Citronella

Citronella (Cymbopogon nardus and Cymbopogon winterianus): A Comprehensive Reference

1. Identity, Botanical Classification, and Natural Sources

"Citronella" designates the essential oil and, by extension, the aromatic grasses from which it is distilled. Belonging to the Poaceae family, the most common species are Cymbopogon nardus and Cymbopogon winterianus, known for their aromatic and repellent properties. Oil of citronella is the volatile oil obtained from the steam distillation of freshly cut or partially dried grasses, Cymbopogon nardus (Rendle) and Cymbopogon winterianus (Jowitt).

There are two closely related species of citronella grass that have been cultivated over the years to produce citronella essential oil: Cymbopogon nardus, known in the oil industry as the "Ceylon type," and Cymbopogon winterianus, known as the "Java type." In the early 1900s, C. nardus was cultivated almost exclusively in Sri Lanka (formerly known as Ceylon), and C. winterianus was produced on the Indonesian island of Java. In the 19th century, a man named Winter (supposedly an important distiller in Ceylon) recognized the taxonomic differences between several varieties of Cymbopogon nardus and raised a separate population of the variety 'Maha Pengiri,' now called C. winterianus.

It is an evergreen, perennial, clump-forming aromatic grass with multiple upright culms emerging from a short rhizome that is vigorous, aromatic, and multi-harvest. The culms can reach a height of 2.5 m. The plant is popularly known as "citronella grass," originating from Ceylon and India, and is used in Indonesia as a soothing and digestive tea.

The annual world production of citronella oil is estimated at 5,000 tonnes. Most citronella oil is Java citronella oil; production of Ceylon citronella oil is restricted to Sri Lanka and is only 200 tonnes per year.

Commercial Trade Types

• Ceylon type (C. nardus): Consists of geraniol (18–20%), limonene (9–11%), methyl isoeugenol (7–11%), citronellol (6–8%), and citronellal (5–15%).
• Java type (C. winterianus): Consists of citronellal (32–45%), geraniol (11–13%), geranyl acetate (3–8%), and limonene (1–4%).

This differentiation is due to the variance in their chemical compositions, mainly in their geraniol content (higher percent in the Ceylon type) and citronellal content (higher percent in the Java type), and also accounts for the difference in their aromas. The alcohol concentration of Java citronella oil is higher (90–95%) than that of Ceylon citronella oil (60–71%).

Common Forms and Preparations

Citronella essential oil is widely used in the production of food, drink, perfumery, soap, body care products, and pharmaceutical products. Oil of citronella is also commonly found in foods and beverages as a flavoring agent. It is Generally Recognized as Safe (GRAS) as a food additive by the Food and Drug Administration. Commercially available preparations include topical sprays and lotions for insect repellency, candles, incense sticks, diffuser oils, and compounded essential oil blends. The essential oil of C. nardus is prepared using the steam distillation method, and its chemical composition is analyzed by gas chromatography-mass spectroscopy (GC-MS).

2. Traditional and Historical Use

This tropical fragrant grass has been employed as both a spice and a medicine for thousands of years throughout Africa, China, India, and southern Asia. The history of citronella dates back thousands of years, when ancient Asian civilizations discovered its numerous benefits. The plant was used in various forms: its leaves were burned to ward off insects, and the extracted oils were used in medicine for their antibacterial and anti-inflammatory properties.

Infusions of citronella were taken internally to treat fevers, colds, intestinal parasites, digestive ailments, headaches, and menstrual problems, and externally poultices of leaves were used for inflammation, rheumatism, and infections. In traditional medicine, poultices of the leaves are applied to treat minor cuts and bruises, while extracts are used against internal disorders and as a vermifuge. The extracts are also mildly astringent and stomachic.

For centuries, citronella oil has been a natural medicinal remedy and a food ingredient in China, Sri Lanka, and Indonesia. It is traditionally used as a flavoring agent in culinary applications, a soothing agent for pain, infections, rashes, and inflammation, a non-toxic insect-repelling agent, a natural and fragrant household cleaning agent, and an ingredient in perfumery, soaps, detergents, scented candles, and cosmetic products.

While citronella has been used traditionally for centuries throughout tropical Asia, its formal distillation began in earnest during British colonial rule, when it was cultivated in plantations and exported for use in candles, soaps, and early medicinal preparations. In the 19th century, citronella oil gained attention as an insect repellent during the colonial era, especially in regions where malaria and other mosquito-borne diseases were prevalent. Oil of citronella was initially registered as an insect repellent by the US EPA in 1948.

In Thailand, a preparation of crude citronella oil mixed with leaves of Azadirachta indica and rhizomes of Alpinia galanga is applied as a bio-insecticide in vegetable production and in citrus orchards.

3. Key Constituents and Active Compounds

Oil of citronella is a mixture of many components. The exact composition varies by grass variety. However, the main components are citronellol, citronellal, and geraniol.

A detailed GC-MS analysis of C. nardus essential oil from one study found: seventeen constituents, with the components found in higher amounts being the monoterpenes geraniol (33.88%), citronellal (27.55%), and citronellol (14.40%). A separate analysis confirmed: the partially dried grass contained 35 components; the oil contains 16 monoterpenes (79.8%), nine sesquiterpenes (11.5%), and four non-terpenic compounds (1.4%). The prominent monoterpenes were citronellal (29.7%), geraniol (24.2%), γ-terpineol (9.2%), and cis-sabinene hydrate (3.8%).

For the Java type (C. winterianus): 23 compounds were identified (96.50% of total oil). The main components were citronellal (27.00%), trans-geraniol (22.78%), and citronellol (10.09%).

The major individual compounds and their known or proposed activities:

• Citronellal (3,7-dimethyloct-6-en-1-al): The predominant aldehyde. A monoterpenoid exhibiting notable neurological activity, including anxiolytic and anticonvulsant effects, primarily through GABAergic pathways. It is also central to the oil's antifungal and insect-repellent properties (see below).
• Geraniol: A major monoterpene alcohol present in both types. Geraniol (GNL), a natural monoterpene, is found in many essential oils including lavender, citronella, lemongrass, and other medicinal and aromatic plants. It is commonly used by the cosmetic and food industries and has shown a wide spectrum of pharmacological activities including anti-inflammatory, anticancer, antimicrobial, antioxidant, and neuroprotective activities. It represents a potential anti-inflammatory agent and a promising cancer chemopreventive agent, as it has been found to be effective against a broad range of cancers including colon, prostate, breast, lung, skin, kidney, liver, and pancreatic cancer — though this evidence is predominantly preclinical.
• Citronellol: A naturally occurring monoterpenoid alcohol. Citronellol has the molecular formula C₁₀H₂₀O and exists in two enantiomeric forms: (+)-Citronellol and (–)-Citronellol. Citronellol, a major part of citronella oil, was ranked as the number one mosquito repellent by Bunker and Hirschfelder (1925) in their study from 1925.
• Methyl eugenol: Present at 7–11% in Ceylon-type oil. Its presence carries regulatory significance discussed under safety below.
• Limonene, geranyl acetate, elemol, and β-elemene are among other identified constituents. The major chemical components of Java citronella oil are citronellal, geraniol, elemol, geranyl acetate, limonene, β-elemene, citronellyl acetate, and eugenol.

4. Established Mechanisms of Action

4.1 Insect Repellency

Oil of citronella is a volatile, liquid oil derived from dried cultivated grasses. It works on insects without harming or killing them. It has a distinctive odor which may make it difficult for some pests to locate a host. The precise olfactory mechanism is thought to involve disruption of host-seeking behavior through masking or overriding the chemical cues (carbon dioxide, lactic acid, and skin-borne volatiles) that biting insects use to locate warm-blooded hosts.

4.2 Antimicrobial Mechanisms

Carveol, citronellal, and carvone essential oils were shown to modify hydrophobicity and disrupt membrane integrity, leading to the leakage of K⁺ ions. This disruption of the bacterial cell membrane is considered a primary bactericidal mechanism. In one in vitro study, the essential oil of C. nardus (EOCN) produced a 41 mm halo on S. aureus, with MIC values of 0.5 mg/mL for the EOCN and 0.25 mg/mL for geraniol, both with bactericidal effect. The antibiofilm action was confirmed, with EOCN and geraniol reducing the biofilm biomass of S. aureus up to 100% between 0.5 and 4 mg/mL concentrations.

4.3 Antifungal Mechanisms

A dedicated mechanistic study examined how citronellal, a typical terpenoid of Cymbopogon nardus essential oil, acts on ergosterol to exhibit its antifungal activity against Penicillium digitatum. Citronellal inhibited the growth of P. digitatum with a minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) of 1.36 and 2.72 mg/mL, respectively. In citronellal-treated cells, the membrane integrity and ergosterol contents significantly decreased, whereas lanosterol, which serves as a precursor for ergosterol biosynthesis, massively accumulated. Addition of 150 mg/L of exogenous ergosterol decreased the inhibitory rate of citronellal, restoring the ergosterol content and membrane structure to normal levels. This indicates that inhibition of the ergosterol biosynthesis pathway is a key antifungal mechanism.

Citronellal was also capable of inhibiting the mycelial growth and spore germination of Penicillium digitatum in a dose-dependent manner, by deteriorating the plasma membrane of P. digitatum spores and leading to a higher extracellular conductivity and release of cell constituents.

4.4 Anti-inflammatory Mechanisms

Due to the monoterpenes found in its composition, citronella essential oil has anti-inflammatory activities, reported to include inhibition of lipoxygenase. Geraniol induces apoptosis and cell cycle arrest, modulates multiple molecular targets including p53 and STAT3, activates caspases, and modulates inflammation via transcriptional regulation. These mechanisms have primarily been characterized in cell-based and animal models.

4.5 Sedative and CNS Mechanisms

Citronellal exhibits notable neurological activity, including anxiolytic and anticonvulsant effects, primarily through GABAergic pathways. Researchers have explored its sedative potential using in vitro, in vivo, and in silico approaches through the GABAergic pathway.

5. Scientific Evidence by Area of Use

5.1 Insect Repellency

Regulatory and classification status: Oil of citronella was first registered in the United States in 1948. It is currently on the U.S. Environmental Protection Agency's (EPA) list of minimum risk pesticides. The US FDA considers citronella oil as GRAS.

Efficacy compared to DEET: In general, studies show that citronella-based repellents are less effective than DEET repellents. Citronella provides a shorter protection time, which may be partially overcome by frequent reapplication of the repellent. In 1997, after analyzing available data on the repellent effect of citronella, the EPA concluded that citronella-based insect repellents must contain the following statement on their labels: "For maximum repellent effectiveness of this product, repeat applications at 1-hour intervals."

Duration of protection: A comparative study using Korean Food and Drug Administration (KFDA) guidelines tested DEET, citronella oil, and fennel oil on human volunteers: The repellency of citronella oil decreased over time, from 97.9% at 0 h to 71.4% at 1 h and 57.7% at 2 h. In contrast, the repellency of DEET remained over 90% for 6 h. The complete protection time (CPT) of DEET (360 min) was much longer than the CPT of citronella (10.5 min).

Randomized controlled field trial (Nepal, 2013–2015): This study was conducted using a real-life randomized controlled pilot trial to confirm the effectiveness and applicability of locally-produced citronella oil as a mosquito repellent. A repellency activity test was performed with 100% citronella oil (Cymbopogon winterianus) in the Tikapur Municipality of the Kailali district, Nepal. The test was divided into two trials: an indoor exposure (IE) test (N=101) and an outdoor exposure (OE) test (N=140), conducted from 5:00 pm to 7:00 pm. The results regarding the frequency of mosquito bites and the repellency percentage of citronella oil, when applied topically, could provide a sufficient protective effect against mosquito bites in both indoor and outdoor conditions. The study concluded that topical application of citronella oil can be employed as an easily-available, affordable, and effective alternative mosquito repellent to prevent mosquito-borne diseases in rural areas such as Tikapur, Nepal.

Citronella candles: One study compared the efficacy of commercially available 3% citronella candles, 5% citronella incense, and plain candles to prevent bites by Aedes species mosquitoes under field conditions. Subjects near the citronella candles had 42% fewer bites than controls that had no protection (a statistically significant difference). However, citronella and geraniol candles are widely sold as outdoor repellents; field studies against mixed populations of nuisance mosquitoes show reductions in biting of around 50%, although they do not provide significant protection against mosquito bites.

Efficacy variability: Data on the efficacy of citronella-based products are conflicting, varying greatly depending on the study methodology, location, and species of biting insect tested. Lindsay et al. (1996) did not find that citronella candles had any effect on mosquito biting pressure. Fradin and Day (2002) found that citronella-based spray-on repellents only protected for very short times from mosquito bites. The duration of effectiveness of essential oil repellents including citronella is estimated to be between 30 minutes and 2 hours.

Formulation effects: A mixture of citral, myrcene, and citronellal oil (C:M:Ci = 6:4:1) greatly affected and inhibited host-seeking behavior. Compared with the result for DEET (44%, 22% with 400 μL), adjusting the composition formulas of citronella oil had a synergistic effect for more effective repellency against Ae. aegypti.

Overall evidence strength: The insect repellent activity of topical citronella preparations is moderately supported by human studies, but these consistently show shorter duration of protection and lower overall efficacy compared to DEET. Evidence is strongest for topical application. Evidence for environmental dispersal (candles, diffusers) is mixed and limited.

5.2 Antimicrobial Activity

The essential oil of C. nardus (citronella) was shown to have broad-spectrum fungistatic activity against Aspergillus, Chaetomium, Myrothecium, Penicillium, and Trichoderma spp. Billerbeck et al. (2001) claimed that essential oil of C. nardus at a concentration of 400 mg/L could inhibit 80% of Aspergillus niger growth. Meanwhile, Oussalah et al. (2006) reported that the essential oil showed antimicrobial activity at a concentration of 4 mg/mL against Pseudomonas putida CRDAV 372 isolated from fresh beef.

A total of 22 chemical compounds were identified in C. nardus essential oil, representing 93.1% of the detected compounds; citronellal was the major compound (29.6%), followed by 2,6-octadienal, 3,7-dimethyl- (E)- (11.0%) and cis-2,6-dimethyl-2,6-octadiene (6.9%). The MIC values of the citronella oil ranged from 0.244 µg/mL to 0.977 µg/mL when tested against bacterial isolates from aquatic animals.

Citronellal has a strong inhibitory effect against fungi. It was reported to be effective in inhibiting the growth of three pathogenic Penicillium strains including P. digitatum, P. italicum, and P. ulaiense, with the inhibition zone ranging from 31 to 43 mm².

Antibiofilm evidence: The antibiofilm action of citronella essential oil was confirmed in vitro; EOCN and geraniol reduced the biofilm biomass of S. aureus up to 100% between 0.5 and 4 mg/mL concentrations.

Overall evidence strength: Antimicrobial and antifungal activities of citronella oil are well-documented in vitro, with demonstrated MIC values across multiple bacterial and fungal species. There are no published human clinical trials assessing citronella oil as a stand-alone antimicrobial therapeutic for systemic or topical human infections. Evidence is limited to laboratory and preclinical settings.

5.3 Anti-inflammatory Activity

Due to the monoterpenes found in its composition, citronella also exhibits anticancer activities, inhibiting the proliferation of LNCαP and HeLα cells. Additionally, citronella essential oil has been reported to have anti-inflammatory activities by inhibiting lipoxygenase. The essential oil of Cymbopogon nardus has shown antioxidant and anti-inflammatory properties in addition to its antimicrobial action.

Overall evidence strength: Anti-inflammatory activity has been demonstrated at the molecular and cellular level in preclinical models. No human clinical trials specifically assessing citronella essential oil for inflammatory conditions have been identified in the published literature.

5.4 Antifungal Activity Against Candida

A 2024 study evaluated geraniol, linalool, and citronellal — monoterpenes sharing the molecular formula C₁₀H₁₈O — for antifungal activity, cytotoxicity, and antibiofilm structure-activity relationships, as well as the influence of geraniol on the Candida spp.-induced dysregulated inflammatory axis and in vivo toxicity. All work was conducted in cell and animal models, with no human clinical outcomes reported.

5.5 Sedative and Anxiolytic Activity

Citronellal exhibits notable neurological activity, including anxiolytic and anticonvulsant effects, primarily through GABAergic pathways. Its sedative potential has been explored using in vitro, in vivo, and in silico approaches. In vitro GABAergic activity of citronellal was assessed via colorimetric assay, while acute toxicity was evaluated in Swiss mice per OECD guidelines with doses up to 2000 mg/kg. Sedative effects were assessed using thiopental sodium (TS, 40 mg/kg)-induced sleep protocols. Citronellal was administered at 62.5, 125, and 250 mg/kg doses, alone or combined with diazepam or flumazenil.

Overall evidence strength: Sedative and anxiolytic activities are supported by animal studies and in vitro assays only. No human clinical trials have been conducted to evaluate these effects.

5.6 Antioxidant Activity

Studies report that citronella also has antioxidant properties. This is attributed to its high content of monoterpenes, although lower than that of gallic acid, which is used as a standard. Geraniol scavenges free radicals and preserves the activity of antioxidant enzymes.

Overall evidence strength: Antioxidant activity is demonstrated in vitro, primarily using DPPH and FRAP assays. No controlled human trials have evaluated clinical antioxidant benefit.

5.7 Larvicidal Activity

The larvicidal action of citronella essential oil (OECN) was tested against the third and fourth larval stages of Aedes aegypti at concentrations (2.5, 5.0, 7.5, and 10 μL/20 mL) for 24 to 48 hours. This represents a potential application in vector control, although human trials are absent from the literature.

6. Body Systems and Health Areas

• Integumentary system (skin): Topical application as insect repellent; traditional use for wounds, bruises, rashes, and infections. Risk of contact sensitization (see Safety).
• Immune/Inflammatory system: Preclinical evidence for inhibition of lipoxygenase and anti-inflammatory pathways; no clinical evidence in humans.
• Gastrointestinal system: Traditional use internally as a digestive tea and stomachic in Indonesia and parts of Asia; no clinical trial evidence.
• Central nervous system: Preclinical evidence for GABAergic-mediated sedative and anxiolytic activity via citronellal; no human trials.
• Infectious disease / vector-borne disease prevention: Best-studied human application; moderate clinical evidence for topical repellency against mosquitoes, with important limitations on duration and field effectiveness.
• Mycological (antifungal): In vitro evidence against Aspergillus, Penicillium, Candida, and other fungi; no human clinical trials.

7. Dosage Forms and Reported Dosages

As citronella is regulated as a pesticide repellent rather than a drug supplement in most jurisdictions, formal therapeutic dosing guidelines do not exist. The following dosages are those reported specifically within cited studies:

• Topical repellent (human study, Nepal): 100% citronella oil was tested topically in a randomized controlled pilot trial.
• Topical repellent (laboratory/human): A 1.5 mL aliquot of repellent solution was applied evenly on the right forearm between the wrist and elbow.
• Citronella candles (field study): 3% citronella candles and 5% citronella incense were tested against plain candles under field conditions.
• Antibacterial in vitro: The essential oil showed antimicrobial activity at a concentration of 4 mg/mL against Pseudomonas putida. MIC values ranged from 0.244 µg/mL to 0.977 µg/mL in aquatic pathogen testing.
• Antifungal (in vitro, Aspergillus niger): 400 mg/L of C. nardus essential oil inhibited 80% of Aspergillus niger growth.
• Antifungal (citronellal MIC, P. digitatum): Citronellal inhibited the growth of P. digitatum with an MIC of 1.36 mg/mL and a minimum fungicidal concentration (MFC) of 2.72 mg/mL.
• Sedative/anxiolytic (animal study): Citronellal was administered in mice at doses of 62.5, 125, and 250 mg/kg, alone or combined with diazepam (2 mg/kg) or flumazenil (0.1 mg/kg).
• Animal feed additive (EFSA assessment): For short-living animals, the additive was considered of no concern at concentrations of 18 mg/kg in chickens for fattening, 24 mg/kg in turkeys for fattening, 20 mg/kg for piglets and pigs for fattening, veal calves, cattle for fattening, sheep/goats for meat production, horses for meat production and rabbits for meat production, and 30 mg/kg for salmonids.

8. Safety Considerations

8.1 General Toxicology

In studies using laboratory animals, oil of citronella shows little or no toxicity. The only concern is skin irritation. Although essential oils are exempt from registration through the EPA, they can be irritating to the skin and their repellent effect is variable, dependent on formulation and concentration. Repellents containing only essential oils in the absence of an active ingredient such as DEET should not be recommended as repellents for use in disease endemic areas, and those containing high levels of essential oils could cause skin irritation, especially in the presence of sunlight.

8.2 Skin Sensitization

Oil of citronella derived from the "Ceylon type" is a weak dermal sensitizer. The essential oil should be considered as irritant to skin and eyes and as a dermal sensitizer. When handling the essential oil, exposure of unprotected users to methyleugenol may occur. Therefore, to reduce the risk, the exposure of the users should be minimized.

If applied to the skin, oil of citronella can cause skin irritation or allergic reactions in some people. Therefore, some oil of citronella products should not be used on children less than six months old unless directed by a doctor.

8.3 Methyl Eugenol: A Specific Constituent Concern

The Canadian regulatory concerns with citronella as an insect repellent are due to a lack of safety data and the presence of methyl eugenol. Methyl eugenol is categorized as a Group 2B substance, possibly carcinogenic to humans, by the IARC (2013), as there is sufficient evidence in experimental animals indicating carcinogenicity, although no data are available for humans. In addition, methyl eugenol is "reasonably anticipated to be a human carcinogen" by the US National Toxicology Program (NTP), due to its carcinogenicity in rodents.

Citronella oil is regarded as a category 3 substance regarding the presence of methyl eugenol, and is not sold as an insect repellent within the EU since 2006. Recently, citronella is not being used as an insect repellent in Canada and Europe. The Canadian regulatory concerns with citronella as an insect repellent are due to a lack of safety data and the presence of methyl eugenol. Methyl eugenol is primarily concentrated in the Ceylon type (C. nardus), which contains 7–11% methyl isoeugenol, whereas the Java type has considerably lower levels.

8.4 Regulatory Status Summary

• United States (EPA): Oil of citronella was first registered in the United States in 1948 and is currently on the U.S. EPA's list of minimum risk pesticides.
• United States (FDA/food use): Oil of citronella is commonly found in foods and beverages as a flavoring agent and is Generally Recognized as Safe (GRAS) as a food additive by the FDA.
• European Union: Citronella oil is regarded as a category 3 substance and is not sold as an insect repellent within the EU since 2006. In Europe, Ceylon-type citronella oil is placed on the category 3 list, with some safety concern regarding methyl eugenol.
• IFRA (fragrance use): The International Fragrance Association (IFRA) provides recommended limits for citronellol use in different product categories to minimize sensitization risks.
• EFSA (animal feed): The use of citronella oil in animal feed is expected to be of no concern for consumers and for the environment at the concentrations assessed.

8.5 Environmental Safety

Because of the low toxicity and limited uses of oil of citronella, it is not harmful to the environment. No adverse effects are expected for use around the home.

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