Efficacy and safety of catnip (Nepeta cataria) as a novel filth fly repellent

Abstract

Catnip (Nepeta cataria) is known for its pseudo-narcotic effects on cats. Recently, it has been reported as an effective mosquito repellent against several Aedes and Culex species, both topically and spatially. Our laboratory bioassays showed that catnip essential oil (at a dosage of 20 mg) resulted in average repellency rates of 96% against stable flies, Stomoxys calcitrans (L.) and 79% against houseflies, Musca domestica (L.), respectively. This finding suggested that the application of repellent could be used as part of filth fly management. Further evaluations of catnip oil toxicity were conducted to provide a broad-spectrum safety profile of catnip oil use as a potential biting and nuisance insect repellent in urban settings. Acute oral, dermal, inhalation, primary dermal and eye irritation toxicity tests were performed. The acute oral LD(50) of catnip oil was found to be 3160 mg/kg body weight (BW) and 2710 mg/kg BW in female and male rats, respectively. The acute dermal LD50 was > 5000 mg/kg BW. The acute inhalation LD50 was observed to be > 10,000 mg/m3. Primary skin irritation tested on New Zealand white rabbits showed that catnip oil is a moderate irritant. Catnip oil was classified as practically non-irritating to the eye. In comparison with other U.S. Environmental Protection Agency-approved mosquito repellents (DEET, picaridin and p-menthane-3,8-diol), catnip oil can be considered as a relatively safe repellent, which may cause minor skin irritation.

Full text

Medical and Veterinary Entomology (2009) 23, 209–216
Efficacy and safety of catnip (
Nepeta cataria
)asa
novel filth fly repellent*
J. J. Z H U1,X.-P.ZENG
2,D.BERKEBILE
1,H.-J.DU
3,Y.TONG
2and
K. Q I A N2
1Agroecosystem Management Research Unit, U.S. Department of Agriculture, Agricultural Research Service, University of
Nebraska, Lincoln, Nebraska, U.S.A, 2Institute of Disinfection and Pest Control, Beijing Centre for Disease Prevention and
Control, Beijing, China and 3Institute of Toxicology, Beijing Centre for Disease Prevention and Control, Beijing, China
Abstract. Catnip (Nepeta cataria) is known for its pseudo-narcotic effects on cats.
Recently, it has been reported as an effective mosquito repellent against several Aedes
and Culex species, both topically and spatially. Our laboratory bioassays showed
that catnip essential oil (at a dosage of 20 mg) resulted in average repellency rates
of 96% against stable flies, Stomoxys calcitrans (L.) and 79% against houseflies,
Musca domestica (L.), respectively. This finding suggested that the application of
repellent could be used as part of filth fly management. Further evaluations of catnip
oil toxicity were conducted to provide a broad-spectrum safety profile of catnip oil
use as a potential biting and nuisance insect repellent in urban settings. Acute oral,
dermal, inhalation, primary dermal and eye irritation toxicity tests were performed.
The acute oral LD50 of catnip oil was found to be 3160 mg/kg body weight (BW) and
2710 mg/kg BW in female and male rats, respectively. The acute dermal LD50 was
>5000 mg/kg BW. The acute inhalation LD50 was observed to be >10 000 mg/m3.
Primary skin irritation tested on New Zealand white rabbits showed that catnip oil is
a moderate irritant. Catnip oil was classified as practically non-irritating to the eye.
In comparison with other U.S. Environmental Protection Agency-approved mosquito
repellents (DEET, picaridin and p-menthane-3,8-diol), catnip oil can be considered as
a relatively safe repellent, which may cause minor skin irritation.
Key words. Musca domestica ,Nepeta cataria ,Stomoxys calcitrans , biting fly indoor
bioassay, catnip, housefly, repellency, stable fly, toxicity evaluation.
Introduction
Flies and mosquitoes constitute two major groups of nuisance
species in rural and urban environments worldwide. Many fly
species are also called filth flies because of their association
with contaminated substances, including food wastes, animal
manure and carrion. The two major urban and livestock pest fly
species in the world are houseflies, Musca domestica (L.), and
stable flies, Stomoxys calcitrans (L.) (also called ‘dog flies’ or
‘beach flies’). These are of considerable importance to people,
pets and the tourist industry. Flies are also capable of becoming
contaminated with more than 100 different pathogens (such as
Salmonella and Campylobacter) that cause enteric infections
Correspondence: Dr J. J. Zhu, USDA-ARS Agroecosystem Management Research Unit, 305 Entomology Hall, University of Nebraska, East
Campus, Lincoln, Nebraska 68583, U.S.A. Tel: + 1-402-472-7525; Fax: + 1-402-437-5712; E-mail: Jerry.Zhu@ars.usda.gov
* This article reports the results of research only. Mention of a proprietary product does not constitute an endorsement or a recommendation for
its use by the U.S. Department of Agriculture.
and lead to the development of diseases in both human and
animals (Bonnefoy et al., 2008).
Use of insect repellents is one of the most efficient ways to
prevent disease transmission by biting insects, particularly by
mosquitoes (Curtis, 1992; Gupta & Rutledge, 1994). In many
circumstances, application of repellents to the skin can be an
effective way to directly reduce vector–host contact, which
minimizes the incidence of disease transmission as well as the
discomfort of insect bites (Fradin, 2001).
Fly control in urban areas is usually achieved by trapping
techniques (baited, sticky traps and ultraviolet light traps),
together with insecticide application technologies. However,
there have been limited studies on the control of flies using
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210 J. J. Zhu et al.
repellents. Although some insecticide-impregnated ear tags
have been observed to be repellent to biting flies (mainly horn
flies), very few have so far been reported for stable flies. Com-
mercially available insect repellents can be divided into two
categories: synthetic chemicals and plant-derived compounds
made from essential oils. The most widely used mosquito repel-
lent is N,N-diethyl-m-toluamide (DEET). A DEET analogue,
N,N-diethylphenylacetamide, has been reported to repel stable
flies in cage studies (Parashar et al., 1993).
Many plant extracts have been identified as having repellent
effects (Coats, 1994; Isman, 2006). Researchers are now turn-
ing to folk remedies for novel chemistries and new repellent
activities. Several reports have demonstrated varying degrees of
mosquito repellency among plant oils, including clover, pepper-
mint, geranium, neem and turmeric, etc. (Sharma et al., 1993;
Kant & Bhatt, 1994; Mafong & Kaplan, 1997; Isman, 2006;
Krajick, 2006;). Catnip (Nepeta cataria) is a herbaceous mint
native to Eurasia and North Africa, which is also found in
most of North America. It has been reported recently that top-
ical application of catnip essential oil can effectively prevent
biting by several disease-transmitting mosquito species, and
there is additional evidence of spatial repellency (Peterson,
2001; Bernier et al., 2005; Trongtokit et al., 2005; Zhu et
al., 2006). Although the effectiveness of catnip oil has been
demonstrated on mosquitoes, the toxicity of this natural prod-
uct repellent is largely unknown. Toxicity evaluations of most
plant-based repellents have focused only on registered commer-
cially available repellent products (U.S. Environmental Pro-
tection Agency [EPA] Biopesticide Registration Documents
011550 and 7505C).
The present paper reports the results of catnip repellency
tests on two fly species (stable flies and houseflies), and the
outcomes of catnip toxicity studies including acute oral, dermal
and inhalation toxicity tests, as well as evaluation tests of
primary skin, eye and dermal sensitization.
Materials and methods
Repellent
Catnip essential oil was purchased from Bramble Berry Inc.
(Bellingham, WA, U.S.A.). The purity of the oil was deter-
mined by gas chromatography-mass spectrometry; it comprised
the major ingredient compounds of (Z,E)-nepetalactone and
(E,Z)-nepetalactone (90%), and caryophyllene (10%) (Schultz
et al., 2004).
Insects
Stable flies and houseflies used for laboratory repellency tests
were sourced from colonies maintained at the U.S. Department
of Agriculture, Agricultural Research Service, Agroecosystem
Management Research Unit (Lincoln, NE, U.S.A.). The flies
were maintained at 23 ±2◦C with variable humidity (30 – 50%
relative humidity [RH]) and a light : dark (LD) photoperiod of
12:12 h. Adult stable flies were fed with citrated bovine blood
(3.7 g sodium citrate/L) by soaking a feminine hygiene pad
(Stayfree®; McNeil-PPC Inc., Skillman, NJ, U.S.A.) in blood
and placing it on top of the cage. Houseflies were fed with a
dry sugar–dry milk mixture made from products obtained from
local grocery stores. Water was supplied ad libitum.
Repellency assays
The laboratory bioassay for testing repellent efficacy on
fly biting and feeding consisted of a six-well feeding reser-
voir system similar to the K&D module (Klun & Debboun,
2000). A cage was set up with either stable fly or housefly
pupae supplied with 10% sugar water. The sugar water was
removed 24–48 h before the repellency test began. On the
day of the test, small squares of the feminine hygiene pad
(3.75 ×4.75 cm) were cut to fit into the wells of the module.
When testing stable flies, the pads were soaked with ∼5mL
of citrated bovine blood (sourced from a local abattoir). The
outer layers of the feminine hygiene pads, which comprised
two layers of 100% cotton flannel and a layer of ultra-thin
nylon, were cut and coated with catnip oil. Catnip oil mea-
sured at three dosages, 0.2 mg, 2 mg and 20 mg, respectively,
was dissolved in 300 μL of hexane (Burdick & Jackson Lab-
oratories, Inc., Muskegon, MI, U.S.A.), then topically applied
onto the outer layer evenly (4 ×5 cm). After air drying, it was
placed on top of the blood-soaked pad. Approximately three to
five flies (stable flies starved for 48 h and houseflies starved
for 24 h) were collected from the fly cages and transferred
into each testing cell. For houseflies, a 10% sugar solution
was used instead of the bovine blood. The solution was dyed
with neutral red (1 g/100 mL; Sigma-Aldrich, St. Louis, MO,
U.S.A.) to aid in the determination of feeding status at the
end of the trial. After 4 h, both stable flies and houseflies
(anaesthetized with CO2)were checked for feeding status by
squashing their abdomen to determine the presence of blood
(stable flies) or red dye (houseflies) after the trials. Repellent
assays were conducted daily at room temperature for ≥4h.
Flies in the repellent bioassay were exposed to randomized
treatments (different dosages), which were repeated until at
least five replicates had been completed. Percentiles of repel-
lency ([number of flies fed on control–number of flies fed on
catnip]/number of flies fed on control ×100) were determined
and transformed to arcsine square-root values for analyses of
variance (ANOVA). Significant differences at P=0.05 (SAS Ve r -
sion 10; SAS Institute, Cary, NC, U.S.A.) were determined by
analyses performed on the least-square means as a result of the
unequal number of observations among the treatments.
Animals
Adult Kunming mice (closed strain, see details in Yue et al.,
2003; 18–21 g), Wistar rats (209 – 332 g) and New Zealand
house rabbits were obtained from Beijing Laboratory Animal
Centre, Institute of Microbiology and Epidemiology, Beijing.
The animals were individually housed in wire cages (20 ×20 ×
22 cm) containing bedding material (low-dust wood shavings)
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Natural product repellent for filth flies 211
and given feed (laboratory animal food provided by the animal
seller) and water ad libitum except during the experiment.
Animal rooms were maintained at a constant temperature
(20–23◦C) and humidity (46 –56% RH) and at LD 12:12 h.
All animal experiments were performed in compliance with
the Good Laboratory Practice (GLP) requirements of the
Organization for Economic Cooperation and Development
(OECD) Guidelines for Testing of Chemicals, and followed
the standard protocols for the treatment of laboratory animals
of the Beijing Centre for Disease Control and Prevention in
accordance with an ethical review.
Acute oral toxicity
In the acute oral toxicity study, a single dose of catnip
oil, diluted in corn oil, was given by oral gavages to mice
of either sex at 1000 mg/kg, 2150 mg/kg, 4640 mg/kg and
10 000 mg/kg body weight (BW). The rats were fasted for
16 h prior to dosing and returned to feeding 3 h after dosing.
The mice were observed daily for 14 days for signs of toxicity
and deaths. No differences in weights were found among the
tested animals before and after the experiments. The analysis
of mortality involved the calculation of the median lethal
concentration (LD50)of catnip oil using the method described
in Zhu et al. (2006).
Acute dermal toxicity
In this study, a single high-limit dose of catnip oil,
5000 mg/kg BW, was applied to the clipped backs (5 ×5cm)
of 10 male and 10 female Wistar rats, respectively. Catnip
oil was injected under an occlusive rubber sleeve which sur-
rounded the clipped trunk of each rat. The sleeve was removed
after 24 h, and the area was gently cleansed of any residual
testing substance with mild warm water. Rats were examined
daily for toxic signs during the 14-day experimental period.
Acute inhalation toxicity
In the acute inhalation study, Kunming mice (10 per sex)
were exposed by the whole-body exposure technique for mice
for 2 h at a concentration of 10 g/m3in a 0.1-m3exposure
chamber equipped with a fan-based ventilation system. During
the first 3 min of exposure, a heating system was activated.
Animals were removed from the exposure chamber and pro-
vided with normal living conditions for 2 weeks to observe for
signs of toxicity caused by the exposure.
Primary skin irritation
A primary skin irritation test was conducted on rabbits to
determine the potential for catnip oil to produce an irrita-
tion after a single topical application. Four healthy female
New Zealand White (NZW) rabbits (2.3–2.5 kg) were treated
dermally with 0.5 mL of undiluted catnip oil by direct appli-
cation to shaved intact skin of the left dorsal and trunk area
(3 ×3 cm). The same area of shaved skin on the right side
of the animal was used as the control (no application). On
the day before application, the test areas were covered with
a semi-occlusive dressing for 4 h. After the exposure period,
the patches were removed and residual test material was wiped
from the exposed skin using gauze moistened with distilled
water. On the day of dosing, prior to application, the animals
were given a health check and their skin condition was exam-
ined for any abnormalities. No pre-existing skin irritation was
observed.
Individual evaluation of test dose sites was scored according
to the Draize Scoring System (Table 1) at approximately 1 h
after the removal of catnip oil (Draize et al., 1944; Draize,
1965) during a 14-day experimental period. The degree of
irritancy was classified according to the descriptive rating for
the mean primary dermal irritation index illustrated by Shara et
al. (2005). The animals were also observed for signs of gross
toxicity and behavioural changes at least once daily during the
test period.
Primary eye irritation
Three healthy female NZW rabbits (2.3–2.4 kg) were treated
with 0.1 mL of catnip oil. The upper and lower lids were
gently held together for about 1 s before releasing to mini-
mize loss of the test catnip oil. The treated (left) eyes of the
animals were rinsed with physiological saline approximately
30 s after instillation of the test material. The non-treated right
eyes were used as controls. The animals were observed at 1 h,
24 h, 48 h, 96 h and 128 h post-instillation for any exhibited
corneal opacity, iritis or conjunctival irritation (Table 2). For
ocular observation, we used sodium fluorescein and ultraviolet
light to detect corneal abnormalities (Kay & Calandra, 1962).
Rabbits assigned to the study had no observable pre-existing
abnormalities.
Tab le 1. Scoring criteria for dermal reactions in the primary skin
irritation study of catnip oil in New Zealand White rabbits.
I Erythema formation Value
No erythema 0
Very slight erythema (barely perceptible with no defined
edges)
1
Slight erythema (pale red in colour with definable edges) 2
Moderate to severe erythema (defined by colour with
well-defined area)
3
Severe erythema (coloured crimson red) 4
II Oedema formation
No oedema 0
Very slight oedema (barely perceptible with no defined edges) 1
Slight oedema (edges of area well defined by definite raising) 2
Moderate oedema (raised approximately 1 mm) 3
Severe oedema (raised >1 mm and extending beyond area of
exposure)
4
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212 J. J. Zhu et al.
Tab le 2. Scoring criteria for ocular irritation in primary eye irritation study of catnip oil in New Zealand White rabbits.
ICornea
(A) Area of cornea involved
No ulceration or opacity 0
>0%, ≤25% 1
>25%, ≤50% 2
>50%, ≤75% 3
>75%, ≤100% 4
Score =A×3Maximum total =12
II Iris
(A) Values
Normal 0
Markedly deepened rugae, congestion, swelling, circumcorneal injection (any or all of these
or combination thereof), iris still reacting to light (sluggish reaction is positive)
1
No reaction to light, haemorrhage, gross destruction (any or all of these) 2
Score =A×3Maximum total =6
III Conjunctivae
(A) Redness (refers to palpebral and bulbar conjunctivae excluding cornea and iris)
Blood vessels normal 0
Some blood vessels definitely hyperaemic (injected above) normal 1
Diffuse, deeper crimson colour, individual vessels not easily discernible 2
Diffuse beefy red 3
Score =A×3Maximum total =9
Sum of all scores obtained Maximum total score possible =27
Results
Feeding repellency
The repellency of catnip oil at three concentrations was
tested on stable flies and houseflies using the modified K&D
module in the laboratory. Results are shown in Fig. 1. At
20 mg and 2 mg, catnip oil demonstrated significant repellent
activity against the feeding of both houseflies and stable flies,
in comparison with that of control (in which feeding rates
of 96–100% were observed). The strongest repellency was
observed at 20 mg for stable flies. However, houseflies treated
with 2 mg catnip oil were repelled from feeding as effectively
as those treated with 20 mg. When the dosage was decreased to
0.2 mg, no significant repellency was found for either species.
No differences in repellency were found between the two fly
species when tested under the same dosages (Student’s t-test,
t=2.36–2.77, P>0.05).
Acute oral toxicity
Catnip oil, at doses of 1000–2150 mg/kg BW, did not
cause mortality and did not induce any signs of toxicity in
100
90
80
70
60
50
40
% of Repellency
30
20
10
020 mg 2 mg 0.2 mg
aa
A
B
b
House flies
Stable flies
C
Fig. 1. Percentiles of feeding observed
in starved flies treated with three different
dosages of catnip oil in a modified K&D mod-
ule. Means with different letters are signifi-
cantly different at P<0.05 (SAS Ver s i o n 9 . 1 ,
performed on the least-square means).
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Natural product repellent for filth flies 213
the treated male and female mice following dosing (during
the observation period of 14 days thereafter), except for the
death of one male mouse. No gross pathological alterations
were evident at terminal necropsy in any of the tested rats.
Dose levels >4640 mg/kg BW caused 100% mortality. The
primary clinical signs of toxicity observed were decreased
activity, wobbly gait and hyperhidrosis, respectively, in all
female and male mice tested with dosages >4640 mg/kg BW.
One male mouse in the 2150-mg/kg group was found dead, but
no abnormality was observed at necropsy. In the 4640-mg/kg
BW group, most female mice died in the first 2 days and male
mice died in the first day after dosing. In the group of mice
treated at a dosage of 10 000 mg/kg BW, both sexes died in the
first day. However, necropsy revealed no abnormality. Based
on these results, the median lethal dose (LD50 )of catnip oil was
3160 mg/kg BW in female and 2710 mg/kg BW in male mice.
Acute dermal toxicity
The acute dermal toxicity test was conducted using a single
dose of topical application (5000 mg/kg BW) on Wistar rats
to determine the toxic potential of catnip oil. All animals
survived and appeared active and healthy after the test. There
were no signs of gross toxicity, dermal irritation, adverse
pharmacological effects or abnormal behaviour. No gross
abnormalities were noted for any of the animals at necropsy.
Therefore, the acute dermal LD50 of catnip was found to be
>5000 mg/kg BW.
Acute inhalation toxicity
Neither death nor clinical signs of intoxication were observed
to occur in groups of mice exposed to catnip oil at a
concentration of 10 g/m3. No gross abnormalities were noted
in any of the treated mice after the 2-week testing period. The
LC50 of acute inhalation was >10 g/m3for both sexes of mice.
Primary skin irritation
During the first 2 days following application of 0.5 g catnip
oil, none of the four NZW rabbits were observed to show any
signs of erythema or oedema. Slight erythema appeared on
the treated area of one animal starting from day 3 and on the
rest of the tested animals from day 4. The erythema remained
throughout the entire testing period (14 days) and no decrease
in symptoms was observed. However, no oedema was noted in
any tested animals during the period of the exposure test. The
overall incidence and severity of irritation scores are presented
in Table 3. No irritation on the skin of the control area was
observed.
Primary eye irritation
None of the three rabbits exhibited signs of corneal opacity
and iritis. Conjunctival irritation was revealed 1 h after test
Tab le 3. Primary dermal irritation scores in female New Zealand
White rabbits after exposure to catnip oil.
Time
post-instillation,
days Erythema Oedema Mean score∗
1000.00 ±0.00
2000.00 ±0.00
3100.25 ±0.25
4401.00 ±0.00
5–14 4 0 1.00 ±0.00
∗Mean values (n=4).
Tab le 4. Incidence, severity and reversibility of ocular irritation in
femaleNewZealand White rabbits after exposure to catnip oil (n=3).
Time
post-instillation, h
Corneal
opacity
Iritis Conjunctivitis MMT value∗
1 0/3 0/3 3/3 1.0
24 0/0 0/0 0/0 0.0
48 0/0 0/0 0/0 0.0
72 0/0 0/0 0/0 0.0
96 0/0 0/0 0/0 0.0
168 0/0 0/0 0/0 0.0
∗MMT, maximum mean total.
material instillation, but did not persist for 24 h. All control
eyes remained free of any symptoms of eye irritation. The
maximum mean total scores for catnip oil are listed in Table 4.
No other signs of gross toxicity were observed during the
testing period.
Discussion
The primary goal of the present study was to determine whether
catnip essential oil can be used to effectively repel filth flies,
including houseflies and stable flies, as it has been reported to
do against mosquitoes. Furthermore, the study also aimed to
evaluate the safety profile of catnip oil as a repellent before
designating it as a safe alternative repellent against biting
insects for practical application on humans and animals.
Stable flies and houseflies are the two major urban fly
pests in the world. The management of these two fly species
relies heavily on sanitation and pesticide application, which
are difficult to implement because of high labour costs and
the ineffectiveness of the available control products. Repellents
have been used widely for personal protection against mosquito
bites and the diseases they transmit. However, so far there have
been very few studies conducted on the use of repellents against
urban and agricultural fly pest species. The present study
demonstrated that catnip essential oil has a significant repellent
effect against these two urban fly pests. We observed repellency
rates of >96% against stable flies at a dosage of 20 mg and
of 86% against houseflies at a dosage 10 times lower, at 2 mg.
This is the first study into the use of a plant-based essential oil
to repel fly pests. Although the results from the K&D module
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214 J. J. Zhu et al.
tests suggested a deterrent activity against feeding flies, the
further tests conducted in our greenhouse cage study showed
a spatial repellency, particularly against gravid stable flies in
oviposition assays (unpublished data).
This study established a laboratory bioassay to screen
substances that might serve as potential repellents against biting
flies without the burden of using animals. The K&D module
was originally designed for the purpose of testing mosquito
repellents in vitro. However, during the tests on flies in this
study, no flies were observed to feed on either the blood or
sugar water through the membranes (Baudruche membrane
or Edical collagen) originally used for testing in mosquito
research. Starved flies were observed to probe the membrane
aggressively during the testing period, but were incapable
of penetrating the layer of Baudruche or Edical collagen
membranes employed in the K&D module for mosquitoes.
This may reflect the result of differences between mosquitoes
and muscid flies in mouth part structures. The mouth parts
of stable flies were easily able to cut through the material
used in the outer covering layer of the feminine hygiene pad
for bloodfeeding. The control flies showed feeding rates of
96–100%. The development of such an in vitro bioassay will
contribute significantly to the future testing of other novel
repellents for integrated fly management involving the push-
and-pull strategy.
Although the use of botanically based repellents against
mosquito bites has been widely accepted by the public, it is
still very uncertain whether they are really safe for human
and animal use. So far, only a very few plant-based repellents
have been evaluated for toxicity. Most of them are EPA-
registered commercial products. Catnip oil has been reported
as an effective alternative insect repellent against several
disease-transmitting urban insect pests, including mosquitoes
and cockroaches (Peterson, 2001; Schultz et al., 2004; Bernier
et al., 2005; Trongtokit et al., 2005; Zhu et al., 2006). In
addition, catnip has been known historically as a folk remedy
used to repel insects, and Eisner (1964) reported that it repelled
at least 13 families of insects. The results from the present
study demonstrated that catnip also effectively repels filth flies.
Further toxicity tests showed no acute dermal toxicity or acute
inhalation toxicity; therefore catnip falls into the category of
‘virtually non-toxic’. The median lethal dose of catnip for acute
oral toxicity in mice ranged from 2000 mg/kg to 3690 mg/kg
BW (with a 95% confidence interval), indicating very low
toxicity. In the skin irritation study, no deaths or other signs
of toxicity were observed, except that catnip caused slight
erythema on tested rabbits. However, no oedema or other
dermal findings were induced. This categorized catnip oil as
being slightly irritating to the skin. The eye irritation test
indicated that catnip could cause slight irritation, but evidence
showed that all tested animals were free of ocular irritation
within 24 h.
The practice of using plant derivatives or botanically based
insecticides and repellents in agriculture dates back at least two
millennia to ancient China, Egypt, Greece and India (Schultz
et al., 2004; Isman, 2006). Even in Europe and North Amer-
ica, the documented use of botanicals extends back more
than 150 years, dramatically predating discoveries of the major
Tab le 5. Acute toxicity comparisons of several selected mosquito repellents and their rankings
Acute toxicity to rats and rabbits
U.S. Environmental Protection Agency categories and warning labels
Oral LD50,DermalLD
50, Inhalation LD50,
mg/kg mg/kg mg/L Eye effects Skin effects Category PAN narrative rating Warning label
DEET 2170 – 3664 >2000 5.95 Irritation cleared
in <7days
Minimal irritant 3 Slightly toxic Caution
Picaridin 2236– 4743 >2000 >4364 Irritation cleared
in 8–21 days
Not an irritant 2 –3 Slightly to moderate toxic Caution to warning
p-Menthane
3,8-diol
>5000 >5000 2.17 Corrosive Slight irritant 1– 3 Slightly to highly toxic Caution to danger
Catnip oil 2710– 3160 >5000 >10 000 Irritation cleared
within 24 h
Moderate irritant 3– 4 Not acute toxic to slight toxic None to caution
PAN, Pesticide Action Network
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Natural product repellent for filth flies 215
classes of synthetic chemical insecticides. Recent studies have
further proven their effectiveness as alternative mosquito repel-
lents (Sukumar et al., 1991; Barnard, 1999; Thacker, 2002;
Ware & Whitacre, 2004; Trongtokit et al., 2005). So far, toxi-
city tests have been performed on only two plant-associated
repellent compounds, para-menthane-3,8-diol (derived from
the Australian lemon-scented gum tree) and picaridin (a syn-
thetic derivative of pepper) (U.S. EPA Biopesticide Registra-
tion Documents 011550 and 7505C). Citronella oil, repre-
senting another group of botanically based insect repellents,
was exempted from regulation under the Federal Insecticide,
Fungicide and Rodenticide Act of 1996 because of its very
low or non-toxicity, but it provided only very short-lived pro-
tection (Sukumar et al., 1991). Based on the EPA-published
acute toxicity data for picaridin, para-menthane-3,8-diol and
DEET (U.S. EPA DEET Registration Document), a compar-
ative summary table (Table 5) was presented to show acute
toxicity rankings based on the evaluation criteria of the U.S.
EPA categories and warning labels adapted from the acute
toxicity categories of the Pesticide Action Network Pesticide
Database (http://www.epa.gov/oppsrrd1/REDs/0002red.pdf). In
general, acute toxicity of catnip oil appeared to be extremely
low because almost no gross signs of toxicity were noted. Cat-
nip could be considered the least toxic of the four repellents
compared. However, erythema occurred in all tested animals
4 days after topical application, which suggests that catnip
oil may cause skin irritation. More extensive animal toxicol-
ogy studies in conjunction with human clinical studies will
need to be performed to establish the safety of catnip oil in
humans and other animals before it can be fully accepted
for use in personal and animal protection against biting
insects.
Acknowledgements
We express our deep gratitude to T. Weinhold and L. Ma for
their technical help with this study. This work was performed
in co-operation with the Institute of Agriculture and Natural
Resources, University of Nebraska-Lincoln, and supported
partly by Regional Project 1030.
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