The Phytochemical Composition of Melia volkensii and Its Potential for Insect Pest Management

Due to potential health and environmental risks of synthetic pesticides, coupled with their non-selectivity and pest resistance, there has been increasing demand for safer and biodegradable alternatives for insect pest management. Botanical pesticides have emerged as a promising alternative due to their non-persistence, high selectivity, and low mammalian toxicity. Six Meliaceae plant species, Azadirachta indica, Azadirachta excelsa, Azadirachta siamens, Melia azedarach, Melia toosendan, and Melia volkensii, have been subject to botanical pesticide evaluation. This review focuses on Melia volkensii, which has not been intensively studied. M. volkensii, a dryland tree species native to East Africa, has shown activity towards a broad range of insect orders, including dipterans, lepidopterans and coleopterans. Its extracts have been reported to have growth inhibiting and antifeedant properties against Schistocerca gregaria, Trichoplusia ni, Pseudaletia unipuncta, Epilachna varivestis, Nezara viridula, several Spodoptera species and other insect pests. Mortality in mosquitoes has also been reported. Several limonoids with a wide range of biological activities have been isolated from the plant, including volkensin, salannin, toosendanin, trichilin-class limonoids, volkendousin, kulactone among others. This paper presents a concise review of published information on the phytochemical composition and potential of M. volkensii for application in insect pest management.


Introduction
The continuous and indiscriminate use of synthetic pesticides in crop protection has led to an increase in pest resistance, health and environmental concerns [1]. This has led to a renewed interest in natural products as alternative sources for insect pest control [1]. One of the most promising options is the use of secondary metabolites produced by plants, many of which are toxic to a wide spectrum of insect pests [2]. Plant extracts can offer a solution to insect pest control because they are environmentally friendly, easily biodegradable, and are target-specific [3]. that both fruit powder and its cake could be used as safe ruminant feed supplement [21]. Its use as a fodder crop underscores its safety in mammals [20], and traditionally, it is used for the treatment of diarrhea, pain, skin rashes, and eczema [22]. Aqueous extracts of M. volkensii have also traditionally been used to control ticks and fleas in goats [19]. M. volkensii offers a key indigenous tree species that can be used to mitigate against desertification in arid and semi-arid lands [23], while also offering a high economic potential for the rural community in these regions [24]. This paper presents a concise review of published information on the phytochemical composition and potential application of M. volkensii in insect pest management.
Plants 2020, 9,143 3 of 12 indicating that both fruit powder and its cake could be used as safe ruminant feed supplement [21]. Its use as a fodder crop underscores its safety in mammals [20], and traditionally, it is used for the treatment of diarrhea, pain, skin rashes, and eczema [22]. Aqueous extracts of M. volkensii have also traditionally been used to control ticks and fleas in goats [19]. M. volkensii offers a key indigenous tree species that can be used to mitigate against desertification in arid and semi-arid lands [23], while also offering a high economic potential for the rural community in these regions [24]. This paper presents a concise review of published information on the phytochemical composition and potential application of M. volkensii in insect pest management.

Biological Activity of Melia volkensii Extracts Against Insects
Crude fruit extracts from M. volkensii have been reported to pose activity towards a broad range of insect orders including Diptera, Lepidoptera, Coleoptera among others [19] as shown in Table 1. The methanolic fruit extracts were first reported to have antifeedant effects against Schistocerca gregaria Forsk. (desert locusts) [25]. Repellency effect, decreased mobility, retarded development and reduced fecundity were observed against S. gregaria when seed extract was applied to their preferred host plants mainly Schouwia thebaica Webb, Fagonia olivieri DC (fagonbush plant) and Hyoscyamus muticus Linnaeus (Egyptian henbane) in a field trial experiment [26]. Although the mode of action of the extracts is still unknown, it is postulated that the active compounds in M. volkensii extracts could affect hormone levels in S. gregaria larvae [27]. In fifth-instar nymphs of S. gregaria, 80% mortality was recorded 48 hours after injection with crude ethanolic and methanolic extracts at a concentration of 30 µg/g of the insect [19]. When sprayed on third-to fifth-instar S. gregaria, M. volkensii and neem oil have been reported to cause mortality of up to 91% and 92%, respectively, after 14 days in a comparative study [26]. In contrast to synthetic pesticides, these botanicals do not have a knock-down effect, but their slow response is similar to inhibitors of chitin synthesis [26].
Antifeedant and larval growth inhibitory effects of fruit extracts have been observed in Trichoplusia ni Hübner (cabbage looper) and Pseudaletia unipuncta Haworth (true armyworm) [25] and [28]. Crude seed extracts are also an effective growth inhibitor against T. ni (dietary EC50 = 7.6 ppm) and feeding deterrent (DC50 = 0.9 µg/cm 2 ) [29]. Prolonged exposure to M. volkensii extracts has been observed to lead to a decrease in antifeedant response when tested against T. ni implying that the insect could develop tolerance to the extracts [30]. However, when tested against Plutella xylostella Linnaeus (diamondback moth) and P. unipuncta, there was no significant decrease in feeding deterrent response to the extracts following continuous exposure [31]. It has been postulated that triterpenoids from seed kernels of M. volkensii are responsible for the insecticidal activity in T. ni [11]. Comparative efficacy has been observed with M. volkensii extracts, other Meliaceae plant extracts (A. indica, A. excelsa, M. azedarach, and Trichilia americana Sessé & Mocino) and commercial botanical insecticides (ryania, pyrethrum, rotenone and essential oils of rosemary and clove leaf) when tested against T. ni and P. unipuncta [32].

Biological Activity of Melia volkensii Extracts Against Insects
Crude fruit extracts from M. volkensii have been reported to pose activity towards a broad range of insect orders including Diptera, Lepidoptera, Coleoptera among others [19] as shown in Table A1 (Appendix A). The methanolic fruit extracts were first reported to have antifeedant effects against Schistocerca gregaria Forsk. (desert locusts) [25]. Repellency effect, decreased mobility, retarded development and reduced fecundity were observed against S. gregaria when seed extract was applied to their preferred host plants mainly Schouwia thebaica Webb, Fagonia olivieri DC (fagonbush plant) and Hyoscyamus muticus Linnaeus (Egyptian henbane) in a field trial experiment [26]. Although the mode of action of the extracts is still unknown, it is postulated that the active compounds in M. volkensii extracts could affect hormone levels in S. gregaria larvae [27]. In fifth-instar nymphs of S. gregaria, 80% mortality was recorded 48 hours after injection with crude ethanolic and methanolic extracts at a concentration of 30 µg/g of the insect [19]. When sprayed on third-to fifth-instar S. gregaria, M. volkensii and neem oil have been reported to cause mortality of up to 91% and 92%, respectively, after 14 days in a comparative study [26]. In contrast to synthetic pesticides, these botanicals do not have a knock-down effect, but their slow response is similar to inhibitors of chitin synthesis [26].
Antifeedant and larval growth inhibitory effects of fruit extracts have been observed in Trichoplusia ni Hübner (cabbage looper) and Pseudaletia unipuncta Haworth (true armyworm) [25,28]. Crude seed extracts are also an effective growth inhibitor against T. ni (dietary EC 50 = 7.6 ppm) and feeding deterrent (DC 50 = 0.9 µg/cm 2 ) [29]. Prolonged exposure to M. volkensii extracts has been observed to lead to a decrease in antifeedant response when tested against T. ni implying that the insect could develop tolerance to the extracts [30]. However, when tested against Plutella xylostella Linnaeus (diamondback moth) and P. unipuncta, there was no significant decrease in feeding deterrent response to the extracts following continuous exposure [31]. It has been postulated that triterpenoids from seed kernels of M. volkensii are responsible for the insecticidal activity in T. ni [11]. Comparative efficacy has been observed with M. volkensii extracts, other Meliaceae plant extracts (A. indica, A. excelsa, M. azedarach, and Trichilia americana Sessé & Mocino) and commercial botanical insecticides (ryania, pyrethrum, rotenone and essential oils of rosemary and clove leaf) when tested against T. ni and P. unipuncta [32].
M. volkensii fruit extracts when tested at concentrations ranging from 1 to 50 µg/µL showed feeding deterrence, growth disruption and mortality against Nezara viridula Linnaeus (stink bug), a polyphagous pest which attacks a variety of crops, including nuts, corn, cotton, grains and tomatoes [16]. The disruption of the molting process led to eventual mortality in N. viridula [16]. Furthermore, deformities and malfunctions like shortened or missing antennae, legs failing to detach from the exuvium, absent or shortened hemelytra, notching, and lack of symmetry have been observed in N. viridula when exposed to fruit extracts, with 10 µg/µL causing malformation in up to 85.70% of surviving adults [16]. A delay of the imaginal molt was observed in immature Coranus arenaceus Walker even though there were no deformities in resultant adults after topical application of the M. volkensii extracts at 1, 5, and 10 µg/µL [16].
Dried M. volkensii fruit extracts have shown growth-inhibiting activity against Aedes aegypti Linnaeus (yellow fever mosquito) larvae at 2 µg/mL in water, whilst recording high mortality during the molting and melanization process with LC 50 of 50 µg/mL in 48 h [13]. At a high dose (100 µg/mL), the extracts caused acute toxicity, while at a low dose, the lethal effect took a long time, indicating the presence of compounds with an acute toxic effect at a high concentration and a growth-inhibiting effect at a low concentration [20]. Growth inhibiting and disrupting effects in A. aegypti could be a result of synergistic effects of a plethora of limonoid compounds or a single active compound exerting these effects [20].
A column chromatography-purified fraction of M. volkensii fruit kernel extract showed growth-inhibiting activity against Anopheles arabiensis Giles with an LC 50 of 5.4 µg/mL in 48 h [13]. Mortality (LC 50 of 34.72 µg/mL in 48 h) and oviposition deterrence was observed in second-instar larvae of Culex quinquefasciatus Say (Southern house mosquito) when treated with refined methanolic fruit extracts [33]. The granular formulation of M. volkensii fruit acetone extract showed S-and U-shaped postures and frequent stretching in C. quinquefasciatus; such postures and stretching are a characteristic of mosquito larvae reared in M. volkensii fruit extract [34]. The test granules also caused 86% mortality in third-and fourth-instar larvae of C. quinquefasciatus within 36 h [34]. Acetone extracts from M. volkensii seeds have recorded growth inhibitory effects and equal toxicity (LD 50 of 30 µg/mL) for larvae and pupae of C. pipiens f. molestus Forskål (London underground mosquito) [17]. M. azedarach seed extracts recorded lower toxicity (LD 50 of 40 µg/mL) while pure azadirachtin A recorded higher toxicity (LD 50 of 1-5 µg/mL) against C. pipiens when compared with M. volkensii extracts [17]. The water solubility of the acetone seed extract from M. volkensii may indicate the presence of saponins as toxic principles thus making it an interesting candidate for application against aquatic insects such as mosquitoes and other vectors of diseases [17].

Phytochemistry and Insect Bioactivity of Melia volkensii
Insect antifeedants have been found in major classes of secondary metabolites-alkaloids, phenolics, and terpenoids [35]. However, it is in the terpenoids that the greatest number and diversity of Plants 2020, 9, 143 5 of 11 antifeedants, and the most potent, have been found. Most well-documented antifeedants are triterpenoids [35]. Effective insect antifeedants have been isolated from various parts of M. volkensii, as shown in Figure 2 and Table A2 (Appendix B), although azadirachtin, the major ingredient in neem seeds, does not occur in M. volkensii. This indicates that insect control bioactivity is, therefore, based on other compounds than azadirachtin [25]. It is postulated that the major active compound in M. volkensii fruit is more lipophilic than azadirachtin [20]. Botanical antifeedants are easily degraded after application thereby causing little environmental impact [36].
fruit is more lipophilic than azadirachtin [20]. Botanical antifeedants are easily degraded after application thereby causing little environmental impact [36].

Further Phytochemical Composition and Biological Activity of Melia volkensii
Other compounds have also been isolated from M. volkensii with different biological activities. These include volkensinin, as isolated from ethanolic extracts of M. volkensii root bark [44], which showed weak bioactivity in the brine shrimp lethality test BST (LC50 = 57 µg/mL) and weak cytotoxicity against six human tumor cell lines with ED50 values of 27.90, 28

Conclusions
Extracts and pure compounds isolated from M. volkensii have proved to be effective insect antifeedants and growth inhibitors. Extensive research has been done on mosquito control using M. volkensii; however, more research needs to be done on insect pests of agricultural importance. M. volkensii has no reported adverse effect on the environment or mammals, making it a potential botanical pesticide for the biosafe application in integrated pest management. The availability of renewable resources from the tree, such as fruits, stem bark, and leaves makes this plant a potential candidate for insect control with minimal interference on the plant. In this regard, M. volkensii could be further exploited as a source of natural insecticide.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.