Patent Literature on Mosquito Repellent Inventions which Contain Plant Essential Oils – A Review

Table of Contents

Abstract

Bites of mosquitoes belonging to the genera Anopheles Meigen, Aedes Meigen, Culex L. and Haemagogus L. are a general nuisance and are responsible for the transmission of important tropical diseases such as malaria, hemorrhagic dengue and yellow fevers and filariasis (elephantiasis). Plants are traditional sources of mosquito repelling essential oils (EOs), glyceridic oils and repellent and synergistic chemicals. A Chemical Abstracts search on mosquito repellent inventions containing plant-derived EOs revealed 144 active patents mostly from Asia. Chinese, Japanese and Korean language patents and those of India (in English) accounted for roughly 3/4 of all patents. Since 1998 patents on EO-containing mosquito repellent inventions have almost doubled about every 4 years. In general, these patents describe repellent compositions for use in topical agents, cosmetic products, incense, fumigants, indoor and outdoor sprays, fibers, textiles among other applications. 67 EOs and 9 glyceridic oils were individually cited in at least 2 patents. Over 1/2 of all patents named just one EO. Citronella [Cymbopogon nardus (L.) Rendle, C.winterianus Jowitt ex Bor] and eucalyptus (Eucalyptus L'Hér. spp.) EOs were each cited in approximately 1/3 of all patents. Camphor [Cinnamomum camphora (L.) J. Presl], cinnamon (Cinnamomum zeylanicum Blume), clove [Syzygium aromaticum (L.) Merr. & L. M. Perry], geranium (Pelargonium graveolens L'Hér.), lavender (Lavandula angustifolia Mill.), lemon [Citrus × limon (L.) Osbeck], lemongrass [Cymbopogon citratus (DC.) Stapf] and peppermint (Mentha × piperita L.) EOs were each cited in > 10 % of patents. Repellent chemicals present in EO compositions or added as pure “natural” ingredients such as geraniol, limonene, p-menthane-3,8-diol, nepetalactone and vanillin were described in approximately 40 % of all patents. About 25 % of EO-containing inventions included or were made to be used with synthetic insect control agents having mosquito repellent properties such as pyrethroids, N,N-diethyl-m-toluamide (DEET), (±)-p-menthane-3,8-diol (PMD) and dialkyl phthalates. Synergistic effects involving one or more EOs and synthetic and/or natural components were claimed in about 10 % of all patents. Scientific literature sources provide evidence for the mosquito repellency of many of the EOs and individual chemical components found in EOs used in patented repellent inventions.

Key words

botanical repellents - natural products - plant‐based repellents - repellent essential oils - mosquito repellents

Introduction

Mosquito vectors of severe tropical diseases

Severe human tropical diseases such as malaria, dengue and yellow fevers and filariasis are transmitted by the bites of infected hematophagous female mosquitoes belonging to the genera Aedes Meigen, Anopheles Meigen, Culex L. and Haemagogus L. (Diptera: Culicidae). For example, about 3.3 billion people − 1/2 of the world's population – are at risk of contracting malaria. In 2008, there were more than 247 million cases and more than 1 million deaths caused by malaria mainly in African children [1]. Human malaria is caused by infections by unicellular protozoan parasites Plasmodium falciparum Welch, P. vivax Grassi & Feletti, P. malariae Feletti & Grassi and P. ovale Stephens which are transmitted by about 20 Anopheles spp. Another important disease is dengue hemorrhagic fever which is a viral infection caused by several Flavivirus spp. (Flaviviridae) whose most important vector is Aedes (Stegomyia) aegypti L. Dengue and dengue hemorrhagic fevers threaten an estimated 2.5 billion people − 2/5 of the world's population – and an estimated 50 million people contract the disease per year. Around 500 000 dengue patients, most of whom are children, require hospitalization each year and around 2.5 % of those affected die [2]. Another serious tropical disease which threatens about 1 billion people in 80 countries is filariasis or elephantiasis. This disease already affects an estimated 120 million people and severely incapacitates and deforms 40 million people worldwide. Filariasis is caused by infections by several roundworm species of which Wuchereria bancrofti Cobbold (Filariidea: Onchocercidae) is the most important and is transmitted by the bites of the common house mosquito Cx. pipiens L. complex, Cx. quinquefaciatus Say, Aedes and Anopheles spp. [3], [4]. Yellow fever is an arbovirus of the Flavivirus genus (Flaviviridae) which is transmitted from monkeys in the jungle to humans and then from human to human by mosquitoes. The most significant yellow fever mosquito vector is Ae. aegypti. Despite the existence of effective vaccines, there are an estimated 200 000 cases of yellow fever and approximately 30 000 deaths attributed to this disease each year [5]. In public health initiatives which aim to limit or eradicate these and other tropical diseases, mosquito vector control methods such as repellence figure prominently among those which are employed.

Synthetic mosquito repellents DEET and alkyl phthalates

For more than 50 years, the synthetic compound DEET (N,N-diethyl-m-toluamide), has been the most effective single repellent for mosquito species and is the basis for many commercial repellent products on the market. Despite reports of severe toxic properties which can dramatically affect adults and especially young children including dermatitis, allergic reactions, neurological (seizures, coma) and cardiovascular toxicity, the risk of serious toxic effects from DEET is considered slight. Nevertheless, DEET should always be used at the lowest effective dose possible. Also, dimethyl and di-n-butyl phthalates (DMP and DBP, respectively), which are effective mosquito repellents and were widely used in the last century, are no longer generally recommended for use as mosquito repellents due to their toxicity [6].

Plants as sources of mosquito control agents

Plants have historically been valuable sources of agents for the control of insects [3], [7]. They are the sources of the natural insecticidal and larvicidal substances nicotine (Nicotiana L. spp.), quassin (Quassia amara L.), rotenone and rotenoids (Derris Lour. spp. and Lonchocarpus Kunth spp. roots), pyrethrins like chrysanthamic acid and its derivatives present in pyrethrum [extracts of Chrysanthemum cinerariifolium (Trevir.) Vis. flowers] and azadirachtin (Azadirachta indica A. Juss. seed kernel). These and other natural insect control agents have served as the basis for the development of the structurally-related synthetic pyrethroid, nicotinoid and rotenoid classes of insecticides and piperonyl butoxide synergist. Also, pyrethroids and piperonyl butoxide synergism are the basis for a number of commercially available mosquito control products in use today [3].

Botanical repellents

Citronella essential oils (EOs) are obtained mainly from varieties of Cymbopogon nardus (L.) Rendle (Ceylon citronella) and C. winterianus Jowitt ex Bor (Java citronella). They have been used in mosquito repellency for more than a century in much of the world and are the most widely used natural repellents today [3]. Also, EOs of Eucalyptus L'Hér. spp. are widely used to repel insects, including mosquitoes, and contain insecticidal and repellent components p-menthane-3,8-diol (PMD), 1,8-cineole, α-pinene, p-cymene and γ-terpinene among other active compounds [8]. According to a recent review of the scientific literature, the most frequently studied repellent EOs are those obtained from species belonging to the genera Cymbopogon Spreng., Ocimum L. and Eucalyptus L'Hér. spp. and a number of mosquito repellent EOs have been identified in recent years having known active repellent chemical components [7]. Mosquito repellency is believed to be due to the synergistic interactions of the chemical components in EOs. Furthermore, strong synergistic effects between EOs and isolated natural or synthetic substances have been reported [7].

The United States Environmental Protection Agency [9] has registered citronella EO, eucalyptus EO and other plant oils as safe and effective ingredients for use in topical insect repellents. However, caution is recommended in the use of EOs in general due to a number of potential toxic effects [7]. Among the important toxic effects of EOs are mutagenicity and genotoxicity. Another toxic effect is the allergenicity of EO chemical components which are controlled in the European Community and elsewhere. Interestingly, a number of repellent and insecticidal plant EOs and their chemical constituents have been evaluated using a variety of methods and are believed to be non-mutagenic [10].

A number of commercial repellent products have been developed over the past decades which utilize derivatives of plants such as EOs, fractions and their isolated chemical components and synthetic components. Thus, it is important to have a comprehensive knowledge of the commercially significant uses of EOs in mosquito repellent inventions and to the best of our knowledge the patent literature on this topic has not been reviewed. The aim of the present review is to explore and analyze patent literature on mosquito repellent inventions which make use of or are based wholly on plant EOs and/or their chemical components. A secondary aim is to analyze the scientific bases and relevancy of the use of plant EOs and chemical components of these oils in patented mosquito repellent formulations.

Chemical Abstracts Search Criteria

A Chemical Abstracts search of the patent literature for the period of 1991 through May, 2010 was performed using SciFinder Scholar® [11]. Combinations of the key words “repellent”, “mosquito” and “essential oil” generated an initial set of approximately 160 patents. This set was further refined. Patents describing mosquitocidal/larvicidal inventions, but having no stated claim or use as mosquito repellents were eliminated. Also, patented mosquito repellent inventions which did not make use of at least one commercially-obtained, plant-derived oil [a volatile (essential) oil, a concrete, or a pressed oil] or which did not describe the preparation of an EO by physical means for use within the patent were eliminated. This approach eliminated patents presenting solvent extraction performed on mixtures of several plant materials followed, for example, by distillation and evaporation as a means of obtaining the mosquito repellent invention. Also, by this criterion, incense and other smoke-generating inventions made from mosquito repellent plants but having no (added) EO in the composition were eliminated. Applying the above criteria led to a data set of 144 patents describing plant oil-containing mosquito repellent inventions which was the basis for the analysis presented below.

Patenting Countries and Patent Families: Trends

The contribution of China, Japan, Korea and India to the overall number of patented EO-containing mosquito repellent inventions is significant. These countries accounted for, respectively, 37, 15, 13 and 8 % (73 %) of all patents ([Fig. 1]). An interesting trend is that all 53 Chinese, 17 of 18 Korean, 20 of 22 Japanese and 9 of 11 Indian patents have only been deposited in their countries of origin and in general have not been followed-up by later patents. Also, all Belgian, Brazilian, German and Polish patents followed the rule of single deposits without follow-up patents. In contrast, industrial, academic and other patents originating in Australia, Canada, the United Kingdom and the United States gave rise to larger patent families, made greater use of the World Patent System and led to deposits of patents on EO-containing repellents in multiple countries as a rule. Finally, more than 75 % of all patented repellent inventions are indexed as agrochemical bioregulators and over 80 % describe chemical compositions (often together with preparation details) ([Fig. 1]).

Fig. 1 Analysis of EO-containing patented mosquito repellent inventions by patenting country, patent language, patent type and utility.

EOs Used in Patented Mosquito Repellent Inventions

Plant taxonomic information [12], extraction methods, chemical composition, literature sources [8], [10], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95], [96], [97], [98], [99], [100], [101], [102], [103], [104], [105], [106], [107], [108], [109], [110], [111], [112], [113], [114], [115] and frequency of use for 67 EOs and 9 glyceridic oils which were individually cited in at least 2 (1.4 %) of the total number of patents are presented in [Table 1]. Approximately 60 % of all patents named just one plant EO. One patent claimed to use up to 44 plant oils in formulations. Citronella (34.7 %) and eucalyptus (30.6 %) EOs were each named in about 1/3 of all patented repellent inventions followed by (species, % of patents): lavender (Lavandula angustifolia Mill., 21.5 %), peppermint (Mentha × piperita L., 16.7 %), clove [Syzygium aromaticum (L.) Merr. & L. M. Perry, 15.3 %], lemongrass [Cymbopogon citratus (DC.) Stapf, 14.6 %], cinnamon (Cinnamomum zeylanicum Blume, 12.5 %), geranium (Pelargonium graveolens L'Hér., 11.8 %), camphor [Cinnamomum camphora (L.) J. Presl, 11.1 %] and lemon [Citrus × limon (L.) Osbeck, 11.1 %] EOs.

Table 1 Botanic information, extraction methods, major chemical components, literature sources and frequency (%) of use of plant essential and glyceridic oils in patented mosquito repellent inventions, 1991 – May, 2010.

Oil	Species	Family	Part	Method	Major components	Lit.	% Patents
Essential Oils
Ambrette	Abelmoschus moschatus Medik.	Malvaceae	whole seed	steam distilled	2E,6E-farnesyl acetate, Z-7-hexadecen-16-olide, β-farnesene	[13], [14]	1.4
Angelica	Angelica archangelica L.	Apiaceae	root	steam distilled	ligustilide, α & β -pinene, carvacrol, 3-carene, limonene, β-phellandrene, 15-pentadecanolide	[15], [16]	1.4
Anise	Pimpinella anisum L.	Apiaceae	fruit, seed	steam distilled	trans-anethole	[17]	4.2
Artemisia	Artemisia argyi H. Lév. & Vaniot	Asteraceae	leaf	steam distilled	germacrene D, α-phellandrene, α -myrcene, 1,8-cineole, borneol, terpinol, spathulenol	[18], [19]	4.9
Basil	Ocimum basilicum L.	Lamiaceae	leaf, flower top	steam distilled	estragole, limonene, fenchone, linalool, eugenol E-methyl cinnamate, 1,8-cineole	[20], [21], [22]	4.9
Bay laurel	Laurus nobilis L.	Lauraceae	leaf	steam distilled	1,8-cineole, sabinene, α-terpinyl acetate, linalool	[23], [24]	1.4
Bergamot	Citrus × bergamia Risso	Rutaceae	fresh or dried peel	pressed	limonene, linalyl acetate, β –pinene, γ -terpinene, linalool	[24]	1.4
Camphor	Cinnamomum camphora (L.) J. Presl	Lauraceae	wood, bark, leaf	steam distilled	1,8-cineole, α -terpineol, α -pinene, linalool, camphor, sabinene	[10]	11.1
Cassia	Cinnamomum cassia (L.) C. Presl	Lauraceae	leaf, bark, stalk	steam distilled	bark: E -cinnamaldehyde, methyl o -salicylate; leaf: 3-methoxy-1,2-propanediol, E -cinnamaldehyde, o-methoxycinnamaldehyde	[25], [26]	2.1
Catnip, catmint	Nepeta cataria L.	Lamiaceae	dry leaf, stem	steam distilled	nepetalactone, 1,8-cineole, α-humulene, α-pinene, E -geraniol, β-caryophyllene, citronellol	[20], [27], [28]	4.9
Cedar	Cedrus Trew (Cupressus L., Juniperus L.) spp.	Pinaceae (Cupressaceae)	wood	steam distilled	thujopsene, eudesmol, E-(+)-α-atlantone; α,β & γ-himachalenes; α- & β-cedrenes; limonene, β-phellandrene, α & β -pinene, 3-carene; p-methyl-Δ-3-tetrahydro & p-methyl acetophenones; hinokitiol, carvacrol	[29]	9.7
Chamomile	Chamaemelum nobile (L.) All. Matricaria recutita L.	Asteraceae	seed, leaf, flower	steam distilled	Roman: isobutyl, isoamyl & 2-methylpentyl angelates, α -pinene German: E-β-farnesene, E,E-α-farnesene, α -bisabolol, α-bisabolol oxides A & B	[20], [28], [30]	2.1
Chrysanthemum	Chrysanthemum indicum L.	Asteraceae	dry flower	steam distilled	verbenol, 2-(2,4-hexadiynylidene)-1,6- dioxaspiro[4.4]non-3-ene, 1,8-cineole, α -pinene, camphor, borneol, bornyl acetate	[31], [32]	2.8
Cinnamon	Cinnamomum zeylanicum Blume	Lauraceae	bark, leaf	steam distilled	eugenol, cinnamaldehyde	[25], [33], [34]	12.5
Citronella	Cymbopogon nardus (L.) Rendle, C. winterianus Jowitt ex Bor	Poaceae	leaf	steam distilled	citronellal, geraniol, citronellol, geranyl acetate	[35], [36]	34.7
Citrus	Citrus × limon (L.) Osbeck, C. × sinensis (L.) Osbeck, C. × aurantifolia L.	Rutaceae	peel	pressed, steam distilled	see orange, lemon, lime oil compositions	–	1.4
Clove	Syzygium aromaticum (L.) Merr. & L. M. Perry	Myrtaceae	flower bud	steam distilled	eugenol, caryophyllene, eugenyl acetate	[37]	15.3
Coriander	Coriandrum sativum L.	Apiaceae	fruit, seed	steam distilled	linalool, geraniol, geranyl acetate, 2-decenal, 3-dodecenal	[20], [38], [39]	2.1
Cypress	Cupressus sempervirens L.	Cupressaceae	needle, twig	steam distilled	sabinene, α -pinene, terpinen-4-ol, limonene	[40], [41]	4.2
Dill	Anethum graveolens L.	Apiaceae	seed, leaf, stem	steam distilled	carvone, limonene, α-phellandrene, α -pinene, cis-dihydrocarvone	[42], [43]	1.4
Eucalyptus	Eucalyptus L'Hér. spp.	Myrtaceae	leaf	steam distilled	1,8-cineole, p -menthane-3,8-diol, α -pinene, p -cymene, γ -terpinene, eucamalol, allo-ocimene, citronellol, α -terpineol	[8]	30.6
Fennel	Foeniculum vulgare Mill.	Apiaceae	fruit	steam distilled	E-anethole, (+)-fenchone, α-phellandrene, (±)-limonene, estragole	[24]	4.9
Garlic	Allium sativum L.	Amaryllidaceae	bulb	steam distilled	diallyl disulfide, diallyl trisulfide, methyl allyl trisulfide	[44], [45]	6.3
Geranium	Pelargonium graveolens L'Hér.	Geraniaceae	leaf, stem	steam distilled	2-phenylethanol, geraniol, citronellol, geranyl acetate	[46]	11.8
Ginger	Zingiber officinale Roscoe	Zingiberaceae	rhizome	steam distilled	geranial, α-zingiberene, E,E-α-farnesene, neral, ar-curcumene, geraniol	[47], [48]	1.4
Grapefruit	Citrus reticulata Blanco	Rutaceae	peel	pressed	limonene, geranial, neral	[49]	2.1
Guaiac wood	Bulnesia sarmientoi Lorentz ex Griseb.	Zygophyllaceae	wood	steam distilled	bulnesol, guaiol, α-bulnesene	[50]	1.4
Hiba	Thujopsis dolabrata (Thunb. ex L. f.) Siebold & Zucc.	Cupressaceae	wood	steam distilled	sabinene, 4-terpineol, thujopsene, hinokitiol, α-thujaplicine, carvacrol	[51], [52], [53]	2.1
Ho leaf	Cinnamomum camphora (L.) J. Presl	Lauraceae	leaf	steam distilled	1,8-cineole, α -terpineol, linalool, camphor, safrole, sabinene, nerolidol	[10], [54], [55]	1.4
Hyssop	Hyssopus officinalis L.	Lamiaceae	leaf, flower	steam distilled	sabinene, pinocamphene, isopinocamphene, isopinocamphone, pinocarvone, cis &  trans-pinocamphones, β -pinene, 1,8-cineole, camphor, linalool	[56], [57], [58]	1.4
Jasmine	Jasminum officinale L.	Oleaceae	flower	solvent extracted	linalool, benzyl acetate, methyl & benzyl benzoates, methyl anthranilate, Z-jasmone, eugenol	[59], [60]	3.5
Juniper	Juniperus communis L.	Cupressaceae	fruit	steam distilled	α -pinene, myrcene, sabinene, germacrene D	[61]	1.4
Lady-of-the-night	Cestrum nocturnum L.	Solanaceae	flower	solvent extracted, steam distilled	phenylethyl alcohol, benzyl alcohol, eugenol	[62]	2.1
Lavender	Lavandula angustifolia Mill.	Lamiaceae	flower	steam distilled	linalool, linalyl acetate, lavandulyl acetate, α -terpineol, geranyl acetate, terpinen-4-ol, 1,8-cineole	[63], [64]	21.5
Lemon	Citrus × limon (L.) Osbeck	Rutaceae	peel	pressed	limonene, β-pinene, γ-terpinene	[65]	11.1
Lemon eucalyptus	Eucalyptus citriodora Hook.	Myrtaceae	leaf, twig	steam distilled	citronellal, citronellol	[8], [66]	8.3
Lemongrass	Cymbopogon citratus (DC.) Stapf	Poaceae	leaf	steam distilled	geranial, neral, myrcene	[41]	14.6
Lemon tea tree	Leptospermum petersonii F. M. Bailey	Myrtaceae	leaf	steam distilled	neral, geranial, γ -terpinene, geraniol, geranyl acetate, α -pinene, citronellal, terpinolene	[67]	1.4
Lime	Citrus × aurantifolia L.	Rutaceae	peel	steam distilled, pressed	D-dihydrocarvone, D -limonene, α -terpineol	[68]	2.1
Manuka	Leptospermum scoparium Forst. & Forst.	Myrtaceae	leaf, stem	steam distilled	leptospermone, trans-calamenene, flavesone, 1,8-cineole, α -pinene	[69], [70]	1.4
Marjoram	Origanum majorana L.	Lamiaceae	leaf, flower	steam distilled	p -cymene, γ -terpinene, terpinen-4-ol, linalool, cis-sabinene hydrate	[71], [72]	2.1
May chang/Litsea	Litsea cubeba (Lour.) Pers.	Lauraceae	fruit	steam distilled	neral, R-(+)-limonene, geranial, citronellal	[46], [73], [74]	4.9
Melaleuca/Tea tree	Melaleuca alternifolia Cheel	Myrtaceae	leaf	steam distilled	terpinen-4-ol, γ -terpinene, α -terpinene, 1,8-cineole	[75]	9.7
Mint, mentha	Mentha L. spp.	Lamiaceae	leaf, flower	steam distilled	menthone, menthol, 1,8-cineole, 4-terpineol	[76]	9.7
Orange	Citrus × sinensis (L.) Osbeck	Rutaceae	peel	pressed	limonene, myrcene	[77]	4.2
Palmarosa	Cymbopogon martini (Roxb.) Will.Watson	Poaceae	dry leaf	steam distilled	geraniol, geranyl acetate, geranial	[78]	4.9
Parsley (Curl leaf)	Petroselinum crispum (Mill.) Fuss	Apiaceae	leaf, stem, seed	steam distilled	β-phellandrene, myristicin, α & β -pinene, myrcene	[79]	1.4
Patchouli	Pogostemon cablin (Blanco) Benth.	Lamiaceae	dry, fermented leaf	steam distilled	(−)-patchoulol, α-guaiene, seychellene, β-caryophyllene, α & β-patchoulenes, selinene, α-bulnesene, norpatchoulenol, pogostol	[80]	4.2
Pepper	Piper nigrum L.	Piperaceae	fruit	steam distilled	E-β-caryophyllene, limonene, β -pinene	[81], [82]	2.1
Peppermint	Mentha × piperita L.	Lamiaceae	aerial part	steam distilled	isomenthol, p -menthone, isomenthyl & menthyl acetates	[83], [84]	16.7
Pine	Pinus sylvestris L.	Pinaceae	twig, bud	steam distilled	3-carene, α & β -pinene, α-cadinol, camphene	[85], [86]	8.3
Rose	Rosa × damascena Mill., R. × centifolia L.	Rosaceae	petal	steam distilled, solvent extracted	2-phenethyl alcohol, citronellol, geraniol, linalool, nonadecane	[30], [87]	6.3
Rosemary	Rosmarinus officinalis L.	Lamiaceae	flower	steam distilled	verbenone, camphor, borneol, bornyl acetate, α -terpineol, terpinen-4-ol	[41], [88]	9.0
Salvia (Sage)	Salvia L. spp.	Lamiaceae	leaf, flower, aerial part	steam distilled	camphor, 1,8-cineole, α -pinene, camphene, limonene, linalool, myrcene, β-caryophyllene, caryophyllene oxide, spathulenol, viridiflorol, 3-carene, α -bisabolol, geraniol, linalyl acetate, γ-muurolene, α -thujone, manool	[89], [90], [91], [92]	1.4
Sandalwood	Santalum L. spp.	Santalaceae	heartwood	steam distilled	α, β & epi-β-santalenes; α, β & epi-β-santalals; α −santalol; spiro, β, E-β & epi-β santalols, trans-α-bergamotene, trans-α-bergamotol	[80], [93]	3.5
Sour (bitter) orange	Citrus × aurantium L.	Rutaceae	peel	pressed	limonene	[94]	3.5
Spearmint	Mentha spicata L.	Lamiaceae	flower	steam distilled	carvone, limonene	[95]	2.8
Star anise	Illicium verum Hook. f.	Schisandraceae	seed	steam distilled	E-anethole, 4-allylanisole	[96], [97]	see anise*
Tagetes	Tagetes minuta L.	Asteraceae	leaf, stalk, flower	steam distilled, solvent extracted	E & Z-β-ocimenes, limonene, α -terpineol, dihydrotagetenone, Z-tagetone, E &  Z-tagetenones	[98]	2.1
Tarragon	Artemisia dracunculus L.	Asteraceae	leaf, flower	steam distilled	sabinene, elemicine, methyl eugenol	[99]	1.4
Thyme	Thymus vulgaris L.	Lamiaceae	flower, leaf	steam distilled	p -cymene, geraniol, cis -thujone, thymol, carvacrol	[41]	7.6
Turmeric	Curcuma longa L.	Zingiberaceae	rhizome	steam distilled, solvent extracted	α-phellandrene, 1,8-cineole, terpinolene, zingiberene, β-sesquiphellandrene, α & β-turmerones	[41]	2.8
Verbena	Lippia triphylla (L'Hér.) Kuntze	Verbenaceae	leaf	steam distilled	geranial, neral, limonene, geraniol	[100]	1.4
Vetiver	Vetiveria zizanioides (L.) Nash	Poaceae	root	steam distilled	khusimol, isonootkatool, β-vetivenene, α & β-vetivones	[101]	2.1
Violet	Viola odorata L.	Violaceae	leaf, flower	solvent extracted	nona-2,6-dienal, cis-hex-3-en-1-ol, hexadec-1-ene, pentadec-3-enal, octadec-1-ene, hexadecanoic & octadeca-9,12-dienoic acids	[102]	1.4
Wintergreen	Gaultheria procumbens L.	Ericaceae	leaf	steam distilled	methyl salicylate	[103]	6.3
Ylang-Ylang	Cananga odorata (Lam.) Hook. f. & Thomson	Annonaceae	flower	steam distilled	linalool, benzyl acetate, benzyl benzoate, benzyl salicylate	[41]	2.8
Glyceridic Oils
Castor	Ricinus communis L.	Euphorbiaceae	seed	pressed	ricinolic, linoleic, oleic acids	[104]	4.9
Evening primrose	Oenothera biennis L.	Onagraceae	seed	pressed	linalool; palmitic, stearic, oleic, linoleic &  γ-linolenic acids	[105], [106]	1.4
Ligusticum/Chuanxiong	Ligusticum chuanxiong Hortorum ex Qiu, et al. in Qiu	Apiaceae	root	CO2 extracted	butylphthalide, 2-propenyl phenoxyacetate, 3-isobutyliden phthalide, palmitic &  octadecenoic acids	[107]	1.4
Mustard	Brassica L. spp.	Brassicaceae	seed	Pressed	erucic, oleic, linoleic, linolenic, palmitic & stearic acids	[108]	2.1
Neem	Azadirachta indica A. Juss.	Meliaceae	seed	pressed	salanin, nimbinin, nimbin, nimbidiol, azadirachtin; palmitic, stearic, oleic, linoleic acids	[104], [109]	8.3
Olive	Olea europaea L.	Oleaceae	fruit	pressed	oleic, palmitic, linoleic & stearic acids	[110]	2.1
Perilla	Perilla frutescens (L.) Britton	Lamiaceae	seed	pressed	oleic, linoleic & linolenic acids, S-limonene, perillaldehyde, elsholzia- ketone, naginataketone, perillaketone, myristicin, dillapiol, elemicin, isoegomaketone, perillene, egomaketone, shisofuran	[111], [112], [113]	1.4
Sesame	Sesamum indicum L.	Pedaliaceae	seed	pressed	linoleic, oleic, palmitic & stearic acids	[114]	3.5
Soybean	Glycine max (L.) Merr.	Fabaceae	seed	solvent extracted	linoleic, oleic, palmitic, α-linolenic & stearic acids	[115]	4.2
Note: chemical components in bold text have proven mosquito repellent and deterrent properties according to literature and patent sources which are summarized in [Table 4]). Tropicos, botanical information system at the Missouri Botanical Garden [12] was used as a general reference on taxonomic information in this table

A number of EOs were cited in only one patented repellent invention (and were not included in [Table 1]). Given the importance of potentially novel sources of mosquito repellent botanicals it is important to mention the following EOs: abies [Abies spectabilis (D. Don) Spach], almond [Prunus dulcis (Mill.) D. A.Webb], Australian yuzu (Citrus junos Siebold ex Tanaka), black pine (Pinus nigra Arnold), Blumea lacera (Burm. f.) DC., calamus (Acorus calamus L.), Canada fleabane [Conyza canadensis (L.) Cronquist], cardamom [Elettaria cardamomum (L.) Maton], cork tree (Phellodendron amurense Rupr.), Elsholtzia hunanensis Hand.-Mazz., flos lonicerae japonicae or jin yin hua (Lonicera japonica Thunb. ex Murray flower), fructus forsythiae [Forsythia suspensa (Thunb.) Vahl], galbanum (Ferula galbaniflua Boiss. & Buhse), herba schizonepetae [Schizonepeta (Benth.) Briq. sp.], hibiscus (Hibiscus L. sp.), larch (Larix Mill. sp.), lemon balm (Melissa officinalis L.), lovage (Levisticum officinale W. D. J. Koch), lilac (Syringa vulgaris L.), Limnanthes alba Hartw. ex Benth. seed, linaloe wood (Bursera delpechiana Poiss. ex Engl.), Michelia × alba DC. leaf, myrtle (Myrtus communis L.), Ocimum canum Sims, onion (Allium cepa L.), oregano (Origanum vulgare L.), peach [Prunus persica (L.) Batsch], pennyroyal (Mentha pulegium L.), pepper (Capsicum annuum L. var. annuum), petitgrain (Citrus aurantium L. var. amara), pimento [Pimenta dioica (L.) Merr.], pine needle (Pinus L. sp.), Rhodomyrtus tomentosa (Aiton) Hassk., rue (Ruta graveolens L.), Stephania sinica Diels, tansy (Tanacetum L. sp.), Torreya grandis Fortune ex Lindl. and valerian (Valeriana L. sp.). Nevertheless, we will not systematically go into details of the composition and repellency of these EOs in this review.

Generally, patent literature on mosquito repellent inventions treats fragrant extracts as EOs whether these extracts are produced using an initial solvent extraction step (such as in the production of concretes), by steam distillation, hydrodistillation, direct distillation or pressing (resulting in proper EOs). Herein, the term EO refers broadly to fragrant extracts obtained using these different extraction techniques. Importantly, fragrant oils and partially volatile balsamic oils which have volatile and non-volatile (glyceridic) chemical components in their compositions ([Table 1]) and mosquito repellent properties, such as evening primrose (Oenothera biennis L.), perilla [Perilla frutescens (L.) Britton], ligusticum (Ligusticum chuanxiong Hortorum ex Qiu, et al. in Qiu) and copaiba (Copaifera L. spp.) oils are used in mosquito repellent patented formulations together with plant EOs.

Glyceridic Oils Used in Patented Mosquito Repellent Inventions

Importantly, plant glyceridic oils, several of which are known to possess mosquito repellency, were used in patented formulations as carriers or active ingredients which were associated with prolonged repellent action. The most used glyceridic oils in patented EO-containing repellent inventions were neem/margarosa (Azadirachta indica A. Juss., 8.3 %), castor (Ricinus communis L., 4.9 %), soybean [Glycine max (L.) Merr., 4.2 %] and sesame (Sesamum indicum L., 3.5 %) oils ([Table 1]).

Synergist Effects Associated with EO-Containing Compositions

Synergist interactions were claimed to be operating in 8 % of EO-containing patents ([Table 2]) [116], [117], [118], [119], [120], [121], [122], [123]. For example, EO of a species of Lippia L. and any one of a number of EOs formulated into a slowly evaporating hydrocarbon soluble composition was said to modify neuronal activity in invertebrates such as adult mosquitoes and produce repellent activity comparable to commercial pyrethroids [123]. Another example of a synergist formulation involved almost equal amounts of eucalyptus (Eucalyptus spp.) and cassia [Cinnamomum cassia (L.) C. Presl] EOs together with an emulsifying agent and butyl acetate solvent which was meant to be used with pyrethroids in incense formulations [119]. Still another example of a synergist repellent meant for use with pyrethroids (allethrin, dimefluthrin) used EOs of wintergreen (Gaultheria procumbens L.) 30–50 (w/w) and camphor (Cinnamomum camphora (L.) J. Presl) 10–35 (w/w), emulsifier and solvent [117] ([Table 2]).

Table 2 Uses and applications, EOs and other components of patented mosquito repellent inventions which claimed synergist effects.

Use or technology	Plant EO	Plant species	Non-EO component	Source
Applied films	–	Backhousia citriodora F. Muell.	–	[116]
		Melaleuca ericifolia Sm.
Coatings	Wintergreen	Gaultheria procumbens L.	pyrethroids	[117]
	Camphor	Cinnamomum camphora (L.) J. Presl
	Floor wash	Eucalyptus			Eucalyptus L'Hér. spp.	–	[118]
Lemon eucalyptus		Eucalyptus citriodora Hook.
Citronella		Cymbopogon winterianus Jowitt ex Bor
Incense		Eucalyptus	Eucalyptus L'Hér. spp.	pyrethroids	[119]
		Cassia	Cinnamomum cassia (L.) C. Presl
	Microcapsules for textiles	Eucalyptus	Eucalyptus L'Hér. spp.			DEET, isobornyl derivatives	[120]
Not specified	Rose	Rosa L. sp.	natural pyrethrins	[121]
	Jasmine	Jasminum officinale L.
	Limonene	synergist natural product
Spray/Fumigation on grass, shrubs, trees	Citronella	Cymbopogon Spreng. sp.	dimethyl phthalate, allyl sulfate	[122]
	Neem	Azadirachta indica A. Juss.
	Eucalyptus	Eucalyptus L'Hér. spp.
Vaporizer/Fumigant	Lippia	Lippia L. sp.+:	–	[123]
	Geranium	Pelargonium graveolens L'Hér.
	Lemon eucalyptus	Eucalyptus citriodora Hook.
	Basil etc.	Ocimum basilicum L.

Scientific Basis for Mosquito Repellency of EOs in Patented Inventions

The scientific literature on the mosquito repellence of EOs used in patents provides important insights ([Table 3]) [8], [37], [41], [46], [61], [68], [73], [85], [88], [93], [124], [125], [126], [127], [128], [129], [130], [131], [132], [133], [134], [135], [136], [137], [138], [139], [140], [141], [142], [143], [144], [145], [146], [147], [148], [149], [150], [151], [152], [153], [154], [155], [156], [157], [158], [159], [160], [161], [162], [163], [164], [165], [166], [167], [168], [169], [170], [171], [172], [173], [174], [175], [176], [177]. Firstly, citronella (Cymbopogon nardus, C. winterianus), eucalyptus (Eucalyptus spp.) and lemon eucalyptus (E. citriodora Hook.) EOs which were cited in many patented inventions have been the subject of a number of studies in which repellency against species of Culex, Anopheles and Aedes in some cases comparable to DEET have been reported for these oils alone or in formulations [138], [140], [143], [178]. Furthermore, many EOs used in patented inventions have quite significant repellent properties according to published studies such as: bay laurel (Laurus nobilis L.) [137], camphor (Cinnamomum camphora) [132], cassia (Cinnamomum cassia) [132] EOs against Ae. aegypti; lemon (Citrus × limon) EO [128], [152] against An. stephensi Liston, catnip (Nepeta cataria L.) EO [128], lemongrass (Cymbopogon citratus) [128], may chong/litsea [Litsea cubeba (Lour.) Pers.] [128], [179], tagetes (Tagetes minuta L.) [128], violet (Viola odorata L.) EOs [128] against Ae. aegypti, An. stephensi and Cx. quinquefasciatus; peppermint (Mentha × piperita) EO on human skin against An. annularis van der Wulp, An. culicifacies Giles and Cx. quinquefasciatus [160]; sandalwood (Santalum L. spp.) EO formulations against a Culex sp. [177]; geranium (Pelargonium graveolens) oil formulations [137], [144], [147], [148] against species of Culex, Anopheles and Aedes; thyme (Thymus vulgaris L.) EO against Cx. quinquefasciatus [128], [137], [162]; marjoram (Origanum majorana L.) and juniper (Juniperus communis L.) EOs against Cx. pipiens pallens Coquillet [151]; and wintergreen (Gaultheria procumbens) EO against species of Culex and Aedes [131].

Table 3 Scientific evidence for mosquito repellent and related properties of plant EOs and glyceridic oils used in patented repellent inventions.

Plant oil	Plant species	Mosquito repellent properties	Source
Essential oil
Andiroba	Carapa guianensis Aubl.	patented candles commercialized in Brazil as mosquito fumigant-repellents; repellency in Amazon field test of 1 : 1 : 1 andiroba, copaiba and baby oil mixture	[124], [125]
Anise	Pimpinella anisum L.	good repellent against Cx. pipiens	[126]
Artemisia	Artemisia argyi H. Lév. & Vaniot	repels mosquitoes	[127]
Basil	Ocimum basilicum L.	Ae. aegypti (PP 2 h, R% 81), An. stephensi (PP 3.5 h, R% 67), Cx. quinquefasciatus (PP 8 h, R% 100)*; 5 % hairy basil + 5 % vetiver + 10 % citronella EOs in nanoemulsion repel Ae. aegypti 4.7 h; repels Anopheles sp. in human bait test & Cx. pipiens	[128], [129]
Bay laurel	Laurus nobilis L.	spatial repellence against Ae. aegypti, acceptable smell, definite mosquito repellence	[130]
Bergamot	Citrus × bergamia Risso	repellence against Culex & Aedes spp. is comparable to citronella oil	[131]
Camphor	Cinnamomum camphora (L.) J. Presl	Ae. aegypti (PP 2.5 h, R% 32), An. stephensi (PP 8 h, R% 43), Cx. quinquefasciatus (PP 8 h, R% 57)*; EO (0.1 mg · cm−2 on human skin): repellency and duration against Ae. aegypti comparable to DEET	[128], [132]
Cassia	Cinnamomum cassia (L.) C. Presl	bark extract (0.1 mg · cm−2): repels Ae. aegypti comparably to DEET (on human skin), 5 % EO in cream provided 50 min of protection to humans against female Ae. aegypti	[132]
Catnip, catmint	Nepeta cataria L.	Ae. aegypti (PP 8 h, R% 84), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100)*; 1–6 h protection against Ae. albopictus (23 & 468 µg · cm−2); safety pharmacology evaluation: N. cataria oil is safe compared to DEET, p-menthane-3,8-diol, etc. May cause minor skin irritation	[128], [133], [134]
Cedar	Cedrus Trew (Cupressus L., Juniperus L.) spp.	Ae. aegypti (PP 3 h, R% 38), An. stephensi (PP 8 h, R% 38), Cx. quinquefasciatus (PP 8 h, R% 100)*; repels An. stephensi	[127], [128], [135]
Chamomile	Chamaemelum nobile (L.) All.	Ae. aegypti (PP 4 h, R% 65), An. stephensi (PP 8 h, R% 76), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
Cinnamon	Cinnamomum zeylanicum Blume	high repellency (RD95 mg · mat−1) against An. stephensi (49.6), Ae. aegypti (53.9), Cx. quinquefasciatus (44.2)	[136]
	C. verum J. Presl	Ae. aegypti (PP 5.5 h, R% 70), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
Citronella	Cymbopogon nardus (L.) Rendle, C. winterianus Jowitt ex Bor	repellency by 1 : 1 citronella & lavender EOs in 2 oz. castor oil; Ae. aegypti (PP 2 h, R% 76), An. stephensi (PP 8 h, R% 52), Cx. quinquefasciatus (PP 8 h, R% 100)*; C. winterianus + vanillin repels Ae. aegypti, An. dirus, Cx. quinquefasciatus 8 h; + eucalyptus oil in floor cleaner, 6 h repellency; comparable to DEET & N,N-diethyl mandelic acid amide vs. Aedes spp.; 5 % hairy basil + 5 % vetiver + 10 % citronella EOs in nanoemulsion with 4.7 h protection against Ae. aegypti; C. winterianus LC50 & LC95 = 0.5 & 0.9 % for Cx. quinquefasciatus,1.0 & 2.0 % for Ae. aegypti	[37], [128], [129],  [137], [138], [139], [140], [141]
Clove	Syzygium aromaticum (L.) Merr. & L. M. Perry	Cx. pipiens pallens: isoeugenol & eugenol (from clove oil) > repellency than citronella. Clove bud oil + vanillin (long-term protection) > DEET; repels Ae. aegypti, An. dims, Cx. quinquefasciatus for 2–4 h; knockdown LC50 & LC95=0.5 & 0.9 % for Cx. quinquefasciatus, 1.0 & 2.0 % for Ae. aegypti	[46], [61], [141]
Copaiba	Copaifera L. spp.	1 : 1 : 1 andiroba, copaiba & baby oils repels mosquitoes in Amazon field test	[125]
Dill	Anethum graveolens L.	Ae. aegypti (PP 1.5 h, R% 78), An. stephensi (PP 3.5 h, R% 71), Cx. quinquefasciatus (PP 3 h, R% 57)*	[128]
Eucalyptus	Eucalyptus globulus Labill.	Ae. aegypti (PP 1 h, R% 57), An. stephensi (PP 5.5 h, R% 29), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
	E. camaldulensis Dehnh.	EO repels Cx. pipiens adult females	[8]
	Eucalyptus L'Hér. spp.	+ citronella EO in floor cleaning product, 6 h protection; 15 % oil composition ≥ 3 h protection &  composition + vanillin ≥ 5 h protection to humans; EO contg 30 % p-menthanediol repels An. darlingi (97 %)	[126], [140], [142], [143]
	Eucalyptus, broad-leaved	E. dives Schauer	Ae. aegypti (PP 3.5 h, R% 19), An. stephensi (PP 8 h, R% 38), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
	Eucalyptus, narrow-leaved	E. radiata Sieber ex DC.	Ae. aegypti (PP 2.5 h, R% 65), An. stephensi (PP 8 h, R% 43), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
Fennel	Foeniculum vulgare Mill.	5 % in aerosol or 8 % in cream repels Ae. aegypti comparably to citronella & geranium EOs; cream & EO repel Culex, Anopheles & Aedes spp. comparably to geranium & citronella EOs in field; hexane fraction (0.1 mg · cm−2 repels Ae. aegypti 99 %) of fruit methanol extract contains repellent (+)-fenchone &  E-9-octadecenoic acid	[144], [145]
Galbanum	Ferula galbaniflua Boiss. & Buhse	Ae. aegypti (PP 2.5 h, R% 70), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
Garlic	Allium sativum L.	EO and chemical components have strong repellent properties	[146]
Geranium	Pelargonium graveolens L'Hér.	Ae. aegypti (PP 2.5 h, R% 78), An. stephensi (PP 8 h, R% 62), Cx. quinquefasciatus (PP 8 h, R% 100)*; geranium + lemongrass EOs repel Aedes spp.; geranium + sandalwood EOs + soybean oil in burned stick repellents; geranium (25 % geraniol) EO + lemongrass extract product protect against bites of Ae. atlanticus and Ae. mitchellae for 4 h; geranium + sandalwood EOs + soybean oil product repel Culex sp. & other mosquitoes for 3 h in the field; geranium + citronella EOs (1 : 1) in cream product repels Ae. aegypti and Culex, Anopheles & Aedes spp. in the field; in cold creams with repellency to Cx. fatigans in lab &  on humans; various forms of EO definite promise as repellents	[46], [128], [137], [144], [147], [148]
Ginko	Ginkgo biloba L.	edulan (isolated from leaf extract) repels Ae. albopictus	[149]
Hibawood	Thujopsis dolabrata (Thunb. ex L. f.) Siebold & Zucc.	hinokitiol isolated from EO, highly repellent hinokitiol-containing non-woven polyester fabric & coated foot band	[150]
Jasmine	Jasminum grandiflorum L.	Ae. aegypti (PP 4.5 h, R% 14), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
Juniper	Juniperus communis L.	Ae. aegypti (PP 3.5 h, R% 43), An. stephensi (PP 8 h, R% 76), Cx. quinquefasciatus (PP 8 h, R% 100)*; good repellency at 5 µg · cm−2 against female Cx. pipiens pallens	[128], [151]
Lavender	Lavandula angustifolia Mill.	Ae. aegypti (PP 3 h, R% 24), An. stephensi (PP 8 h, R% 81), Cx. quinquefasciatus (PP 8 h, R% 86)*; mosquito repellent: citronella 1: lavender 1 & castor oil 2 oz.	[127], [128]
Lemon	Citrus × limon (L.) Osbeck	Ae. aegypti (PP 1.5 h, R% 68), An. stephensi (PP 7 h, R% 10), Cx. quinquefasciatus (PP 8 h, R% 100)*; repellency to An. stephensi ∼ DEET in animal & human tests	[128], [152]
Lemon eucalyptus	Eucalyptus citriodora Hook.	Ae. aegypti (PP 2.5 h, R% 59), An. stephensi (PP 8 h, R% 52), Cx. quinquefasciatus (PP 8 h, R% 100)*; use (p-menthane-3,8-diol, PMD, as active ingredient) recommended by CDC (Center for Disease Control and Prevention, USA) against West Nile vector; adulticide activity against Cx. quinquefasciatus & Ae. aegypti	[8], [46], [128]
Lemongrass	Cymbopogon citratus (DC.) Stapf	Ae. aegypti (PP 3 h, R% 70), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100)*; field test of 25 % geraniol oil & lemongrass ext. with 4 h repellency against Ae. atlanticus and Ae. mitchellae; mixture containing lemongrass EO + p-menthanediol (PMD) repels An. darlingi & other spp. 95–98 % for 5–6 h in field (repellency > 15–20 % DEET)	[128], [147], [153]
Lemon tea tree	Leptospermum petersonii F. M. Bailey	limited repellency (< DEET) of Ae. aegypti, Cx. quinquefasciatus, Cx. annulirostris	[154]
Leptospermum	L. liversidgei R. T. Baker & H. G. Sm.	repellent: blocks ability of mosquitoes to perceive CO2 emitted by humans	[155]
Lime	Citrus × aurantifolia L.	+ mustard oil carrier as effective mosquito repellent	[68]
Marjoram	Origanum majorana	5 µg · cm−2 good repellent against female Cx. pipiens pallens	[151]
May chang/Litsea	Litsea cubeba (Lour.) Pers.	Ae. aegypti (PP 8 h, R% 73), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100)*; high contact and noncontact repellency to female Ae. aegypti in vitro in cages; formulation to fix on skin with high repellency in human volunteers for up to 8 h & 100 % repellency to An. stephensi, Cx. quinquefasciatus, Ae. aegypti; greater repellence of night-biting mosquitoes An. dirus, Cx. quinquefasciatus and Ae. albopictus than Ae. aegypti	[73], [128], [132], [156]
Melaleuca	Melaleuca alternifolia Cheel	limited repellence of Ae. aegypti, Cx. quinquefasciatus, Cx. annulirostris in lab	[75]
Mint, mentha	Mentha haplocalyx Briq.	d-8-acetoxycarvotanacetone isolated mosquito repellent	[157]
Myrtle	Myrtus communis L.	Ae. aegypti (PP 2.5 h, R% 57), An. stephensi (PP 6.5 h, R% 43), Cx. quinquefasciatus (PP 8 h, R% 86)*	[128]
Palmarosa	Cymbopogon martini (Roxb.) Will.Watson	high geraniol content mosquito repellent	[158]
Patchouli	Pogostemon cablin (Blanco) Benth.	partially repels Ae. aegypti, Cx. quinquefasciatus, An. dims	[159]
Pepper, black	Piper nigrum L.	Ae. aegypti (PP 1.5 h, R% 65), An. stephensi (PP 3 h, R% 62), Cx. quinquefasciatus (PP 8 h, R% 100)*; repels An. dirus, Cx. quinquefasciatus, Ae. albopictus (≥ 4.5 h)	[128], [156]
Peppermint	Mentha × piperita L.	Ae. aegypti (PP 2 h, R% 59), An. stephensi (PP 6.5 h, R% 57), Cx. quinquefasciatus (PP 8 h, R% 100)*; on human skin strongly repels An. annularis (100 %), An. culicifacies (92 %), Cx. quinquefasciatus (85 %); comparable to mylol oil (di-butyl & di-methyl phthalates); repels adult female Cx. pipiens	[126], [128], [160]
Pine	Pinus sylvestris L.	good mosquito repellency	[85]
Rose	Rosa L. sp.	moderate mosquito repellency	[127]
Rosemary	Rosmarinus officinalis L.	Ae. aegypti (PP 5.5 h, R% 43), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100)*; repels An. stephensi, Ae. aegypti & Cx. quinquefasciatus; low repellency against Ae. aegypti in lab, acceptable to humans as final fragrance in repellent formulation; repels Ae. aegypti (avg 90 min)	[88], [128], [130], [136]
Sage	Salvia sclarea L.	Ae. aegypti (PP 2 h, R% 46), An. stephensi (PP 5 h, R% 19), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
Sandalwood	Santalum album L.	Ae. aegypti (PP 2.5 h, R% 59), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
	Santalum L. spp.	repels Cx. pipiens pallens; EO containing cold creams repel Cx. fatigans in the lab & on human skin; area repellency of Culex sp. by commercial sticks (contg 0.5 % EO) (3 h protection); botanical repellent (w/soybean and geranium oil) 100 % effective in field (3-h test period, comparable to DEET)	[93], [148], [151]
	Sour (bitter) orange	Citrus × aurantium L.	limited protection (repellency) against mosquitoes	[137]
Spearmint	Mentha spicata L.	repellent; piperitone oxide from M. spicata (var. viridis) EO repels An. stephensi	[161]
Tagetes	Tagetes minuta L.	Ae. aegypti (PP 1 h, R% 84), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100)*; repels Ae. aegypti (avg 90 min)	[88], [128]
Thyme	Thymus serpyllum L.	Ae. aegypti (PP 2.5 h, R% 57), An. stephensi (PP 7.5 h, R% 33), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
	Thymus vulgaris L.	repels lab-reared adult female Ae. albopictus for 2 h; 0.01 % totally repels Cx. quinquefasciatus	[134], [162]
	Turmeric	Curcuma longa L.	+ 5 % vanillin repels Ae. aegypti, An. dirus and Cx. quinquefasciatus in cage & large room for 8 h; turmeric, citronella & hairy basil EOs + vanillin provide substitute for DEET; repels DEET & IR3535-resistant Ae. aegypti for 4.5 h	[138], [156]
Verbena	Lippia triphylla (L'Hér.) Kuntze	Ae. aegypti (PP 2.5 h, R% 70), An. stephensi (PP 5.5 h, R% 38), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
Vetiver	Vetiveria zizanioides (L.) Nash	stable nanoemulsion of 5 % hairy basil, 5 % vetiver & 10 % citronella EO: repels Ae. aegypti for 4.7 h	[41]
Violet	Viola odorata L.	Ae. aegypti (PP 6 h, R% 68), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 86)*	[128]
Wild verbena (Lippie)	Lippia sidoides, L. javanica, Lippia spp.	sources of perillaldehyde (repels An. gambiae) & perillic acid (repels An. arabiensis & Ae. aegypti)	[163], [164]
Wintergreen	Gaultheria procumbens L.	strongly repels Culex and Aedes spp. in different tests	[131]
Wormwood	Artemisia annua L.	mosquito repellent properties	[165]
Zanthoxylum	Z. piperitum DC.,	repel mosquitoes	[159]
	Z. armatum DC.,
	Z. bungei Planch. & Linden ex Hance
Glyceridic oils
Castor	Ricinus communis L.	best carrier for pyrethrum extracts (long-lasting)	[137]
Mustard	Brassica L. spp.	longer protection (up to 5 h with Zanthoxylum limonella or lime oils) than coconut (Cocus nocifera) oil against Ae. albopictus	[166]
Neem/Margosa	Azadirachta indica A. Juss.	knockdown repellency against Ae. aegypti, Ae. albopictus, An. quadrimaculatus Say; repels female An. stephensi (ED50 0.191–0.156 mg · cm−2) in lab; low repellency against Ae. albopictus & Cx. nigripalpus; 2 % in coconut (Cocus nocifera) oil on exposed body parts of human volunteers provided complete protection for 12 h from all Anopheles spp.; protection from Anopheles spp. (96–100 %), Aedes (85 %), Culex sp. (61–94 %); significant protection by neem cream against adult Ae. aegypti; 1 % in kerosene lamps in preclinical & clinical safety evaluation is safe to humans; 1 % in kerosene burned in lamps effective in 2 field tests. Repellence: Anopheles > Culex; 1–4 % in coconut oil on exposed body parts of humans: 81–91 % protection for 12 h	[167], [168], [169], [170], [171], [172], [173], [174], [175]
Olive	Olea europaea L.	Ae. aegypti (PP 3.5 h, R% 68), An. stephensi (PP 8 h, R% 71), Cx. quinquefasciatus (PP 8 h, R% 71)*; 1 : 1 w/pyrethrum repels mosquitoes for 4 h	[128], [137]
Soybean	Glycine max (L.) Merr.	Ae. aegypti (PP 3 h, R% 54), An. stephensi (PP 8 h, R% 76), Cx. quinquefasciatus (PP 8 h, R% 100)*; oil-based product provided 1.5 h (low) repellency, 24 % DEET provided 5 h protection; good repellency in product w/sandalwood + geranium in burned sticks vs. DEET; commercial product containing 2 % oil repels Ae. albopictus, Cx. nigripalpus, Ae. triseriatus for 5, 8.5 & ≥ 7.3 h, respectively	[128], [169], [176], [177]
Soybean, wild (carotin)	Glycine soja Siebold & Zucc.	Ae. aegypti (PP 3 h, R% 16), An. stephensi (PP 8 h, R% 10), Cx. quinquefasciatus (PP 8 h, R% 100)*	[128]
* PP = protection period (h), R% = percent repellency; 0.1 mL of a 20 % EO solution (in 20 % Genapol, 10 % PEG, 20 % ethanol, 50 % water) applied to 30 cm2 of human forearm; for 20 % DEET solution (in same as above) these values were: Ae. aegypti (PP 6 h, R% 46), An. stephensi (PP 8 h, R% 100), Cx. quinquefasciatus (PP 8 h, R% 100) [24]

A broad-scale screening of plant oils against Ae. aegypti, An. stephensi and Cx. quinquefasciatus evaluated protection periods and percent of repellence on human skin as compared to 20 % DEET [128]. In general, Ae. aegypti was the most difficult species to repel, followed by An. stephensi and finally Cx. quinquefasciatus for both oils and controls. The control (DEET) exhibited a protection period (PP) of 6 h and percent repellency (R%) of 46 % against Ae. aegypti, whereas against An. stephensi and Cx. quinquefasciatus protection was for 8 h at 100 % repellency [128] ([Table 3]). While the most active EOs against all three mosquito species were cited above, the following oils were active against An. stephensi and Cx. quinquefasciatus, but not significantly active against Ae. aegypti: chamomile (Chamaemelum nobile), cinnamon (Cinnamomum verum), galbanum (Ferula galbaniflua), jasmine (Jasminum grandiflorum), lavender (Lavandula angustifolia), pepper (Piper nigrum), rosemary (Rosmarinus officinalis), sandalwood (Santalum album) and soybean (Glycine max) [128]. Another group of oils in this study actively repelled only Cx. quinquefasciatus: cedar (Cedrus, Cupressus and Juniperus spp.), citronella (Cymbopogon nardus, C. winterianus), eucalyptus (Eucalyptus globulus), broad and narrow-leaved eucalyptus (E. dives and E. radiata, respectively), geranium (Pelargonium graveolens), juniper (Juniperus communis), lemon (Citrus × limon), lemon eucalyptus (Eucalyptus citriodora), myrtle (Myrtus communis), peppermint (Mentha × piperita), sage (Salvia sclarea), thyme (Thymus serpyllum) verbena (Lippia triphylla) and wild soybean (Glycine soja) [128]. Differences in the species specificity of the repellence profiles of these and other EOs may explain their use in repellent mosquito products. This may have to do with differences in the local and regional profiles of mosquito species populations and explain the use of mixtures of these EOs to generate broad spectrum formulations for simultaneous repellency of multiple mosquito species.

While Ae. aegypti and other adults may in general be difficult to repel using plant-based products or synthetic repellents, a number of plant EOs have been identified which are effective against this species ([Table 3]). The following repellency effects of EOs against adult Ae. aegypti have been observed: hairy basil (Ocimum basilicum) in stable nanoemulsions with vetiver (Vetiveria zizanioides) and citronella (Cymbopogon nardus and C.winterianus) EOs are a good repellent [129], bay laurel (Laurus nobilis) EO is an acceptable smelling, good spatial repellent [130], camphor (Cinnamomum camphora) EO and cassia (C. cassia) bark extract on human skin are repellents comparable to DEET [137], catmint (Nepeta cataria) EO exhibited 8 h of protection on human skin, cinnamon (Cinnamomum zeylanicum, C. verum) EOs exhibit moderate to good repellency [128], citronella (C. winterianus) EO exhibits knockdown repellency at 1–2 % [141] and C. winterianus EO + vanillin exhibits 8 h of repellency [138], fennel (Foeniculum vulgare) fruit extract/fractions offer complete repellency [145] and EO as an aerosol or cream has comparable repellency to other EO repellents [144], geranium (Pelargonium graveolens) + citronella EO in a cream product exhibited good repellency in the field [46], lemon eucalyptus (E. citriodora) has knockdown repellency/adulticide activity [46], may chang/litsea (Litsea cubeba) EO exhibits contact and noncontact repellency and is a good/excellent repellent in formulations on the human forearm providing protection over 8 h [73], [128], turmeric (Curcuma longa) EO + 5 % vanillin exhibits 8 h of repellency and in formulations with other EOs + 5 % vanillin offers protection from DEET and IR3535-resistant strains of Ae. aegypti over 4.5 h [138], [156] and violet (Viola odorata) EO on human skin exhibited 8 h protection at a good level of repellency [128] ([Table 3]).

Several EOs from Zanthoxylum L. spp. (Z. piperitum DC., Z. armatum DC., Z. bungei Planch. & Linden ex Hance) were cited in patented mosquito repellent inventions. According to recent literature, Zanthoxylum L. spp. EOs have mosquito repellent activity [159], [180]. Also, besides A. argyi H. Lév. & Vaniot (artemisia) EO, which is known to repel mosquitoes [127], the EOs of several other Artemisia L. spp. were used in patented formulations such as A. annua L. (wormwood), A. vulgaris L. and A. apiacea Hance. This is interesting given that A. annua EO has proven insect repellent properties [165].

The Amazon region is a source of plant-derived mosquito repellent oils. For example, Carapa guianensis Aubl. (andiroba) pressed fruit oils or extracts are formulated preferentially into candles during manufacturing as fumigant mosquito repellents [124], burned in kerosene lamps or used in topical repellent formulations which are commercially available in Brazil. Copaifera L. spp. (copaiba) balsam oils or extracts are used in mosquito repellency in Brazil especially in the Amazon region ([Table 3]). Furthermore, a formulation of andiroba, copaiba and baby oils exhibited repellency to mosquitoes in an Amazon field study [125].

Scientific Basis for Mosquito Repellency of Glyceridic Oils

Neem or margarosa oil is obtained by pressing the fruit of the neem tree (Azadirachta indica). Neem oil is burned in 1 % compositions in kerosene lamps as indoor mosquito emitters of chemical repellent-fumigant deterrents which have been evaluated and are considered to be safe [173], [174]. Also, neem oil in mixtures with coconut (Cocos nucifera L.) oil provides good protection for very long periods against Anopheles and Aedes spp. [170], [171], [175]. Castor (Ricinus communis L.), mustard (Brassica spp.), olive (Olea europaea) and other glyceridic oils have important roles in several patented repellent compositions containing pyrethrum extracts and EOs where they act as carriers and can extend the duration of repellent effects for several hours perhaps by slowing the release or evaporation of EOs from surfaces.

There are mixed scientific reports on the effective mosquito repellency of several glyceridic oils. Thus, for soybean (Glycine max) oil low repellency was observed as compared to 24 % DEET formulations [176] and good repellency was observed for the smoke generated from burned sticks which contained soybean oil. Especially interesting is a report from a United States Department of Agriculture laboratory where 4 well-known synthetic mosquito repellents based on 10 % KBR3023 [1-piperidinecarboxylic acid 2-(2-hydroxyethyl)-1-methylpropyl ester], 7.5 % IR3535 [3-(N-butyl-N-acetyl)-aminopropionic acid ethyl ester], 15 % and 7 % DEET and 8 natural product-based repellents based on 2 % soybean oil, 10 % citronella (Cymbopogon spp.) EO, neem oil (Azadirachta indica) and others were tested in the lab against Ae. albopictus Skuse, Cx. nigripalpus Theobald and Ae. triseriatus Say [169]. The 2 % soybean oil formulation exhibited mosquito repellency comparable to both 10 % KBR3023 and 15 % DEET based products each of which provided estimated mean protection time (eMPT) responses averaged over all three mosquito species of ≥ 7.2 h [169]. This study is evidence for the potential of soybean oil as a stand-alone repellent and as a component oil of repellent formulations.

Mosquito Repellent Chemical Components of EOs and Added Isolated Compounds

Approximately 20 % of all EO-containing patents mentioned a non-EO plant derivative and 40 % mentioned a specific EO repellent component or added isolated natural chemical ingredient. Chrysanthamic acid and its derivatives which are components of pyrethrum (Chrysanthemum L. spp. flower extracts) were cited in 12 % of all patented formulations involving EOs. Also, patents cited many “natural” chemical constituents which were either added to formulations or were important chemical components of EOs comprising the formulations. The most widely cited chemicals of plant origin were (% of patents): camphor (8.3), geraniol (7.6), linalool (7.6), menthol (7.6), geranial (7.0), citronellal (6.9), limonene (4.9), citronellol (4.9), borneol (4.2), 1,8-cineole (4.2), p-menthane-3,8-diol (4.2), camphene (3.5) and nepetalactone and derivatives (3.5). Many of these and other chemical components of EOs are potent mosquito repellents ([Table 4]) [131], [137], [143], [145], [151], [163], [169], [181], [182], [183], [184], [185], [186], [187], [188], [189], [190], [191], [192], [193] and are discussed below.

Table 4 Mosquito repellency or deterrency effects of individual major chemical components of EOs used in patented inventions.

EO component	No. EOs†	Repellency	Mosquito	Source
α-Bisabolol	2	d84 % spatial repellency for 180 min, avoidance freq. (contact repellency) = DEET	female Ae. aegypti	[181]
Borneol	3	b1.4 mg (−)-borneol · cm−2 = 70 % repellency	female Ae. aegypti	[182]
		b,cRD50 = 1.7 × 10−3 mg · cm−2	female An. gambiae	[163]
Bornyl acetate	2	b1.4 mg · cm−2 = 15 % repellency	female Ae. aegypti	[182]
Camphene	2	b,cRD50 = 2.2 × 10−3 mg · cm−2	female An. gambiae	[163]
Camphor	6	b,cRD50 = 1.4 × 10−3 mg · cm−2	female An. gambiae	[163]
		b1.4 mg · cm−2 (±)-camphor, (+)-camphor, (−)-camphor = 36, 35 & 47 % repellencies, resp.	female Ae. aegypti	[182]
3-Carene	4	b,cRC50 = 8.6 × 10−4 mg · cm−2	female An. gambiae	[183]
		1 h, 1.92 µg · cm−2 = 68†, 65‡ % repellency	Ae. aegypti, Ae. albopictus	[163]
Carvacrol	4	b,cRC50 = 2.4 × 10−4 mg · cm−2	female An. gambiae	[183]
Carvone	2	b,cRD50 = 1.3 × 10−3 mg · cm−2	female An. gambiae	[163]
		d-carvone & l-carvone: feeding deterrents/repellents, good spatial repellents/inhibitors, contact repellents/deterrents	–	[184]
Cinnamaldehyde	2	definite mosquito repellent potential	–	[137]
		E-cinnamaldehyde (0.051 mg · cm−2) & DEET (0.025 mg · cm−2) provided 87 & 95 % protection, resp., after 30 min	female Aedes aegypti	[184]
1,8-Cineole	15	bcineole 1.4 mg · cm−2 = 21 % repellency	female Ae. aegypti	[182]
		b3 % in vaseline or olive oil = 73 or 120 min protection	1 : 1 male : female Cx. pipiens molestus	[185]
		b,cRD50 = 1.2 × 10−3 mg · cm−2	female An. gambiae	[163]
Citronellal	4	b,cRD50 = 2.2 × 10−4 mg · cm−2	female An. gambiae	[163]
		feeding deterrent/repellent, spatial repellent/inhibitor, contact repellent/deterrent	–	[186]
Citronellol	6	repellent component	–	[145]
		feeding deterrent/repellent, spatial repellent/inhibitor	–	[186]
p-Cymene	3	b,cRD50 = 1.0 × 10−5 mg · cm−2	female An. gambiae	[163]
Eugenol	5	high repellency	female Cx. pipiens pallens	[151]
		b,cRD50 = 1.3 × 10−3 mg · cm−2	female An. gambiae	[163]
Fenchone	2	b,cRD50 = 1.9 × 10−3 mg · cm−2	female An. gambiae	[163]
		0.4 mg (+)-fenchone · cm−2 = 51 % repellency after 1 hb,f	female Ae. aegypti	[145]
Geranial	7	at 0.2 mg · cm−2, blood-sucking by mosquitoes reduced by 10, 15, and 18 % after 1, 2 and 3 h, respectively	Cx. pipiens pallens, Cx. pipiens quinquefasciatus	[187]
Geraniol	11	b,cRD50 = 1.1 × 10−4 mg · cm−2	female An. gambiae	[163]
		spatial repellent: 0.25 µg · cm−2 ca. 100 % (reversible) inhibition of host-seeking after 48 h exposure	female Ae. albopictus	[188]
Geranyl acetate	6	effective feeding deterrent/repellent, effective spatial repellent/inhibitor, contact repellent/deterrent	Mosquitoes	[186]
Limonene	19	b3 % in vaseline/olive oil = 61/78 min protection	1 : 1 male : female Cx. pipiens molestus	[185]
		1 h, 1.92 µg · cm−2 = 70†, 70‡ % repellency	Ae. aegypti, Ae. albopictus	[189]
		b,cRD50 = 1.8 × 10−3 mg · cm−2	female An. gambiae	[163]
Linalool	14*	b1.4 mg (±)-linalool · cm−2 = 67 % repellency	female Ae. aegypti	[182]
		b3 % in vaseline/olive oil = 52/65 min protection	1 : 1 male : female Cx. pipiens molestus	[185]
		b,cRD50 = 1.5 × 10−3 mg · cm−2	female An. gambiae	[163]
p-Menthane-3,8-diol	1	26 % formula more repellent than 4 synthetic & 7 natural product repellents	Ae. albopictus, Cx. nigripalpus, Ochlerotatus triseriatus	[169]
		15 % formula with lemongrass EO (Cymbopogon citratus) = 99 % protection for 5 h in the field; h16 % formula with lemongrass EO = 95 % protection for 6 h in the field	An. darling, Ae. ochlerotatus taeniorhynchus	[143]
Menthone	2	b3 % in olive oil = 35 min protection	1 : 1 male : female Cx. pipiens molestus	[185]
Methyl salicylate	2	most repellent (screening)	–	[131]
Myrcene	6	b,cRC50 = 8.4 × 10−4 mg · cm−2	female An. gambiae	[183]
		b1.4 mg · cm−2 = 20 % repellency	Ae. aegypti	[182]
Nepetalactone	1	mosquito repellent ingredient in formulations	–	[190]
E-Nerolidol	1	d67 % spatial repellency for 180 min, avoidance freq. (contact repellency) < DEET	female Ae. aegypti	[181]
Oleic acid	8**	0.4 mg · cm−2 = 52 % repellency after 1 hb,f	female Ae. aegypti	[145]
Perillaldehyde	1	b,cRD50 = 3.2 × 10−4 mg · cm−2	female An. gambiae	[163]
α-Pinene	15	d-α-pinene, 2 h protection	Mosquito	[191], [192]
		b3 % in vaseline/olive oil = 27/56 min protection	1 : 1 male : female Cx. pipiens molestus	[185]
β-Pinene	8	b3 % in vaseline/olive oil = 22/39 min protection	1 : 1 male : female Cx. pipiens molestus	[185]
		b,cRD50 = 1.6 × 10−3 mg · cm−2	female An. gambiae	[163]
α-Santalol	1	d87 % spatial repellency for 180 min, avoidance freq. (contact repellency) = DEET	female Ae. aegypti	[181]
Spathulenol	2	eproportion not biting = 0.73, 0.75, respectively	Ae. aegypti, An. stephensi	[193]
α-Terpinene	1	1 h, 1.92 µg cm2 = 68†, 55‡ % repellency	Ae. aegypti, Ae. albopictus	[189]
		b1.4 mg · cm−2 = 15 % repellency	female Ae. aegypti	[182]
		b,cRD50 = 2.4 × 10−3 mg · cm−2	female An. gambiae	[163]
γ-Terpinene	6	1 h, 1.92 µg · cm−2 = 60†, 72‡ % repellency	Ae. aegypti, Ae. albopictus	[189]
		b3 % in vaseline/olive oil = 35/48 min protection	1 : 1 male : female Cx. pipiens molestus	[185]
		b,cRD50 = 2.7 × 10−3 mg · cm−2	female An. gambiae	[163]
Terpinen-4-ol	5	1 h, 1.92 µg · cm−2 = 92†, 85‡ % repellency	Ae. aegypti, Ae. albopictus	[189]
		b1.4 mg · cm−2 = 83 % repellency	Ae. aegypti	[182]
		b,cRD50 = 1.5 × 10−3 mg · cm−2	female An. gambiae	[163]
		spatial repellent/inhibitor	–	[186]
α-Terpineol	7	b3 % in vaseline/olive oil = 78/99 min protection	1 : 1 male : female Cx. pipiens molestus	[185]
		feeding deterrent/repellent, spatial repellents/inhibitor, contact repellent/deterrent		[186]
		b,cRD50 = 1.3 × 10−3 mg · cm−2	female An. gambiae	[163]
Terpinolene	2	1 h, 1.92 µg · cm−2 = 55†, 70‡ % repellency	Ae. aegypti, Ae. albopictus	[189]
		b,cRD50 = 2.6 × 10−3 mg · cm−2	female An. gambiae	[163]
(α + β)-Thujone	2	b1.4 mg · cm−2 = 20 % repellency	Ae. aegypti	[182]
		b,cRD50 = 1.5 × 10−3 mg · cm−2	female An. gambiae	[163]
Thymol	1	b,cRC50 = 1.9 × 10−3 mg · cm−2	female An. gambiae	[183]
		b3 % in vaseline/olive oil = 52/65 min protection	1 : 1 male : female Cx. pipiens molestus	[186]
cis-Verbenol	1	b,cRD50 = 7.5 × 10−5 mg · cm−2	female An. gambiae	[163]
Verbenone	1	b,cRD50 = 1.6 × 10−3 mg · cm−2	female An. gambiae	[163]
– = Information not found. † Number of oils in [Table 1] which contain this as their major component. *In evening primrose (Oenothera biennia) glyceridic oil. ** Mosquito repellent component of glyceridic oils. † DEET = 50 % repellency. ‡ DEET = 60 % repellency. a Used as additive in one patent. b Human-bait assay. c Positive control DEET exhibited RC50 = 3.3 ×  10−4 mg · cm− 2. d Applied concentration 78.6 µg · cm−2. Repellency is statistically different from negative control. DEET and turmerone had 83 and 89 % repellencies, respectively. Contact repellency is defined as 100 % of individuals off treated surface. Avoidance frequency of DEET and turmerone = 0.8. e Substance was applied at 25 nmol · cm−2 cloth. Positive control was (1S,2′S)-2-methylpiperidinyl-3-cyclohexen-1-carboxamide (SS-220) at 25 nmol · cm−2 cloth (provided 0.80 and 0.78 non-biting mosquitoes, respectively, for Ae. aegypti and An. stephensi). f 0.2 mg · cm−2 DEET (positive control) provided 97 % repellency after 1 h. g Positive control 15 % DEET (in EtOH) = 92 % protection (average over trial period). h Positive control 20 % DEET (in EtOH) = 64 % protection (average over trial period)

Vanillin in EO containing mosquito repellent inventions

Vanillin was cited as an additive in 4 % of EO-containing patents. According to several scientific publications, it increases the duration and magnitude of the repellent effect of synthetic repellents such as DEET as well as plant EOs. For example, 5 % vanillin in formulations of EOs of turmeric (Curcuma longa L.), citronella (C. winterianus) or hairy basil (Ocimum americanum L.) provided up to 8 h of protection against Ae. aegypti, An. dirus and Cx. quinquefasciatus under cage conditions. Interestingly, 5 % vanillin in formulations with DEET increased the protection time as compared to DEET alone against these three mosquito species (≥ 8 h protection). In another example, 15 % eucalyptus (Eucalyptus spp.) EO and 5 % vanillin provided 5 h of repellency against Ae. albopictus [18] and in other work involving human volunteers, clove (Syzygium aromaticum) bud EO and vanillin mixtures provided long acting repellency against adult female Cx. pipiens pallens [151]. Thus, combinations of vanillin with plant EOs can lead to formulations which are substitutes for DEET [138], [142], [151].

In patents, vanillin is cited as an ingredient in mosquito repellent formulations with: Zanthoxylum armatum DC. and/or Z. piperitum DC. EOs [194], clove bud and leaf (Syzygium aromaticum), juniper berry (Juniperus communis) and/or marjoram (Origanum majorana) EOs [195], lemongrass (Cymbopogon citratus) EO/p-menthane-3,8-diol formulations [196], combinations of citronella, clove, geranium (Pelargonium graveolens), lavender (Lavandula angustifolia), patchouli [Pogostemon cablin (Blanco) Benth.] and peppermint (Mentha × piperita) EOs [197] among others.

Synthetic additives in EO-containing inventions

About 25 % of all EO-containing patented repellents had at least one synthetic repellent component. Thus, DEET (N,N-diethyl m-toluamide), the gold-standard synthetic mosquito repellent [11], was used in 10 % and dimethyl (dialkyl) phthalates were cited in 5 % of EO-containing patented inventions. Also, one or more synthetic pyrethroids (allethrin, cyhalothrin, deltamethrin, dimefluthrin, esbiothrin, metofluthrin, permethrin, tetramethrin and vaporthrin) were used in 10 % of EO-containing mosquito repellents. It must be stressed that pyrethroids can be strong mosquitocides (toxicity), spatial mosquito repellents and mosquito irritants as was shown, for example, for permethrin in field studies [6] and α-cypermethrin, deltamethrin, permethrin in a very recent publication [198]. Only about 3 % of patents made use of the synthetic synergist piperonyl butoxide.

Chemical Composition of EOs Used in Patented Inventions

Information on the major chemical components of plant EOs used in mosquito repellent inventions is presented in [Table 1]. Among these are EOs which are concentrated sources of proven mosquito repellent monoterpenes and phenylpropanoids such as the EOs of angelica (Angelica archangelica), artemisia (Artemisia argyi), basil (Ocimum basilicum), bergamot (Citrus × bergamia), camphor (Cinnamomum camphora), cassia (Cinnamomum cassia), catnip (Nepeta cataria), chrysanthemum (Chrysanthemum indicum), cinnamon (Cinnamomum zeylanicum), citronella (Cymbopogon nardus, C. winterianus), coriander (Coriandrum sativum), cypress (Cupressus sempervirens), dill (Anethum graveolens), eucalyptus (Eucalyptus spp.), geranium (Pelargonium graveolens), grapefruit (Citrus reticulata), ho leaf (Cinnamomum camphora), hyssop (Hyssopus officinalis), juniper (Juniperus communis), lavender (Lavandula angustifolia), lemon (Citrus × limon), lemon eucalyptus (Eucalyptus citriodora), lemongrass (Cymbopogon citratus), lemon tea tree (Leptospermum petersonii), lime (Citrus × aurantifolia), marjoram (Origanum majorana), may chang/litsea (Litsea cubeba), melaleuca/tea tree (Melaleuca alternifolia), mint/mentha (Mentha), orange (Citrus × sinensis), palmarosa (Cymbopogon martini), curl leaf parsley (Petroselinum crispum), pepper (Piper nigrum), pine (Pinus sylvestris), rose (Rosa × damascena, R. × centifolia), rosemary (Rosmarinus officinalis), salvia/sage (Salvia spp.), sour (bitter) orange (Citrus × aurantium), spearmint (Mentha spicata), thyme (Thymus vulgaris), verbena (Lippia triphylla) and wintergreen (Gaultheria procumbens). In these EOs, proven mosquito repellent volatile components camphor, 1,8-cineole, citronellol, eugenol, geranial, geraniol, limonene, linalool, myrcene, α and β-pinenes, γ-terpinene, terpinen-4-ol and α-terpineol are well represented among the major components ([Table 4]).

Chemical composition of glyceridic oils used in patented EO-containing inventions

Glyceridic plant oils are important components of EO-containing mosquito repellent formulations not only for their carrier properties, but because they contain trace or larger amounts of free, long-chain mono- and polyunsaturated fatty acids and derivatives of these. It has been known for some time that insect antennae are highly sensitive to gas-phase long-chain fatty acids. For example, Hwang et al. [199] showed that gravid Cx. quinquefasciatus Say in an olfactometer in contact with room temperature vapors of C₁₄-C₂₄ mono-, di- and polyunsaturated fatty acids were significantly repelled. These authors also demonstrated that fatty acids having at least one Z-configuration olefin bond (analogous saturated and E-configuration fatty acids were less repellent or inactive) were especially effective repellents. One of the most repellent fatty acids was oleic acid (9Z-olefin) which is a common free fatty acid in vegetable oils whose 18 carbon chain was considered to be optimal within the group of fatty acids studied [199]. Independently, female Ae. aegypti were found to be moderately repelled by oleic acid as compared to DEET in a human-bait assay [145]. Thus, free, long-chain, unsaturated fatty acids which are present in glyceridic oils are sufficiently volatile to be mosquito repellent substances in formulations containing plant EOs. This is especially relevant given claims in several patents of the mosquito repellent effects of olive (Olea europaea L.), sesame (Sesamum indicum L.), mustard (Brassica L. spp.), soybean [Glycine max (L.) Merr.] and other oils (for examples, see [200] or [201]).

Mosquito repellent major chemical components of EOs

Among the more widely distributed mosquito repellent major chemical components of EOs used in patented inventions are the cyclic and noncyclic monoterpenes limonene, α-pinene, 1,8-cineole and linalool ([Table 4]). These are major components of, respectively, 28, 22 [including limonene-containing perilla (Perilla frutescens) oil], 22 and 21 % [including linalool-containing evening primrose (Oenothera biennis) oil] of the EOs most used in patented repellent inventions ([Table 1]).

EOs in patented mosquito repellent inventions are composed of a number of chemical components which individually exhibit important mosquito repellency, deterrency and inhibitory effects ([Table 1] and [Table 4]). For example, α-bisabolol and α-santalol provided 3 h of high spatial repellency and contact repellency equivalent to DEET against female Ae. aegypti while E-nerolidol exhibited good spatial repellency for 3 h and less contact repellency than DEET [181]. Also, the following substances singly exhibited good repellency against the mosquito species indicated: carvacrol (RC₅₀ = 2.4 × 10⁻⁴ mg · cm⁻², female An. gambiae) [183], cinnamaldehyde (0.051 mg · cm⁻² against female Aedes aegypti, repellency comparable to DEET) [184], citronellal (RD₅₀ = 2.2 × 10⁻⁴ mg · cm⁻²), p-cymene (RD₅₀ = 1.0 × 10⁻⁵ mg · cm⁻²) and cis-verbenol (RD₅₀ = 7.5 × 10⁻⁵ mg · cm⁻²) against female An. gambiae [163], geranial (0.2 mg · cm⁻², reduces blood-sucking by Cx. pipiens pallens and Cx. pipiens quinquefasciatus over 3 h). The following EO components provided reasonable repellency at ca. 2 µg · cm⁻² against adult Ae. aegypti and Ae. albopictus: 3-carene, limonene, α-terpinene, γ-terpinene, terpinen-4-ol and terpinolene [189].

Several interesting isolated repellent substances are described in patented EO-containing products ([Table 3]). For example, hinokitiol-containing polyester non-woven fabric and hinokitiol-coated polyester foot band are said to provide 100 % repellency against mosquitoes and hinokitiol from hibawood (Thujopsis dolabrata) EO (and other sources) is an important natural repellent [150]. Another example is the isolation of edulan from ginkgo (Ginkgo biloba) EO and the repellent activity of this compound and derivatives against Ae. albopictus are described in a patent in which it is also claimed that these compounds are safe to humans [149]. The mosquito repellent natural substance d-8-acetoxycarvotanacetone was isolated from Mentha haplocalyx Briq. [157]. Also, several Lippia spp. EOs were cited in patents [e.g., L. sidoides and L. javanica (wild verbena or lippie oil)] and are important sources of the mosquito repellent compounds perrilic acid and perillaldehyde [163], [164] ([Table 3]). The latter compound is a component of perilla (Perilla frutescens) oil and exhibited good repellency (RD₅₀ = 3.2 × 10⁻⁴ mg · cm⁻²) against female An. gambiae [163] ([Table 4]). In another patented invention, garlic (Allium sativum) EO and isolated component chemicals of garlic EO were tested for repellency against mosquitoes. More than a dozen allyl sulfide, allyl disulfide and allyl polysulfide components of garlic EO applied to human skin were found to deter Ae. aegypti, An. gambiae and Cx. quinquefasciatus landing and blood feeding [202].

Geraniol

Geraniol is a major component of 16 % of the more used EOs in patented inventions ([Table 1]) and its mosquito repellency has been studied. As an isolated chemical, it exhibited good repellency on human skin in the lab [163] and reversible spatial repellency/protection from bites [188]. A 25 % formulation of this substance was the basis for a commercial product which exhibited effective repellency in the lab [93], [167], [169] but was less effective as compared to other products at repelling mosquitoes according to an early field study [143]. In a recent field study, a 25 % geraniol and lemongrass (Cymbopogon citratus) composition offered superior protection against the bites of Ae. atlanticus and Ae. mitchellae (4 h) than repellent formulations based on 12 % EOs (1 h) and EOs, benzophenone-3, octinoxate and octisalate mixture (1.5 h) [147].

p-Menthane-3,8-diol

The mosquito repellency of the component of Eucalyptus spp. EOs, p-menthane-3,8-diol (PMD), is firmly established [8], [143], [178]. As a repellent additive in formulations, 15–16 % PMD/lemongrass formulations provided 5–6 h of excellent repellency (95–99 %) against Anopheles and Aedes spp. [153]. Indeed, in a U. S. Dept. of Agriculture study, 26 % PMD-containing product was more repellent in the lab to Ae. albopictus, Cx. nigripalpus and Ae.triseriatus than synthetic products based on 10 % KBR3023, 7.5 % IR3535, 15 % and 7 % DEET, and 7 natural product-based repellents based on 2 % soybean oil, 10 % citronella EO, neem oil, 25 % geraniol, etc. [169].

Mosquito repellent minor components of EOs

Some minor or lower concentration components of EOs are worth mentioning because they exhibit significant mosquito repellency. For example, citral (geranial + neral) and fenchyl alcohol are minor components of citronella (Cymbopogon nardus, C. winterianus) and rose (Rosa damascena) EOs, respectively, and exhibit moderate repellency to female An. gambiae (RD₅₀ = 1.3 × 10⁻³ and 1.4 × 10⁻³ mg · cm⁻², respectively) [163]. Also, linalool oxide, a minor component of lemon eucalyptus (E. citriodora) EO was found to be repellent (RC₅₀ = 6.5 × 10⁻⁴ mg · cm⁻²) to female An. gambiae [183] and Tripathi et al. [161] found that the peppermint (Mentha × piperita) EO component piperitone oxide was highly repellent to adult An. stephensi. In other works, farnesol [a component of chamomile, jasmine and rose (Rosa damascena) EOs], elemol (a component of catnip, citronella and hyssop EOs) and hedycaryol (a component of black pepper EO) exhibited 69, 89 and 95 % spatial repellency over 180 min to adult female Ae. aegypti and avoidance frequency (contact repellency) greater than or equal to that of DEET [181]. Cinnamyl alcohol, a minor constituent of cassia (Cinnamomum cassia) EO, exhibits less repellency to female Ae. aegypti than the major component, cinnamaldehyde, but still this repellency is significant (at 0.051 mg · cm⁻², 86 % protection after 30 min) and worth mentioning here [184]. Also, a minor component of EO of catmint (Nepeta L. spp.) EOs [28], dihydronepetalactone, exhibits important repellency against arthropods, including mosquitoes [203]. Citronellyl acetate is a minor component of eucalyptus (Eucalyptus spp.) [165], lemon eucalyptus (E. citriodora) [66], rose (Rosa damascena) [87] and tarragon (Artemisia dracunculus) [99] EOs and is a feeding deterrent/repellent and mosquito spatial repellent/inhibitor. Also, the minor component of salvia/sage EO [92], hydroxy-p-cymene, is both a feeding deterrent/repellent and mosquito contact repellent/deterrent [186]. Lastly, isoeugenol (a component of clove EO) was found to be highly repellent to female Cx. pipiens pallens [151].

Synthetic EOs: Additive Effects/Synergism versus Suppression/Dilution

In some cases, patented formulations in effect improve upon the natural repellency of an essential oil by mimicking or synthesizing an oil which ideally contains components which together contribute to the repellent effect and eliminates those which counteract (attract mosquitoes), make no contribution or suppress the repellency of other components. Work done by Odalo et al. [183] nicely illustrates this process. An initial observation was that major components of 6 EOs when tested singly were less repellent than the natural EOs in which these components are found. Synthetic EOs were prepared by mixing pure major components in the same proportion in which they occur in the natural EOs. Repellencies of synthetic EOs ranged from comparable to up to three times the activity of the corresponding natural EOs against An. gambiae in the human-bait test based on RC₇₅ values [183]. The activity of synthetic EOs substantiates the additive and/or synergistic nature of the interaction of blended EO components (and also suppressive or diluting/repellency diminishing effects of nonactive components). The same principles of addition/synergism and suppression/dilution are operational in the process of formulating EOs and isolated components into patented repellent inventions.

Enantiomeric Composition of EOs

Scientific (and patent) literature on the composition of EOs is often based on non-chiral column gas chromatography-mass spectrometry (GC‐MS) and compound retention time/index analysis. Thus, information is not systematically available on the enantiomeric purity of the EO components in [Table 1]. Enantiospecificity in mosquito repellency is an important issue. For example, Gu et al. [189] found that commercial R-(−)-terpinen-4-ol exhibited > 90 % repellency against Ae. aegypti and Ae. albopictus adults at a concentration of 1.92 µg · cm⁻² in a test lasting 1 h. S-(+)-terpinen-4-ol of high enantiomeric purity is found in lavender (Lavandula angustifolia) EOs [204]. No mosquito repellency data were found in the literature for S-(+)-terpinen-4-ol. In principle, enantiomers may not be equally repellent to mosquitoes and their organoleptic and other properties (toxic, allergic, etc.) are not equally desirable for use as repellents. More research is needed on the comparative mosquito repellency of enantiomerically pure EO components such as S-(+)-terpinen-4-ol and R-(−)-terpinen-4-ol against different mosquito species.

Allergenicity of EO Chemical Components

Limonene, benzyl alcohol, linalool, citronellol, geraniol, citral, anisyl alcohol, cinnamaldehyde, eugenol, isoeugenol, coumarin, farnesol, benzyl benzoate, benzyl salicylate among other volatile components are known allergens. As described above, these components are major or minor components of EOs used in repellent inventions ([Table 1] and references cited therein). These and other volatile allergens are easily detected by GC‐MS and are controlled in the European Union and elsewhere.

Conclusion

Patents on plant EO and volatile component-containing mosquito repellent inventions make use of scientifically substantiated claims to repellency to mosquitoes based on lab and field experiences. Our experience is that mosquito repellents and other insect control products, must be evaluated in the environments and settings in which they are to be used. Repellents must be developed based on plant oils and isolated chemical components by targeting Aedes, Anopheles, Culex and other mosquito species which are found locally and regionally. Repellency approaches should be multipronged and make use of sprays, fumigants, paints and varnishes, incense, candles, etc. in domestic settings, fumigation and spraying in outdoor settings and topical repellents, clothes made of repellent fabrics, repellent wristbands among other available products for individual protection.

Acknowledgements

The authors are grateful to Joanne Bero and Joëlle Quetin-Leclercq for the invitation for this review. Our thanks also go to the anonymous referee for many invaluable corrections and suggestions. The authors are thankful for grants and/or scholarships received from Brazilian agencies MCT/INPA, CNPq 561559/2008-2 and 555.669/2009-2 (Brazilian Malaria Network), CNPq Bionorte Network grant 2009, FAPERN.

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