The Science of Cinnamon: Repellency of Essential Oils and Plant-Derived Compounds

Article
Repellency of Essential Oils and Plant-Derived Compounds
Against Aedes aegypti Mosquitoes
April D. Lopez 1

, Sophie Whyms 2

, Hailey A. Luker 1

, Claudia J. Galvan 3

, F. Omar Holguin 3

and Immo A. Hansen 1,*

1 Department of Biology, New Mexico State University, Las Cruces, NM 88003, USA;
aprlopez@nmsu.edu (A.D.L.); hailey13@nmsu.edu (H.A.L.)
2 NatPro Centre, School of Pharmacy and Pharmaceutical Sciences, Trinity College Dublin, 2 Dublin, Ireland;
whymsso@tcd.ie
3 Department of Plant and Environmental Sciences, New Mexico State University, Las Cruces, NM 88003, USA;
cjgalvan@nmsu.edu (C.J.G.); frholgui@nmsu.edu (F.O.H.)
* Correspondence: immoh@nmsu.edu

Simple Summary: Plant-derived essential oils are a complex mixture of various com-
pounds. They have been used as an alternative to synthetic mosquito repellents since ancient times. Currently, there are limited and at times contradictory scientific data on their efficacy.

The aim of this study was to explore the mosquito repellency of two essential oils—clove and cinnamon—and four pure plant-derived oil compounds—2-PEP,geraniol, eugenol, and eugenyl acetate.

We tested these oils at concentrations up to 10% and in different combinations, with the exception of eugenyl acetate that was only tested at a 10% concentration. We found a sigmoidal relationship between concentration and protection from mosquito bites for all these oils, except for eugenyl acetate. We found that concentrations of more than 5% often did not result in longer protection times than 5% concentrations. Mixing two effective repellents did not provide longer protection times than those of individual oils. Our findings are an important contribution to our understanding
of plant-derived mosquito repellents and stress the importance of the scientific efficacy
testing of plant-derived mosquito repellent products.
Abstract: Plant-based oils have a long history of use as insect repellents. In an earlier
study, we showed that in a 10% concentration, geraniol, 2-phenylethl propionate, and the
plant-based essential oils clove and cinnamon effectively protected from mosquito bites for over 60 min. To expand on this study, we reanalyzed our GC–MS data to identify the short organic constituents of these oils. We then used an arm-in-cage assay to test the repellency of different concentrations and combinations of these oils and pure compounds. We found a sigmoidal relationship between the complete protection time from mosquito bites and the concentration of these oils. The complete protection times we recorded for combinations of these oils suggest an absence of additive effects. The results of this study can inform the development of novel, effective, and plant-based insect repellents.
Keywords: repellent; essential oils; concentrations; mixtures; arm-in-cage; clove; cinnamon;
eugenol; geraniol; 2-PEP

1. Introduction
Mosquitoes transmit major vector-borne and re-emerging infectious diseases around
the world [1,2]. Specifically, the yellow fever mosquito, Aedes aegypti (Linnaeus, 1762), is a

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Insects 2025, 16, 51 2 of 14
vector of diseases such as yellow fever, dengue, chikungunya, and Zika [3–6]. Ae. aegypti
is distributed across all continents, except Antarctica, and is heavily present in tropical
and subtropical zones [7]. Due to their rapid geographical expansion, the global risk
for mosquito-borne diseases continues to increase despite advancements in mosquito
population control measures [8,9].
Among control measures, the proper use of personal insect repellents can decrease the
risk of contracting mosquito-borne diseases [10,11]. Effective repellents available commercially
include synthetically derived active ingredients such as DEET, picaridin, and IR3535 [12].
Although these synthetic repellents are highly effective, there is growing consumer interest in
alternatives that are plant-derived and considered more “natural” [13–15].
Mosquito repellents interact with chemoreceptors and alter mosquito host-seeking
behaviors [16]. These receptors are located in their antennae, maxillary palps, labella,
and tarsi [17,18]. Different classes of receptors located in these organs are critical for
detecting human volatile compounds [19,20]. When a mosquito approaches a host, these
chemoreceptors are used to determine host suitability [21]. The exact mode of action of
many active ingredients in both synthetic and natural repellents is unknown [22].
In 1987, the United States Environmental Protection Agency (EPA) released a list
of active ingredients eligible for minimum-risk pesticide status, which includes several
plant-based oils [23]. The EPA 25(b) list of active ingredients eligible for minimum-risk
pesticide products is a published document containing ingredients that are exempt from
the FIFRA (Federal Insecticide, Fungicide, and Rodenticide) act.

There is abundant literature on the use of essential oils as mosquito repellents. Cinna-
mon oil, clove oil, and geraniol have been tested as mosquito repellents by many different

groups [24–27]. Using a common homemade extraction protocol for three different clove
extracts, Tan et al., 2019 used a surface, landing, and feeding assay to study repellency
against Culex quinquefasciatus (Say, 1823) and Ae. aegypti [27]. Their study found eugenol,
ß-caryophyllene, and eugenyl acetate to be the major constituents of the clove oil extracts.

Using the equivalent of a 6–7% concentration for arm-in-cage assays, they found signifi-
cantly lower repellency against both Cx. quinquefasciatus and Ae. aegypti compared with a

1% DEET control. Using the same clove oil formulations in an arm-in-cage assay, Miot and
coworkers found similar results [28]. Several studies have investigated the repellency and

insecticidal effects of cinnamon oil and its major constituents: eugenol and cinnamalde-
hyde [29,30]. Using a modified arm-in-cage assay, Uniyal et al. found 5% cinnamon oil

from Cinnamomum zeylancium (Presl, 1823) to be an effective mosquito repellent for up to
one hour [31]. The plant-derived compound “geraniol” has been shown to have significant
repellency against mosquitoes in both spatial and contact repellency assays [32,33]. A
study by Hao and coworkers demonstrated that Aedes albopictus (Skuse, 1894) mosquitoes,
after a long exposure to geraniol, had altered host-seeking and blood-feeding behavior in
an arm-in-cage assay [34]. A study conducted in 1999 by Barnard tested the repellency
of different concentrations and combinations of essential oils from the EPA 25(b) list on
Aedes aegypti and Anopheles albimanus (Wiedemann, 1820) mosquitoes [35]. A modified
arm-in-cage assay was used to measure complete protection times (CPTs) against mosquito
bites. The authors tested concentrations of essential oils from 5 to 100% using ethanol as
carrier. Among all concentrations and combinations tested, the only mixture that repelled
Ae. aegypti as well as a 25% DEET solution was a mixture of 75% clove oil and 25% thyme oil.
In 2023, our group tested 21 active ingredients from the EPA 25(b) list on Ae. aegypti using
an arm-in-cage assay [36]. This assay was used to measure the complete protection times
(CPTs) from mosquito bites conferred by these active ingredients at 10% concentrations.
We identified seven active ingredients that significantly protected from mosquito bites. Of
these, four provided CPTs of over 60 min—cinnamon oil, clove oil, 2-PEP, and geraniol. A

Insects 2025, 16, 51 3 of 14
chemical analysis using GC–MS was performed to identify individual compounds within
the tested active ingredients.
In the current study, we re-analyzed the GC–MS data from our previous study to
investigate correlations between compound composition and CPT. We further expanded
our investigation of cinnamon oil, clove oil, 2-PEP, and geraniol. First, we explored the
impact of the concentration of these ingredients on CPT. We tested these ingredients in
arm-in-cage assays at 1, 3, 4, 5, and 10% concentrations. We found that concentrations
over 5% did not confer any significantly longer CPTs, except for 2-PEP. Next, we studied
the effect of combining these top-performing active ingredients together in a 1:1 ratio at a
10% total concentration. We observed no significant additive effects. Lastly, we selected
and further investigated two single compounds—eugenol and eugenyl acetate—found
in high concentrations in cinnamon and clove oil. We measured the CPTs conferred by
these two compounds at different concentrations. Our results suggest that eugenol is the
major repellent compound in clove and cinnamon oil, while eugenyl acetate provided no
significant protection from mosquito bites.
The findings from this study provide useful information for the future development
and formulation of mosquito repellents derived from plant-based oils.
2. Materials and Methods
2.1. Mosquito Rearing
Two to three-week-old adult female Ae. aegypti mosquitoes from the strain UGAL
(University of Georgia Laboratory) were used exclusively for all experiments. The UGAL
strain was received from Alexander Raikhel’s laboratory at the University of California
Riverside. Approximately 600 mosquito eggs were hatched in 32 × 42 × 6 cm3 pans filled
with 2.5 L of DI water. The larvae were fed cat food pellets (Special Kitty, Walmart Stores
Inc., Bentonville, AR, USA) ad libitum, and the water was changed as needed. The larvae
were stored in an incubator set to 27.8 ◦C and 74% relative humidity. Pupae were transferred

to a 200 mL dish filled with water that was stored in a 30 × 30 × 30 cm3 BugDorm-1 Insect-
Rearing cage (Bug Dorm Company, Taichung, Taiwan). Adult mosquitoes were allowed to

feed ad libitum on a 20% sucrose solution delivered in a 100 mL Erlenmeyer flask with a
sucrose-solution-saturated cotton wick at the top. Sucrose solution flasks were replaced
on a weekly basis. Adult mosquitos in rearing cages were stored in an insectary room
set at a temperature of 27 ◦C, relative humidity of 80%, and light/dark cycle of 14/10 h,
respectively. The mosquitoes used in this study were not blood-fed at any time.
2.2. Sample Preparation
Samples were prepared by diluting individual plant-based oils (Table 1) in an organic
lotion base (chemistrystore.com, accessed on 8 August 2023). All samples were made to a
total volume of 5 mL in 15 mL centrifuge tubes. The dilution series treatments were made
v/w, plant-based oil to organic lotion base. The organic lotion base was measured in grams
to provide the most accurate and consistent measurement. Individual plant-based oils were
measured by volume and added to the organic lotion base. Each treatment was well mixed
using a vortex for 60 s immediately before testing.
2.3. Re-Analysis of Gas Chromatography/Mass Spectrometry (GC–MS) Data
The GC–MS data reported in a previous study were re-analyzed. Kovats retention

indices were calculated according to the method described by Luker et al. (2023) [36]. Com-
pound identifications were putatively assigned by spectral matching to the GC/MS library

using an 80% similarity cutoff and a retention index (RI) tolerance of 50. Following this, a
quantitative analysis of identified compounds was performed by peak area normalization.

Insects 2025, 16, 51 4 of 14

The area of all identified peaks was assessed, and the quantity of each individual compo-
nent in each oil was calculated and reported as percentage concentration. Every identified

component was then sorted in terms of its compound class. The following classes were
detected in our analysis: monoterpene hydrocarbon (MH), oxygenated monoterpene (OM),
sesquiterpene hydrocarbon (SH), oxygenated sesquiterpene (OS), oxygenated diterpene
(OD), ester (E), ketone (K), aldehyde (A), and other compound (OC). The re-analyzed
GC–MS data can be found in Supplemental File S1.
Table 1. Oils and compounds used in this study.

Common Name Millipore Sigma Number CAS Number
Cinnamon oil (Ceylon Type)
(Cinnamomum verum (Presl, 1823)) W229202 8015-91-6

Clove oil

(Syzygium aromaticum (L.) Merr, L.M. Perry, 1939) C8392 8000-34-8
Eugenol W246719 97530
Eugenyl Acetate (Chavibetol acetate) W246905 93-28-7
Geraniol 163333 106-24-1
2-Phenylethyl propionate (2PEP) W286702 122-70-3

2.4. Composition Analysis
Graphical representations of compound quantity and compound type were generated
in Excel using the scatter plot function overlaid with an exponential trend line and the
stacked bar plot charts, respectively (Figure 1b,c).

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Figure 1. Plant-based oils with long CPTs are composed of few compounds/compound types. (a)
Rank chart of CPTs of plant-based oils. Shown are plant-based oils ranked from lowest to highest
based on mean complete protection time determined by Luker and coworkers [25]. Plant-based oils
were ordered by CPT from highest to lowest. (b) Diagram of compound quantities plotted against
CPT. Shown are the number of compounds identified in each plant-based oil with its corresponding
CPT in minutes over laid with a polynomial trend line R2 = 0.54. (c) The relative distribution of

compound types in plant-based oils. Shown are stacked bar graphs of percentages of different com-
pound types identified in each oil.

22–27 °C and an approximate humidity of 40% was maintained throughout all experi-
ments. The number of trials for each treatment was distributed evenly between male and

female volunteers, with at least two males and two females per treatment. An elbow-
length polyethylene glove (RoyalPaper.com, Royal, Dayton, OH, USA) was prepared by

cutting out an 8.5 × 10 cm2 area in the forearm region of the glove. This cut-out region was
the only part of the volunteer’s skin the mosquitoes had direct contact with during this
assay. The volunteer used the scentless “Free & Clean” hand soap (seventh generation®)
to wash their forearm and hand. The area was well-rinsed with water, then sanitized using
70% ethanol, and thoroughly dried with a clean paper towel. The volunteer then inserted
their arm into the pre-cut glove. Two layers of cloth medical tape were used to adhere the
Figure 1. Plant-based oils with long CPTs are composed of few compounds/compound types.
(a) Rank chart of CPTs of plant-based oils. Shown are plant-based oils ranked from lowest to highest
based on mean complete protection time determined by Luker and coworkers [25]. Plant-based oils
were ordered by CPT from highest to lowest. (b) Diagram of compound quantities plotted against
CPT. Shown are the number of compounds identified in each plant-based oil with its corresponding
CPT in minutes over laid with a polynomial trend line R2 = 0.54. (c) The relative distribution of
compound types in plant-based oils. Shown are stacked bar graphs of percentages of different
compound types identified in each oil.

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2.5. Arm-in-Cage Assay
The arm-in-cage assay was conducted following a modified version of the published
EPA product performance guidelines for insect repellents [37,38]. Mosquitoes between
2 and 3 weeks old were sugar-starved for two to four hours prior to experimentation. Then,
25 female mosquitoes were transferred to a 30 × 30 × 30 cm3 Bug Dorm insect-rearing cage
that was modified with a clear plexiglass side for observation. An ambient temperature of
22–27 ◦C and an approximate humidity of 40% was maintained throughout all experiments.
The number of trials for each treatment was distributed evenly between male and female
volunteers, with at least two males and two females per treatment. An elbow-length
polyethylene glove (RoyalPaper.com, Royal, Dayton, OH, USA) was prepared by cutting
out an 8.5 × 10 cm2 area in the forearm region of the glove. This cut-out region was the
only part of the volunteer’s skin the mosquitoes had direct contact with during this assay.
The volunteer used the scentless “Free & Clean” hand soap (seventh generation®) to wash
their forearm and hand. The area was well-rinsed with water, then sanitized using 70%
ethanol, and thoroughly dried with a clean paper towel. The volunteer then inserted their
arm into the pre-cut glove. Two layers of cloth medical tape were used to adhere the edges
of the cut-out area to the volunteer’s arm. Adequate biting pressure was established by
inserting the volunteer’s gloved arm into a pre-prepared mosquito-infested Bug Dorm
cage. Upon gloved-arm insertion into the cage, a stopwatch was started. If the volunteer
received a mosquito bite within 60 s, the time was recorded as the control and the cage
was used for treatment experiments. If the above requirement was not met, the cage was
discarded. All bites were visually confirmed as proboscis insertion into the skin while all
four mosquito tarsi had direct skin contact. The volunteer was instructed to immediately
shake off a biting mosquito.
For experiments, 170 μL of a sample (2 μL/cm2

) was applied to the volunteer’s
exposed skin by reverse pipetting the emulsion with a 1 mL pipette with the tip cut off. The
emulsion was spread evenly throughout the treatment area with the same pipette tip. As
soon as the treatment area was fully coated, a stopwatch was started. The volunteer initially
inserted their treated, gloved arm into the mosquito-filled cage for 15 min or until treatment
failure, defined as the first mosquito bite. If a first bite did not occur during this time,
the volunteer removed their arm from the cage and re-inserted it at the 30 min time mark
for 5 min and then in regular 30 min intervals for 5 min at a time until treatment failure.
Treatment failure was measured as complete protection time (CPT), defined as the entire
time from treatment application up to the first bite in minutes. After the first bite, a second
bite (confirmation bite) within 30 min of the first bite was necessary for the first bite to be
valid. If a confirmation bite did not occur within 30 min after the first bite, the volunteer
continued to reinsert their arm in the regular 30 min intervals for 5 min at a time or until
another first bite was recorded and confirmed by a second bite in the same manner as stated
above. We tested clove oil, cinnamon oil, geraniol, 2-phenylethyl propionate, and eugenol
in 1, 3, 4, 5, and 10% concentrations. We only tested eugenyl acetate at a 10% concentration.
We combined clove oil, cinnamon oil, geraniol, and 2-phenylethyl propionate in 1:1 binary
mixtures, not to exceed 10% total EO concentration, and compared them to 5% and 10%
single oil concentrations. All raw data can be found in Supplemental File S2.
2.6. Ethics Declaration
All experiments conducted in this study have been reviewed and approved by the New
Mexico State University Institutional Review Board (IRB). We confirm that we followed
all guidelines mentioned in our current application—(22010) “Insect and Tick-Repellent
Research”, which expires 09/2029. The Environmental Protection Agency’s (EPA’s) protocol
“OPPTS 810.3700. Insect repellents for human skin and outdoor premises.” was used as

Insects 2025, 16, 51 6 of 14
a guide to perform our arm-in-cage assay [39]. All volunteers were given and signed an
informed consent form. Vulnerable persons (i.e., minors, pregnant and nursing women,
prisoners, immune-compromised individuals, and people with severe reactions to mosquito
bites) were excluded from this study [40]. Participants were advised to avoid alcohol,
tobacco, and any scented products at least 12 h prior to this study. Four to five volunteers
with a balanced ratio of males and females were recruited and performed the assay for
each control and experimental group. Volunteers were between 21 and 54 years old. The
recruitment of volunteers is addressed in our ethics declaration.
2.7. Statistical Analyses
Complete protection times (CPTs) in minutes were analyzed using GraphPad
Prism10.0.0 [41]. For the dilution series, the data were entered using the XY table format.
For all other analyses, a one-way ANOVA followed by Dunnett’s multiple comparisons
test were used and followed by the compact letter display function [42,43].
3. Results
3.1. GC–MS Analysis of Plant-Based Oils
3.1.1. Chemical Composition of Oils
We re-analyzed a GC–MS-derived dataset from our previous study on plant-based
oils and individual compounds from the EPA 25(B) list [36]. We identified a total of
72 compounds across 13 different plant-based oils/compounds (Supplemental File S1).
We found that many of the analyzed oils had complex constituent profiles. Four of the
oils had relatively few constituents identified. After a cut off threshold of 1% was applied
to the relative percent concentration, three constituents were identified in clove oil—87.49%
eugenol, 10.82% eugenyl acetate, and 1.68% aromadendrene. In cinnamon oil, we identified
a total of three compounds—95.63% eugenol, 2.26% benzyl benzoate, and 2.12% eugenyl
acetate. Geraniol, a monoterpene, and phenylethyl propionate (2-PEP), an ester, were the
pure compounds used in this study.
3.1.2. Repellency Profiling and Chemical Class Composition of Oils

Plant-based oils from the EPA 25(B) list were previously tested by Luker and col-
laborators at a 10% (v/v) concentration in an organic lotion base using the arm-in-cage

assay [36]. DEET protected from mosquito bites for 6 h, at which point the experiment
was ended. The mean CPTs ranged from 1.73 min for sesame oil to 111.45 min for clove
oil (Figure 1a). Figure 1b shows the CPTs of individual oils plotted against the number
of individual compounds we identified in these oils. The four oils with the highest CPTs,
clove, cinnamon, geraniol, and 2-PEP, are composed of relatively few compounds. The
compound classes per plant-based oil showed a similar trend (Figure 1a).
3.2. Repellency Efficacy of Plant-Based Oil Dilutions

We chose two oils and two pure compounds to further test because they were previ-
ously shown to confer CPTs of over 60 min from mosquito bites. We measured the CPTs

of these oils at lower concentrations up to 10% (Figure 2). A 0% concentration refers to
the solvent, organic lotion base alone. A non-linear regression model was fitted to the
dilution series data for each tested plant-based oil. Our model shows a low CPT prior to a
steep increase around 4% concentrations and a plateau to 10% concentrations for all four
plant-based oils tested.

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Figure 2. The relationship between plant-based oil concentration and CPT. (a–e) Complete protec-
tion times were measured using the arm-in-cage assay. (a–d) A one-way ANOVA followed by Dun-
nett’s multiple comparison test were used to calculate the statistical significance (p < 0.05) between

the mean and standard deviation of each group. Experimental groups that share the same letter
indicate no statistical difference, while groups that do not share the same letter indicate a statistically
significant difference (p < 0.05) between the groups. (a) Clove oil dilution series. (b) Cinnamon oil

dilution series. (c) 2PEP dilution series. (d) Geraniol dilution series. (e) Individual data points over-
laid with a four-parameter sigmoidal curve model of the relationship between the concentration of

plant-based oils to CPT. EC50 values are as follows: clove = 4.3%, cinnamon = 4.4%, geraniol = 5.0%,
and 2PEP = 3.4%. The Hillslope values are as follows: clove = 6.2, cinnamon = 6.9, geraniol= 13.6, and
2PEP = 15.03. The goodness of fit was determined based on corresponding R2 values: clove R2 = 0.92,
cinnamon R2 = 0.93, geraniol R2 = 0.96, and 2-phenylethyl propionate (2PEP) R2 = 0.80.
3.3. Mosquito Repellent Efficacy of Different Mixtures of Oils
Next, we tested the repellent efficacy of different mixtures and concentrations of

clove and cinnamon oil, geraniol, and 2PEP. All combinations we tested produced signif-
icantly greater CPTs than the organic lotion base control (Figure 3). Clove oil combined

with geraniol had the longest CPT out of all the combinations. This combination had a
significantly longer CPT than 5% geraniol but not 5% clove oil alone. Furthermore, 5%
Figure 2. The relationship between plant-based oil concentration and CPT. (a–e) Complete protection
times were measured using the arm-in-cage assay. (a–d) A one-way ANOVA followed by Dunnett’s
multiple comparison test were used to calculate the statistical significance (p < 0.05) between the mean
and standard deviation of each group. Experimental groups that share the same letter indicate no
statistical difference, while groups that do not share the same letter indicate a statistically significant
difference (p < 0.05) between the groups. (a) Clove oil dilution series. (b) Cinnamon oil dilution series.

(c) 2PEP dilution series. (d) Geraniol dilution series. (e) Individual data points overlaid with a four-
parameter sigmoidal curve model of the relationship between the concentration of plant-based oils to

CPT. EC50 values are as follows: clove = 4.3%, cinnamon = 4.4%, geraniol = 5.0%, and 2PEP = 3.4%.
The Hillslope values are as follows: clove = 6.2, cinnamon = 6.9, geraniol= 13.6, and 2PEP = 15.03.
The goodness of fit was determined based on corresponding R2 values: clove R2 = 0.92, cinnamon
R
2 = 0.93, geraniol R2 = 0.96, and 2-phenylethyl propionate (2PEP) R2 = 0.80.
3.3. Mosquito Repellent Efficacy of Different Mixtures of Oils
Next, we tested the repellent efficacy of different mixtures and concentrations of clove
and cinnamon oil, geraniol, and 2PEP. All combinations we tested produced significantly

greater CPTs than the organic lotion base control (Figure 3). Clove oil combined with geran-
iol had the longest CPT out of all the combinations. This combination had a significantly

longer CPT than 5% geraniol but not 5% clove oil alone. Furthermore, 5% 2PEP combined
with 5% cinnamon significantly increased the CPT when compared with 5% 2PEP alone.
These combinations were not significantly different than 5% clove oil or 5% cinnamon oil.

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2PEP combined with 5% cinnamon significantly increased the CPT when compared with
5% 2PEP alone. These combinations were not significantly different than 5% clove oil or
5% cinnamon oil.

Figure 3. CPTs of different combinations of plant-based oils. CPTs were measured using an arm-in-
cage assay. A one-way ANOVA followed by Dunnett’s multiple comparison test were used to cal-
culate the statistical significance (p < 0.05) between the mean and standard deviation of each group.

Experimental groups that share the same letter indicate no statistical difference, while groups that
do not share the same letter indicate a statistically significant difference (p < 0.05) between the
groups.
3.4. Mosquito Repellency Assessment of Major Clove and Cinnamon Oil Constituents
The major components in clove oil are eugenol, an oxygenated monoterpene, and
eugenyl acetate, an ester (see Supplemental File S1). We tested a 10% eugenyl acetate
emulsion and eugenol at various concentrations in the arm-in-cage assay (see Figure 4).
Eugenyl acetate did not provide a CPT significantly different from the organic lotion base
control. A 10% eugenol formulation provided a CPT statistically similar to clove oil but
not cinnamon oil.

Figure 3. CPTs of different combinations of plant-based oils. CPTs were measured using an arm-
in-cage assay. A one-way ANOVA followed by Dunnett’s multiple comparison test were used to

calculate the statistical significance (p < 0.05) between the mean and standard deviation of each group.
Experimental groups that share the same letter indicate no statistical difference, while groups that do
not share the same letter indicate a statistically significant difference (p < 0.05) between the groups.
3.4. Mosquito Repellency Assessment of Major Clove and Cinnamon Oil Constituents
The major components in clove oil are eugenol, an oxygenated monoterpene, and
eugenyl acetate, an ester (see Supplemental File S1). We tested a 10% eugenyl acetate
emulsion and eugenol at various concentrations in the arm-in-cage assay (see Figure 4).
Eugenyl acetate did not provide a CPT significantly different from the organic lotion base
control. A 10% eugenol formulation provided a CPT statistically similar to clove oil but not
cinnamon oil.

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Figure 4. Repellent efficacy of the compounds eugenol and eugenyl acetate. CPT was measured

using the arm-in-cage assay. (a) Eugenyl acetate and a dilution series of eugenol were tested. A one-
way ANOVA followed by Dunnett’s multiple comparison test were used to calculate the statistical

significance (p < 0.05) between the mean and standard deviation of each group. Experimental groups
that share the same letter indicate no statistical difference, while groups that do not share the same
letter indicate a statistically significant difference (p < 0.05) between the groups. (b) Eugenol curve
model fit. Individual data points (red triangles) are overlaid with a four-parameter sigmoidal curve
model fitted to the data for a dilution series of eugenol (red line). EC50 = 6.2. Hillslope = 2.2. The
goodness of fit was determined based on corresponding R2 = 0.65.
4. Discussion
Effective mosquito repellents are heavily relied on for the protection of consumers

from mosquito bites and mosquito-borne diseases [44]. There is growing interest in plant-
based “natural” repellents as opposed to “chemical”, synthetic products with active in-
gredients like DEET, picaridin, or IR3535. While consumer interest is growing in plant-
based oils as mosquito repellents, the current gold standard for protection from mosquito

bites continues to be DEET. The repellent efficacy of novel products that rely on the repel-
lency of plant-based essential oils or other ingredients should be scientifically evaluated

to inform their labels. An aim of this study was to provide scientific data to support the
development and formulation of novel mosquito repellents from “natural” ingredients.
Our first goal was to identify lead compounds that contribute to the repellency of

these plant-based oils [36]. The chemical composition of different batches can differ sig-
nificantly, even when these oils are isolated from the same plants [45,46]. The variables

responsible for this diversity are the method of distillation, the plant parts used for extrac-
tion, plant strains, season and climate during the growth season, and the geographic re-
gion that the plants are sourced from [47–52]. For example, the cinnamon oil we sourced

from Sigma Aldrich (St. Louis, MO, USA.) (Table 1) contained a relatively high
Figure 4. Repellent efficacy of the compounds eugenol and eugenyl acetate. CPT was measured

using the arm-in-cage assay. (a) Eugenyl acetate and a dilution series of eugenol were tested. A one-
way ANOVA followed by Dunnett’s multiple comparison test were used to calculate the statistical

Insects 2025, 16, 51 9 of 14

4. Discussion
Effective mosquito repellents are heavily relied on for the protection of consumers from
mosquito bites and mosquito-borne diseases [44]. There is growing interest in plant-based
“natural” repellents as opposed to “chemical”, synthetic products with active ingredients
like DEET, picaridin, or IR3535. While consumer interest is growing in plant-based oils as
mosquito repellents, the current gold standard for protection from mosquito bites continues

to be DEET. The repellent efficacy of novel products that rely on the repellency of plant-
based essential oils or other ingredients should be scientifically evaluated to inform their

labels. An aim of this study was to provide scientific data to support the development and
formulation of novel mosquito repellents from “natural” ingredients.
Our first goal was to identify lead compounds that contribute to the repellency of these
plant-based oils [36]. The chemical composition of different batches can differ significantly,
even when these oils are isolated from the same plants [45,46]. The variables responsible
for this diversity are the method of distillation, the plant parts used for extraction, plant
strains, season and climate during the growth season, and the geographic region that the
plants are sourced from [47–52]. For example, the cinnamon oil we sourced from Sigma
Aldrich (St. Louis, MO, USA.) (Table 1) contained a relatively high concentration of eugenol,
indicating it is likely derived from the leaves and not the bark, which typically contains a
high amount of cinnamaldehyde [53,54]. On the other hand, the clove oil we also sourced
from Sigma Aldrich contained characteristic compounds such as eugenol and eugenyl
acetate in similar quantities found in clove oil from other sources and lacked the expected
abundance of caryophyllene [55,56]. We hypothesize that variations in specific compound
concentrations can change the efficacy of any given essential oil as a mosquito repellent.
This might explain the large number of conflicting reports on the effectiveness of citronella
essential oil as a mosquito repellent [14,57,58].
When we re-analyzed the composition of plant-based essential oils (see Supplemental

File S1), we found that most of the analyzed oils were complex mixtures of various com-
pounds. Interestingly, the oils that provided the longest protection from mosquito bites

were the least complex amongst the oils (see Figure 1). Notably, clove oil and cinnamon
oil only consist of three compounds while geraniol and 2-PEP are pure compounds. Our
results suggest that the repellency of these plant-derived essential oils is due to active
components that are in low abundance.
Plant-based essential oils are the most expensive ingredient of essential oil-based
mosquito repellents [59,60]. Commercial repellents therefore often contain relatively low
concentrations of these oils, typically between 1 and 10%. In addition, some essential oils
can be skin irritants at high concentrations [61]. There is very little scientific literature
that has systematically investigated the impacts of low concentration of 25b essential oils
and their repellent efficacy. One study from 1999 conducted by Barnard investigated the
repellent efficacy of oils from the EPA 25(B) list in a 5–100% concentration range using
ethanol as a carrier [35]. He reported that clove oil at concentrations of 5 and 10% provided
no protection from mosquito bites and that the higher the concentration, the longer the
complete protection, with 25% protecting for 90 min and 100% protecting for 225 min.
We hypothesize that the carrier used with plant-based oils is critical for their efficacy
as mosquito repellents. We found in the current and in other studies that clove oil in an
organic lotion base can confer protection from mosquito bites for over an hour at 10%
concentrations. The results from our dilution series experiments (Figure 2) in which we
used an organic lotion base as a carrier support this hypothesis because the CPTs were
significantly longer than those found by Barnard and coworkers when using ethanol
as a carrier [35].

Insects 2025, 16, 51 10 of 14
Our findings suggest that the concentrations of individual plant-based oils are critical
for the length of the CPT. We observed sigmoid curves when we plotted the CPTs of clove
oil, cinnamon oil, 2-PEP, and geraniol against their concentrations in the repellent mixtures.
We found that the tested oils had a sharp increase in CPT once a minimum effective dose
(MED) was reached. For example, when testing dilutions of geraniol, we observed a 16-fold
increase in CPT between 4% and 5% concentrations. We also found some evidence to
suggest that these oils have an optimal concentration that confers the longest protection
from mosquito bites. This was seen with clove oil, where the 10% concentration protected
for the same length of time as the 5% concentration. This suggests that 5% clove oil in an
organic lotion base is the optimal concentration to confer the longest CPT. The research
conducted by Deng and coworkers in 2023 supports our findings. When they tested clove
oil at concentrations higher than 10%, they found no increase in CPT [62]. We predict
that once this concentration is reached, higher concentrations will not confer longer CPTs.
However, it is important to note that the use of other carriers will likely result in different
optimal concentrations.
Many commercially available essential oil-based mosquito repellents contain mixtures
of two or more different essential oils. While the rationale for mixing different essential
oils in these repellents is not explained on any label we analyzed, it is likely that some
sort of positive combination effect is expected. There is abundant literature supporting
additive and synergistic effects after combining different plant-based oils for a variety of
applications including insect repellency [63–66]. Our experiments using binary mixtures of
essential oils/compounds were designed as a preliminary screen for potential additive and
antagonistic effects. Surprisingly, we found neither effect (Figure 3). Our combinations of
oils did not provide any significant increases in CPT when compared with their constituent
individual oils at a 5% concentration.
Lastly, we tested the hypothesis that the repellent activity of clove and cinnamon oil
is due to a specific constituent shared by both oils. The two major constituents of clove
and cinnamon oil are eugenol and eugenyl acetate. Eugenol is a phenolic oxygenated
monoterpene with well-known antibacterial, acaricidal and insecticidal activities [67–69].
Interestingly, when testing eugenol in a dilution series, we did not see a sharp increase in
CPT between specific consecutive concentrations as we did in clove, cinnamon, 2PEP, and
geraniol. Instead, we saw a very linear increase in CPT as eugenol concentration increased
(Figure 4). We also showed that 10% cinnamon oil conferred significantly longer CPTs
than 10% eugenol. To expand on this finding, we tested the hypothesis that the compound
eugenyl acetate has repellent properties. However, 10% eugenyl acetate did not show
any bioactivity in our tests. We predict that this compound may work in synergy with
eugenol to confer the CPT of cinnamon and clove oil. More focused studies are necessary to
answer this question. The results of our study suggest that the repellent activity of specific
essential oils is caused by only a single or a small number of active compounds in these
oils. When mixed, different compounds did not show additive or synergistic effects when
tested as repellents. Therefore, mixing different essential oils might not be a useful strategy
when creating new formulations of plant-based mosquito repellents. In order to better
understand these findings, we suggest a controlled study of individual active ingredients,
addressing their evaporation profiles in a repellent mixture as well as their skin absorption
and release patterns when applied to human skin. Determining the optimal concentration
for individual essential oils in specific carriers will be valuable for the formulation of novel
essential oil-based repellents.

Insects 2025, 16, 51 11 of 14

5. Conclusions
Our study highlights the importance of optimizing concentrations and combinations
of plant-based oils and their carriers when formulating novel mosquito repellents that are
based on active ingredients from the EPA 25(b) list. The fact that the chemical composition
of plant EOs can vary significantly depending on plant parts, season, geographical location,
and method of extraction must be considered. Our results suggest that the repellent efficacy
of plant-based oils is more likely due to a small number of active compounds rather than to
additive or synergistic effects in complex mixtures.
Supplementary Materials: The following supporting information can be downloaded at https://www.
mdpi.com/article/10.3390/insects16010051/s1, File S1: GCMS profiling data; File S2: Arm-in-cage
raw data.

Author Contributions: Conceptualization, A.D.L., H.A.L. and I.A.H.; methodology, A.D.L.; valida-
tion, A.D.L.; formal analysis, S.W. and A.D.L.; data curation, A.D.L., C.J.G. and F.O.H.; original draft

preparation, A.D.L. and S.W.; writing—review and editing; A.D.L., H.A.L., F.O.H. and I.A.H. All
authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by NIH grant number R35GM144049.
Institutional Review Board Statement: All experiments conducted in this study have been reviewed
and approved by the New Mexico State University Institutional Review Board (IRB). We confirm that
we followed all guidelines mentioned in our current application—(22010) “Insect and Tick-Repellent
Research”, which expires 09/2029. The Environmental Protection Agency’s (EPA’s) protocol “OPPTS
810.3700. Insect repellents for human skin and outdoor premises.” was used as a guide to perform
our arm-in-cage assay [28,39].
Informed Consent Statement: All volunteers were given and signed an informed consent form.
Vulnerable persons (i.e., minors, pregnant and nursing women, prisoners, immune-compromised
individuals, and people with severe reactions to mosquito bites) were excluded from this study [40].
Participants were advised to avoid alcohol, tobacco, and any scented products at least 12 h prior to
this study. Four to five volunteers with a balanced ratio of males and females were recruited and
performed the assay for each control and experimental group. Volunteers were between 21 and
54 years old. The recruitment of volunteers is addressed in our ethics declaration.
Data Availability Statement: All data generated or analyzed during this study are included in this
publication and its Supplemental Files.
Acknowledgments: We acknowledge our laboratory technicians Maria Chacon, Ilse Tolentino,
Brittney Frietze, and Carolyn Armendariz for rearing the mosquitoes used in this study.
Conflicts of Interest: The authors declare no conflict of interest.

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The Science of Cinnamon: Repellency of Essential Oils and Plant-Derived Compounds