Abstract: The long-term applications of synthetic insecticides have resulted in residues accumulating in different environmental components. They have adverse effects on non-target organisms, ecosystems and human health. Therefore, bio-insecticides “Green pesticides” have posted as an alternative to synthetic insecticides in agriculture and public health sectors. The study presented here focuses on the prospects of essential oils (EOs) as bio-insecticides for insect pest management. In fact, many EOs have insecticidal, fumigant, antifeedant, attractive and repellent activities against a broad spectrum of insects with some selectivity. The EOs are a complex of chemical compounds with multiple modes of action that enhances their activity due to the synergistic action between constituents. Due to their volatility in nature, EOs are used as a fumigant against agriculture and storage food insects. Consequently, EO-based insecticides are very important for control stored insects because they are active against a variety of insects, fast penetrating and no toxic residues in the treated products. In contrast, some problems (e.g., volatility, solubility and oxidation) of EO-based insecticides were recorded, which plays an important role in the EOs activity, application and persistent. For this reason, new formulations with nanotechnology “Nanoformulation” can resolve these problems and offer numerous advantages. So, encapsulating the EOs has a considerable perspective as commercial insecticide products. Finally, EOs-based insecticides are low toxic, environmental persistence and eco-friendly. Therefore, they are not having worker re-entry and harvest restrictions after treating crops. They are compatible with biological control programs and indigenous natural enemies of pests.
INTRODUCTION
Essential oils (EOs) are known as essences, volatile oils, etheric oils or aetheroleum. They founded in aromatic plants as a mixture of volatile components produced as secondary metabolites1. They are extracted from plant through steam or water distillation. Generally, EOs have densities lower than those of water2,3. They contain hundreds of individual compounds and their mixtures that give physical, chemical and biological characteristics of them4. Additionally, aromatic plants could be extracted by organic solvents to give oleoresins and by carbon dioxide to yield high-quality extracts free of solvents. Nevertheless, the solvent extraction is more hard and complicated processes than steam distillation. The EOs extracted by organic solvent produces volatile EOs and non-volatile flavor components that used in wide applications in food, agriculture and pharmaceutical industries4.
Recently, there has been an increasing interest in studying and evaluating the botanical insecticides (e.g., EOs) for pest management in both developing and developed countries as a result of insect resistant to the traditional insecticides5. Also, the long-term applications of synthetic insecticides have resulted in residues accumulating in different environmental components (e.g., water, food, air and soil)6. They have adverse effects on non-target organisms, ecosystems7 and human health8. Therefore, bio-insecticides (e.g., EOs) have posted as an alternative to synthetic insecticides in agriculture and public health sectors9. The use of nature-oriented as bio-pesticide (e.g., plant extracts, EOs, etc.) to control insect are called "Green pesticides". The term "Green pesticides" include all natural materials that can reduce the pest population and increase food production. Consequently, EOs are playing an essential role of pest control in organic food production globally10,11. According to Rozman et al.12 the botanical insecticide presented by 1% of global insecticide market. They have lesser extent of environmental and human health impacts than most synthetic pesticides.
It has been reported that about 17500 aromatic plant species growing worldwide in tropical environments and more than 3000 constituents have been identified13. Three hundred of these EOs are commercially used for pharmaceutical drugs, cosmetics and perfume industries3 beside pesticides14, 15. However, the highest plant families bear EOs is Apiaceae, Asteraceae, Combretaceae, Geraniaceae, Gramineae and Lamiaceae. In addition to Myrtaceae, Meliaceae, Piperaceae, Rutaceae, Verbenaceae and Zingiberaceae plant families16. The ability of angiosperms and gymnosperms plants to accumulate EOs is quite excessive in both plants. However, the most sources of important commercially EOs are related to angiosperms (flowering plants)17,18.
In the past few decades, various studies have investigated the insecticidal activity of EOs and their potential uses as bio-insecticides against important insect pests19-28.
The study presented here focuses on the prospects of essential oils as bio-insecticides for insect pest management.
CHEMISTRY OF ESSENTIAL OILS
Essential oils are naturally found in plants as secondary metabolites. They play an important role in plant defense system against microorganisms, insects, herbivores and allelopathic interactions29-31. Previous studies have reported that volatile compounds of EOs can be classified into four groups: (a) Terpenes, (b) Benzene derivatives, (c) Hydrocarbons and (d) Other miscellaneous compounds32. Based on the number of isoprene units in the chemical structures of terpenes, EOs are classified into (a) Hemiterpenes (1 unit, C5), (b) Monoterpenes (2 units, C10), (c) Sesquiterpenes (3 units, C15), (d) Diterpenes (4 units, C20) and (e) Consequently. It has been found that most terpenes in EOs are monoterpenes (C10H16) and sesquiterpenes (C15H24). However, monoterpenoids are the most terpenes and represented by 90% of essential oils33-35. Monoterpenes have a chemical structure varies greatly along with various functions. Monoterpenes and their related chemical structure compounds have 10-carbone hydrocarbon. These chemical structures divided into (1) Acyclic alcohols such as linalool, geraniol and citronellol, (2) Cyclic alcohols such as menthol, isopulegol and terpeniol, (3) Bicyclic alcohols such as borneol and verbenol, (4) Phenolic compounds such as thymol, carvacrol, (5) Ketones such as carvone, menthone and thujone, (6) Aldehydes such as citronellal, citral, (7) Acids such as chrysanthemic acid and (8) Oxides such as cineole34.
ESSENTIAL OIL EXTRACTION TECHNIQUES
Essential oils are found in flowers and other plant parts associated with other substances, e.g., gums and resins. However, different methods have reported for extracting and preparation essential oils from different parts of the plant. The methods or isolating techniques used for EOs extraction were playing an important role in differentiating EOs chemical compositions. These changes in the EOs chemical profile contain changing in the number of chemical compounds and the stereochemical types of molecules extracted34. In laboratory scale techniques, steam distillation is the common methods used for EOs extraction. In this method, Cleavenger apparatus was used for EOs extraction. However, under certain conditions of distillation process, many reactions may occur. These reactions include isomerization, saponification and other chemical reactions. These changes lead to alteration in the composition of the isolated oil36. In addition, other methods are used for isolation and extraction of EOs such as extraction by solvent and simultaneous distillation. Other methods are also used such as extraction by supercritical carbon dioxide and microwave ovens. Infact, many factors can affect the quality and the quantity of the product and their chemical composition, e.g., plant age, part, vegetative cycle stage, soil, fertilizer and climate37,38.
Steam distillation method is the most common and economical one used for the production of EOs. Nearly, most EOs from different sources are extracted by distillation techniques. There are three types of distillation operation, namely: (1) Water distillation that recommended mostly for dried plant material. Simultaneously, the plant material is in direct contact with boiling water, (2) Water and steam distillation that’s used for both fresh and dried plant materials (the plant material is at the bottom of the still and the plant material is in contact with saturated steam only), (3) Direct steam distillation that used for treat quantities of fresh plant material39,40. However, during steam distillation, certain components of the volatile oil tend to hydrolyze, whereas, other constituents of EOs that sensitive to heat and prolonged action of steam in the stillness can cause degradation and decomposition with the high temperature. Also, polymerization and resinification might also take place. Some components of essential oils dissolved in distilled water can’t be easily recovered41. Therefore, other methods are used to solve these components such as (1) Expression method used for Citrus peel sp., oil extraction, e.g., peel of the orange, lemon and bergamot, (2) Solvents extraction method used for oil extraction decomposed by the action of steam or oils are found in small quantities in the plant, (3) Carbon dioxide method, this new method for extraction of EOs using liquid and liquefied supercritical carbon dioxide gas42. (4) Continuous subcritical water extraction method, in this method, the EOs has been extracted by either steam distillation or solvent and (5) Microwave extraction method.
TECHNIQUES APPLIED FOR ANALYSIS OF ESSENTIAL OILS
It is well known that the composition of EOs is represented by terpenes as major constituents in addition to other compounds. These constituents contain monoterpenes, sesquiterpenes and their oxygenated derivatives. In addition to aliphatic aldehydes, alcohols, esters and phenol ethers. The most structurally varied classes of plant natural products are terpenes that derived from the repetitive fusion of branched five-carbon units (isoprene unit)43. The analytical methods applied in the characterization of EOs mainly depend on the number of molecular species. In addition, the essential oil, chemical profile is closely related to the extraction procedure employed. Therefore, the choice of the appropriate extraction method becomes crucial. Most of the methods applied in the analysis of essential oils rely on chromatographic procedures. These methods can enable the separation and identification of components44. The most frequent methods applied to EOs analysis is Gas Liquid Chromatography (GLC). In this technique, detectors such as a Flame Ionization Detector (FID), Thermal Conductivity Detector (TCD) and others can be used for EOs analysis. Another technique for EOs analysis and identification is gas chromatography-mass spectrometry (GC-MS). This is the most common, simple and easy for identification by comparison of the required unknown mass spectra with those identified in reference MS library45.
INSECTICIDAL PROPERTIES OF ESSENTIAL OILS
The chemical composition of EOs is lipophilic, which can enter into insect and cause biochemical dysfunction and mortality46. The toxicity of EOs does not only depend on the chemical compounds that act as toxins but also on many other factors playing an important role in the toxicity. The point of entry of the toxin, molecular weights and the mechanisms of action are factors of EOs to induce toxicity. It has been reported that common essential oils with insecticidal activities can be inhaled, ingested or skin absorbed by insects47. Previous studies have confirmed that terpenoids such as monoterpenoids have insecticidal activity against different insects19-28,46,48-50.
Literature survey on the potential use of EOs as bio-pesticides indicated that EOs obtained from the plant families, including Asteraceae, Myrtaceae, Apiaceae, Lamiaceae and Rutaceae have insecticidal activity. They are active as a repellent, fumigant, larvicidal and adulticidal against insects in Lepidoptera, Diptera, Coleopteran, Hemiptera and Isopteran orders34.
| Fig. 1: | Chemical structure of some monoterpenes found in essential oils with insecticidal activities |
For example, EOs obtained from Artemisia species have insecticidal activity against some insects such as coleopteran beetles. They have vapor toxicity and repellent action against these insects51-53. Similarly, the numbers of monoterpenes were evaluated for their insecticidal activity. Monoterpenes (e.g., β-thujaplicin) obtained from the wood of Western crimson cedar (Thuja plicata) has insecticidal activity against larvae of the old house borer (Hylotrupes bjulus)54. In contrast, the low insecticidal activity from α and β-thujaplicin was observed against the termite, Reticulothermes flavipes. The (+)-3-thujone and (-)-3- iso-thujone accounted to 80-90% of the leaf EOs of this plant and have antifeedant activity against the white pine weevil54. Thymol has a contact toxicity to larva Agriotes obscures (L.) (Coleoptera: Elateridae)55. In this respect, several reports have shown insecticidal activity of terpenes such as citronellal56,57, camphor53,58,59, 1,8-cineole60,61, a-terpineol62 and α-pinene63. In addition, the insecticidal activities of EOs constituencies were evaluated. For example, pure geraniol, citronellol, citral and carvacrol have insecticidal activity. Also, cuminaldehyde, limonene, d-limonene, menthone, trans-anethole, thymol, 1,8-cineole and cinnamaldehyde have insecticidal activity against different insects19,56,57,64,65,66. Many compounds have been isolated, identified and evaluated for their insecticidal activities, e.g., cineole, geraniol and piperidine67, linalool and limonene68. Some studies have reported insecticidal activity of piperitone against Callosobruchus maculates69 and antifeedant activity of trans-ethyl cinnnamate70. Abdelgaleil et al.71 has reported that both piperitone and trans-ethyl cinnamate obtained from Artemisia judaica L., EO showed antifeedant activity against Spodoptera littoralis (Boisd). However, Fig. 1 shows some chemical structure of terpenes found in some plant essential oil with insecticidal activities.
ESSENTIAL OILS AS BIOINSECTICIDES
Essential oils and other biopesticides are becoming a new trend for pest management in modern farming and organic agriculture. For 2 decades, EOs are used as insecticides to control insects, however, they have not reached their full potential because they are rapidly volatile and low residual activity. They are considered safe, environment-and-eco-friendly, compatible with biological control programs and have low mammals toxicity. Also, EOs are available worldwide for their low to moderate cost. The main active constituents with insecticidal activity are monoterpenes, sesquiterpenes and related phenylpropenes. The EOs can be applied as pesticides as they are or as their active components "Active ingredient" or as co-adjutants in pesticide formulations. However, they can be acted as a contact insecticide, causing changes in the pathways of biochemical metabolism of the insect, knockdown and rapid death72, fumigants73, repellents74 and antifeedant71.
Repellent activity: The repellent activity of EOs from numerous plants is well documented. This activity is related to major active compounds and other chemical constituents7,75. The repellent active compounds isolated from Limnophila geoffrayi and Schizonepeta tenuifolia are pulegone, linalool, eugenol, thymol and methyl chavicol76,77. While, other repellent compounds such as a-pinene, b-pinene, D-limonene, (E)-3, 7-dimethyl-, 2, 6-octadienal were isolated from Armoracia rusticana, Pimpinella anisum, Allium sativum, Laurelia sempervirens and Drimys winteri78,79. However, most of repellent activity studies were conducted on pests belonging to Coleoptera and Diptera species. Abd-Elhady80 reported that EOs obtained from Artemisia judaica L., has repellent activity against the cowpea weevil, Callosobruchus maculatus (Fab.). It is concluded that EO at concentrations of 4.0, 8.0, 15.9, 31.9 and 63.7 μg cm–2 induced reduction in egg laying of C. maculates by 12.5, 42.7, 61.8, 86.0 and 92.5%, respectively. Garlic and mint EOs were evaluated against the cowpea aphid (Aphis craccivora Koch.)81, which caused 100% repellent at a concentration of 4.0% on adult aphids, while the effect was decreased in a concentration-dependent manner. They also concluded that garlic oil has higher repellent than mint oil. This repellent action of EOs may be related to the major constituents, e.g., piperitone, camphor and (E)-ethyl cinnnamate.
An essential oil from Citrus limonum and Litsea cubeba have repellent activity against adults of Tenebrio molitor82. The authors reported that the repellent activity of these EOs may be due to the presence of D-limonene and 3,7-dimethyl-6-octenal of C. limonum EO and (E)-3, 7-dimethyl-2, 6-octadienal and (E)-cinnamaldehyde of L. cubeba. Zapata and Smagghe79 showed that four EOs from the leaves and bark of Laurelia sempervirens and Drimys winteri have highly repellent activity against Tribolium castaneum. The repellent activity of L. sempervirens oil was 90% at low concentration 0.032 μL cm–2, while the same effect was observed by higher concentration (3-10 times) of D. winteri oils after 4 h of exposure.
Pavela83 evaluated 10 EOs for their repellent activity against Meligethes aeneus. They reported that EOs isolated from Carum carvi and Thymus vulgaris had the highest repellent activity and the repellent index accounted to 65.6 and 63.8%, respectively, while other EOs has a repellent index lower than 15%. Abdel-Sattar et al.84 showed that EOs of Schinus molle (fruit and leaf) had insect repellent activity against Trogoderma granarium and Tribolium castaneum. They found that p-cymene was identified as a major component in fruits and leaf oils. In addition, GC-MS analysis demonstrate that monoterpenes presented the highest amounts and accounted for 80.43 and 74.84% in fruits and leaves, respectively. They also concluded that the repellent activity of the leaf and fruits of S. molle L. The EOs were 75.02 and 71.89%, respectively at a concentration of 1000 μL cm–2. In this regard, the repellent activity of EO obtained from the leaves induced higher activity than that of the fruit.
The repellent activity of EOs obtained from Mentha longifolia and Thymus kotschyanus was evaluated against Callosobruchus maculates85. The repellent values at a concentration of 800 ppm were recorded 90 and 73.33% for M. longifolia and T. kotschyanus against C. maculates, respectively. Negahban and Moharamipour53 found that EO of Artemisia sieberi at 1.5 ppm was more repellent to Tribolium castaneum than Sitophilus oryzae and Callosobruchus maculatus. The repellent values amounted 65.90, 59.70 and 55.80%, respectively. In contrast, A. sieberi EO showed a strong repellence activity against T. castaneum, S. oryzae and C. maculatus 86. The repellent activity recorded 63.80, 62.01 and 48.57% for T. castaneum, S. oryzae and C. maculatus, respectively. Nikooei and Moharramipour87 found that EO of Salvia mirzayanii at a concentration of 3 ppm had a strong repellent activity against Tribolium confusum (86.66%) than on C. maculatus (70.0%). In addition, EOs of Anethum graveolens L. (Apiaceae), T. vulgaris and Rosmarinus officinalis produced a strong repellent activity against T. confusum than on C. maculatus. The repellent values of these EOs were 100, 100 and 93.33%, respectively88. Saeidi et al.89 showed that the EO of Citrus reticulate has higher repellent activity than C. limon and C. aurantium against Callosobruchus maculatus.
| Table 1: | EOs of some plants with repellent activity |
The repellent values of C. reticulata (peel essential oils) at a concentration of 1, 3, 5 and 7 ppm were 26.66, 33.33, 33.66 and 40.0%, respectively. The researchers confirmed that at concentration 7 ppm, the repellent activity of C. reticulata EO was higher than that of C. aurantium. The strong repellent activity of Penstemon acaulis EO was found against S. oryzae, C. maculatus and T. castaneum. The highest concentration 2 μL mL–1 of P. acaulis EO shows high repellence with values of 83.6, 71.6 and 63.6% for adult insects of S. oryzae, C. maculatus and T. castaneum, respectively90.
The EOs extracted from plants with higher repellence included many active compounds. Examples of these active compounds are citronella, cedar, verbena, pennyroyal, geranium and lavender76-78. The repellent action was induced by pine, cinnamon, rosemary, basil, thyme, peppermint, p-cymene and pulegone. Also, linalool, eugenol, thymol, cymol and methyl chavicol have repellent activity against several insects76-78,91. Natural terpenoids found in crushed basil (Ocimum basilicum L.) leaves repels some thrips species92. These results show that linalool and eugenol have repellent activity. Also, the high repellent activity was shown by caryophyllene and 1,8-cineole. Some active compounds obtained from different EOs include methyl salicylate93 and salicylaldehyde94 have repellent activity. Table 1 illustrates examples of plants with repellent activity.
Insecticidal activity: The insecticidal activity of many EOs has been evaluated against a number of insects (Table 2). Previous studies have shown that the toxicity of EOs against a variety of insects is related to terpenes. They represent major of EOs components, especially monoterpenoids and sesquiterpenes57,98-101. Many EOs were analysis by GC/MS and the main active components were identified and tested on a variety of insects. For example, camphor (over 400 ppm) the major component of Coriandrum sativum and Carum carvi L. The EOs have high toxicity to Rhyzopertha dominica and Cryptolestes pusillus. While linalool (1617 ppm) found in coriander seed EO is toxic to Sitophilus oryzae. Carvone, limonene and (E)-anethole are the major active component found in caraway EO. Carvone (972 ppm) had high insecticidal activity against Sitophilus oryzae57,98,99.
The insecticidal toxicity of many EOs and their chemical constituents were evaluated. For example, 1,8-cineole, carvacrol and eugenol have insecticidal activity57,98-100. In addition, limonene, α-pinene and thymol have a toxic effect against some insects57,98-100. The volatile toxicity of Coriandrum sativum and Carum carvi L., EOs were evaluated in the laboratory.
| Table 2: | EOs of some plants with insecticidal activity |
Where they show strong insecticidal activity against S. oryzae, Rhyzopertha dominica and Cryptolestes pusillus. The high activity of these oils can be due to the occurrence of linalool (1617 ppm of the oil) as a major compound57,98-102. Also, camphor-rich fractions (over 400 ppm) were very toxic to Rhyzopertha dominica and Cryptolestes pusillus57,98-102. The insecticidal activity of neemix® (4.5% EC, neem oil produced from Azadirachta indica) and basil oil (Ocimum basilicum) against Aphis craccivora Koch was evaluated22. They have shown insecticidal activity against A. craccivora Koch when the faba bean plants were treated systematically or by contact. They caused toxicity to adult stage of A. craccivora Koch and the accumulative mortality reached 100% after 7 and 8 days of treatment by basil oil and neemix oil, respectively. It has been reported that methyl chavicol, linalool and geranial are the major active component in basil oil. Therefore, the high insecticidal activity of this oil may be due to the presence of these active compounds99. In addition, many studyies have reported the insecticidal activity of essential oil against economic agriculture insects. Other studies reported toxicity of volatile methyl chavicol, linalool and geranial against insects, e.g., whitefly and cotton leaf worm103,104.
In laboratory studies, EOs of rosemary (Rosemarinus officinalis), sage (Salvia officinalis) and curcuma (Curcuma longa) plants were formulated26. The formulated oils were applied as a soil treatment (systematically) or spray (contact) against sucking aphid (Aphis craccivora Koch) on Vicia faba. The accumulative mortality after 1 week of treatment by curcuma, sage and rosemary at a concentration of 1%, were 49.98, 63.33 and 93.33%, respectively105. Insecticidal activity of some terpenes such as carvone, linalool and terpeniol, which isolates from EOs (garlic and mint) were evaluated against Agrotis ipsilon. In addition to isolated phellandrine and citronellol were tested against Agrotis ipsilon (eggs, larva and pupae). Terpenes had different toxicity on eggs, larvae and pupae with different effects on growth and development. The high toxicity to egg and pupal stages were induced by phellandrine, while nerol acted as a stomach or contact poison to the second larval instar. They stated that combination exposure to citronellol plus garlic oil or mint oil increased their toxicity105. After 8 days of treatment with Salvia officinalis EO at concentration 4%, the mortality was 75% of Agrotis iIpsilon adult. The EO caused a high reduction in egg deposition (67.4%) and egg hatchability (69.4%) at sub-lethal concentration. Consequently, they induced more than 31.2% sterility in females25. It is reported that the insecticidal activity of Salvia officinalis might be due to the presence of terpenes and sesquiterpenes. The active components are camphene, α-pinene, β-pinene, myrcene and limonene. Moreover, 1,8-cinieole, α-thujone, β-thujone, camphor, linalool, bornylacetate and borneol have a high toxic effect100.
The insecticidal activity of EO obtained from clove buds (Syzygium aromaticum) and their major constituents were evaluated against Callosobruchus chinensis106. At laboratory scale, their active components such as eugenol, eugenol acetate and β-caryophyllene were studied in the laboratory stage. The contact toxicity of EO and commercial eugenol was 99.00% and β-caryophyllene was 98.00%. The LD50 values for adults of C. chinensis found to be 0.730, 0.673 and 0.708 μg per adult, while that of nymphs were 1.795, 1.668 and 1.770 μg per nymph due to EOs, eugenol and β-caryophyllene, respectively. In comparison, LD50 values of commercial eugenol acetate (98%) were found106 to be 92.66 and 99.42 μg per nymph. Lopez et al.107 reported that Coriandrum sativum oil was very toxic to S. oryzae, R. dominica and C. pusillus and their main active ingredient linalool at a concentration of 1617 ppm. While, the fractions of camphor (over 400 ppm) were highly toxic to R. dominica and C. pusillus. The most effective component of caraway EO is carvone (972 ppm) a monoterpenoids that more toxic to S. oryzae. Sharaby et al.108 evaluated the toxicity of essential oils of garlic, mint and Eucalyptus against the grasshopper (Heteracris littoralis). After 2 weeks of feeding on diet containing different concentrations of EOs, the LC50 values were estimated. The LC50 of these EOs were 0.067, 0.075 and 0.084 mL/100 mL diet of garlic, eucalyptus and mint, respectively. Liu et al.109 reported contact toxicity of Artemisia capillaris and A. mongolica EOs against Sitophilus zeamais, which were 105.95 and 87.92 μg per adult insect, respectively. The insecticidal activity may be due to the main components found in the EO as 1,8-cineole, germacrene D and camphor of A. capillaries EO and α-pinene (12.68%), germacrene D and γ-terpinene of A. Mongolic EO109. Khani and Asghari110 evaluated the EO of Pulicaria gnaphalodes against T. confusum and C. maculatus in the laboratory. They found that the major active components found by GC/MS of EO were linalool (57.57%) and geranyl acetate (15.09%). The toxicity of EO was studied by using the older adults (1-7 days) of insect. Mortality of insects was increased with increasing concentrations from 43-357 μL L–1 air along with exposure time from three to 24 h. The LC50 values of EO were 1.34 μL L–1 air on C. maculatus and 318.02 μL L–1 air of T. confusum. The toxicity of P. gnaphalodes EO can be due to their active ingredients, linalool107 and camphor111.
Origanum vulgare L. (oregano) and Thymus vulgaris L. (thyme) EOs were tested against Nezara viridula. Their active ingredients were p-cymene for oregano and thymol for thyme oil112. Their LC50 were 1.7 and 3.5 μg cm–2 for oregano and thyme for nymphs and 169.2 and 48.8 μg cm–2 for adults, respectively. Other studies confirmed that Triaenops persicus had insecticidal activity against adults of T. castaneum and S. oryzae91. However, the mechanism of toxicity of oregano and thyme may be due to their neurotoxic effect. In this regard, EO and their active compound thymol can interact with neuromodulator octopamine. They also can induce neurotoxicity via effects on gated chloride channels GABA. Thymol reported also to induce high toxicity to some insects such as Lipaphis pseudobrasicae113, Spodoptera litura56 and S. oryzae12. Another volatile oil of Artemisia judaica L., was specific against the cowpea weevil, C. maculatus. The seeds of the cowpea were treated with A. judaica EO with sub-lethal concentration, that has insecticidal activity against C. maculatus insect80. Their toxic effect was attributed to piperitone (32.4%), camphor (20.6%) and (E)-ethyl cinnnamate (8.2%). The previous compounds are monoterpenoids which are lipophilic have a fast penetration properties into insects which consequently interfere with biochemical and physiological functions114. Oils of S. hortensis, T. serpyllum and O. creticum at a dose of the 100 μg per larva were investigated. They induced more than 90% mortality in larval of tobacco cutworm, S. litura after 24 h of treatment. The LD50 values of S. hortensis and T. ulgaris were 48.4 and 46.9 μg per larva, respectively. The insecticidal action of T. ulgaris and S. hortensis species can be due to the presence of monoterpenoids, phenols, i.e., thymol and carvacrol115. However, lethal and sub-lethal doses of EOs can cause mortality and change in the fertility of insects56,116.
The EOs of Origanum onites, Satureja thymbra at concentration 9 and 25 μL L–1 air was investigated against E. kuehniella and P. Interpunctella. After 24 h of treatment, the mortality was 100% for both insects5. Pavela117 evaluated the toxicity of 34 EOs against S. littoralis larvae. The insecticidal activity of Nepeta cataria and Thuja occidentalis EOs against S. littoralis larvae were investigated by Wang et al.82. They found that the LC50 of these EOs were ≥10.0 mL m–3. The EOs from other plants such as Mentha citrata, Salvia sclarea, Origanum vulgare, Origanum compactum, M. officinalis, T. mastichina and L. angustifolia have highly toxic effect against S. littoralis with LC50≤0.05 μL per larva82.
Fumigants activity: Essential or volatile oils naturally are liquid at room temperature and get easy to change to vapors at room or with slightly higher temperature without any decomposition91. Therefore, the volatile oils are frequently used as a fumigant against insects in agriculture (greenhouse) and stored food5,118. Fumigants of EOs are very important for control-stored insects. The EOs have a broad activity spectrum against a variety of insects, by fast penetrating and non-toxic residues in the treated products118.
| Table 3: | EOs of some plants with fumigant activity |
For example, EO of Carum copticum has fumigant toxicity against S. oryzae and T. castaneum119. The high toxic effect of C. copticum EO was recorded at concentration 185.2 μL L–1. They have fumigant toxicity against both insects and the LC50 were 0.91 μL L–1 for S. oryzae and 33.14 μL L–1 for T. castaneum. The mortality of S. oryzae reached 100% after 12 h of exposure time. The fumigant toxicity of C. copticum could be attributed to the presence of thymol and other monoterpenoids found as major constituents. Monoterpenoids are volatile and they induced toxic action quickly as fumigant due to their fast penetration into insects61. Some examples of plants EOs has fumigant activity are shown in Table 3.
It has been reported that pulegone, linalool and limonene have fumigant toxicity against S. oryzae. In addition, linalool and linalayl acetate, that found in Mentha citrata EO are fumigate toxicity against this insect120. However, Apiaceae family is one of the most promising fumigant agents. The fumigant activity of some EOs of this family has reported against several stored product insects. For example, Kim et al.121 found that over 90% mortality of S. oryzae and C. chinensis was induced by Foeniculum vulgare after 3 or 4 days of treatment. Fumigant toxicity of 22 EOs against Acanthoscelides obtectus was tested122. They found that monoterpenoids such as thymol, carvacrol and terpineol are effective in inhibiting insect reproduction. Chaubey123 evaluated the fumigant toxicity of EOs from A. graveolens and C. cyminum against C. chinensis. The LC50 values were 10.8 and 11.0 μL L–1 for adult of C. chinensis after 24 h of exposure. Park et al.77 pointed out that the EO of S. tenuifolia at concentration 12.5 μg mL–1 air caused 96.6% mortality in larvae of L. ingénue. They reported that the fumigant toxicity of S. tenuifolia can be attributed to pulegone, menthone and limonene as a major active compound of volatile oil of S. tenuifolia. The LC50 values of these components were 1.21, 6.03 and 15.42 μg mL–1, respectively. In addition, more than 90% mortality in J. termite, R. speratus Kolbe was recorded after 3 days of fumigant treatment with EO of O. japonica at concentration 3.5 μL L–1 air124. In contrast, during the first 2 days of treatment, 100% mortality was recorded of I. verum, C. roborowskik, S. tenuifolia, A. cepa, A. sativum and E. caryophyllata EO at concentration 2.0 μL L–1 air.
Various studies have reported strong fumigant activity of EOs against storage insects. Fang et al.125 reported that EO of caraway had fumigant toxicity against S. zeamais and T. castaneum.
| Table 4: | EOs of some plants with antifeedant activity |
The LC50 was 3.37 and 2.53 mg L–1 for S. zeamais and T. castaneum, respectively. Fumigate toxicity of some EOs against T. castaneum adults were studied. For example, the LC50 of EOs were 6.84 mg L–1 and LC50 4.51, 19.47, 20.50, 11.39 and 9.0-10.5 μL L–1 of M. exotica126, T. polycephalum127, L. sempervirens, D. winteri79 and A. vestita128, respectively. Motazedian et al.129 studied the fumigant toxicity of Z. multiflora, N. cataria, T. minuta and A. sieberi EOs against the adults of B. brassicae. All EOs were toxic in a concentration-dependent manner. After 24 h of treatment with N. cataria EO, the mortality were 24, 36, 46, 52, 76 and 94% of concentrations 3, 8, 16, 31, 63 and 126 μL L–1 air, respectively. The fumigant toxicity of N. cataria EO may be due to -α, 7-β, 7a-α-nepetalactone which accounted 76.8% of this EO as shown by GC/MS analysis. Also, Tayoub et al.100 recorded the fumigant toxicity of EO obtained from S. officinalis against T. granarium. They reported that the activity of S. officinalis can be due to their occurrence of monoterpenes (57.3%) and sesquiterpenes (41.7%). The EOs of P. gnaphalodes and A. wilhelmsii were tested for fumigant toxicity against C. maculates and T. castaneum110. They found that the LC50 values ranged from 1.54-2.65 μL L–1 air against C. maculates. While, LC50 were 10.02 μL L–1 air of A. wilhelmsii and M. longifolia against T. castaneum, respectively.
Antifeedant activity: Antifeeding are compounds or substances that reduce insect feeding or consumption. This substance induces deter feeding by modification the insect behavior, through a direct action on peripheral sensilla of insects130. The antifeedant activity of many EOs has been tested against a number of insects as shown in Table 4. Huang et al.131 reported that pogostone which isolated from Pogostemon cablin EO has highly antifeedant activity against Spodoptera litura and Spodoptera exigua. At concentration, 250 mg L–1 the time for feeding was shorter. Also, antifeedant activities were found 92.03 and 99.32% at concentration 4000 mg L–1 against S. litura and S. exigua, respectively. Abdelgaleil et al.71 evaluated two antifeeding compounds (piperitone and trans-ethyl cinnamate) from EO of A. judaica against the third instar larvae of S. littoralis. Antifeedant activity (100% inhibition) was recorded at concentration 1000 μg mL mg L–1. Sousa et al.132 evaluated the antifeedant activity of EOs from A. graveolens, P. crispum and C. cyminum and 11 pure compounds against Pseudaletia unipuncta. They found that EOs from P. crispum and A. graveolens fruit and two pure compounds (trans-anethole and cuminaldehyde) have antifeedant activity and the inhibition of the larvae feeding account more than 70%. The antifeedant activity of Cymbopogon nardus, C. flexuosus and C. martini was determined against larva of A. fusca and E. elaeasa. Strong antifeedant activity was observed at concentrations between 0.002-0.600 μL cm–2 of these EOs133.
Shukia et al.134 evaluated the antifeedant activity of E. adenophorum and A. nilagirica against adults of R. ferrugineus. After 96 h of treatment, both EOs shown significant antifeeding effects at concentrations 1000 ppm. The feeding prick-marks of adult insect were 60.13±11.31 and 60.13±7.94 for both EOs, respectively. The antifeedant activity of EOs can be due to its main constituents, camphor. Moreover, they reported that the minor constituents of EOs play an important role in changing the activity by synergistic effects. However, it has been reported that EOs plays an important role in plant defense systems. In general, the mixture of the chemical composition of EOs is more effective than that of individual pure compounds. Therefore, synergistic effects between EOs components are playing an essential role in the EOs activity135,136. Saroukolai et al.137 evaluated the antifeedant activity of some EOs from Satureja khuzistanica, O. basilicum and Myrtus communis in addition to EOs from T. daenensis, M. spicata and E. caryophyllus against L. decemlineata. They found that the EO of S. khuzistanica was the most antifeeding activity compared to the other EOs against 4th instar larvae and adult insect. Ebdadollahi138 studied the antifeedant activity of Eucalyptus globulus and Lavandula stoechas EOs against T. castaneum. All the tested EOs caused significant reductions in the feeding of insects. Also, either the oils or their vapors had antifeedant activity and the effect was increased by increasing EOs concentrations.
Bekircan et al.139 found the strong antifeedant activity of EO obtained from Thymus leucotrichus against Aphrophora alni after 3 days of treatment. The EO at concentration 2000 ppm caused the highest antifeedant index (AFI). Also, the AFI value was 41.055 after 2 days of treatment. Another study by Rana et al.140 showed the antifeedant activity of Vitex negundo EO against C. chinensis and S. oryzae. They found that EO at concentration 0.062-0.5% possess moderate antifeedant effects against C. chinensis and S. oryzae. While, at concentration 0.25% and upto 0.58 and 1.69% had strong antifeedant activity against both insects.
The highest antifeedant activity was observed after treatment with the Vinca rosea and Callistemon lanceolatus EOs and their combinations at ratio 1:1, 1:3 or 3:1 v/v against Helicoverpa armigera141. The percent of feeding inhibition (AI50) was highest for V. rosea and C. lanceolatus EOs. The AI50 values were 5536 ppm and 6346 ppm for V. rosea and C. lanceolatus after 1 day of treatment. Strong antifeedant activity was recorded after treatment with the EOs combination at ratio 1:1. The AI50 was 4808 ppm of C. lanceolatus at concentration 10%, which induced 100% deterrence of feeding of H. armigera. After 4 days of exposure, the weight gain of the larva was decreased. Saroukolai et al.137 determined the feeding index of EOs of S. khuzistanica, O. basilicum, M. communis, Thymus daenensis, M. spicata and Eugenia caryophyllus against Leptinotarsa decemlineata. All EOs shownantifeedant activity against 4th instar larvae and adults of insect. The EO of S. khuzistanica show highest antifeedant activity than other EOs. The antifeedant activity can be due to the main constituent, carvacrol, which has an insecticidal activity against important insects of agriculture and stored products114,142. It has been found that the eugenol obtained from Laurus nobilis (Lauraceae) EO has antifeedant activity against Mythimna unipuncta143. Moreover, some EOs such as Artemisia tridentata, P. tridentate and Chrysothamnus nauseosus shown antifeeding properties against L. decemlineata144. The EO of S. officinalis at concentration 4% had antifeeding activity (63.6%) against 2nd instar larvae of A. ipsilon23.
Insect Growth Regulators (IGR) activity: Previous studies reported that several EOs and their constituents have properties similar to juvenile hormone and act as IGR (Table 5). They cause disruption in growth and affect the reproduction of insects101,145,146. For example, EOs obtained from several plants such as A. graveolens, C. cyminum and I. verum have reproductive activity against some insects. Also, EOs from M. fragrans, N. sativa, P. nigrum and T. ammi were induced changes in growth and reproduction of C. chinensis123. This disruption in growth of insects could be due to the inhibition of different biosynthetic processes of insects at different growth stages145. Abbas et al.147 reported that EO of C. reticulata induced growth inhibition and decline of R. domonica insect population. Several essential oils are good inhibitors of pest’s oviposition in that way they lead to disturbing the general growth of the populations. The EO of E. cardamomum has high activity on oviposition deterrence of C. maculatus. Therefore, treatment with this EO was reduced the numbers of insects in the treated grain148. In the same way, EO obtained from Citrus peels caused a high reduction in oviposition of insects e.g., C. maculates135,149. Both neemix and basil oil caused the prolongation of the nymphal duration and reduce the number of adult stage of A. craccivora22. Anshul et al.150 studied the effect of EO of A. annua on the reproduction of larvae of H. armigera. The diet contains EO-induced 69.71% reduction in larva weight after feeding on the diet. Also, after treatment with A. annua EO, the larva stage was extended to 19.5 days.
| Table 5: | EOs of some plants with IGR activity |
Maggi et al.151 reported that the reduction in growth and weight of larva might due to the disruption of insect hormone. In addition, EO of P. interpunctella inhibited the fertility of adult A. annua. The change in fecundity and fertility may be due to decrease in biochemical parameters. For example, protein and lipid were decreased after larva treatment with M. azedarach152. Basil oil caused changes in biochemical biomarkers (AST, ALT and ALP) in A. craccivora22. Several studies have been shown the oviposition deterrence of various EOs such as T. kotschyanus on C. maculatus85. In addition, T. vulgaris EO caused oviposition on C. maculates153. In the laboratory, oviposition deterrence was recorded of C. copticum and V. pseudonegundo EOs on C. maculates154. Jeyasankar155 observed that larva of Agrotis ipsilon exposed to gaultheria oil shows the highest deformed development of larvae, pupae and adults.
Attractants activity: Essential oils are a complex mixture of different groups of chemical compounds. They contain monoterpenes, sesquiterpenes and phenylpropanoid compounds. The EOs have numerous activities against insects such as toxic, fumigant, repellent, ovicidal, larvicidal and antifeedant activities. Moreover, many volatile compounds in EOs such as monoterpenes and other are documented attracts activity which helpful for the control and monitoring the insect pests121. For example, Hernandez-Sanchez et al.158 reported that p-cymene and limonene that isolated from Mangifera indica fruit had attractant activity. They have a high attractants activity to male and female Ceratitis capitata. Katerinopoulos et al.159 found that 1,8-cineole is the main constituent of Rosmarinus officinalis essential oil. Frankliniella occidentalis is attracted by 1,8-cineole. Also, it is reported that 1,8-cineole, p-anisaldehyde, linalool and salicylaldehyde can be used for control or monitoring of F. occidentalis on greenhouse crops. It has been reported that essential oil from the peels of Citrus limonum that contains a number of terpenes like geraniol has attracted the insects. They attract thrips and Japanese bettles. Also, they attract several insects when the EOs adds to traps, for example, thrips, leafminer, aphid and whitefly91. In addition, cis-jasmone attracted adult Lepidoptera160 and sandalwood oil, basil oil and grapefruit oil has shown a high attractiveness for whitefly161.
MECHANISM OF ACTION OF ESSENTIAL OILS
The mechanism of toxicity and site of action of EOs and their chemical compounds as bio-insecticides were investigated130,162. The biological activity of EOs and their active components against insects were reported. They have insecticidal57,98-102, repellent75-81, antifeedant130-133 and IGR101,145,146 activities. This fact shows that the EOs can disrupt insect physiology in different ways. However, the activity of EOs as insecticides shows to be the action of their effect on the insect nervous system.
| Fig. 2: | Octopamine, acetylcholine and γ-aminobutyric acid neurotransmitter |
| Fig. 3: | Proposed mechanism of EOs-induced toxicity to insects |
This effect, including the neurotoxic action of EOs by inhibition of acetylcholinesterase (AChE) or by blocking the octopamine receptors163. Also, γ-aminobutyric acid (GABA) receptor was suggested as one of the mechanisms of EOs induce toxicity in insects. They can interfere with GABA-gated chloride channels in insects164. For example, some monoterpenes such as thujone can induce the neurotoxic effects by acting GABA receptors in insects165,166. Besides, these modes of action of EOs on insects, some effects on the hormone and pheromone system and cytochrome P450 monooxygenase were reported167. However, some insect neurotransmitter is shown in Fig. 2.
Infact, octopamine is a neurotransmitter, neurohormone and circulating neurohormone-neuromodulator. Its disruption results in stop working of the nervous system in insects163. The octopamine receptor was not found in vertebrates. Therefore, EOs as bioinsecticides has selective toxicity to mammalian. Concerning the anti-cholinesterase effect of EOs, the mechanism of neurotoxic action and symptoms are similar to that induced by organophosphates and carbamates insecticides130. The symptoms show hyperactivity, convulsions, tremors and then paralysis163.
Several studies point out that EOs and monoterpenoids induce mortality by inhibiting AChE activity in insects. The neurotoxic effect of EOs may be due to the inhibition of AChE at the hydrophobic site168. However, some monoterpenoids have anticholinesterasic e.g., pulegone162, 1,8-cineole169, fenchone, carvone and linalool170. In addition, the toxicity of EOs constitutions has resulted in the octopaminergic nervous system of insects163. In contrast, Emekci et al.171 reported that monoterpenes can effects on many targets in insects with numerous mechanism of toxicity. However, proposed mechanism of EOs-induced toxicity to insects are illusterated in Fig. 3.
STRUCTURE-ACTIVITY RELATIONSHIPS
The relation between chemical structures of EOs constitutes and their biological activity were studied. It has been reported that slight change in molecular structure can make strong changes in their biological activity. For example, the modifications of the chemical structure of monoterpenoids from EOs can lead to enhanced the biological action of EOs and monoterpenoids that contain the functional groups. The activities of EOs and their constituents are dependent on the functional group (nature and position) and chemical properties such as volatility and molecular weights172.
| Fig. 4: | Chemical structures of L-menthyl and its derivatives |
In the course of the importance of studying the relationships between chemical structure and biological activity, the knowledge of important functional groups and best chemical structure can be illustrated through biorational design of the derivatives according to the results obtained from bioassay experiments172.
Some researchers have studied the correlation between the chemical structure of EOs constituents and their insecticidal activity. For example, EOs from M. arvensis has been reported to have insecticidal activity. The L-menthol isolated from this EO and seven of its acyl derivatives were evaluated against storage insects173. They reported that menthyl propionate and L-menthol were high insecticidal activity. The high activity of derivatives (menthyl propionate) as compared to L-menthol can be due to increasing the number of methyl groups in the side chain. Due to the nucleophilic properties of the methyl group (electron-donating), increasing the number of the methyl groups on the side chain cause decreasing in the positive charge (increase negative charge) on the carbon atom. Therefore, high activity of menthyl propionate may be due to increasing the negative charge on the carbon atom because of the presence of two methyl groups (Fig. 4). However, the increase of electrophilic groups such as methyl groups in the chain leads to increase the activity of a function carbon atom. For example, the low insecticidal activity of format derivative was found because it didn’t contain the methyl group. Depending on the number of methyl groups, acetate derivative (-CH3) have the slight activity while menthyl propionate (-(CH3)2) have high insecticidal activity. Low activity of menthyl benzoate was observed because they do not have methyl group and the benzene ring was attached directly to menthyl carbon. While, moderate activity was observed of menthyl cinnnamate because they have a double bond and benzene ring not directly attached to the carbon atom34,173.
Nine benzene derivatives (eugenol, isoeugenol, methyl eugenol, safrole and isosafrole) and terpenes (cineole, limonene, p-cymine and α-pinene) were tested for insecticidal activity174. It is observed that derivatives of benzene have higher insecticidal activity than that of terpenes. This toxicity may be due to the active groups that found in benzene derivatives. For example, the insecticidal activity of eugenol and its chemical analogues isoeugenol and methyl eugenol. The presence of a double bond in the side chain of aromatic ring and the substitution of the methoxy group is playing an important role in the toxicity of these analogues. The knock down was increased in methyl-eugenol due to the further methoxy group. The order of contact toxicity (knock down) of these compounds was methyl-eugenol>isosafrole = eugenol>safrole. In contrast, when the double bond of the side chain is nearer to aromatic ring, the fumigant toxicity was decreased. Therefore, safrole is a more fumigant activity than isosafrole.
| Fig. 5: | Chemical structures of some benzene derivatives and terpenes |
| Table 6: | Insecticidal activity of some essential oil constituents against Spodoptera litura |
However, the benzene derivatives have more insecticidal activity than monoterpenes174. In the mean time, chemical structures of some benzene derivatives and terpenes are shown in Fig. 5.
A good example of the correlation between the chemical structure of EOs constituents and their insecticidal activity was shown in Table 6. The insecticidal activity of thymol, pulegone, trans-anethole and eugenol were evaluated against Spodoptera litura56. Thymol has a higher toxicity than pulegone, trans-anethole and eugenol against S. litura. Their LD50 values were 25.4, 51.6, 65.5 and 157.6 μg per larva for thymol, pulegone, trans-anethole and eugenol, respectively. The order of toxicity of these compounds was thymol>pulegone>trans-anethole>eugenol. The high toxic effect of thymol as compared to other compounds attributed to the presence of methyl groups (-CH3) in the side chain. This effect can be due to the electron-donating of methyl groups which decrease the positive charge on the carbon atom of the side chain. Also, some factors are effective on the toxicity the position of the double bond and the methoxy groups (Table 6).
SYNERGISTIC ACTION OF EOs
Various studies have investigated the synergistic effect of EOs, active components and their combinations against insects. Abbassy et al.175 reported that terpinen-4-ol and c-terpinene isolated from M. hortensis EO has a synergistic effect. They increased the insecticidal activity of profenofos and methomyl by 2-4 folds against larva of S. littoralis. Another study by Regnault-Roger176 showed that M. hortensis EO has a highly insecticidal activity than that of α-terpinene and terpinen-4-ol active compounds. They reported that the high toxic effect of EO against S. littoralis and A. fabae might be due to the synergistic effect of EO active compounds. Also, some studies have reported a synergistic effect of some EOs from Ocimum spp., Conyza newii and Plectranthus marruboides against some insects177,178. The synergistic effects of these EOs may be due to the different mechanism of action of their chemical constitutes. For example, the mechanism of toxicity of monoterpenes and some EOs is inhibiting AChE and some is neurotoxic142. In addition, monoterpenes can effects on many targets in insects with numerous mechanism of toxicity171. Jiang et al.179 evaluated the insecticidal activity of EOs from L. pungens, L. cubeba and the mixture of major constituents against Trichoplusia ni larvae. The major active constituents of both EOs are 1,8-cineole and γ-terpinene from L. pungens and L. cubeba, respectively. The LD50 values of both EOs were 87.1 and 112.5 μg per larva, respectively. Also, the toxicity of individual major active constituents was lower than the toxicity of EOs. This result indicated that the synergistic effect of artificial mixtures of active and inactive compounds. Therefore, all active and inactive components are required to complete full toxicity. Faraone et al.180 reported that the important role of second compounds and their synergistic effect with the major component in the toxicity of EOs. They found that the synergistic action of the mixtures of imidacloprid and EOs from L. angustifolia and T. vulgaris against insects increased the toxicity by 16-20 fold. The mixtures of imidacloprid and two active compounds linalool and thymol occurred dissimilarity and lower synergistic effect. Khalfi et al.181 studied the insecticidal activity of spearmint EO and two active compounds 1,8 cineole and carvone against Rhyzopertha dominica. They found that the activity is due to the synergistic action of 1,8 cineole and carvone. It has been reported that EOs combination or in mixtures with botanical and synthetic insecticides has shown synergistic activity against variety insects56,179.
ESSENTIAL OIL-BASED INSECTICIDES FROM RESEARCH TO MARKET
Aromatic plants have been used traditionally for control stored product insects worldwide. Consequently, it is normal to re-evaluate the EOs to be used against insect-pests. Some EOs and active substances with a special status where it can be used without the requirement of expensive and takes a long time for production. These requirements include toxicological and ecotoxicological tests that are necessary for registering commercial products. Dependent on the exception from federal regulatory approval in the USA. The EcoSMART technologies in the USA introduced some pesticides based on EOs182. They used EOs from rosemary, peppermint, cinnamon, lemongrass and thyme in 1998. Currently, Prentiss Inc. markets some qualified products, e.g., urban pest management such as pest controls. While, agricultural products are marketed by Brandt Consolidated Inc. In addition, Mycotech corporation produced an aphidicide/miticide/fungicide formulation. They use for greenhouse, horticultural crops and tree fruits. These formulations are based on cinnamon oil with cinnamaldehyde (30% EC). The EcoSMART technologies have introduced insecticides EcoPCOR. They contain eugenol and 2-phenethyl propionate as active ingredients. They were used against crawling and flying insects and for pest control professionals. The EcoTrolTM formulation is based on rosemary oil as the active ingredient. They used as an insecticide/miticide on horticultural crops. Also, garlic oil-based insecticide was produced in the US. These formulations contain mint oil as the active ingredient. They are used in the home and garden for pest control183. However, EOs have some properties that make them suitable for insect management182. These contain:
| • | EOs is produced in large global scale |
| • | They have broad activity against many insects due to their multiple modes and site of action |
| • | They have many activities such as insecticidal, repellent, fumigant, antifeedant and attractive |
| • | EOs and their active constituents are nontoxic to mammals. The LD50 for rats were 800-3,000 mg kg–1 for pure compounds and ≥5,000 mg kg–1 for formulating insecticides |
| • | Due to their volatility, the oils and active compounds are environmentally non-persistent |
| • | They are effective under low pest pressure |
| • | They mix with synthetic insecticides (tank-mixed) and are suitable for biological control programs |
| • | They do not have to harvest or worker re-entry restrictions for treating crops |
| • | They have a short residual half-life on plants |
NANOINSECTICIDES BASED ON ESSENTIAL OILS
Although, the promising activities of EOs against many insects, some problems were registered. For example, EOs volatility, water solubility and oxidation, that playing an important role in the EOs activity, application and persistent. Therefore, these problems must be resolved before using the EOs as an alternative to synthetic pesticides for pest control184. New formulations with nanotechnology "Nanoformulation" can resolve these problems. The new trend for using nanoformulation lead to protect the EOs from degradation, increase their residue half-life by reducing the evaporation. They can achieve a controlled release of EOs and ease of application and handling185. These nanoformulations can enhance EOs activity due to the small particle size. It had a high surface area, solubility and mobility. Due to their solvent elimination, they have low toxicity to mammals186-188. However, polymeric nanoparticles are the most promising for EOs nanoformulations188.
Anjali et al.189 reported that the insecticidal activity of neem oil was increased in nanoemulision formulation. This effect can be due to the smallest droplet size of EO nanoemulision (31.03 nm). Yang et al.190 showed that loaded nanoparticles with garlic EOs are active to control Tribolium castaneum. New nanotechnology methods were planned to control H. armigera191. As a result of this new method EO of Artemisia arborescens was stable. This stability can be due to built-in of EO with solid lipid nanoparticles and developed an emulsion. This emulsion has gotten better stability and insecticidal activity192. Nanoencapsulation shown high repellent activity than EOs193. For example, nanoencapsulation of Artmisia oil shown more activity than oil at concentration 1.9 ppm. The activity of nanocapsule was 80 and 62% for pure oil against P. xylostella.
SAFETY OF ESSENTIAL OILS
In general, the common use of plant essential oils in drugs and foods strength expectant the EOs are without significant toxicity to mammals. As shown by Isman et al.182, most of the EOs and their active constituents are nontoxic to mammals. The LD50 for rats were 800-3,000 mg kg–1 for pure compounds and ≥5,000 mg kg–1 for formulating bioinsecticides. Moreover, due to the safety of some EOs and their active substances, they can be used without toxicological and ecotoxicological studies which are essential for registering commercial products182. On the other hand, some important exceptions were recorded. For example, pennyroyal oil was considered as poisonings to human and animal. The pulegone isolated from this oil is toxic to rats with LD50 150 mg kg–1 i.p.194. Also, α-thujone from wormwood oil is very toxic to rats with LD50 = 45 mg kg–1 i.p.165.
CONCLUSION
It can be concluded that many essential oils are toxic to a wide range of insect pests. They have insecticidal, fumigant, antifeedant, attractive and repellent activities against a broad spectrum of insects with some selectivity. The EOs are a complex of chemical compounds with multiple modes of action that enhances their activity due to the synergistic action between constituents. Due to their volatility in nature, EOs are used as a fumigant against agriculture and storage food insects. Consequently, essential oil-based insecticides are very important for control stored insects because they are active against a variety of insects, fast penetrating and non-toxic residues in the treated products. The EOs have some properties that made them suitable for insect management. Therefore, EcoSMART technologies in the USA introduced some insecticides based on EOs. In addition, some qualified products were used to control insect pests in urban pest management, greenhouse, horticultural crops and tree fruits. In contrast, some problems (e.g., volatility, solubility and oxidation) of essential oil-based insecticides were recorded, which plays an important role in the EOs activity, application and persistent. For this reason, new formulations with nanotechnology "Nanoformulation" such as the merging of the EOs in controlled release through nanocapsule formulations can re-solve these problems and offer numerous advantages. So, encapsulating the EOs has a considerable perspective as commercial insecticide products. Finally, EOs-based insecticides are low toxic, environmental persistence and eco-friendly. Therefore, they are not having worker re-entry and harvest restrictions after treating crops. So, they are compatible with biological control programs and indigenous natural enemies of pests.
ACKNOWLEDGMENTS
The author would like to thank the Permanent Scientific Committee for Staff Promotion in Diseases, Pests and Plant Protection, National Research Centre, Egypt for review this manuscript. The author also would like to thank Dr. Souad Elgengaihi Professor of Medicinal and Aromatic Plants, National Research Centre, Dokki, Giza Egypt for review this manuscript.