HerbalEGram: Volume 9, Number 10, October 2012

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

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2. West Nile virus: What you need to know. Centers for Disease Control website. Available at: www.cdc.gov/ncidod/dvbid/westnile/wnv_factSheet.htm. Accessed September 26, 2012.
   
   
3. The insect repellent DEET. Environmental Protection Agency website. Available at: www.epa.gov/opp00001/factsheets/chemicals/deet.htm. Accessed September 25, 2012.
   
   
4. Reregistration eligibility decision (RED): DEET. Environmental Protection Agency website. Available at: www.epa.gov/oppsrrd1/REDs/0002red.pdf. Accessed September 28, 2012.
   
   
5. Maia MF, Moore SJ. Plant-based insect repellents: a review of their efficacy, development and testing. Malaria Journal. 2011;10(Suppl 1):S11. Available at: www.ncbi.nlm.nih.gov/pmc/articles/PMC3059459/. Accessed September 21, 2012.
   
   
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8. Infectious disease information: mosquito-borne diseases. Centers for Disease Control website. Available at: www.cdc.gov/ncidod/diseases/list_mosquitoborne.htm. Accessed September 27, 2012.
   
   
9. R.E.D. facts: oil of citronella. Environmental Protection Agency website. Available at: www.epa.gov/oppsrrd1/REDs/factsheets/3105fact.pdf. Accessed September 26, 2012.
   
   
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