Residual and ovicidal efficacy of essential oil‐based formulations in vitro against the donkey chewing louse Bovicola ocellatus

Essential oils have shown good experimental potential as novel veterinary ectoparasiticides. However, if they are to be used as veterinary products, they must be available in formulations that are suitable for practical application against specific ectoparasites. Here, the efficacies of formulations containing 5% (v/v) lavender or tea tree oil, in combination with two emulsifiers [a surfactant, 5% (w/v) N‐lauroylsarcosine sodium salt (SLS), and a soluble polymer, 5% (w/v) polyvinylpyrrolidone (PVP)], with or without 10% coconut oil, were tested in contact bioassays against the donkey chewing louse Bovicola ocellatus (Piaget) (Phthiraptera: Trichodectidae). Residual activity was quantified in open and closed containers; ovicidal efficacy was also examined. Exposure to either of 5% (v/v) lavender or tea tree oils with SLS or PVP resulted in louse mortality of 100%, but when coconut oil was included as an excipient, significantly lower efficacy was recorded. However, the formulations became significantly less effective after 2 h in open containers and 40 h in closed containers. The results confirm that the residual activity of essential oils is relatively transitory and the addition of 10% coconut oil does not prolong the period of insecticidal activity by slowing essential oil evaporation. Too short a period of residual activity is likely to be a significant impediment to the effective practical use of essential oils. However, unlike many synthetic pediculicides, the essential oils tested here were highly ovicidal, which suggests that prolonged residual activity may not be essential to kill newly hatched nymphs after treatment.     

Antimicrobial activity of lavender,tea tree and lemon oils in cosmetic preservative systems

Aims: The aim of the study was to verify the antimicrobial activity of commercial essential oils: lavender, tea tree and lemon as the components of a preservative system in oil in water body milks. Methods and Results: The inhibition efficacy of essential oils alone (0·5%), in mixtures (1%) as well as combined with the synthetic preservative 1,3‐dimethylol‐5,5‐dimethylhydantoin and a 3‐iodo‐2‐propynyl butyl carbamate mixture (0·1% and 0·2%) was tested against Staphylococcus aureus ATCC 6538, Pseudomonas aeruginosa ATCC 9027, Candida sp. ?OCK 0008 and Aspergillus niger ATCC 16404 in compliance with the standards of the European Pharmacopoeia Commission. The in vitro activity of oils determined by an impedimetric method was also compared with their activity in cosmetic preparations. Criterion A for bacteria (reduction in the inoculum by 3 logarithmic units within 7 days with no increase up to the 28th day) and fungi (reduction in the inoculum by 2 logarithmic units within 14 days with no increase up to the 28th day) was fulfilled for cosmetic formulations containing the tested essential oils with 0·2% of the synthetic preservative. The preservative concentration could be decreased to 0·1% (with preserving the same efficacy) in combination with lavender and tea tree oils at a concentration of 0·5% each. Conclusions: In all combinations of essential oils with the synthetic preservative, a synergistic effect of the preservative system components was observed, which made it possible to reduce the usable level of the synthetic preservative up to 8·5 times. Significance and Impact of the Study: To develop an effective preservative system in cosmetics in which a synthetic chemical preservative is replaced by natural essential oils.     

Effects of essential oil inhalation on objective and subjective sleep quality in healthy university students

Aromatherapy with essential oils is one of the most popular complementary medical tools for improving sleep quality. However, only a few reports have objectively measured the effects of essential oils on sleep. Here, we used objective and subjective measures to analyze the effects of the essential oils of lavender (Lavandula angustifolia) and sweet orange (Citrus sinensis) on the sleep quality of healthy university students. The participants were monitored for 15 consecutive nights as they inhaled lavender oil and sweet orange oil, in a crossover design. Their sleep was monitored objectively by actigraphy, and total sleep time (TST), sleep efficiency, sleep latency, and wake after sleep onset (WASO) were analyzed. Their sleep was analyzed subjectively using Oguri–Shirakawa–Azumi (OSA) sleep inventory scores. Inhalation of an essential oil improved sleep measures only in participant whose sleep quality was poor in the control condition. Lavender seemed more effective than sweet orange in objective measures, especially in improving sleep latency. In the subjective sleep analysis, the essential oils improved sleep maintenance, dreaming, and sleep length in subjects who had poor sleep quality. Sweet orange seemed more effective than lavender in the subjective sleep measures. The difference between the two oils suggests that expectancy bias had little effect on the hypnotic effect of lavender on objective sleep. Although no obvious effect was observed in good sleepers, the inhalation of lavender oil could be effective for helping poor sleepers improve objective sleep quality.

     

Non‐host plant essential oil volatiles with potential for a ‘push‐pull’ strategy to control the tea green leafhopper,Empoasca vitis

The tea green leafhopper, Empoasca vitis Göthe (Hemiptera: Cicadellidae), is an economically important pest of tea crops, Camellia sinensis (L.) O. Kuntze (Theaceae), in China. The use of non‐host plant essential oils for manipulation of E. vitis was investigated for potential incorporation into a ‘push‐pull’ control strategy for this pest. The effectiveness of 14 plant essential oils in repelling E. vitis was investigated in laboratory assays. Rosemary oil, geranium oil, lavender oil, cinnamon oil, and basil oil repelled leafhoppers in a Y‐shaped olfactometer. We also compared the efficacy of these five plant essential oils to repel E. vitis in the presence of a host plant volatile‐based leafhopper attractant, (Z)‐3‐hexenyl acetate, in a tea plantation. In the treatment combination, four plates (north, south, east, and west) treated with an essential oil surrounded a central sticky plate treated with (Z)‐3‐hexenyl acetate. Fewer E. vitis were found on the plates treated with rosemary oil (12.5% reduction) than on the four water‐sprayed control treatment plates surrounding a central plate with (Z)‐3‐hexenyl acetate. We compared the distribution of E. vitis on the plates, and the relative numbers of E. vitis on each plate were compared with similar plates in the control treatment. When four plates treated with rosemary oil surrounded a central (Z)‐3‐hexenyl acetate‐treated plate, the distribution of E. vitis on the different plates changed significantly compared with that of the control. Relatively fewer E. vitis were found on the east (13.0% reduction) rosemary oil‐treated plates and more E. vitis (11.3% increase) were found on the central attractant‐treated plate. Our findings indicate that rosemary oil is a promising leafhopper repellent that should be tested further in a ‘push‐pull’ strategy for control of E. vitis.     

Repellent effectiveness of seven plant essential oils,sunflower oil and natural insecticides against horn flies on pastured dairy cows and heifers

Plant essential oils (basil, geranium, balsam fir, lavender, lemongrass, peppermint, pine and tea tree), mixed with either sunflower oil or ethyl alcohol, were applied at 5% concentrations to the sides of Holstein cattle. Pastured cattle treated with essential oils diluted in sunflower oil had less flies than the untreated control for a 24‐h period. However, the essential oil treatments were not significantly different than the carrier oil alone. Barn‐held heifers treated with essential oils and sunflower oil alone had significantly less flies than the untreated control for up to 8 h after treatment. Basil, geranium, lavender, lemongrass and peppermint repelled more flies than sunflower oil alone for a period ranging from 1.5 to 4 h after treatments applied to heifers. All essential oils repelled > 75% of the flies on the treated area for 6 and 8 h on pastured cows and indoor heifers, respectively. Geranium, lemongrass and peppermint stayed effective for a longer duration. Essential oils mixed with ethyl alcohol demonstrated less repellence than when mixed with the carrier oil. Safer's soap, natural pyrethrins without piperonyl butoxide and ethyl alcohol alone were not efficient at repelling flies. Essential oils could be formulated for use as fly repellents in livestock production.     

Antimicrobial Activities of the Essential Oils of Various Plants against Tomato Late Blight Disease Agent Phytophthora infestans

The aim of this study was to find an alternative to synthetic fungicides currently used in the control of devastating oomycete pathogen Phytophthora infestans, causal agent of late blight disease of tomato. Antifungal activities of essential oils obtained from aerial parts of aromatic plants such as oregano (Origanum syriacum var. bevanii), thyme (Thymbra spicata subsp. spicata), lavender (Lavandula stoechas subsp. stoechas), rosemary (Rosmarinus officinalis), fennel (Foeniculum vulgare), and laurel (Laurus nobilis), were investigated against P. infestans. Both contact and volatile phase effects of different concentrations of the essential oils used were determined by using two in vitro methods. Chemical compositions of the essential oils were also determined by GC-MS analysis. Major compounds found in essential oils of thyme, oregano, rosemary, lavender, fennel and laurel were carvacrol (37.9%), carvacrol (79.8), borneol (20.4%), camphor (20.2%), anethole (82.8%) and 1,8-cineole (35.5%), respectively. All essential oils were found to inhibit the growth of P. infestans in a dose-dependent manner. Volatile phase effect of oregano and thyme oils at 0.3 μg/ml air was found to completely inhibit the growth of P. infestans. Complete growth inhibition of pathogen by essential oil of fennel, rosemary, lavender and laurel was, however, observed at 0.4–2.0 μg/ml air concentrations. For the determination of the contact phase effects of the tested essential oils, oregano, thyme and fennel oils at 6.4 μg/ml were found to inhibit the growth of P. infestans completely. Essential oils of rosemary, lavender and laurel were inhibitory at relatively higher concentrations (12.8, 25.6, 51.2 μg/ml respectively). Volatile phase effects of essential oils were consistently found to be more effective on fungal growth than contact phase effect. Sporangial production was also inhibited by the essential oil tested. Light and scanning electron microscopic (SEM) observation on pathogen hyphae, exposed to both volatile and contact phase of oil, revealed considerable morphological alterations in hyphae such as cytoplasmic coagulation, vacuolations, hyphal shrivelling and protoplast leakage.     

Essential Oil of Lavender Inhibited the Decreased Attention during a Long-Term Task in Humans

This study examined the effects of odors on sustained attention during a vigilance task. Two essential oils (lavender and eucalyptus) and two materials (l-menthol and linalyl acetate) were compared with a control. The increase in reaction time was significantly lower with lavender than with the control. The results suggest that the administration of lavender helped to maintain sustained attention during the long-term task.     

Essential oils against multidrug resistant gram-negative bacteria

Multi-drug resistant uropathogens are responsible for urinary tract infections. The antibacterial activity of seven essential oils, oregano, thyme, clove, arborvitae, cassia, lemongrass, tea tree) was investigated by agar diffusion method, followed by determination of minimum inhibitory (MIC) and bactericidal (MBC) concentrations against five multidrug resistant isolates namely Pseudomonas aeruginosa, Escherichia coli, Enterobacter cloaceae, Morganella morganii, Proteus mirabilis. Oregano, thyme, cassia had antibacterial activity with inhibition zones ranging 25–39 mm; clove, arborvitae, tea tree and lemongrass 12–15 mm. The essential oils showed antibacterial activities with MICs ranged from 0.005% (w/v) to 0.5% (w/v). Thyme had the same MIC and MBC on all strains. The effects of the vapors of the essential oils were also tested by placing the oils on the underside of the Petri dish lid. Thyme, oregano and cassia essential oils strongly inhibited the growth of the clinical strains of bacteria tested in vapor phase. This study demonstrates the potential of investigated essential oils as natural alternatives for further application in hospital therapies in order to retard or inhibit the bacterial growth. For the first time antibacterial effects of essential oils (clove, arborvitae, tea tree, lemongrass, and cassia) were evaluated against Enterobacter cloaceae and Morganella morganii clinical isolates.     

Grooming site preferences determined by lice infection among Japanese macaques in Arashiyama

I investigated the effect of the density of louse eggs (Pedicinus obtusus andP. eurygaster) on grooming site preferences in Japanese macaques (Macaca fuscata). Louse eggs were more often found on the outer side of the body (upper back, lower back, outer arms, and outer legs) than on the inner side of the body (chest, belly, inner arms, and inner legs). Japanese macaques were more likely to be groomed on the outer side than the inner side of the body by allogrooming and autogrooming. Such grooming site preferences correlated with the distribution of louse eggs but not with the areas of body parts. Thus, the ecology of lice might affect grooming behavior of Japanese macaques. Five hundred and fifty louse eggs were estimated to parasitize an adult female Japanese macaque. Considering the intrinsic rate of natural increase of lice, monkeys need to be groomed almost every day. This suggests that Japanese macaques need grooming partners and form social bonds with others for everyday grooming.     

Essential Oil Compositions and Antibacterial and Antioxidant Activities of Five Lavandula stoechas Cultivars Grown in Thailand

The essential oils of five Lavandula stoechas cultivars grown in Thailand were characterized for their volatile compounds using GC‐FID and GC/MS methods as well as screened for antibacterial and antioxidant activities. Dried aerial parts, including flowers and stems from each cultivar, were subjected to hydrodistillation for 4 h. The essential oil yields were 0.18 %–0.82 % w/w. Of the 95 compounds detected and identified, 1,8‐cineole, fenchone, and camphor were considered the major compounds. Essential oil from each cultivar demonstrated different patterns of antibacterial activity and a variety of antioxidant properties. The highest antibacterial activity, MIC=0.39 mg mL^?1, was observed from the essential oil of L. stoechas ‘major’ (against Klebsiella pneumoniae and Salmonella typhimurium) and the essential oil of L. stoechas ‘white lavender’ (against S. typhimurium). The essential oil of L. stoechas×viridis ‘St. Brelade’ possessed the highest antioxidant capacity, as determined by the DPPH and ABTS assays (IC₅₀ of 67.65 and 89.26 mg mL^?1, respectively). The results indicated that some of these essential oils could be used as key ingredients in lavender oil products in Thailand to increase their therapeutic efficacy, depending on their intended application.     

Comparison of solid‐phase cytometry and the plate count method for the evaluation of the survival of bacteria in pharmaceutical oils

Aim: To compare the survival of four bacterial strains (Escherichia coli, Proteus mirabilis, Staphylococcus aureus, Pseudomonas aeruginosa) in pharmaceutical oils, including jojoba oil/tea tree oil, carbol oil, jojoba oil and sesame oil. Methods and Results: Oils were spiked with the test bacteria in a concentration of 10⁴ CFU ml^?1. Bacteria were extracted from oils with phosphate‐buffered saline containing 0·5% Tween 20. Aliquots of the pooled water layers were analysed by solid‐phase cytometry and plate counting. Plate counts dropped to zero for all test strains exposed for 24 h to three of the four oils. In contrast, significant numbers of viable cells were still detected by SPC, except in the jojoba oil/tea tree oil mixture and partly in sesame oil. Conclusions: Exposure of bacteria for 24 h to the two oils containing an antimicrobial led to a loss of their culturability but not necessarily of their viability. The antibacterial activity of the jojoba oil/tea tree oil mixture supersedes that of carbol oil. Significance and Impact of the Study: These in vitro data suggest that the jojoba oil/tea tree oil mixture more than carbol oil inhibits bacterial proliferation when used for intermittent self‐catherization.     

Essential oils: effects of application rate and modality on potential for combating northern fowl mite infestations

The northern fowl mite (NFM), Ornithonyssus sylviarum (Mesostigmata: Macronyssidae), is the primary blood‐feeding ectoparasite found on poultry in the U.S.A. Three experiments were conducted in vitro to test the acaricidal properties of cade, garlic, lavender, lemongrass, pine and thyme essential oils against NFM, and to evaluate whether these effects are altered by adjusting oil application rates and application modality (direct vs. vapour contact). Applied at the rate of 0.21 mg/cm², the essential oils of cade, thyme, lemongrass and garlic resulted in higher NFM mortality at 24 h post‐application than lavender and pine oils, and the untreated and ethanol‐treated controls. Cade and thyme were the most consistent and fast‐acting of the essential oils in terms of toxicity to NFM. Cade applied at 0.21 mg/cm² and 0.11 mg/cm² and thyme applied at 0.21 mg/cm² were effective in eliminating NFM within 2 h through direct contact. The modality of application did not affect the efficacy of cade and thyme essential oils. The results suggest that essential oils may be utilized as alternatives to chemical pesticides and could be used as fumigants for the control of NFM.     

Laboratory evaluation of aromatic essential oils from thirteen plant species as candidate repellents against <Emphasis Type="Italic">Leptotrombidium</Emphasis> chiggers (Acari: Trombiculidae), the vector of scrub typhus

Scrub typhus, a rickettsial disease transmitted by several species of Leptotrombidium chiggers (larvae), is endemic in many areas of Asia. The disease is best prevented by the use of personal protective measures, including repellents. In this study commercially produced aromatic, essential oils of 13 plant species and ethanol (control) were tested in the laboratory for repellency against host-seeking chiggers of Leptotrombidium imphalum Vercammen-Grandjean and Langston (Acari: Trombiculidae). A rapid, simple and economic in vitro test method was used by exposing the chigger for up to 5 min. Repellency was based on relative percentages of chiggers attracted to test and control substances. Four of the 13 essential oils showed promise as effective repellent against L. imphalum chiggers. Syzygium aromaticum (clove) oil exhibited 100% repellency at 5% concentration (dilution with absolute ethanol), whereas Melaleuca alternifolia (tea tree) oil exhibited 100% repellency at 40% concentration. Undiluted oils of Zingiber cassamunar (plai) and Eucalyptus globules (blue gum) exhibited 100% repellency. Of the remaining nine essential oils, only 100% Pelargonium graveolens (geranium) exhibited >50% repellency (viz. 57%). Styrax torkinensis (benzoin) oil did not exhibit any repellency. These findings show that several aromatic, essential oils of plants may be useful as chigger repellent for the prevention of scrub typhus. Syzygium aromaticum oil may be safer and more economical to prevent chigger attacks than commercially available synthetic chemicals, such as DEET that may have harmful side effects.     

Decreased Immunoreactivity of Hepatitis E Virus Antigen Following Treatment with Sakhalin Spruce (Picea glehnii) Essential Oil

The hepatitis E virus (HEV) causes a common infectious disease that infects pigs, wild boars, deer, and humans. In most cases, humans are infected by eating raw meat. Some essential oils have been reported to exhibit antiviral activities. In this study, in order to investigate the anti-HEV properties of essential oils, the immunoreactivities of HEV antigen proteins against the relevant antibodies were analyzed after the HEV antigens underwent treatment with various essential oils. The essential oils extracted from the tea tree, which was previously reported to exhibit antiviral activity, lavender, and lemon had strongly reduced activity. We found that treatment with the essential oil prepared from Sakhalin spruce was associated with the strongest reduction in immunoreactivity of HEV antigen protein(s) among the tested substances. The main volatile constituents of Sakhalin spruce essential oil were found to be bornyl acetate (32.30 %), α-pinene (16.66 %), camphene (11.14 %), camphor (5.52 %), β-phellandrene (9.09 %), borneol (4.77 %), and limonene (4.57 %). The anti-HEV properties of the various components of the essential oils were examined: treatment with bornyl acetate, the main component of Sakhalin spruce oil, α-pinene, the main component of tea tree oil, and limonene, the main component of lemon oil, resulted in a strong reduction in HEV antigen immunoreactivity. These results indicate that each main component of the essential oils plays an important role in the reduction of the immunoreactivity of HEV antigen protein(s); they also suggest that Sakhalin spruce essential oil exhibits anti-HEV activity. In a formulation with the potential to eliminate the infectivity of HEV in foodborne infections, this essential oil can be applied as an inactivating agent for meat processing and cooking utensils, such as knives and chopping boards.     

11种植物精油对6种植物病原真菌的抑菌活性研究

为筛选有效的植物杀菌成分,采用菌丝生长法,测定了香茅油、薰衣草油、菊花油、月桂油、柠檬油、广藿香油、肉桂油、天竺葵油、迷迭香油、茶树油、薄荷油对6种植物病原真菌的抑菌活性。发现在2 g/L的浓度下,上述11种精油对6种供试病菌均有明显的抑制作用,其中香茅油、肉桂油、天竺葵油、月桂油、茶树油和薄荷油对6种病原真菌的抑制率均为100%。剂量效应试验表明,肉桂油对灰葡萄孢（Botrytis cinerea）和禾谷镰孢菌（Fusariumgraminearum）的EC50值分别为29.05μg/mL和42.96μg/mL,而天竺葵油对灰葡萄孢（Botrytis cinerea）和禾谷镰孢菌（Fusarium graminearum）的EC50值分别为34.02μg/mL和68.48μg/mL。     

Efficacy of plant essential oils against two major insect pests of coffee (Coffee berry borer,Hypothenemus hampei,and antestia bug,Antestiopsis intricata) and maize weevil,Sitophilus zeamais

Coffee berry borer, antestia bug and maize weevil are serious pest of coffee and maize, respectively. Bioassays of plant essential oils were conducted with coffee berry borer, antestia bug and the maize weevil. Essential oils of Thymus vulgaris, Aloysia sp., Ruta chalepensis, Chenopodium ambrosioides and Cymbopogon nardus resulted in 80%–90% mortality of coffee berry borer, whereas essential oils of C. ambrosioides, T. vulgaris and R. chalepensis achieved 87.5%–92.5% mortality of antestia bug. Essential oils of C. ambrosioides caused significantly the highest percentage mortality of the maize weevils (95%) with “Lethal Dose” (LD₅₀) values of 2.202 ml 100 ml^?1 within 24 h after treatment, whereas essential oils of Mentha spicata, T. vulgaris and R. chalepensis resulted in 82.5%, 77.5% and 73.5% mortality, respectively. The potential of plant essential oils for the management of coffee berry borer, antestia bug and maize weevil for the small-scale farmers is discussed.     

Evaluation of commercial essential oil samples on the growth of postharvest pathogen Monilinia fructicola (G. Winter) Honey

Aim: To assess the effect of several commercial essential oils samples Australian lemon myrtle (Backhousia citriodora), cinnamon bark (Cinnamomum zeylanicum), oregano (Origanum vulgare), thyme oil (Thymus vulgaris), clove bud (Eugenia caryophyllata), valerian (Valeriana officinalis) and Australian tea tree oil (Melaleuca alternifolia) on mycelium growth and spore germination of Monilinia fructicola. The effectiveness of lemon myrtle essential oil as a fumigant for the control of brown rot in nectarines was evaluated. Methods and Results: Monilinia fructicola exhibited a different level of sensitivity to each tested essential oil with results suggesting that the essential oils provide excellent control of the pathogen with respect to mycelium growth and spore germination at very low concentrations, whereas for others higher concentrations are needed to reduce significant fungal growth. In vivo application of lemon myrtle essential oil effectively reduced the incidence of M. fructicola on noninoculated fruit. Fumigation of nectarines following inoculation did not reduce the incidence of brown rot in comparison with the inoculated control treatment. No evidence of phytotoxicity on the fruit was recorded. Conclusions: Lemon myrtle essential oil exhibited the strongest antifungal activity against M. fructicola, in vitro and to a lesser extent, under in vivo conditions. Significance and Impact of the Study: The results demonstrate that lemon myrtle essential oil, in particular, has potential as an antifungal agent to control M. fructicola.     

Efficacy of essential oils and biopesticides on Phytophthora infestans suppression in laboratory and growth chamber studies

Control of late blight by alternative products is important for reduction of fungicide inputs and potato production costs. The efficacy of essential oils and biopesticides for inhibition of growth of Phytophthora infestans on media and suppression of late blight on potato plants in growth chambers was examined. Growth of pathogen isolates of diverse genotypes was evaluated on Rye B media amended with essential oils (lavender, thyme, thyme borneal, and oregano) and the biopesticide Serenade (Bacillus subtilis strain QST 713). Over 90% inhibition of pathogen growth was achieved with oregano and Serenade amendments. The protective foliar application of Serenade, an aerated compost tea (ACT), Effective Microorganism mix (EM), and oregano, resulted in disease suppression of 5-40% relative to the untreated control. ACT had no significant suppressive effects (0-15% reduction), EM resulted in mild suppression (15-30% reduction), and oregano and Serenade consistently resulted in moderate disease reduction (20-40%). No oil or biological treatment produced disease control comparable to the chemical control chlorothalonil, which resulted in disease reductions of 80-98%. Both oregano and Serenade resulted in some phytotoxicity at high doses. These results suggest that the natural products and biological amendments tested are not sufficient for effective late blight control by themselves; however, when used in combination with other established disease control practices, these approaches may contribute to improved, integrated, and more sustainable management options for late blight.     

Characterization of radicals arising from oxidation of commercially-important essential oils

Inappropriate use of essential oils may entail risks to human health due to mutational events, carcinogenic effects, genetic damages and sensitizing effect caused by generation of reactive oxygen species. In order to detect radicals that are expected to form during their oxidation, we measured the electron spin resonance (ESR) spectra of a standard reaction mixture (I) containing 25?μM flavin mononucleotide, 0.018% several essential oils (or 0.015% geraniol), 1.9 M acetonitrile, 20?mM phosphate buffer (pH 7.4), 0.1 M α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) and 1.0?mM FeSO₄(NH₄)₂SO₄ irradiated with 436?nm visible light (7.8 J/cm²). The ESR peak heights of the standard reaction mixture (I) of the essential oils increased in the following order: tea tree?>?palmarosa?>geranium?>?clary sage?>?petitgrain?>?lavender?>?bergamot?>?frankincense?>?ravintsara?>?ylang ylang?>?lemongrass?>?niaouli?>?eucalyptus globulus?>?peppermint. The ESR peak height of the standard reaction mixture (I) of geraniol, a main component of palmarosa, was comparable to the one of palmarosa (97?±?19% of palmarosa). Furthermore, high performance liquid chromatography (HPLC)-ESR analyses of the standard reaction mixture (I) of palmarosa and geraniol gave the same peaks. The results suggest that the radicals formed in the standard reaction mixture (I) of palmarosa are derived from geraniol. HPLC-ESR-mass spectrometry analyses detected m/z 294 ions, 4-POBN/5-hydroxy-3-methyl-3-pentenyl radical adducts and m/z 320 ions, 4-POBN/C₇O₂H₉ radical adducts in the standard reaction (I) of geraniol. The 5-hydroxy-3-methyl-3-pentenyl and C₇O₂H₉ radicals may be implicated in the sensitizing effect of palmarosa.