ORIGINAL RESEARCH ARTICLE
Oral toxicity of essential oils and organic
acids fed to honey bees (Apis mellifera).
Timothy A Ebert1*, Peter G Kevan2, Bert L Bishop3, Sherrene D Kevan2, and Roger A Downer1
1Laboratory for Pest Control Application Technology, Ohio Agricultural Research and Development Center, The Ohio State University,
1680 Madison Ave. Wooster, OH 44691, USA.
2Enviroquest Ltd., 352 River Road, Cambridge, ON N3C 2B7, Canada.
3Computing and Statistical Services, Ohio Agricultural Research and Development Center, The Ohio State University, 1680 Madison
Ave. Wooster, OH 44691, USA.
Received 27 March 2007, accepted subject to revision 8 June 2007, accepted for publication 17 June 2007.
*Corresponding author. Email: firstname.lastname@example.org
Natural plant products have been studied for potential use as in-hive fumigants for suppression of parasitic mites and other pests.
A more direct application through direct feeding of bees would avoid problems with fumigant volatility in cold climates and provide
a more systemic route of exposure for the target pest.However, there must be a balance between toxicity to hive pests and toxicity
(safety) to the bees. We focused on adult bee toxicity when testing ten products: cineole, clove oil, formic acid,marjoram oil,
menthol, oregano oil, oxalic acid, sage oil, thymol, and wintergreen. Each product was tested at several concentrations in a sugar
syrup fed to bees over several days, and dead bees were counted daily. Oxalic acid was the most toxic of the products tested.
Menthol and cineole had mortality levels no different from controls fed plain syrup after 8 days of treatment. At 14 days of
treatment, wintergreen was the least toxic, but neither menthol nor cineole were a part of the testing that went to 14 days. Our
results indicate that the tested products could all be used safely for treating bees orally if dose is carefully managed in the hive.
Toxicidad oral de aceites esenciales y ácidos orgánicos en la
alimentación de la abeja de la miel (Apis mellifera)
Los productos naturales de plantas han sido estudiados para su uso potencial como agentes fumigantes de represión de ácaros
parásitos y otras plagas. Una aplicación más directa a través de la alimentación de las abejas evitaría problemas como la volatilidad de
los fumigantes en climas fríos y proporcionaría una vía más sistémica de exposición para las plagas. Sin embargo, debe haber un
equilibrio entre la toxicidad para las plagas y la toxicidad (seguridad) para las abejas. Nosotros nos hemos centrado en la toxicidad
sobre abejas adultas de diez productos: eucaliptol, aceite de clavo, ácido fórmico, aceite de mejorana, mentol, aceite de orégano,
ácido oxálico, aceite de salvia, timol y aceite esencial de wintergreen (salicilato de metilo).Cada producto fue probado con
diferentes concentraciones en un jarabe de glucosa que alimentó a las abejas durante varios días, las abejas muertas fueron contadas
diariamente. El ácido oxálico fue el producto más tóxico de todos los analizados. El mentol y el eucaliptol presentaron niveles de
mortalidad similares a los controles, que fueron alimentados únicamente con jarabe después de 8 días de tratamiento. Tras 14 días
de tratamiento, el aceite esencial wintergreen fue el menos tóxico, pero ni el mentol ni el eucaliptol se incluyeron en el análisis a los
14 días. Nuestros resultados indican que todos los productos testados pueden ser utilizados con seguridad por vía oral para el
tratamiento de las abejas si la dosis es administrada cuidadosamente en la colmena.
Keywords: Medicaments, oral toxicity, natural plant products, mortality, miticides, protectants
Journal of Apicultural Research and Bee World 46(4):220–224 (2007) © IBRA 2007
Oral toxicity of essential oils and organic acids fed to honey bees 221
Various essential oils and organic acids have been evaluated as
materials to manage mite populations afflicting honey bees. In
general, these products are proposed to be used as in-hive
fumigants (Imdorf et al. 1999) or as contact treatments (Amrine
et al. 1996). In-hive fumigants have the disadvantage of needing
warmth for sublimation or evaporation, and therefore they are
less effective in colder weather (Scott-Dupree & Otis 1992).
Nevertheless, organic acids like formic acid, and essential oils like
thymol, have been found to be effective in management against
mite pests in honey bee hives (Imdorf et al. 1999). Oral
application circumvents the problems of fumigation in cold
weather, but this application strategy has been mostly neglected.
Menthol can be administered orally to honey bees, in
microencapsulated formulation, with beneficial effects in
suppressing population growth of tracheal mites (Acarapis woodi)
(Kevan et al. 1997;2003). If essential oils and organic acids are to
be considered as potential medicaments, rather than fumigants,
one must be sure that the target animals (e.g. honey bees) are
not poisoned. As with the research on menthol, we investigated
oral toxicity of various essential oils and organic acids with the
aim of assessing the potential problem of poisoning the patients
with the active ingredient of the medicine (Kevan et al. 1999). As
a control, we also used a plant compound, amygdalin, known to
be poisonous to honey bees (Kevan & Ebert 2005), and
reassessed menthol as a compound known to be innocuous to
honey bees (Kevan et al. 1999). Menthol provides a positive
control that is useful in evaluating the toxicity of other
This research was on the toxicity of various natural plant
compounds to honey bees with the intent to use these
compounds to treat hives for various hive pest problems. It is
likely that such treatments will involve exposing the colony to the
compound for weeks or months. It is therefore necessary to
assess how both the dose and the length of exposure influence
Materials and Methods
Honey bees (Apis mellifera ligustica) were obtained from hives at
the Ohio Agricultural Research and Development Center
(OARDC) Honey bee Lab, and placed in cages similar to those
used by Kulencevic & Rothenbuhler (1973), and previously
described in Kevan & Ebert (2005). Each cage had an average of
48 bees (+/-15 S. D.; range 20–134). Fed bees were given sugar
syrup (69% sucrose), or a 69% sucrose syrup spiked with one of
10 possible natural plant products: cineole (CAS 470-82-6), clove
oil, formic acid (CAS 64-18-6), marjoram, L-menthol (CAS 2216-
51-5), DL-menthol (CAS 1490-04-6), oregano oil, oxalic acid
(CAS 144-62-7), sage oil, thymol (CAS 89-83-8), or natural
wintergreen oil (CAS 119-36-8). The clove oil was a commercial
extract from Eugenia caryophyllata Thunb. (Myrtaceae). The main
chemical components of clove oil are eugenol, eugenol acetate,
iso-eugenol and caryophyllene
oregano oil was a commercial extract from Origanum vulgare L.
(Lamiaceae). The main chemical components are carvacrol,
p-cymene, y-terpinene, and b-caryophyllene (Chorianopoulos et
al. 2004). The marjoram oil was a commercial extract from
Origanum marjorana L. (Lamiaceae). The main chemical
constituents are sabinene, a-terpinene, y-terpinene, p-cymene,
terpinolene, linalool, cis-sabinene hydrate, linalyl acetate, terpinen-
4-ol and y-terpineol (http://www.essentialoils.co.za/essential-
oils/marjoram.htm). The sage oil was a commercial extract from
Salvia sclarea L. (Lamiaceae). The main chemical components of
sage oil are linalool, a-terpenol, linalyl acetate, neryl acetate, and
sclareol (Pitarokill et al. 2002). All solutions, including the control,
had 5ml ethanol added to bring the total syrup volume to 100ml.
Insoluble potential medicaments were dissolved in the ethanol
first, then mixed with the syrup to bring the volume up to 100ml.
In addition to these treatments, we included an unfed control,
with no food or water. The starvation treatment was necessary
because it was rumored that some of these medicaments would
reduce feeding. We needed a starvation treatment to
differentiate between toxicity and death due to starvation or
Bees were fed ad libitum for the duration of the experiments.
No additional water was provided, except what they could get by
feeding on the syrup solution. We will call this days of treatment
(DOT), since the bees are continually exposed for the entire
duration. Each treatment was replicated four times. Mor tality was
checked daily, and dead bees were removed. Bees were
considered dead when they would no longer move in response
to poking with forceps. At the end of the experiment, the live
bees were frozen and then counted. Cages were kept in open
laboratory conditions that ranged from 17 to 25°C with a R.H.
(Relative Humidity) from 18 to 37%.
We present the results from two tests. The first test
evaluated the toxicity of all potential medicaments every day over
an 8 day period at concentrations of 100 and 1000 ppm. The
second test evaluated the toxicity of clove oil, formic acid, oxalic
acid, oregano oil, and sage oil along with a fed and a starved
control over a 14 day period. Potential medicament
concentrations in the second test were at 100, 500, 1000, 5000,
10000, and 100000 ppm. Note; the 100000 ppm solutions for all
potential medicaments tended to separate, or crystals developed
in the solution. It is likely that the bees never experienced a
potential medicament at 100000 ppm, even though we tried
mixing the potential medicament back into the sugar syrup by
shaking up the bottles once per day. Also note that we tested
both D and DL menthol because sometimes a particular isomer
is more toxic than others. However, we could not find any
evidence of such a difference in the toxicity of these products.
Therefore, our discussion of menthol will be the results from the
combined data of the L-menthol and DL-menthol treatments.
Data were analyzed in SAS using a time-dose-mortality
analyses and probit analyses. The time-dose-mortality model was
a complimentary log-log model using a SAS program that was
written as an implementation of the work by Priesler & Robinson
(1989). However, the Hosmer-Lemeshow goodness-of-fit test
(Nowierski et al. 1996) was highly significant for all models. We
suggest that this lack-of-fit was caused by long tails in the data.
When individual time intervals were of special interest, we used
222 Ebert, Kevan,Bishop, Kevan, Downer
Proc Probit to estimate the LD50 values for that day. Although the
Proc Probit corrected for control mortality, the results from the
time-dose-mortality analysis did not correct for control mortality.
For this reason, the time-dose-mortality analysis overestimates
the toxicity of these products.
We have included the results from the amygdalin (known to be
toxic to honey bees) trials for comparison (Kevan & Ebert 2003).
Relative to amygdalin, all of the materials we tested were
innocuous. Although oxalic acid is quite toxic relative to the other
materials (Table 1), Table 2 shows it to be somewhat less than
half as toxic as amygdalin on a molecule per molecule basis.
Starved bees do not live long, with 40% dying in the first 24
H, and all the bees dead within four days (Table 1). No other
treatment had higher first day mortality rates. High
concentrations of oxalic acid were the most toxic treatment, and
differed from the starvation treatment by having relatively low
first day mortality. Mortality was 100% at the highest oxalic acid
concentration, but bees at the lowest concentration only had
about 60% mortality after 14 days (there was 40% mortality in
the controls by this time). The second most toxic was formic acid,
for which the last survivors died eight days post treatment at the
highest concentration tested. However, mor tality was only 30%
after 14 days at the lowest concentration. A few other potential
medicaments also showed high mortality levels at the maximum
concentration tested. In contrast to these products, we conclude
that cineole, menthol, marjoram, and thymol are non-toxic to
bees because their mortality levels never exceeded background
mortality at any concentration tested (Table 2).
All products tested were much less toxic than amygdalin on a
molecular basis. Although the LT50 for oxalic acid and amygdalin
are similar, it takes over twice the number of molecules of oxalic
acid to achieve an equivalent level of toxicity (Table 2). However,
when comparing LD50s at 8 days of treatment (DOT), oxalic acid
appears more toxic (Table 3). The reason the model is non-
significant (α<_0.01) in this case is because mor tality in all
treatments was too high and too variable (average mortality in
lowest dose was 50% +/- 40% S. D.). The reason the model for
sage is also non-significant (α<_0.01) is because, even at the
highest dose, mortality was only 60% +/- 40% S. D.
The critical feature in the potential utility of all these products
is their long term effects on bee health. This was assessed by
keeping bees in the cages as long as possible. Treatment with sage
oil resulted in highly variable mortality, for which we have no
explanation. Oxalic acid, with the lowest LD50 value, was most
toxic and wintergreen was the least (Table 4).
All LD50 levels decline over time. However, most show a rapid
decline in LD50 values within the first few days, followed by a
leveling off. In part this trend reflects ever increasing levels of
background mortality. However, most cages of bees had a few
individuals that lived many days longer than their sisters. This
made the tails of the mortality distribution long, and probably
accounted for most of the significance in the lack-of-fit tests.
Day 1 Number
Potential Average Average % mortality of Bees
8 DOT 14DOT
Medicament % mortality 1000 ppm 100,000 ppm*Tested
Amygdalin 0 79** 221
Cineole 2 11 214
Clove oil 3 28 96 927
Control – fed 1 10 40 1189
Control – unfed 46 100*** 100*** 495
Formic acid 1 33 100 1438
Marjoram oil 2 34 308
Menthol DL 1 25 256
Menthol L 1 21 239
Oregano oil 1 41 92 953
Oxalic acid 6 96 100 1217
Sage oil 3 21 87 1289
Thymol 8 43 201
Wintergreen 1 24 99 1308
Table 1. Twenty four hour mortality and average mortality at 8 days and 14 days exposure (DOT) for various essential oils and
organic acids fed to honey bees, together with the total number of bees tested (sum of all replicates and dosages).
*Cells with missing data are left blank. ** Dosage for amygdalin was 2250 ppm. *** All bees were dead in 4 days.
Oral toxicity of essential oils and organic acids fed to honey bees 223
Table 2. Estimated LT50 for potential medicaments at 1000 ppm. In this analysis, data from tests 1 and 2 were combined to estimate
the LT50. Except as noted, all terms in the models were significant (α<_0.01), and all lack-of-fit tests were also significant (α<_0.01).
Material Molarity*LT 50 Lower 95% Upper 95%
in days Fudicial Fudicial
Amygdalin** 0.0049 4.6 3.0 6.3
Cineole 0.0065 NS
Clove Oil 11.2 10.2 12.8
Controls – Fed 17.0 15.9 18.7
Controls – unfed 1.9 1.7 2.1
Formic Acid 0.0217 11.8 8.1 30.3
Marjoram Oil 27.0 15.0 3935.0
Menthol 0.0064 NS
Oregano Oil 10.8 9.9 12.1
Oxalic Acid 0.0111 4.8 2.6 5.9
Sage Oil 11.5 10.6 12.7
Thymol 0.0067 NS
Wintergreen 0.0029 14.4 12.6 17.7
NS = no significant model. Mortality in these tests did not reach 50%, so accurate estimates cannot be made.
* These plant extracts are blends of several compounds so it is not possible to calculate the molarity of each component.
** Amygdalin concentration was 2,250 ppm, the lowest concentration tested by Kevan and Ebert (2005).
Table 3. Estimated LD50 for the tested essential oils and organic acids at 8 Days exposure (DOT).
Material LD50 Lower 95% Upper 95%
in ppm Fudicial Fudicial
Amygdalin 1600 1300 1800
Clove Oil 7800 2300 18100
Formic Acid 5600 3500 8100
Oregano Oil 14900 0 43400
Oxalic Acid NS
Sage Oil NS
Wintergreen 13500 8500 21700
NS = model non-significant (α<_0.01). Oxalic acid was too toxic and mortality by Sage Oil was too variable.
Table 4. LD50 values 14 days exposure (DOT).
Material LD50 Standard Lower 95% Upper95%
in ppm Error of Fudicial Fudicial
Log10 (dose) Limit Limit
Clove Oil 240 0.1483 80 710
Formic Acid 450 0.0653 280 720
Oregano Oil 600 0.1327 230 1620
Oxalic Acid 80 0.0635 50 130
Wintergreen 800 0.0632 500 1270
*dose was not a significant variable in the time-dose-mortality model.
Looking at mortality in Table 1, we conclude that wintergreen,
menthol, sage oil, and cineole all are relatively innocuous, and that
marjoram oil is quite benign. At the other extreme, oxalic acid
and, as expected, amygdalin were the most toxic, and has the
potential to cause high levels of mortality if the dosage is high.
These results also demonstrate that development of hive
treatment protocols require balancing the exposure time and the
dose given to the hive. Our results are sufficient for dosing bees
for up to 14 days, but we expect that dosage would have to be
more limited if the exposure period is lengthened. If continuous
dosing is the best treatment option, then additional research
would be needed to assess toxicity during the over-wintering
period and the effects these products may have on egg laying
viability of the queen, and development of larvae. However, all of
these products have a low enough toxicity that they could be
used as ingested medicines (rather than fumigants) with minimal
effects on the hive. Clearly, the next phase is to determine if safe
doses of these products can be fed to bees to effectively manage
hive pests, and whether it is better to shock the hive with a short
term massive dose, or to try long term exposures at low dosages.
We would like to caution readers that these results are most
relevant to adult worker health. It is possible that the adult
workers could feed the medicaments to larvae, and that the
larvae may be more sensitive. It is also possible that the
medicaments fed to the queen or drones could affect their
reproductive capacity. However, exposure of these individuals is
buffered through the workers. Unless the only source of food for
the entire hive is the treated sugar water, there will be a dilution
effect where the treated sugar water is mixed in the hive with
nectar from outside sources. A queen that gets one drop of
treated syrup from one worker followed by a droplet of nectar
from outside the hive is effectively consuming a nectar at half the
dosage of the treated syrup. Consequently, workers will be
exposed to greater dosages of these medicaments than will other
members of the hive. Therefore, testing the toxicity of the
medicaments to the workers is a natural first step, and it may be
the only necessary step unless other problems occur as these
products are developed.
We thank SBIR (Small Business Innovation Research) of the
United States Department of Agriculture for support to Robert
A. Stevens and Betterbee Inc. (Greenwich, NY). Alison Skinner
and team of the Ontario Beekeepers’ Association assisted with
bee wrangling and technical instruction. Jim Tew and the Ohio
State University Honey bee Lab supplied the bees for the
research in Wooster. Rebecca Eber t provided additional
laboratory support in Wooster.
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