inconsistent and influenced by the specific type of pollutant involved⁴¹. Pollution from dust from roadsides, cement plants, and mining operations causes gloom, closure of the stoma, and reduction in the influx of CO₂.

Edaphic factors: Soil type and composition are determinant factors in secondary metabolite composition, particularly phytochemical composition. Many plant species rate of survival and growth are severely hampered in poorly aerated and drained soils, significantly reducing crop and essential oil yields and altering phytochemical composition. The addition of three nutrients (NPK) to the soil has resulted in an oil yield increase, however, the individual supplementation of these nutrients resulted in variations in both the composition and yield of the essential oils. Low Ca²⁺ levels can lead to the development of soils that are acidic which suppresses growth. Additionally, plant row spacing that is too close can have a negative impact due to allelopathic factors⁴².

Genetic influence: Studies in genetics and hybridization have revealed that genes are a major determinant of the chemical constituent of essential oils. Research on the essential oils of various Mentha species indicates that the CC or Cc genotypes facilitated the transformation of -terpineol into limonene, which was then oxidized to produce carvone. Conversely, the cc genotype led to the production of menthadiene, which was subsequently converted into pulegone and menthol⁴³. The processes driving the development of phytochemical production in plants involve several mechanisms: (a) Duplication of genes with subsequent divergence, where the original enzyme function is preserved A novel function arises as a result of gene duplication; (b) Convergent evolution, the independent emergence of similar functional traits across distinct evolutionary lineages; (c) Modification of an existing gene without gene duplication, resulting in the development of a novel enzymatic function, often resulting in a reduction or loss of the original function and (d) Complete loss of the function of the enzyme. These processes, in any of these cases, result in gene expression changes. Furthermore, few enzymatic structural changes can occur which can be increased via enzyme exposure to constitutive or inducible conditions. If new metabolite production provides an adaptive advantage for the plants, there is retention and an increase in metabolite production. Expression of protein changes may not always result in an enzyme loss, rather, they may cause the synthesis of secondary metabolites in a different vegetative or reproductive structure³⁵.

Storage: The storage method may also influence the amount of phytochemicals gotten from plants. Sunlight, moisture content, temperature, age, contaminations, oxidation, and other factors all have an impact on the yield and composition of phytochemicals. As stated previously, the secretory structure also influences the production and availability of phytochemicals. Secretory structures inside the plant are less susceptible to volatile losses while trichomes are more vulnerable to mechanical damage caused by transportation, and storage by crushing in piles. Species that show a yield decrease and phytochemicals composition changes with storage or drying are Anethum graveolens, Cananga odorata, Carum carvi, Matricaria recutita, Jasminum grandiflorum, Narcissus poeticus, Ocimum basilicum, Sassafras albidum, and Zingiber officinale³⁵.

Environmental changes: Thakore⁴⁴ stated that an element that remains beyond human control, is the environment. Essential oils and phytochemicals production is heavily influenced by weather conditions. Floods, droughts, cyclones, and hurricanes are examples of atmospheric conditions that have had varying degrees of severity for phytochemicals availability. According to Ileke et al.⁴⁵ stress caused by drought increased the total amount of resins and terpenes in the Pinus sylvestris plant. Drought stress was observed to be directly related to an increase in phytochemicals in species like Artemisia dracunculus, Ocimum basilicum, and Anethum graveolens whereas yield increases in Artemisia annua, Coriandrum sativum, and Thymus vulgaris were only achieved with normal or higher irrigation. Drought stress has been found to reduce photosynthetic rates and the uptake of nutrient such as sugar and carbon in plants. Monoterpene emission rates in Pinus elliotii increased exponentially with temperature, particularly those of myrcene and limonene between 20 and 46°C⁴⁶.

PLANT RESPONSES TO ENVIRONMENTAL CHANGES

Plant chemicals, such as primary and secondary metabolites, are very important in the interaction between plants and insects. The composition and concentration of plants secondary metabolites (PSM) in plant tissues as well as volatile emissions can be altered by changes in the environment through biosynthetic pathways changes and individual compound physiochemical properties. Recent studies have shown that increased temperature exert different effects on PSMs. An increase in total phenolics, including condensed tannins, flavonoids, saponins, and alkaloids, was observed in black locust (Robinia pseudoacacia) seedlings exposed to elevated temperatures⁴⁷. Multiple studies have also reported increases in secondary metabolites, including lignin⁴⁸, saponins⁴⁹, and volatile terpenes⁵⁰, while others have shown decreases in PSMs, including condensed tannins⁴⁷ and lignin⁵¹ under stress condition induced by warming. Additionally, increase in total phenolics such as some tannins and flavonoids were observed in some plants grown under increased CO₂ and O₃ condition⁵². Decrease in terpenoids content was observed under increased CO₂ and increased O₃ conditions⁵³. Hormones in plants like ethylene influence plant chemical defenses in response to elevated CO₂ and O₃. The CO₂ enrichment tends to reduce jasmonic acid and ethylene regulated chemical defenses while increasing defenses that are dependent on salicylic acid. These chemical defense changes exert a negative, neutral or beneficial effect on plant-insects interaction⁵², making it difficult to predict future changes in insect control. Increased salicylic acid biosynthesis and the upregulation of defense compounds such as some phenolic compounds involved in the shikimic acid pathway can be associated with increased O₃ levels⁵³. A reduction in the production of phenols, flavonoids and terpenoids was observed in Ficus insipida leaf growing under a high O₃ environment.

IMPACT OF ENVIRONMENTAL CHANGES ON THE CHEMICAL COMPOUNDS OF SOME PLANTS COMMONLY USED FOR MOSQUITO CONTROL

Occurrences solely due to nature and anthropogenic activities cause drastic changes in the environment by transforming and transporting vast amounts of resources and power^54,55. Natural systems capture solar energy to produce biomass, driving transformative processes by cycling matter through geological, biological, oceanic, and atmospheric pathways while humans use energy to satisfy their needs and desires. Historically, the impact of human activities on these natural processes was minimal. However, today, human actions are altering these flows on an unprecedented scale. Humans and natural processes fix almost as much nitrogen and Sulphur in the environment⁵⁶. Fossil fuels burned by humans release vast amounts of carbon into the atmosphere which consequently disrupts the carbon cycle. Additionally, anthropogenic activities contribute to the release of metals like zinc, cadmium, nickel, mercury, arsenic, and vanadium at levels that are two times or more than those that are naturally occurring⁵⁷. Environmental changes, which stem from a variety of sources, have become especially pronounced in recent centuries. One notable example is the increase in CO₂ concentrations, largely due to the burning of fuels and cement production⁵⁸. From preindustrial levels of around 270-280 parts per million (ppm), carbon dioxide concentrations have now surged to over 374.9 ppm⁵⁹. Over the past century, the Earth’s climate has undergone an average warming of about 0.8°C, with a notable increase of 0.6°C occurring in the last three decades⁵⁸. Environmental stress can disrupt various pathways of metabolism, leading to the generation of various metabolite byproducts. The UV-B, for example, causes changes in the phenylpropanoid biosynthetic pathway and the production of essential oils in Curcuma plants⁶⁰. Cornus officinalis bioactive compound accumulation is entirely dependent on weather conditions⁶¹.

Azadirachta indica and Eucalyptus globulus (neem and gum tree): According to Akiode et al.⁶², low temperatures increased the amount of alkaloid obtained from the neem and gum tree leaves and barks, while hot temperatures increased the amount of flavonoid obtained from these leaves. Under hot conditions, the phenol content of Azadirachta indica and E. globulus leaves was also significantly higher. Under high temperatures, neem bark produces a significant amount of phenol, whereas E. globulus bark and leaf produce a significant amount under optimum conditions. Akiode et al.⁶² also reported that low temperatures produced significantly high amounts of saponin and tannin from both leaves and bark of Neem and gum trees while a remarkably low amount of tannin was discovered under hot conditions for both Azadirachta indica and E. globulus. Mckiernan et al.⁶³ reported no changes in terpenes and tannins content and a reduction in the total phenolic concentrations of gum tree exposed to drought stress, with leaves from plants in the moderate water treatment containing 17% less than controls, and leaves from low-water plants showing a 19% reduction in concentrations compared to controls with concentrations of tannins remaining unaffected by drought treatments.

Allium sativum (garlic): Atif et al.⁶⁴ reported that extended daylight and increased temperature led to a greater accumulation of phenols which could influence the mosquitocidal properties of Allium sativum. Increased nutritional traits indicate that the bulbs developed under hot conditions, resulting not only in increased phenols but also in large amounts of proteins and sugars, which amplified their overall mosquitocidal properties. Higher growth temperatures led to an increase in total phenolics, flavonoids, soluble proteins, and sugars in garlic cultivars. Previous research has also associated elevated phenol levels with enhanced plant defense and growth protection⁶⁵. Therefore, garlic bulbs grown in extended daylight and increased temperature conditions have higher insecticidal properties, which is an effective mosquito control measure. In addition to genetic factors, fluctuations in the environment also impact phenol levels⁶⁶. Ecological factors have a stronger impact on phenolic content than genotypic variations⁶⁷. Results indicated that garlic experiences a reduction in phenolic compounds under salinity-induced drought stress, as demonstrated by increased phenol leakage and a lower total phenolic content in stressed plants compared to controls⁶⁸.

Zanthoxylum species: Leaves of the Zanthoxylum species are more sensitive to seasonal fluctuations, including water stress, sunlight exposure, and developmental stages. The oil obtained from trunk bark has little chemical variability, which is likely due to the slow formation of trunk bark, which is less influenced by factors in the environment⁶⁹. Tanoh et al.⁷⁰ reported that the composition of phytochemicals in Zanthoxylum varies due to climatic conditions, as certain compounds, which were previously present in only small amounts, were found in higher concentrations under certain conditions. Tanoh et al.⁷⁰ also discovered a decrease in tri dean-2-one production at low temperatures. This was also the case with e-ocimene, whose content was less than 4% at low temperatures but 23.57% at slightly lower temperatures. Undecan-2-one was also abundant at 8% at both low and high temperatures. Dendrolasin was found in significant amounts (4-16.4%) at both high and low temperatures, but not at slightly low temperatures. In the case of e-ocimene, 4% insignificant amount was observed at very low temperatures, while 23.57% was discovered at slightly low temperatures. They also discovered 13.30% thymol, an oxygenated monoterpene, at high temperatures. Higher temperatures are associated with large amounts of Limonene and Copaene. On the other hand, cooler temperatures with minimal fluctuations encourage the production of Linalool. In the same environment, compounds like α-Terpineol, Caryophyllene, L-α-Terpineol, Terpinolene, α-Phellandren, d-Limonene, and Geranyl acetate are a lot sensitive to ecological factors, while β-Pinene and Limonene levels are less influenced by climatic variables⁷¹.

Jatropha curcas (barbados nut): Barbados nut plant has been discovered to be toxic to insects, microbes and parasites⁷². The crude leaf extract of Jatropha curcas extracted with petroleum ether demonstrated larvicidal activity against vector mosquitoes, including Culex, Anopheles and Aedes mosquito⁷³. Leaves of Jatropha curcas has been reported to exhibit variation in terms of tannins, phenolic compounds, and amino acids when subjected to different temperatures. Maximum levels of these compounds were found in leaves collected under high temperatures and least under low temperatures however, phytic acid contents were greater under low temperatures⁷⁴. Iwuala et al.⁷⁵ reported low levels of alkaloid, flavonoids, tannins and steroids contents in J. curcas plants exposed to drought conditions while an increase in saponin content was recorded.

Ocimum gratissimum (holy basil): Seasonal changes influenced the compounds of essential oils in Ocimum plants⁷⁶, while variations in Ocimum gratissimum were linked to the time of day⁷⁷. The phenyl-propanoid eugenol and the monoterpene 1,8-cineole concentrations were highest at low and high temperatures. Low temperatures promote the production of sesquiterpene components, whereas high temperatures produced no monoterpene linalool. The study’s findings revealed that the highest essential content of phytochemicals was produced at low temperatures. High temperatures resulted in the lowest percentage. In terms of plant parts, the leaves had the largest amount of essential oil, followed by the inflorescence. The stem had the lowest concentration of essential oil phytochemicals. The chemical constituent limonene has a highly variable behavior at different temperatures. The highest value was obtained at a low temperature. This is similar to the phytochemicals eugenol and linalool, which were also produced in high amounts at low temperatures and low amounts at high temperatures. The chemical constituent methyl chavicol is highly favoured by slightly low temperatures while the lowest value was recorded under high temperatures. The camphor content was slightly affected by the changed seasons with comparisons to other chemical constituents⁷⁸. According to Rahimi et al.⁷⁹, drought stress increased the content of phenolic and flavonoid compounds in basil. Essential oil composition changes change with drought stress with the conversion of some compounds into others, a decrease of some compounds, and an increase for others. For example, increases in linalool and 1.8-cineol, with a decrease in the geranial compound. The amount of methyl chavicol was also observed to increase with increased drought stress.

CONSEQUENCES OF THE CHEMICAL COMPOSITION CHANGES ON MOSQUITO CONTROL AND THE ENVIRONMENT

Altered mosquito-repelling properties: Changes in phytochemistry have an impact on the repellent properties of these plants. Many of these plants are effective because of phytochemicals, serving a vital role in their activities against mosquitoes. However, as environmental conditions change, the production and concentration of these bioactive compounds are influenced. Changes in temperature, precipitation patterns, and exposure to elevated levels of solar rays, for example, have an impact on the synthesis and efficacy of these phytochemicals. These chemical changes in mosquito-repelling plants can result in a reduced ability to repel mosquitos effectively⁸⁰.

Increased disease transmission: Reduction in the efficacy of mosquito control measures heightens the likelihood of disease transmission, especially in mosquito-borne disease endemic areas. In regions heavily impacted by diseases transmitted by mosquitoes, any reduction in the efficacy of mosquito control measures has the potential to worsen the existing public health challenges. Reduced ability to repel or deter mosquitoes may result in increased human-mosquito interactions, potentially increasing disease transmission⁸¹.

Public health inequities: Inconsistent mosquito control measures may inadvertently worsen disparities in community health outcomes due to the shifting variety of plants that repel mosquitoes and how well they work. This is especially true in areas that are endemic with diseases vectorized by mosquitoes and where the effectiveness of mosquito control measures can signify the difference between health and illness. Communities dealing with public health disparities frequently bear the brunt of these challenges. Inconsistent mosquito control measures can increase exposure to mosquito-borne diseases, affecting vulnerable populations disproportionately. Socioeconomic factors, healthcare access, and environmental conditions can all exacerbate these disparities. For example, marginalized communities with limited access to healthcare resources may be less prepared to manage and treat mosquito-borne diseases, resulting in worse health outcomes⁸².

Increased reliance on chemical insecticides: Changes in the phytochemicals of mosquitocidal plants can increase reliance on chemical insecticides if these plants fail to control mosquitoes. Synthetic insecticides, while effective in curbing mosquito populations and reducing disease transmission, are not without their drawbacks. One significant concern is their environmental impact. The application of these chemicals can lead to consequences, such as water pollution, disruption of non-target species, and the development of insecticide resistance in mosquito populations. These consequences can resonate throughout ecosystems, causing imbalances and more environmental challenges. Prolonged or improper exposure to these chemicals can result in adverse health effects, including skin irritations, respiratory problems, and, in extreme cases, more severe health conditions⁸³.

Environmental degradation: The increased reliance on chemical insecticides as a primary tool in mosquito control strategies has destructive environmental consequences. The contamination of water bodies, via wash-off and effluents from chemicals can occur when they are carried into nearby streams, rivers, and lakes by runoff from treated areas, resulting in water pollution. This contamination affects not only aquatic life but also the quality of drinking water. Chemical insecticides cause further disruption in ecosystems, affecting beneficial microorganisms in soil and aquatic environments. These microorganisms are important for nutrient cycling, soil fertility, and ecological balance. Their disruption can cause ecosystem imbalances, threatening not only the environment’s health but also its ability to support life and biodiversity. The persistence of chemical insecticides in the environment is one of the most harmful long-term consequences. These chemicals can remain in the soil, water, and air for long periods, accumulating over time. This persistence can result in chronic environmental degradation, with consequences that may not be immediately apparent but may emerge years or decades later⁸⁴.

Emerging disease hotspot: Changes in the chemical composition of mosquitocidal plants can create an environment that is more conducive to mosquito breeding. These plants’ phytochemicals may become less effective at repelling or deterring mosquitoes as they evolve in response to changing weather patterns. As a result, mosquitoes may become more prevalent in areas where these plants were once a natural barrier to their development. Such changes can lead to the establishment of new mosquito breeding grounds, contributing to the emergence of disease hotspots. With increased mosquito populations, the risk of disease transmission increases, and regions previously thought to be low-risk for mosquito-borne illnesses may now face an increased threat⁸¹.

WAYS OF MITIGATING THESE EFFECTS FOR EFFECTIVE PLANT BASED CONTROL

Plant breeding: This involves the development of breeding programs to select and propagate mosquitocidal plant varieties with a genetic makeup that allows them to withstand the various stressors associated with changes in the environment, including shifts in temperatures precipitation patterns and pest pressures. Not only does breeding for resilience protect the efficacy of mosquito control, but it also contributes to more sustainable and environmentally friendly mosquito management. This involves breeding plants with increased drought tolerance, enhanced photosynthetic efficiency, and the capacity to produce robust levels of phytochemicals in response to varying temperature and light conditions⁸⁵.

Selective pruning: Selective pruning is a valuable horticultural technique used to maximize the production of plant parts with desired chemical compositions. This practice is critical in mosquito control because it ensures the availability of plant materials high in phytochemicals known to repel mosquitos. Selective pruning allows for the long-term and targeted enhancement of phytochemical content, providing a versatile and environmentally friendly approach to improving the efficacy of mosquito control strategies. Selective pruning involves removing specific plant parts’ carefully, such as leaves, stems, or branches, to improve other plant parts growth with the desired phytochemical composition. This is done in a bid to direct the plant’s growth materials toward the production of phytochemicals with insecticidal or mosquito-repelling properties, such as alkaloids, terpenoids, phenolics, and essential oils⁸⁶.

Microclimate control: It involves the creation of local environments with optimum resources needed to nurture these plants. Microclimate control shields plants from the adverse effects of extreme weather events and temperature variations by managing the immediate environment surrounding them. This involves manipulating factors like temperature, humidity, and light exposure within a confined area. Methods of microclimate control include greenhouses, shade structures, or rain shelters which protect plants against these extreme weather conditions. Methods like heating, cooling, and insulation can be used to maintain an optimal temperature condition. Also, misting systems, irrigation control, and humidity sensors are some methods that enable growers to maintain the ideal humidity conditions for mosquitocidal plants. Light exposure can also be regulated with the use of shade nets, artificial lighting, or adjustable shading systems which helps to protect plants from overexposure to radiation while receiving an optimal amount of sunlight⁸⁷.

Permaculture: Permaculture involves imitating natural ecosystems. It aims to replicate functional relationships by observing how ecosystems function such as inter-cropping, integrating plants and animals, and incorporating beneficial interactions within the system. Companion planting, for example, is a common permaculture practice in which plants mutually beneficial plant species together to optimize ecological functions and increase resilience. Additionally, permaculture often incorporates swales and terraces to manage the flow of water during heavy rainfall, preventing soil erosion and flooding. These practices safeguard agricultural productivity and minimize damage caused by harsh environmental conditions⁸⁸.

Conservation tillage: It is a practice that involves the deliberate reduction or elimination of traditional tillage, which is the mechanical preparation of soil through plowing or digging before planting. The retention of soil moisture is important for crop growth and agricultural sustainability during drought. Conservation tillage helps achieve this by reducing soil disruption. Traditional tillage breaks up soil structure, increasing the rate of water evaporation and reducing the soil’s ability to retain moisture while conservation tillage leaves crop residues on the field’s surface, providing a natural mulch that acts as a barrier against water loss. This organic mulch shields the soil from direct sunlight reduces evaporation and helps maintain consistent soil moisture levels, ensuring that crops have optimum water availability⁸⁵.

CONCLUSION

Mosquitocidal plants, known for their natural mosquito-repellent, adulticidal, and larvicidal properties, face threats from environmental changes like rising temperatures, UV-B radiation, elevated CO₂, and pollution, disrupting phytochemical biosynthesis. These disruptions reduce their efficacy, increasing reliance on chemical insecticides and environmental degradation. Future research should focus on understanding environmental stressors’ effects on phytochemicals, developing resilient plant varieties, adaptive cultivation practices, and leveraging biotechnological tools. Promoting sustainable cultivation and community involvement is vital to preserving their role in pest management and public health.

SIGNIFICANCE STATEMENT

This study highlights the critical role of mosquitocidal plants in sustainable mosquito control and their reliance on phytochemical efficacy, which is increasingly threatened by environmental changes. By examining the impacts of factors such as climate change, pollution, and anthropogenic activities on phytochemical production, the study provides valuable insights into the vulnerabilities of these plants. The findings reveal the urgent need for adaptive strategies to reduce environmental stressors and ensure the continued utility of mosquitocidal plants as eco-friendly alternatives to chemical insecticides. Furthermore, the study emphasizes the importance of integrating scientific advancements, sustainable cultivation practices, and community engagement in preserving the efficacy of these plants. This research holds significant implications for global public health, environmental conservation, and the development of resilient pest management systems in the face of changing environmental conditions.

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