Applications of Plant Essential Oils in Pest Control and Their Encapsulation for Controlled Release: A Review

Abstract

Essential oils (EOs) are volatile products derived from the secondary metabolism of plants with antioxidant, antimicrobial, and pesticidal properties. They have traditionally been used in medicine, cosmetics, and food additives. In agriculture, EOs stand out as natural alternatives for pest control, as they show biocidal, repellent, and antifeedant effects. However, they are highly volatile compounds and susceptible to oxidation, which has limited their use as pesticides. This has led to exploring micro- and nano-scale encapsulation to protect these compounds, improving their stability and allowing for a controlled release. Various encapsulation techniques exist, such as emulsification, ionic gelation, and complex coacervation. Nanoemulsions are useful in the food industry, while ionic gelation and complex coacervation offer high encapsulation efficiency. Materials such as chitosan, gelatin-gum-Arabic, and cyclodextrins are promising for agricultural applications, providing stability and the controlled release of EOs. Encapsulation technology is still under development but offers sustainable alternatives to conventional agrochemicals. This article reviews the potential of EOs in pest management and encapsulation techniques that enhance their efficacy.

1. Introduction

Essential oils (EOs) are volatile and odorous phytochemical products of plants secondary metabolism. Since ancient times, they have been widely used in folk and alternative medicines to treat pain and diverse ailments [1,2,3,4,5] as insecticides and repellents [6,7]. They are highly appreciated in cosmetics and perfumery due to their pleasant aroma [8]. The food industry also takes advantage of the properties of essential oils as additives for food flavoring [9] and food preservatives to prevent oxidation, microbial spoilage [10,11,12], and the growth of food poisoning-causing bacteria [13,14].

One of the first examples of the usage of EOs for microbial and pest control comes from the mummification process in ancient Egypt, where it was the belief that existence in the afterlife required the body to be preserved in a lifelike form [15], meaning corpse decomposition by bacteria and insects must be minimized. Analysis with gas chromatography–mass spectroscopy allowed for identifying several compounds found in embalming materials, including monoterpenes, sesquiterpenes, phenols, and naphthalene; derived from the oily materials used for the preservation of the bodies and tombs. Said compounds are present in cedar oil, pine oil, and juniper cones [16,17].

Several studies have assessed the biological properties of essential oils, being particularly interested in their activities as radical-scavenger antioxidants [18,19,20], antimicrobial [21,22], antifungal [11,23,24], and antiviral agents [18,25], as well as their sedative, analgesic, and anti-inflammatory properties [1,26,27,28]. Moreover, producing EOs in plants is a stress response, mainly to, but not limited to, deterrent herbivores [29], hence their potential as natural pesticides and repellents for crop plagues.

About a third of the global crop production is lost annually due to plagues and plant diseases [30]; additionally, the primary control for pests is synthetic pesticides, which have become less effective mainly due to plagues developing resistance to their active compounds, forcing farmers to search for alternative control. On the other hand, during the last few decades, there has been an increased concern related to the environmental damage and the potential impact on human health of exposure to synthetic pesticides; consequentially, there has been a heightened interest in the search for natural alternatives to be used in the control of plagues and pests. Several EOs and their components have been studied regarding their biocide and repellent properties against insects, arthropods, nematodes, larvae, and other plagues, with promising results [31,32,33,34,35]. However, the agricultural application of EOs faces challenges such as their components’ high volatility and oxidability.

Encapsulated systems provide EOs with protection from interactions with environmental oxygen, photodamage, evaporation, and other alterations that could compromise their biological activities and can facilitate a controlled and continuous release for long-term protection, avoiding the need for constant reapplication. Encapsulation materials, also known as matrixes, are presented in an extensive range of physicochemical characteristics, from synthetic polymers to biodegradable alternatives such as carbohydrates (chitosan, dextrins, cellulose), proteins (gelatin, albumin), gums (Arabic, cashew), and lipids (paraffin, oils, beeswax) [36]. The material selection should respond to the particular need of application and the interactions between the matrix and the encapsulated substance (core).

This article aims to give an overview of EOs as prospective agrochemicals for pest management and the different encapsulation techniques and materials that improve the applications and stability of the EOs.

2. Essential Oils

Essential oils are volatile, hydrophobic liquids made up of a mixture of secondary metabolites (monoterpenes, sesquiterpenes, phenylpropanoids, etc.) (Figure 1), as well as some components that are specific to certain EOs, such as allyl sulfides, found in garlic essential oil [37], or the characteristic indole in citrus blossoms’ volatile oils [38]. However, EOs vary in composition due to genetic causes, climate, sun exposure, rainfall, geographic location, plant age, vegetative parts, and the presence of plagues [39,40].

EOs production is restricted to a limited number of families, including Myrtaceae, Lauraceae, Rutaceae, Lamiaceae, Asteraceae, Apiaceae, Zingiberaceae, and Piperaceae [41]. EOs are produced in morphologically diverse, specialized cells present in different organs in aromatic plants [42]. These secretory tissues are found in flowers (roses), leaves (eucalyptus, mint), woods (sandalwood), barks (cinnamon), roots (valerian), rhizome (ginger), peels (citrus), and seeds (nutmeg). Around 3000 EOs are known, and 300 to be commercially important, primarily destined for flavor and fragrance markets [43,44].

EOs can be obtained by different means. The extraction method can affect the composition and quality of the EO due to the exposure of the components to temperature and pressure or their interaction with the solvent [45,46,47]. Steam distillation is the most common EO extraction method. It involves placing the vegetable material in boiling water and allowing the steam to break down plant cells, releasing the oils. Around 93% of EOs are obtained through this method, and the remaining 7% are extracted via other methods [48], including the traditional methods of enfleurage (extraction with cold fat), solvent extraction (used for labile materials), and mechanical pressing, for citrus EOs [47,49]. Newer extraction techniques such as microwave- and ultrasound-assisted extraction or supercritical fluid extraction, eliminate the annoyances of traditional methods related to time and power consumption and the decomposition of EOs during the process [44,47]; nevertheless, specialized equipment is required.

Figure 1. Common monoterpenes with reported biocidal and repellent activities. The top three rows are monoterpenoids; the Fourth row is monoterpenoid phenols and sesquiterpene; the Lower row is phenylpropanoids [31,33,50,51,52,53,54,55,56,57,58].

Figure 1. Common monoterpenes with reported biocidal and repellent activities. The top three rows are monoterpenoids; the Fourth row is monoterpenoid phenols and sesquiterpene; the Lower row is phenylpropanoids [31,33,50,51,52,53,54,55,56,57,58].

3. Essential Oils for Pest Management

The term pest comprises any organisms that are considered undesirable—commonly, nematodes, mites, insects, and some vertebrates such as rodents [59]. Agricultural pests include those species that cause quantitative and qualitative losses in forestry, agriculture, and stored produce and grains [60], causing waste of around a third of the annual production, translating into a major toll on the global economy. To control pests, farmers usually resort to synthetic pesticides; however, in recent years, the increasing concern for developing more sustainable and safe agricultural practices has raised attention to botanically derived pesticides.

3.1. Essential Oils as Botanical-Sourced Pesticides

Essential oils are, in general, regarded as safe for vertebrates [61], and, as mentioned before, aromatic plants have been used as traditional pest control, with their properties long studied, showing EOs’ broad spectrum of biocidal (Table 1), sublethal, and repellent activities.

One of the most employed oils is Citronella oil (Cymbopogon spp.), widely used as a mosquito repellent. It also exhibits biological activities against other organisms such as nematodes [62,63] and phytopathogenic fungi [64,65]. A 2006 study showed biocidal activity against the fennel aphid and attraction to the plague’s natural predator Cycloneda sanquinea L. (ladybug) [66].

Biocidal activities against phytophagous acari have been reported for several EOs, including those from Chamomilla recuitita, Majorana hortensis [67], Rosmarinus officinalis, Salvia officinalis [68], Salvia fruticosa, Lavandula angustifolia [69], and Senecio glaucus [70]. EOs have been effective against the two-spotted spider mite Tetranychus urticae, also Zataria multiflora, Satureja hortensis agains the strawberry spider mite Tetranychus turkestani [71], and Thymbra spicata, Origanum onites, Mentha spicata and Lavandula stoechas against the carmine spider mite (Tetranychus cinnabarinus) [72].

The effects of EOs on phytopathogenic nematodes have been reported, particularly activities against the root-knot nematodes (Meloidogyne sp.), which are vulnerable to EOs from Piper hispidinervum, which suppressed egg hatching and diminished juvenile infectivity [51], and activity against Artemisia nilagirica, which reduced the root infection and tomato plants and promoted the plant’s growth in greenhouse conditions [73]. Senecio glaucus exhibited nematostatic capacity [70]. Eucalyptus globulas, Carum capticum [74], and the mixture of EOs from Haplophyllum tuberculatum and Plectranthus cylindraceus inhibited the hatching of nematode eggs [75]. Kang and colleagues have also studied the nematocidal activity from EOs-derived phytochemicals [76]. They evaluated 97 compounds (49 monoterpenes, 17 phenylpropenes, 16 sesquiterpenes, and 15 sulfides) from essential oils against the pinewood nematode (Bursaphelenchus xylophilus) via acetylcholinesterase inhibition (AChEI), finding three active monoterpenes, two phenylpropanes, and one sesquiterpene.

Table 1. Essential oils with biocidal activities against some important agricultural pests.

Order	Target	EO Botanical Family	EO	Plant Part	Application Type	Reference
Acari	Tetranychus cinnabarinus (carmine spider mite)	Lamiaceae	Lavandula stoechas (lavender)	Leaves and stems	Fumigant	[72]
		Lamiaceae	Mentha spicata (spearmint)	Leaves and stems	Fumigant	[72]
		Lamiaceae	Origanum onites (oregano)	Leaves and stems	Fumigant	[72]
		Lamiaceae	Thymbra spicata (thyme)	Leaves and stems	Fumigant	[72]
		Zingiberaceae	Curcuma longa (turmeric)	Rhizome	Spray	[77]
Acari	Tetranychus turkestani (strawberry spider mite)	Lamiaceae	Satureja hortensis (summer savory)	Leaves	Fumigant	[71]
		Lamiaceae	Zataria multiflora	Leaves	Fumigant	[71]
Acari	Tetranychus urticae (two-spotted spider mite)	Asteraceae	Chamomilla recutita (chamomile)	Whole plant	Spray on host	[67]
		Lamiaceae	Majorana hortensis (marjoram)	Whole plant	Spray on host	[67]
		Lamiaceae	Rosmarinus officinalis (rosemary)	Aerial	Contact	[68]
		Lamiaceae	Salvia officinalis (sage)	Aerial	Contact	[68]
		Lamiaceae	Lavandula angustifolia (lavender)	Leaves	Fumigant	[69]
		Lamiaceae	Salvia fruticose (Greek sage)	Leaves	Fumigant	[69]
		Lamiaceae	Mentha spicata (spearmint)	Commercial, NS	Fumigant	[78]
		Lamiaceae	Ocimum basilicum (basil)	Commercial, NS	Fumigant	[78]
		Myrtaceae	Callistemon viminalis (callistemo)	Leaves and twigs	Contact	[79]
		Myrtaceae	Eucalyptus bicostata (Victorian blue gum)	Leaves	Contact	[79]
		Myrtaceae	Eucalyptus maidenii (Maiden’s gum)	Leaves	Contact	[79]
		Myrtaceae	Eucalyptus sideroxylm (red ironbark)	Leaves	Contact	[79]
		Myrtaceae	Eucalyptus approximans (Barren Mountain mallee)	Leaves	Contact	[79]
		Verbenaceae	Lippia origanoides (Colombian oregano)	Leaves	Fumigant	[80]
		Verbenaceae	Lippia sidoides (oregano)	Leaves	Fumigant	[81]
Coleoptera	Callosobruchus chinensis (adzuki bean weevil)	Apiaceae	Coriandum sativum (coriander)	Industrial, NS	Fumigant/contact	[82]
		Myrtaceae	Eucalyptus obliqua (eucalyptus)	Industrial, NS	Fumigant/contact	[82]
		Pinaceae	Pinus langifolia (pine)	Industrial, NS	Fumigant/contact	[82]
Coleoptera	Callosobruchus maculatus (cowpea weevil)	Lamiaceae	Rosmarinus officinalis (rosemary)	Aerial	Fumigant	[83]
		Lamiaceae	Mentha piperita (peppermint)	Aerial	Fumigant	[83]
		Rutaceae	Citrus sinensis (sweet orange)	Peel	Fumigant	[84]
Coleoptera	Lasioderma serricone (cigarette beetle)	Cruciferae	Cocholeria armoracia (horseradish)	Commercial, NS	Fumigant	[85]
		Lauraceae	Cinnamomum cassia (cinnamon)	Commercial, NS	Contact	[85]
Coleoptera	Sitophilus granarius (granary weevil)	Lauraceae	Cinnamomum zeylanicum (cinammon)	Commercial, NS	Contact	[54]
		Myrtaceae	Syzygium aromaticum (clove)	Commercial, NS	Contact	[54]
Coleoptera	Ulomoides dermestoides (peanut beetle)	Poaceae	Cymbopogon citratus (lemongrass)	Commercial, NS	Contact	[86]
Diptera	Ceratitis capitata (Mediterranean fruit fly	Lamiaceae	Rosmarinus officinalis (rosemary)	Leaves	Fumigant/Topical	[87]
		Lamiaceae	Lavandula angustifolia (lavender)	Leaves	Fumigant/Topical	[87]
Diptera	Drosophila melanogaster (fruit fly)	Lamiaceae	Mentha pulegium (pennyroyal)	NS	Contact	[31]
		Lamiaceae	Mentha spicata (spearmint)	NS	Contact	[31]
Diptera	Drosophila suzukii (spotted wing drosophila)	Lamiaceae	Mentha piperita (peppermint)	Aerial flowering	Fumigant	[88]
		Lamiaceae	Perilla frutescens (perilla)	Leaves	Fumigant	[88]
Hemiptera	Aphis forbesi (strawberry aphid)	Fabaceae	Tephrosia vogelii (Vogel’s tephrosia)	Flowers	Spray	[89]
Hemiptera	Bemisia tabaci (silverleaf whitefly)	Amaryllidaceae	Allium sativum	NS	Fumigant/contact	[90]
		Lamiaceae	Micromeria fruticosa (white savory)	Aerial	Fumigant	[91]
		Lamiaceae	Nepeta racemose (dwarf catnip)	Aerial	Fumigant	[91]
		Lamiaceae	Origanum vulgare (oregano)	Aerial	Fumigant	[91]
		Lamiaceae	Thymus vulgaris (thyme)	Leaves	Contact	[92]
		Rutaceae	Citrus aurantium (bitter orange)	Peel	Fumigant	[93]
Hemiptera	Hyadaphis foeniculi (fly honeysuckle aphid, fennel aphid)	Lamiaceae	Hyptis suaveolens (alfazema)	NS	Topical	[66]
Hemiptera	Myzus persicae (green peach aphid)	Apiaceae	Cuminum cyminum (cumin)	Schizocarp	Spray on host	[94]
		Asteraceae	Santolina chamaecyparissus (cotton lavender)	Aerial	Spray	[95]
		Asteraceae	Achillea millefolium (yarrow)	Aerial	Spray	[96]
		Cannabaceae	Cannabis sativa (hemp)	Inflorescences	Spray on host	[96]
		Lamiaceae	Mentha pulegium (pennyroyal)	Aerial	Fumigant	[97]
		Lamiaceae	Mentha pulegium (pennyroyal)	Aerial	Spray	[98]
		Lamiaceae	Origanum majorana (marjoram)	Aerial	Spray	[98]
		Lamiaceae	Melissa officinalis (lemon balm)	Aerial	Spray	[98]
Hemiptera	Rhopolasiphum maidis (corn leaf aphid)	Myrtaceae	Syzygium aromaticum (clove)	Buds	Fumigant	[99]
Hemiptera	Trialeurodes vaporariorum (greenhouse whitefly)	Asteraceae	Eupatorium buniifolium (chilca)	NS	Spray on host	[100]
Hemiptera	Trialeurodes vaporariorum (greenhouse whitefly)	Atherospermataceae	Laurelia sempervirens (Chilean laurel)	Leaves	Fumigant	[101]
Hymenoptera	Acromyrmex balzani (leaf-cutter ant)	Lamiaceae	Eplingiella fruticosa	Leaves	Fumigant	[102]
		Myrtaceae	Myrcia lundiana	Leaves	Fumigant	[103]
Lepidoptera	Mediterranean flour moth	Lamiaceae	Origanum onites L. (oregano)	Leaves	Fumigant	[104]
		Lamiaceae	Satureja thymbra L. (savory)	Leaves	Fumigant	[104]
Lepidoptera	Indian meal moth	Lamiaceae	Origanum onites L. (oregano)	Leaves	Fumigant	[104]
		Lamiaceae	Satureja thymbra L. (savory)	Leaves	Fumigant	[104]
Lepidoptera	Spodoptera littoralis (cotton leafworm)	Lamiaceae	Thymus algeriensis (Thyme)	Aerial	Fumigant	[35]
		Apiaceae	Pimpinella anisum (anise)	Schizocarp	Topical	[94]
		Apiaceae	Crithmum maritimum (sea fennel)	Seeds/aereal	Topical	[105]
		Euphorbiaceae	Ricinus communis (castor bean)	Commercia, NS	Fumigant	[106]
		Lamiaceae	Ocimun gratissimum (white wild basil)	Aerial	Topical	[107]
Orthoptera	Schistocerca gregaria (desert locust)	Amaryllidaceae	Allium cepa (onion)	Leaves	Topical	[108]
		Apiaceae	Petroselinum sativum (parsley)	Seeds	Topical	[108]
		Geraniaceae	Pelargonium radula (geranium)	Whole plant	Topical	[108]
		Amaryllidaceae	Allium sativum (garlic)	Commercial, NS	Spray	[109]

NS = Not specified.

Table 1. Essential oils with biocidal activities against some important agricultural pests.

Order	Target	EO Botanical Family	EO	Plant Part	Application Type	Reference
Acari	Tetranychus cinnabarinus (carmine spider mite)	Lamiaceae	Lavandula stoechas (lavender)	Leaves and stems	Fumigant	[72]
		Lamiaceae	Mentha spicata (spearmint)	Leaves and stems	Fumigant	[72]
		Lamiaceae	Origanum onites (oregano)	Leaves and stems	Fumigant	[72]
		Lamiaceae	Thymbra spicata (thyme)	Leaves and stems	Fumigant	[72]
		Zingiberaceae	Curcuma longa (turmeric)	Rhizome	Spray	[77]
Acari	Tetranychus turkestani (strawberry spider mite)	Lamiaceae	Satureja hortensis (summer savory)	Leaves	Fumigant	[71]
		Lamiaceae	Zataria multiflora	Leaves	Fumigant	[71]
Acari	Tetranychus urticae (two-spotted spider mite)	Asteraceae	Chamomilla recutita (chamomile)	Whole plant	Spray on host	[67]
		Lamiaceae	Majorana hortensis (marjoram)	Whole plant	Spray on host	[67]
		Lamiaceae	Rosmarinus officinalis (rosemary)	Aerial	Contact	[68]
		Lamiaceae	Salvia officinalis (sage)	Aerial	Contact	[68]
		Lamiaceae	Lavandula angustifolia (lavender)	Leaves	Fumigant	[69]
		Lamiaceae	Salvia fruticose (Greek sage)	Leaves	Fumigant	[69]
		Lamiaceae	Mentha spicata (spearmint)	Commercial, NS	Fumigant	[78]
		Lamiaceae	Ocimum basilicum (basil)	Commercial, NS	Fumigant	[78]
		Myrtaceae	Callistemon viminalis (callistemo)	Leaves and twigs	Contact	[79]
		Myrtaceae	Eucalyptus bicostata (Victorian blue gum)	Leaves	Contact	[79]
		Myrtaceae	Eucalyptus maidenii (Maiden’s gum)	Leaves	Contact	[79]
		Myrtaceae	Eucalyptus sideroxylm (red ironbark)	Leaves	Contact	[79]
		Myrtaceae	Eucalyptus approximans (Barren Mountain mallee)	Leaves	Contact	[79]
		Verbenaceae	Lippia origanoides (Colombian oregano)	Leaves	Fumigant	[80]
		Verbenaceae	Lippia sidoides (oregano)	Leaves	Fumigant	[81]
Coleoptera	Callosobruchus chinensis (adzuki bean weevil)	Apiaceae	Coriandum sativum (coriander)	Industrial, NS	Fumigant/contact	[82]
		Myrtaceae	Eucalyptus obliqua (eucalyptus)	Industrial, NS	Fumigant/contact	[82]
		Pinaceae	Pinus langifolia (pine)	Industrial, NS	Fumigant/contact	[82]
Coleoptera	Callosobruchus maculatus (cowpea weevil)	Lamiaceae	Rosmarinus officinalis (rosemary)	Aerial	Fumigant	[83]
		Lamiaceae	Mentha piperita (peppermint)	Aerial	Fumigant	[83]
		Rutaceae	Citrus sinensis (sweet orange)	Peel	Fumigant	[84]
Coleoptera	Lasioderma serricone (cigarette beetle)	Cruciferae	Cocholeria armoracia (horseradish)	Commercial, NS	Fumigant	[85]
		Lauraceae	Cinnamomum cassia (cinnamon)	Commercial, NS	Contact	[85]
Coleoptera	Sitophilus granarius (granary weevil)	Lauraceae	Cinnamomum zeylanicum (cinammon)	Commercial, NS	Contact	[54]
		Myrtaceae	Syzygium aromaticum (clove)	Commercial, NS	Contact	[54]
Coleoptera	Ulomoides dermestoides (peanut beetle)	Poaceae	Cymbopogon citratus (lemongrass)	Commercial, NS	Contact	[86]
Diptera	Ceratitis capitata (Mediterranean fruit fly	Lamiaceae	Rosmarinus officinalis (rosemary)	Leaves	Fumigant/Topical	[87]
		Lamiaceae	Lavandula angustifolia (lavender)	Leaves	Fumigant/Topical	[87]
Diptera	Drosophila melanogaster (fruit fly)	Lamiaceae	Mentha pulegium (pennyroyal)	NS	Contact	[31]
		Lamiaceae	Mentha spicata (spearmint)	NS	Contact	[31]
Diptera	Drosophila suzukii (spotted wing drosophila)	Lamiaceae	Mentha piperita (peppermint)	Aerial flowering	Fumigant	[88]
		Lamiaceae	Perilla frutescens (perilla)	Leaves	Fumigant	[88]
Hemiptera	Aphis forbesi (strawberry aphid)	Fabaceae	Tephrosia vogelii (Vogel’s tephrosia)	Flowers	Spray	[89]
Hemiptera	Bemisia tabaci (silverleaf whitefly)	Amaryllidaceae	Allium sativum	NS	Fumigant/contact	[90]
		Lamiaceae	Micromeria fruticosa (white savory)	Aerial	Fumigant	[91]
		Lamiaceae	Nepeta racemose (dwarf catnip)	Aerial	Fumigant	[91]
		Lamiaceae	Origanum vulgare (oregano)	Aerial	Fumigant	[91]
		Lamiaceae	Thymus vulgaris (thyme)	Leaves	Contact	[92]
		Rutaceae	Citrus aurantium (bitter orange)	Peel	Fumigant	[93]
Hemiptera	Hyadaphis foeniculi (fly honeysuckle aphid, fennel aphid)	Lamiaceae	Hyptis suaveolens (alfazema)	NS	Topical	[66]
Hemiptera	Myzus persicae (green peach aphid)	Apiaceae	Cuminum cyminum (cumin)	Schizocarp	Spray on host	[94]
		Asteraceae	Santolina chamaecyparissus (cotton lavender)	Aerial	Spray	[95]
		Asteraceae	Achillea millefolium (yarrow)	Aerial	Spray	[96]
		Cannabaceae	Cannabis sativa (hemp)	Inflorescences	Spray on host	[96]
		Lamiaceae	Mentha pulegium (pennyroyal)	Aerial	Fumigant	[97]
		Lamiaceae	Mentha pulegium (pennyroyal)	Aerial	Spray	[98]
		Lamiaceae	Origanum majorana (marjoram)	Aerial	Spray	[98]
		Lamiaceae	Melissa officinalis (lemon balm)	Aerial	Spray	[98]
Hemiptera	Rhopolasiphum maidis (corn leaf aphid)	Myrtaceae	Syzygium aromaticum (clove)	Buds	Fumigant	[99]
Hemiptera	Trialeurodes vaporariorum (greenhouse whitefly)	Asteraceae	Eupatorium buniifolium (chilca)	NS	Spray on host	[100]
Hemiptera	Trialeurodes vaporariorum (greenhouse whitefly)	Atherospermataceae	Laurelia sempervirens (Chilean laurel)	Leaves	Fumigant	[101]
Hymenoptera	Acromyrmex balzani (leaf-cutter ant)	Lamiaceae	Eplingiella fruticosa	Leaves	Fumigant	[102]
		Myrtaceae	Myrcia lundiana	Leaves	Fumigant	[103]
Lepidoptera	Mediterranean flour moth	Lamiaceae	Origanum onites L. (oregano)	Leaves	Fumigant	[104]
		Lamiaceae	Satureja thymbra L. (savory)	Leaves	Fumigant	[104]
Lepidoptera	Indian meal moth	Lamiaceae	Origanum onites L. (oregano)	Leaves	Fumigant	[104]
		Lamiaceae	Satureja thymbra L. (savory)	Leaves	Fumigant	[104]
Lepidoptera	Spodoptera littoralis (cotton leafworm)	Lamiaceae	Thymus algeriensis (Thyme)	Aerial	Fumigant	[35]
		Apiaceae	Pimpinella anisum (anise)	Schizocarp	Topical	[94]
		Apiaceae	Crithmum maritimum (sea fennel)	Seeds/aereal	Topical	[105]
		Euphorbiaceae	Ricinus communis (castor bean)	Commercia, NS	Fumigant	[106]
		Lamiaceae	Ocimun gratissimum (white wild basil)	Aerial	Topical	[107]
Orthoptera	Schistocerca gregaria (desert locust)	Amaryllidaceae	Allium cepa (onion)	Leaves	Topical	[108]
		Apiaceae	Petroselinum sativum (parsley)	Seeds	Topical	[108]
		Geraniaceae	Pelargonium radula (geranium)	Whole plant	Topical	[108]
		Amaryllidaceae	Allium sativum (garlic)	Commercial, NS	Spray	[109]

NS = Not specified.

3.2. Mechanism of Action of Essential Oils as Insecticides

Although the biocidal mechanism of essential oils and their components is still unclear, the rapid action of EOs on insects and behavioral patterns observed in treated individuals have pointed to a neurotoxic effect [110]. Moreover, due to the observation of synergic and antagonist effects of binary mixtures of essential oils and the divergence of biological activities exhibited by one essential oil and its individual components, it has been proposed that there are a handful of different mechanisms of action for insecticidal activity. Some of the most studied proposed mechanisms are inhibiting the enzyme acetylcholinesterase, modifying gamma-aminobutyric acid (GABA) receptors, and activating octopamine receptors.

3.2.1. Inhibition of Enzyme Acetylcholinesterase (AChE)

Acetylcholinesterase is an essential neuronal enzyme in the cholinergic synapses and neuromuscular junction for vertebrates and invertebrates [111,112]. The extended use of organophosphate and carbamate pesticides, both targeting the cholinergic system, has led to the rapid development of pest resistance [113,114] and increasing interest in botanical pesticides.

Several studies have been conducted to determine the potential of EOs as AChE inhibitors (AChEI); Czerniewicz and colleagues (2018) [95] reported an in vitro reduction in AChE activity after the individual application of EOs from the Asteraceae family, particularly the EOs from Santolina chamaecyparissus and Achillea millefolium. Other EOs that have exhibited AChEI activity include those from the Anthriscus nemerosa root [115], the Citrus aurantium peel [93], the Citrus sinensis peel [84], Echinacea purpurea, Thymus praecox [116], Lippia origanoides [80], Ocimum tenuiflorum [117], Salvia officinalis [118,119], Salvia lavanduleifolia [120], and Rosmarinus officinalis [121].

Individual components of EOs activity as AChE have also been assessed; monoterpenic components have displayed strong inhibitory activities against the enzyme, including (+)-, (−)-α-pinene, β-pinene, limonene, β-phellandrene, Fenchone, S-carvone, L-carvone, linalool, 2-carene, eugenol, pulegone, 1,8-cineole, and 4-terpineol [50,55,76,84,93,117,122,123].

Most studies have used non-invertebrate-sourced AChE for the in vitro assays. Consequently, whether EOs and their components perform as AChEI in insects has not yet been determined.

3.2.2. Modification of GABA Receptors

The amino acid GABA acts as a neurotransmitter in invertebrates. Its receptor comprises multiple binding sites, the most studied GABA-gated chloride channel targeted by many insecticides [124,125,126].

There is minimal information regarding the interaction of EOs with GABA receptors. Nevertheless, some studies show the activity of monoterpenic compounds on this molecular target. Thymol has been reported as an allosteric modulator of the GABA receptors in mammals and insects [127,128,129,130,131]. Tong and Coats [129] also reported carvacrol and pulegone as positive allosteric modulators at insect GABA receptors, as they significantly increased the ³⁶Cl⁻ uptake induced by GABA in arthropods. In another study, linalool, with a synergic effect with methyl eugenol, showed good potential as a GABA agonist and allosteric modulator [58]. Abdelgaleil and colleagues [55] evaluated six natural monoterpenes (1,8-cineole, (−)-citronellal, limonene, α-pinene, pulegone, and 4-terpineol) against GABA-T, with limonene displaying the highest activity, with an IC₅₀ of 11.37 mg/L. In an in silico evaluation conducted by Toledo and colleagues [99], the main components of clove essential oil, eugenol, and β-caryophyllene were found to bind to three molecular targets, including the GABA receptors in the phytophagous aphid Rhopalosiphum maidis. Although these are promising results, the effect of EOs on GABA receptors needs further study.

3.2.3. Interference on Octopamine Receptors

Octopamine is a biogenic amine functionally related to noradrenaline. Invertebrates act as neurotransmitters, neuromodulators, neurohormones, and neuromuscular transmitters and play a part in the regulation of physiological and biological processes, including heartbeat regulation, locomotion, reproduction, and memory [110,132,133].

In 1980, Livingston and colleagues [134] reported that octopamine injections produced behavioral and locomotory effects in lobsters, with a sustained extension of the limbs and abdomen. Enan (2001) [110] described the same effect of the hyperextension of legs and the abdomen on American cockroaches treated with the essential oil components eugenol, α-terpineol, and cinnamic alcohol, reporting immobilization and, lastly, death. Several more studies have attributed EOs and their components’ pesticidal activity to a possible competitive activation of octopaminergic receptors [111,135,136]. The role of octopaminergic interference regarding invertebrates has also been discussed, with some reports linking it to reproductive sterility and feeding behavior changes [133,137,138]. These multi-action mechanisms and octopamine’s lack of functionality in vertebrates make octopamine antagonists effective and selective pesticides [132].

3.3. Repellency and Sublethal Activities

Other widely studied biological activities of EOs that are highly important are their repellency and sublethal effects on pests; the latter are associated with their ability to survive, including locomotion, growth, and reproduction.

3.3.1. Repellency

The repellent properties of EOs against arthropods of public health importance have been widely reported, and the use of these products represents an effective and safe alternative against disease vector organisms worldwide [139,140,141,142,143,144,145]. These findings have progressively been extended to pests of agricultural interest [146,147].

Lacotte and colleagues (2023) [148] evaluated the repellent activity of forty plant essential oils against the pea aphid (Acyrthosiphon pusim). Peppermint oil showed the highest activity; these results coincide with the effect shown on Aphis craccivora [149].

Similarly, the repellent properties of Eos against domestic pest vectors of diseases have been widely described [32,150,151,152,153,154,155], suggesting their potential application as agricultural repellents. EOs from fruits from the genus Zanthoxylu, as well as the Brazilian pepper fruit (Schinus terebinthifolius), display repellency against the whitefly Bemisia tabaci [156,157]; EOs from Artemisia princeps and Cinnamomum camphora and their mixture were repellent against the store grain beetles Sitophillus oryzae and Bruchus rufimanus, with the binary mixture showing significantly higher activity [158]; EOs from red mahogany (Eucalyptus resinifera), star anise (Illicium verum) and croton (Croton anisatum), as well species from the genera Satureja and Zingiber officinale, and the Tunisian endemic plant Ferula tunetana have also exhibited repellent capacity against agricultural pests of high importance [159,160,161,162,163]. Isolated compounds have also been studied for their repellent profile; in 2017, Tak and Isman [164] tested twenty EO-occurring terpenoids against the two-spotted mite, Tetranychus urticae, including carvacrol, thymol, trans-cinnamaldehyde, and α-terpineol.

3.3.2. Antifeedant

Antifeedants deter the insect’s food consumption through feeding behavioral modifications [165]. Some of the most documented antifeedants are triterpenoids and alkaloids. However, some essential oils have displayed a deterrent capacity against agricultural pests. Sousa and colleagues (2013) [166] reported antifeedant activity from EOs of the Apiaceae members A. graveolens and P. crispum against the armyworm’s larvae (Pseudaletia unipuncta), with inhibitions above 70%. In two independent studies, Piper species were reported to be strong antifeedants against some of the most important pests in agriculture (Myzus persicae, Rhopalosiphum padi, and Spodoptera littoralis) [51,167], with P. dilatatum exhibiting the highest activity, with feeding inhibition percentages of 84.4, 98.9, and 65.3 for M. persicae, R. padi, and S. littoralis, respectively [167]. For S. littoralis, EOs from castor and camphor decreased total proteins and carbohydrates in treated larvae, changing their nutritional status [106], and another report showed that EOs from Scutellaria hastifolia have antifeedant activity at 36 ppm, displaying a good deterrent profile [168].

3.3.3. Effects on Reproductive Conduct

Reproduction control is another important tool for pest management. Some reports indicate that essential oils harm insects’ reproduction capacity, acting as oviposition-deterrents. The anti-ovipositor effect of EOs on human disease vector mosquitos has been described in a handful of reports, with EOs from Cinnamomum zeylanicum, Alpinia purpurata, Zanthoxylum limonella, and Sphaeranthus amaranthoides, among others [155,169,170,171]. As for activity on pests of agricultural importance, in a study conducted on four citrus peel oils on the cowpea weevil (Callosobruchus maculatus), the oviposition was inversely proportional to the oil concentration, with percentages of reduction from 29.74 to 71.66%, as well as a significant reduction in emergence from laid eggs [141]. Likewise, when treated with the monoterpenoids E-anethole, estragole, S-carvone, linalool, L-fenchone, geraniol, γ-terpinene, and D-, L-camphor, mated females did not lay eggs, contrary to the control females [172]. Linalool also has shown oviposition-deterrent activity on the significant Mediterranean fruit fly, Ceratitis capitata, in a concentration-independent relationship [173].

Laborda and colleagues (2013) [68] describe ovipositor-deterrent and anti-hatching activities from Salvia officinalis and Rosmarinus officinalis against Tetranychus urticae at concentrations of 0.15–0.25% and after eight days of treatment and at least 50% fewer larvae emerging from eggs in comparison with the control individuals.

Finally, Dos Santos Silva and colleagues (2016) [174] studied citronella oil’s (Cymbopogon winterianus) influence on the reproductive conduct of the fall armyworm (Spodoptera frugiperda), reporting diminished reproduction, with a 90% oviposition reduction, and the non-viability of the eggs derived from treated individuals.

3.4. Non-Target Safety

Although botanical pesticides could be considered a safer alternative, their potential effects on non-target organisms need to be evaluated. Some research groups have reported on the ecotoxicological selectivity of some EOs.

Most of the studies of the effect of EOs on non-target organisms have focused on pollinators such as honeybees, due to their biological and economic importance. Several EOs have been shown to be safe for bees with low mortality rates [175,176] or dose-dependent safe [177], positioning themselves as more environmentally friendly alternatives to non-selective pesticides, not only for agriculture but useful also as a household insecticide and even in beekeeping to control parasitic species.

A study by Sabahi and colleagues (2018) [178] evaluated the effect of lemongrass (Cymbopogon citratus) oil on one of the main parasites of honeybees in the northern United States, the Varroa destructor mite; a strong acaricidal effect on the parasite, which was safe for bees, was observed. Similar results were shown using clove oil against the Varroa jacobsoni mite. In another study, EOs of the Asteraceae family showed no acute toxicity to bees at the concentrations needed to control whiteflies and no effect on the tomato crop yield under greenhouse conditions [100].

Assessing the safety of pesticides against bees is an essential factor to consider in the quest for sustainable agriculture; however, the effect on other non-target organisms needs to be explored.

A study evaluating the effect of citronella [66] and clove oil (Syzygium aromaticum) showed high toxicity against aphid pests but showed no effect on the predatory ladybug [99]. The acute toxicity of EOs for earthworms has been evaluated in several studies, finding high tolerance by these organisms, with adverse effects sometimes shown at doses above 1000 mg [179,180]. Benelli and col. (2019) [107] also reported high earthworm tolerance to basil EO, which incidentally contained a major component of thymol, which is relatively safe for some non-target organisms, including bees and mealworm beetles [131,181,182,183].

Some studies have proposed the need to expand the evaluation of the effects of EOs on organisms used as a biological pest control (polyphagous predators) and soil invertebrates. In a study evaluating the susceptibility of the whitefly (Bemisia tabaci) and its predator (Orius albidipennis) to vapors of the EO of Artemisia sieberi Besser, Pelargonium roseum, and Ferula gummosa, it was observed that the LC₅₀ was 10 times higher for the predator than for the target organism. Other predatory organisms that have been studied regarding the effect of EOs include Nesidiocoris tenuis, a biological control of several tomato pests.

As active as they are, the volatility of EOs discourages their use as pesticides. The constitution of essential oils makes their application very difficult; in recent years, the controlled release of EOs by microencapsulation and nanoencapsulation has been studied and tested, with interesting results.

4. Encapsulation of EOs

As pointed out, EOs comprise highly volatile and oxidable compounds, one of the main restraints against the widespread use of these highly bioactive compounds as pesticides [182].

Micro- (1–1000 µm) and nano-encapsulated (1–100 nm) systems are a way to protect EOs from oxidation and volatilization by their retention in a matrix, allowing for controlled release and the preservation of the qualitative characteristics.

When selecting the matrix, there are some parameters to be considered to achieve successful encapsulation; among others, there is the strength of the interaction between the matrix and the EO, the desired release rate, and the necessities and legislation in the field of application—for example, selecting matrix materials that are allowed in agriculture or organic agriculture could significantly reduce the options.

4.1. Materials and Techniques for the Encapsulation of EOs

Nowadays, a handful of nanomaterials suit different needs and concerns. For essential oils, the material and structure of the encapsulation matrix and their interaction with the core material are responsible for the protection of the bioactive compounds and enable a sustained release of the volatile compounds, which is also dependent on the selected encapsulation technique, the size of the particles, and their morphology and affects the release trigger [184,185]. The release rate, incidentally, can also significantly affect the bioactivity of the EOs, as reported by several authors (Table 2). In the development of EOs products for agricultural use, it is necessary to evaluate the release profile of the active compounds. It has been observed that several factors can alter this release; parameters such as pH, relative humidity, and temperature have been related to the degradation or alteration of the carrier materials [186]. The control of release has also been related to the encapsulation technique, properties, and proportions of the carrier material [187].

Some preferred techniques for generating EOs nanoparticles are emulsification, complex coacervation, and ionic gelation.

4.1.1. Emulsification

The hydrophobic character of essential oils can limit their potential application in biological systems. To favor water solubility and at the same time preserve the biological activity of their active compounds, the nanoemulsion formulation represents an alternative for suitable EO delivery systems [28,188].

Nanoemulsions are nanosized isotropic systems of two immiscible liquids, oily and aqueous, combined in a single, stable formulation [22,189]. Various methods can carry out the generation of nanoemulsions; the most commonly used are (a) high-pressure homogenization, which is a high-energy mechanical method and one of the most efficient methods for producing stable o/w nanoemulsions [190,191]; (b) micro-fluidization, which uses high pressure to force coarse emulsions through microchannels to reduce the size of the particle [191,192]; and (c) the phase-inversion method (Figure 2), which refers to a phenomenon that occurs when an agitated o/w emulsion reverts to a w/o emulsion [193]. This latter method is a low-energy method that produces small particles while requiring no specialized equipment [194,195].

EO-nanoemulsions per se are mainly used in the food industry as preservatives to avoid microbial spoilage, this being one of the leading applications. They are especially useful in the food regions with higher water activities or the liquid–solid interfaces, where microbial growth is favored [196,197,198]. However, nanoemulsions can also be necessary in generating other kinds of EO nanosystems, like nanogels or nanocapsules, which can result in more suitable applications in different areas, including agriculture and medicine. As described below, ionic gelation and complex coacervation techniques observe an emulsion’s preparation stage when encapsulating lipophilic cores.

4.1.2. Ionic Gelation

Ionic gelation is a relatively facile, solvent-free technique based on ionic interactions between oppositely charged groups: a cationic polymer, such as chitosan and sodium alginate, and an anionic molecule, such as tripolyphosphate (TPP), the most widely used non-toxic, crosslinker [199,200].

The overall methodology consists of the cation generation, preparation of the emulsion, the complexation, and the precipitation. For the first of these, the polysaccharide is dissolved in an aqueous acidic solution, followed by adding the surfactant under stirring to ensure proper homogenization; then, the EOs are added, still under homogenization. The polysaccharide and crosslinker solutions are combined dropwise under continuous stirring [191,201,202,203,204], allowing for the ionic gelation and precipitation of forming particles (Figure 3). Centrifugation is then applied to separate the particles from the unreacted compounds, and finally, the particles can be heat-dried [205,206] or freeze-dried [207] and characterized.

It is essential to keep in mind that the proportion of polymer/acid in the solution [199], the selection of the surfactant [191], the polymer/crosslinker ratio [202], as well as the EO content [207] can play a part in the entrapment efficiency (EE) and particle size. Further studies are required to determine adequate conditions for specific requirements.

4.1.3. Complex Coacervation

Complex coacervation is a technique based on the electrostatic interaction between oppositely charged polymers, usually a protein and a polysaccharide [208] present in two separate immiscible phases—one that is polymer-rich, used to coat the particle, and another that is polymer-poor [208,209]. Coacervation presents a high encapsulation efficiency, requires small amounts of wall material, and can be implemented with a variety of biopolymers as matrixes [210], including gelatin-gum Arabic [161,208,211,212,213], gelatin-sodium alginate [210,214,215], gelatin-seed mucilage [209,216], and chitosan-cashew gum [217,218], along with several other proteins of animal and plant origins.

The complex coacervation involves four stages (Figure 4): (i) the preparation of the aqueous solution of the polymers; (ii) the stable emulsification of the hydrophobic phase to the aqueous solution of one polymer, which in most cases is the protein; (iii) the coacervation step, by a change in pH and temperature; and (iv) the hardening of the polymer shell through a chemical crosslinker or elevated temperature [219,220].

The complex coacervation technique is suitable for fabricating NPs or MPs with lipid cores with increased miscibility, deliverability, and the sustained release of the ingredients [221].

Table 2. Matrixes for the encapsulation of essential oils.

Material	Encapsulated EO	Method	Particle Type	Part. Size	EE	Application	Observations	References
Biopolymers
Chitosan	Citronella (Cymbopogon spp.)	Ionic gelation	Microcapsules	11–225 µm	94.7–98.2%	−	Sustained release	[202]
	Satureja hortensis	Ionic gelation	Microcapsules	192 nm	96.17%	Acaricidal	Sustained release Prolonged bioactivity	[204]
	Citrus oils	Ionic gelation	Microcapsules	289.3–8843.2 nm	61.9–68.1%	−	Good release rates. Particle size and EE dependent on the surfactant.	[191]
	Coriandrum sativum	Ionic gelation	Nanocapsules	50–80 nm	26.5–75.99%	Storage product preservation	Controlled release Enhanced bioactivity	[222]
	Eugenia caryophyllata	Ionic gelation	Nanocapsules	40–100 nm	31–45.77%	Antifungal	Controlled release Higher bioactivity	[223]
	Cinnamomum zeylanicum	Ionic gelation	Microcapsules	100–200 nm	88.6%	Antimicrobial	Prolonged stability Improved antimicrobial capacity	[224]
	Mentha piperita	Ionic gelation	Microcapsules	≤563.3 nm	64–70%	Stored food pest control	Improved AChEI and fumigant toxicity	[225]
	Piper nigrum	Ionic gelation	Microcapsules	Av. 527.5 nm	35–40%	Stored food pest control	Improved AChEI and insecticidal activity	[226]
	Eryngium campestre	Ionic gelation	Microcapsules	Av. 157.8 nm	61–75%	Storage product preservation	Improved bioactivity	[227]
Chitosan-Caffeic acid	Cuminum cyminum	EO in nanogel	Nanogel	≤100 nm	85%	Antifungal	Improved antifungal performance by sustained release	[228]
Chitosan–cashew gum	Lippia siodes	Complex coacervation	Nanogel	335–558 nm	70%	Insecticidal	Controlled release Improvement in bioactivity	[217]
Chitosan-Cinnamic acid	Mentha piperita	EO in nanogel	Nanogel	≤100 nm	−	Antifungal	Improved antifungal performance by sustained release	[229]
	Bunium persicum	EO in nanogel	Nanogel	≤100 nm	41–43%	Antifungal and antioxidant	Enhanced antifungal and antioxidant activities	[230]
β-cyclodextrin	Commercial thyme oil (Thymus spp.)	Complexation (kneading)	Complex	582.9 nm	82.55 g/100 g	Antimicrobial	Enhanced antimicrobial activity	[231]
		Complexation (freeze drying)		3226.7 nm	71.27 g/100 g
	Lippia berlanderi	Complexation	Complex	−	71%	Antimicrobial	High entrapment Increased stability	[232]
	Eugenia caryophyllata			−	61%
β-cyclodextrin (HP-β-CD)	Piper nigrum	Complexation (inclusion complex formation)	Complex	−	50.55%	Antioxidant	Increased stability Higher antioxidant activity	[233]
β- & γ-cyclodextrin	Lippia graveolens (thymol and carvacrol chemotypes)	Complexation	Complex	6–13.22 µm	7.4–63.4%	−	Sustained release	[234]
Cornstarch	Thymus vulgaris	Thermoplastic extrusion	Particles	0.01–0.5 cm	69.07%	Repellent	Prolonged residual effect. Improved bioactivity.	[235]
Zein	Syzygium aromaticum	Antisolvent precipitation	Nanoparticles	125 nm	91.4%	Insecticide	Low toxicity to a non-target organism, Caenorhaabditis. elegans Potentialized the bioinsectide activity against Drosophila melanogaster	[236]
Gelatin-gum Arabic (Glutaraldehyde as wall hardener)	Camphor oil with added oil-soluble polystyrene	Complex coacervation	Particles	85.7–299.7 µm	99.6%	−	Sustained release	[211]
Gelatin-chia mucilage	Oregano oil (Lippia spp.)	Complex coacervation/spray dry	Microcapsules	1.65–8.85 µm	82.15–95.6%	−	Enhanced bioactivity Particle size-dependent EE	[209]
Gelatin-Persian gum, Persian gum, and gum Arabic	Satureja hortensis	Complex coacervation	Nanocapsules	81–208 nm	72.1–92.8%	Herbicidal	Improvement in herbicidal activity on encapsulated oil	[161]
Gelatin-sodium alginate	Cymbopogon winterianus	Complex coacervation	Complex	434.06 µm	83.5%	−	Controlled release Usage of waste product gelatin	[215]
	Piper nigrum	Complex coacervation	Complex	−	49.13–82.36%	−	Good retention capacity and preservation of composition	[210]
Maltodextrin-gum Arabic	Schinus molle	Spray drying	Microparticles	0.2–40 µm	96–100%	Insecticidal	Slow release, over 366 h Prolonged insecticidal effect	[237]
Maltodextrin-casein	Thymus vulgaris	Spray drying	Microparticles	Av. 0.87 µm	88.9%	Food preservation	Thermal stability	[238]
Sodium alginate (crosslinker CaCl2)	Satureja hortensis	Ionic gelation	Microparticles	47–117 µm	52–66%	Antibacterial Antioxidant	Controlled release Improved bioactivity	[207]
Whey protein- mesquite gum	Salvia hispanica	Spray drying	Microcapsules	2.54–3.35 µm	71.4–80.7%	−	Good encapsulation efficiency	[239]
Whey protein- gum Arabic				2.33–3.09 µm	70.7–79.8%
Emulsions
o/w Tween 80 as surfactant	Eucalyptus spp.	Sonication	Droplets	Av. 3.8 nm	−	Antimicrobial (topical)	Improvement in wound-healing effectivity	[240]
o/w Tween 20/Triton x−100 as surfactant	Azadiractha indica- Cymbopogon nardus	Constant stir	Droplets	2.8–17.8 nm	−	Antifungal	Stability of components Potent antifungal activity	[64]
Isopropanol Tween 80 as surfactant	Curcuma longa	Sonication	Microemulsion	−	−	Acaricidal	Controlled release Improved acaricidal activity	[77]
o/w/o Palm oil Sunflower oil Soy lecithin as surfactant	Terpene mixture	High-pressure homogenization	Droplets	75–175 nm	−	Antimicrobial	Enhanced activity	[196]
o/w/ polysorbate	Crithmum maritimum	Emulsion phase inversion	Droplets	50–70 nm	−	Agriculture and domestic pest control	Increased anti-ovipository and toxic activity	[241]
Octenyl succinic anhydride (OSA)—starch	Rosmarinus officinalis Zataria multiflora	Spray drying	Microcapsules	461–854 nm	−	Insecticidal	More effective than non-formulated in a long time	[242]
Chitosan-cellulose nanofibers	Citronella essential oil	Ultrasonication	Nanoparticles	426.9 nm	90.8%	Insecticidal	Nano-systems increased the insecticidal activity	[243]
Nanocarriers
SiO2	Crithmum maritimum	Loading of hollow capsule	Nanocapsules	20–78 nm	−	Agriculture and domestic pest control	Increased anti-ovipository and toxic activity	[241]
Zein sta-bilized with Tween 80	Eucalyptus staigeriana Litsea cubeba	Ultrasound-assisted nano-precipitation	Nanoparticles	200 nm	−	Antifungal	Activity against Colletotrichum lindemuthianum	[244]

Part. Size = Particle size; Av.= Average size; o/w = oil/water; o/w/o = oil, water, oil.

Table 2. Matrixes for the encapsulation of essential oils.

Material	Encapsulated EO	Method	Particle Type	Part. Size	EE	Application	Observations	References
Biopolymers
Chitosan	Citronella (Cymbopogon spp.)	Ionic gelation	Microcapsules	11–225 µm	94.7–98.2%	−	Sustained release	[202]
	Satureja hortensis	Ionic gelation	Microcapsules	192 nm	96.17%	Acaricidal	Sustained release Prolonged bioactivity	[204]
	Citrus oils	Ionic gelation	Microcapsules	289.3–8843.2 nm	61.9–68.1%	−	Good release rates. Particle size and EE dependent on the surfactant.	[191]
	Coriandrum sativum	Ionic gelation	Nanocapsules	50–80 nm	26.5–75.99%	Storage product preservation	Controlled release Enhanced bioactivity	[222]
	Eugenia caryophyllata	Ionic gelation	Nanocapsules	40–100 nm	31–45.77%	Antifungal	Controlled release Higher bioactivity	[223]
	Cinnamomum zeylanicum	Ionic gelation	Microcapsules	100–200 nm	88.6%	Antimicrobial	Prolonged stability Improved antimicrobial capacity	[224]
	Mentha piperita	Ionic gelation	Microcapsules	≤563.3 nm	64–70%	Stored food pest control	Improved AChEI and fumigant toxicity	[225]
	Piper nigrum	Ionic gelation	Microcapsules	Av. 527.5 nm	35–40%	Stored food pest control	Improved AChEI and insecticidal activity	[226]
	Eryngium campestre	Ionic gelation	Microcapsules	Av. 157.8 nm	61–75%	Storage product preservation	Improved bioactivity	[227]
Chitosan-Caffeic acid	Cuminum cyminum	EO in nanogel	Nanogel	≤100 nm	85%	Antifungal	Improved antifungal performance by sustained release	[228]
Chitosan–cashew gum	Lippia siodes	Complex coacervation	Nanogel	335–558 nm	70%	Insecticidal	Controlled release Improvement in bioactivity	[217]
Chitosan-Cinnamic acid	Mentha piperita	EO in nanogel	Nanogel	≤100 nm	−	Antifungal	Improved antifungal performance by sustained release	[229]
	Bunium persicum	EO in nanogel	Nanogel	≤100 nm	41–43%	Antifungal and antioxidant	Enhanced antifungal and antioxidant activities	[230]
β-cyclodextrin	Commercial thyme oil (Thymus spp.)	Complexation (kneading)	Complex	582.9 nm	82.55 g/100 g	Antimicrobial	Enhanced antimicrobial activity	[231]
		Complexation (freeze drying)		3226.7 nm	71.27 g/100 g
	Lippia berlanderi	Complexation	Complex	−	71%	Antimicrobial	High entrapment Increased stability	[232]
	Eugenia caryophyllata			−	61%
β-cyclodextrin (HP-β-CD)	Piper nigrum	Complexation (inclusion complex formation)	Complex	−	50.55%	Antioxidant	Increased stability Higher antioxidant activity	[233]
β- & γ-cyclodextrin	Lippia graveolens (thymol and carvacrol chemotypes)	Complexation	Complex	6–13.22 µm	7.4–63.4%	−	Sustained release	[234]
Cornstarch	Thymus vulgaris	Thermoplastic extrusion	Particles	0.01–0.5 cm	69.07%	Repellent	Prolonged residual effect. Improved bioactivity.	[235]
Zein	Syzygium aromaticum	Antisolvent precipitation	Nanoparticles	125 nm	91.4%	Insecticide	Low toxicity to a non-target organism, Caenorhaabditis. elegans Potentialized the bioinsectide activity against Drosophila melanogaster	[236]
Gelatin-gum Arabic (Glutaraldehyde as wall hardener)	Camphor oil with added oil-soluble polystyrene	Complex coacervation	Particles	85.7–299.7 µm	99.6%	−	Sustained release	[211]
Gelatin-chia mucilage	Oregano oil (Lippia spp.)	Complex coacervation/spray dry	Microcapsules	1.65–8.85 µm	82.15–95.6%	−	Enhanced bioactivity Particle size-dependent EE	[209]
Gelatin-Persian gum, Persian gum, and gum Arabic	Satureja hortensis	Complex coacervation	Nanocapsules	81–208 nm	72.1–92.8%	Herbicidal	Improvement in herbicidal activity on encapsulated oil	[161]
Gelatin-sodium alginate	Cymbopogon winterianus	Complex coacervation	Complex	434.06 µm	83.5%	−	Controlled release Usage of waste product gelatin	[215]
	Piper nigrum	Complex coacervation	Complex	−	49.13–82.36%	−	Good retention capacity and preservation of composition	[210]
Maltodextrin-gum Arabic	Schinus molle	Spray drying	Microparticles	0.2–40 µm	96–100%	Insecticidal	Slow release, over 366 h Prolonged insecticidal effect	[237]
Maltodextrin-casein	Thymus vulgaris	Spray drying	Microparticles	Av. 0.87 µm	88.9%	Food preservation	Thermal stability	[238]
Sodium alginate (crosslinker CaCl2)	Satureja hortensis	Ionic gelation	Microparticles	47–117 µm	52–66%	Antibacterial Antioxidant	Controlled release Improved bioactivity	[207]
Whey protein- mesquite gum	Salvia hispanica	Spray drying	Microcapsules	2.54–3.35 µm	71.4–80.7%	−	Good encapsulation efficiency	[239]
Whey protein- gum Arabic				2.33–3.09 µm	70.7–79.8%
Emulsions
o/w Tween 80 as surfactant	Eucalyptus spp.	Sonication	Droplets	Av. 3.8 nm	−	Antimicrobial (topical)	Improvement in wound-healing effectivity	[240]
o/w Tween 20/Triton x−100 as surfactant	Azadiractha indica- Cymbopogon nardus	Constant stir	Droplets	2.8–17.8 nm	−	Antifungal	Stability of components Potent antifungal activity	[64]
Isopropanol Tween 80 as surfactant	Curcuma longa	Sonication	Microemulsion	−	−	Acaricidal	Controlled release Improved acaricidal activity	[77]
o/w/o Palm oil Sunflower oil Soy lecithin as surfactant	Terpene mixture	High-pressure homogenization	Droplets	75–175 nm	−	Antimicrobial	Enhanced activity	[196]
o/w/ polysorbate	Crithmum maritimum	Emulsion phase inversion	Droplets	50–70 nm	−	Agriculture and domestic pest control	Increased anti-ovipository and toxic activity	[241]
Octenyl succinic anhydride (OSA)—starch	Rosmarinus officinalis Zataria multiflora	Spray drying	Microcapsules	461–854 nm	−	Insecticidal	More effective than non-formulated in a long time	[242]
Chitosan-cellulose nanofibers	Citronella essential oil	Ultrasonication	Nanoparticles	426.9 nm	90.8%	Insecticidal	Nano-systems increased the insecticidal activity	[243]
Nanocarriers
SiO2	Crithmum maritimum	Loading of hollow capsule	Nanocapsules	20–78 nm	−	Agriculture and domestic pest control	Increased anti-ovipository and toxic activity	[241]
Zein sta-bilized with Tween 80	Eucalyptus staigeriana Litsea cubeba	Ultrasound-assisted nano-precipitation	Nanoparticles	200 nm	−	Antifungal	Activity against Colletotrichum lindemuthianum	[244]

Part. Size = Particle size; Av.= Average size; o/w = oil/water; o/w/o = oil, water, oil.

4.2. Prospects for Application in Agriculture

In their application in agriculture, EOs required a sustained release rate determined by their bioactivity efficiency as the minimum rate and their phytotoxicity as the maximum [187]. Moreover, environmental legislation limits the use of certain materials in farming and agriculture [245] due to concerns about their environmental and biological impact.

Biodegradable biopolymers are viable for applications in fields, greenhouses, and storage, mainly maintaining the integrity of the active compounds in EOs.

Natural carrier materials like polysaccharides have been well documented [246,247,248,249,250,251,252,253,254]. Other materials that have also shown effective controlled release include protein- and lipid-based encapsulations. Recently, various studies have researched material conjugation during the encapsulation process to enhance properties [255,256,257]. For example, electrostatic attractions in polysaccharide–protein systems provide greater protection to active ingredients [257] These systems have also shown improvements in encapsulation efficiency, solubility, and stability [139,258,259].

4.2.1. Chitosan

Chitosan (Ch) is a biopolymer product of the deacetylation of chitin from marine waste and insects; it is low-cost, highly biodegradable, and biocompatible and presents low toxicity [246,247,248].

This biopolymer was largely mentioned in Table 2 as a candidate for the encapsulation of EOs with pest management purposes. As stated by Ahmadi and colleagues [204], the obtention of ChNP via ionic gelation using TPP as a crosslinker results in a high EE, more than 96%, and a prolonged release for at least 25 days, translating into prolonged acaricidal activity with longer residual effects. Das made a similar observation [222]; he reported an initial burst (46.86% in the first eight hours) followed by a slow release of EO and an enhanced inhibition of store grain fungus. In another report, peppermint oil-chitosan nano- and microcapsules displayed higher toxicity against two stored-product pests, with almost 50% lower LC50 and a similar release behavior: an initial burst followed by a slow release rate. However, 38.5% to 62.4% of the EO was released in the initial burst, and the assays were carried out for only 72 h [225]. Rajkumar (2020) also reported the enhanced bioactivity of black pepper oil encapsulated in chitosan compared to free oil against stored-product pests, noting a decrease in the LC50 values [226].

Nanogels based on chitosan and cashew gum, obtained by complex coacervation, exhibited an EE of 70%, with a release rate dependent on the cashew gum content in the matrix: increased concentrations of the gum appear to favor the nanogel’s hydrophilic character, promoting a faster release of the EO [217].

Chitosan remains the right matrix candidate for EONPs for agricultural purposes. However, it is relevant to note that, even for different oils, release rates seem to follow a pattern of an initial burst and a posterior slow release, which may not suit the application’s specific needs and requires further experimentation to determine more adequate conditions for the desired release.

4.2.2. Gelatin-Gum Arabic

The gelatin-gum Arabic complexes profile is a promising candidate for encapsulating EO for agricultural applications. Both polymers are biodegradable, and the combination of polysaccharide and protein as well as the use of wall-hardening crosslinkers, commonly glutaraldehyde or transglutaminase, confer certain thermostability [212,249,250], a desirable quality for open field applications; high EE, up to 99.6% [198], and good release rates exist under colder and drier conditions [250,251,252].

Several factors can affect this matrix’s efficiency, as reported by Lv [250]. pH played a role in the size and thermal stability of gelatin-gum Arabic/ jasmine EO nanocapsules, obtained via complex coacervation using transglutaminase as a wall-hardening crosslinker. Incidentally, they displayed relative thermal protection when exposed to a water bath at 80 °C, where wall material keeps the core compounds’ integrity for up to 5 h.

The selection of crosslinkers was reported to affect the release control, as the chemical treatment with glutaraldehyde favors retention the most, in contrast with its enzymatic counterpart, transglutaminase [251]. Prata also reported a lower retention rate under wet conditions due to wall swelling, which was not observed under dry conditions.

Further experiments are needed to achieve a higher thermostability and controlled release; however, the protein–polysaccharide matrix remains a good prospect for EO encapsulation.

4.2.3. Cyclodextrins

Cyclodextrins (CDs) are toroidal-shaped natural oligosaccharides with hydrophobic cavities and hydrophilic exteriors, which makes them suitable for the encapsulation of lipophilic cores via complex inclusion [232,253,254]. Among CDs, the seven-sugar unit, β-cyclodextrin, is most widely used due to its simple synthesis and availability [253].

EO-CDs nanosystems have been studied and reported, positioning this matrix as an interesting alternative; some authors have observed improved thermal stability of the labile components of Eos [260], good EE [232,254,261], sustained release [234,253], and enhanced biological activity [233,261].

However, cyclodextrins are a more expensive alternative than chitosan and gelatin, which could affect the large-scale production of EO-CD NPs.

5. Conclusions

The biological activities of ECs are well documented, and there are several reports on their pest control properties. Recent efforts to implement more sustainable agricultural practices have drawn attention to EOs as potential pesticides and repellents of biological origin. However, the main challenge for their application as agrochemicals is the deterioration and high volatility of these compounds.

As nanotechnology is developed in diverse fields, the agrochemical industry is starting to rely on micro- and nanosystems as potential vehicles for bioactive compounds. In vitro, assays of nanosystems of essential oils display interesting release profiles, which translate into higher and prolonged biological activities. Encapsulating materials such as chitosan, gelatin-gum systems, and cyclodextrins are some of the most effective matrixes for EO cores. Other materials that have also shown effective controlled release include protein- and lipid-based encapsulations. Recently, to enhance properties, various studies have researched material conjugation during the encapsulation process, providing them with protection and good release kinetics while being environmentally friendly options due to their biodegradability and low toxicity.

The sub-microscopic encapsulation of EOs is still in need of further study. Several factors could influence the success of the synthesis of nanosystems, including the com-position of the oil, which varies in function of several factors and determines the strength of the interaction with the matrix; it is also important to set realistic release rates and release triggers that work for the specific application field (open field, greenhouse, food storage) and carry out the correspondent ecotoxicological evaluations.

In conclusion, although encapsulation technology is the most recent approach in essential oils, where the generation of biodegradable micro- and nanosystems of EOs is still a developing technology, it is an interesting topic for researchers for literature studies, as they appear to be a viable and more environmentally friendly alternative to conventional agrochemicals, with low soil leaching, more balanced doses, and generally low toxicity to non-target and vertebrate organisms.