At
the end of September 2012, the US Centers for Disease Control (CDC) had
reported nearly 150 West Nile virus-related deaths in the United States. With
more than 3,500 reported cases in 2012 — the most in nearly a decade1
— health officials are urging individuals to take certain precautions to avoid
being bitten by mosquitoes, which can carry the virus. In addition to ensuring
properly fitting screens on windows and doors, wearing pants and long-sleeved
clothing when outdoors, and avoiding pools of stagnant water where mosquitoes
lay eggs, the CDC recommends using an insect repellent with an active
ingredient registered by the Environmental Protection Agency (EPA).2
Of
EPA-registered active ingredients, the most widely recognized and studied
synthetic compound is DEET (N,N-diethyl-meta-toluamide), an
insect repellent developed by the US Department of Agriculture (USDA) in 1952
and approved for public use in the late 1950s.3 Extensive testing
supports the use of DEET as an effective method of preventing mosquito and tick
bites, but mild skin and eye reactions have been reported, as well as several
cases of seizures in individuals who frequently used DEET. EPA requires all
DEET-containing products to contain detailed instructions for safe use, which
includes avoiding over-application of the repellent and washing any treated
skin or clothing after use.
According to the EPA’s 1998 re-registration eligibility decision, “DEET is not
believed to be acutely toxic nor carcinogenic, significantly developmentally
toxic nor mutagenic at the doses tested.” Further, the EPA concluded that the
“available data do not support a direct link between exposure to DEET and
reported seizure incidences (14 cases).”4
In recent years, however,
much research has been conducted on plant-based alternatives to these synthetic
formulas, which may be preferable to consumers with chemical sensitivities or
those who wish to avoid synthetics.
“[Plant-based insect repellents] may be more
cosmetically appealing, more widely available or producible, including in
tropical countries where the public health value of repellents is especially
important,” said Scott P. Carroll, PhD, a scientist in the department of
entomology at the University of California-Davis’s Center for Population
Biology (email, September 25, 2012). “Plants are great biochemical synthesists,
and we are well adapted to plants, so it’s obviously functionally prosperous
for investigation.”
Reasons for Choosing Plant-Based Insect Repellents
Plants produce chemicals that act as natural
deterrents to pests, and they have been used since ancient times to repel
insects, most commonly by burning plant material.5 The established
use of plants as insect repellents in part contributes to their acceptability
among consumers.
“There are quite a few natural products (oils and
single compounds) that demonstrate repellent efficacy,” said Ulrich R. Bernier, PhD, a research chemist in the Mosquito and Fly
Research Unit at the USDA’s Center for Medical, Agricultural, and Veterinary Entomology
(email, September 21, 2012). “One advantage of using a
plant-based botanical is user acceptability. People tend to favor natural
products over synthetics.”
Plant-based active ingredients are also thought
by some to pose fewer risks to users. And since repellents are often applied
directly to the skin, consumers tend to favor products without harsh chemical
smells. Although, according to some experts, an effective active ingredient is the
most important consideration when choosing an insect repellent, personal
preference plays a role in repellent selection as well.
“There is a
strong perception that natural actives are safer than synthetics,” said Sarah J. Moore, PhD, a lecturer in
the Department of Disease Control at the London School of Hygiene and Tropical
Medicine, and a co-author of a 2011 review of plant-based insect repellents published
in Malaria Journal (email, September 21, 2012). “Plant-based odours are scents that people feel comfortable
with as they are natural. My research on [multiple] continents (North and South
America, South-east Asia and Africa) has highlighted this same perception.”
Perhaps more importantly, insect
repellents derived from plants can be an inexpensive, sustainable method of
preventing disease in high-risk regions of the world. “If plant-based
repellents are ethically sourced and produced then they can bring trade [to] developing
countries and are less damaging to the environment,” said Dr. Moore.
Repellents Derived
from the Lemon Eucalyptus Tree
In April 2005, the CDC approved botanically based
para-menthane-3,8-diol, or PMD, as an effective insect repellent.6 PMD,
which is derived from leaves of the lemon eucalyptus tree (Corymbia citriodora), was discovered in the 1960s by
researchers conducting chemical screenings for potential insect-repelling
properties of plants used in traditional Chinese medicine. In fact, as plants
are occasionally named in accordance with their traditional uses, the Chinese
name for lemon eucalyptus, quwenling,
translates roughly to “effective mosquito repeller.”7
Although there are multiple plant-based active ingredients registered with the
EPA, researchers have reported that PMD is "the only plant-based repellent that has been advocated for use in
disease endemic areas by the CDC, due to its proven clinical efficacy to
prevent malaria and is considered to pose no risk to human health. [PMD]
provides very high protection from a broad range of insect vectors over several
hours.”5
PMD is not to be confused with what is frequently referred to as “oil of lemon
eucalyptus.” In distilling the essential oil from leaves of the lemon
eucalyptus tree, PMD is left over as a waste product.5 It is this
waste product that has been shown to be effective in repelling mosquitoes, more
so than the oil itself. Although insect repellent products containing oil of
lemon eucalyptus are available, Dr. Moore cautions against using anything but CDC-
and EPA-recommended active ingredients, such as PMD, in areas with disease risk.
Depending on the concentration, PMD formulas can
last up to 8 hours,7 and have been shown to be almost as effective
as those containing DEET. In some cases, PMD has been shown to be more effective than DEET in repelling
certain species of mosquitoes.
“[Some plant-based ingredients can] better repel certain vectors, as in the case of PMD's evident
superiority to DEET in repelling Anopheles malaria vectors,” said Dr.
Carroll. In a 2006 study published in the Journal
of the American Mosquito Control Association of which Dr. Carroll was a
co-author, he wrote that “PMD has shown unprecedented repellency and
consistency for a botanical.”7
Similarly,
Dr. Moore, who conducts field research around the world, has found that PMD is consistently
effective. Although her research primarily concerns the prevention of malaria,
the same mosquito-repelling properties will protect against other
mosquito-borne diseases such as West Nile fever and encephalitis, dengue fever,
and yellow fever.8
“I can attest to the fact that PMD repellents are highly
effective from both my research where they demonstrate good efficacy and the
research of others who all show a consistently good effect in preventing bites
from disease-vector insects,” she said. “I have lived in Tanzania for the past 6 years in a highly
malarious area where we use PMD every evening in conjunction with long clothing,
and we screen our home and use an insecticide-treated bed net as recommended
best practise. I have never had a negative skin reaction or a vector-borne
disease. Malaria is a preventable disease and we have effective tools, both
synthetic and natural, to combat it."
Citronella-Based Insect Repellents
Although PMD is a widely studied natural insect
repellent, the plant-based ingredient citronella is arguably more recognized as
a mosquito repellent. Citronella oil has been shown to be less effective than
DEET, but it can still be a useful tool for repelling mosquitoes in areas
without disease risk.
“Citronella has found its way into many commercial
preparations through its familiarity, rather than its efficacy,” said Dr.
Moore. “Citronella-based repellents only protect from host-seeking mosquitoes
for about 2 hours.”
Citronella was first registered by the EPA in
1948 and was originally used in perfumes for its pleasant scent.4
Today, citronella candles are ubiquitous in American backyards, although it
also is used in creams, lotions, and sprays. According to the EPA, citronella
is classified as a biopesticide and registered as an insect repellent or
feeding depressant and also as an animal repellent. Oil of citronella comes
from 2 species of aromatic grasses, Ceylon citronella (Cymbopogon nardus) and Java citronella (C. winterianus). Citronella is regarded as a highly safe repellent;
in a 1997 reregistration memo, the EPA concluded that “based on available data, the use of currently registered
products containing oil of citronella in accordance with their approved
labeling will not pose unreasonable risks or adverse effects to humans or the
environment.”9
Citronella and PMD are just 2 of the hundreds of
plants or plant-based ingredients that have been studied for their
insect-repelling properties. In addition to citronella, PMD, and DEET, Health
Canada — the governmental body responsible for national public health in Canada
— has approved products containing soybean (Glycine
max) oil for mosquito-repelling purposes for up to 3.5 hours, although less
research has been conducted on this particular ingredient. Other commonly-cited
botanicals used to repel mosquitoes include species in the mint family (Lamiaceae;
including catnip [Nepeta cataria], basil
[Ocimum basilicum], and peppermint [Mentha x piperita]), 2-undecanone (an extract from tomato plants [Solanum lycospericum]), neem oil (Azadirachta indica), lemongrass (Cymbopogon citratus), fennel (Foeniculum vulgare), and rue (Ruta graveolens).10-12
Issues Associated with Plant-Based Repellents
As a scientific term, volatility refers
to a chemical’s tendency to evaporate. Bruised or damaged plants will release
volatile odors into the environment, which can offer protection from pests at a
distance.4 However, when these chemicals are formulated into insect
repellents that are applied to the skin, volatility becomes a problem.
PMD is unique in that it has an
especially low vapor pressure, which causes it to evaporate slower than other
plant-based insect repellents. Citronella, however, upon initial application is
just as effective as DEET, but its high volatility quickly decreases its
effectiveness.
“Some plant-based molecules are incredibly effective in the short term but
quickly evaporate and for this reason they don't last as long as the synthetic
molecules,” said Dr. Moore.
According to Dr. Bernier, volatility is one of
the challenges to formulating an effective and long-lasting botanical insect
repellent. “One the greatest problems with naturally based repellents is the
volatility of those oils or compounds within oils that contribute to the
observed repellency,” he said. “There are a number of natural compound
repellents on the market and some of these do repel for a short time.”
However, there are some ways to mitigate the
effects of the high volatility of plant-based active ingredients. One option is
to combine plant-based chemicals with larger molecules that evaporate more
slowly. Vanillan, a relatively large molecular component of the vanilla bean (Vanilla spp.), has been added with some
success to botanical insect repellent formulas to reduce the formulas’
volatility.5 And in recent years, advances in nanotechnology have
provided even more options to increase the duration of plant-based repellents.
These techniques have been used in some citronella formulas. For example, “[e]ncapsulated
citronella oil nanoemulsion [can be] prepared by high-pressure homogenization …
to create stable droplets that increase the retention of the oil and slow down
release.”5
To counteract the high volatility of
some plant-based active ingredients, some mosquito repellents contain higher concentrations
of these ingredients. And as with any dermatological applications of chemicals
— plant-based or synthetic — there is some risk of a reaction.
“Some essential oils can cause skin irritation,” said Dr. Moore. “Read the
label — if there are high concentrations of essential oils they are unlikely to
be suitable for those with sensitive skin. If you have a reaction, immediately
discontinue use and consult a physician if you have a dermatitis that does not
resolve after a few days.”
As consumer interest in plant-based
products grows, scientists will continue to study plants to learn more about
their insect-repelling properties. Botanical formulas, when used appropriately,
provide an alternative to synthetic repellents. Although plant-based insect
repellents are often not as effective as DEET, in time, this may change.
“As technology improves so that
formulations make essential oils and other plant based molecules remain on the
skin for as long as DEET, then we will see more effective plant-based
repellents that can be used to prevent disease. I am certainly keen to see this
happen if it means that products can be made in a more environmentally
sustainable way through fair trade,” said Dr. Moore. “It will hopefully help
make repellents such as citronella and PMD more accessible to those living in
disease endemic countries of the tropics because they often cannot afford to
purchase repellents imported from outside. I am keen to see more manufacturers
taking on this challenge.”
—Tyler Smith
References - Division of Vector-Borne Diseases: West Nile Virus. Centers for
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September 25, 2012.
- West Nile virus: What you need to know. Centers for Disease Control
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- The insect repellent DEET. Environmental Protection Agency website.
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- Reregistration
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- Maia MF, Moore SJ. Plant-based insect repellents: a review of their
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Journal. 2011;10(Suppl 1):S11. Available at: www.ncbi.nlm.nih.gov/pmc/articles/PMC3059459/. Accessed
September 21, 2012.
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- Carroll SP, Loye J. PMD, a registered botanical mosquito repellent
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September 27, 2012.
- R.E.D. facts: oil of citronella. Environmental Protection Agency
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September 26, 2012.
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- Bissinger BW, Apperson
CS, Sonenshine DE, Watson DW, Roe RM. Efficacy of the new repellent BioUD®
against three species of ixodid ticks. Exp
Appl Acarol. 2009:48:239-250.
- Polsomboon S, Grieco JP, Achee NL, et al. Behavioral responses of
catnip (Nepeta cataria) by two
species of mosquitoes, Aedes aegypti
and Anopheles harrisoni, in Thailand.
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