by
Jeffrey S. Hoard
Herbal folklore has long included the use of aromatic herbs and oils as insect repellents. Most of us are familiar with the use of citronella oil (Cymbopogon nardus) in candles and in products designed to be used on the skin. Other aromatic herbs used for centuries for their insect repellent qualities include Pennyroyal (Mentha pulegium), Elderflower leaf (Sambucus nigra) which also repels mice, and Mugwort (Artemesia vulgaris) and Rosemary (Rosmarinus officinalis) both known for repelling moths. Natural insecticides such as Tansy (Tanacetum vulgare), Pyrethrum flower (Anacyclus pyrethrum), Anise (Pimpinella anisum) and Birch (Betula alleghaniensis) have also been used since ancient times.
The rise of the use of essential oils has expanded herbal insect repellent possibilities. At our house, where summer gardening is a daily effort, we have at various times used the essential oils of Cajeput (Melaleuca leucadendron L.), Citronella, Eucalyptus (Eucalyptus globulus), Geranium (Pelargonium roseum), Lavender (Lavandula angustifolia), Peppermint (Mentha x piperita), Red Cedarwood (Juniperus virginiana) and Rosemary (Rosmarinus officinalis) with great success. We use these oils either singly or in combinations diluted into hydrosol or water and sprayed liberally on our skin (taking care to avoid the eyes, of course) and clothes.
Now researchers have reported that a component of the essential oil of the Catnip plant (Nepeta cateria) which is so fascinatingly attractive to felines, actually repels mosquitoes 10 times more effectively than DEET, the chemical found in most commercial insect repellents.
Catnip is a perennial herb belonging to the mint family and though it grows wild in most parts of the United States it is also widely cultivated for commercial use. Catnip is native to Europe and was introduced to the United States in the late 18th century. In France the leaves and young shoots were used as a seasoning. Native Americans brewed it into a tea used for colic in infants.
Today it is thought to be useful for many ailments: for migraine headaches, for fevers, for inducing sleep, for inducing perspiration without increasing body temperature, for convulsions in children, for cold prevention, for restlessness, as a pain killer, for fatigue, to improve circulation, to prevent miscarriages and premature births, and for symptoms associated with the flu. Catnip is high in vitamins A, B-complex and C. It also contains magnesium, manganese, phosphorus, sodium and a trace of sulfur.
Entomologist Chris Peterson and Joel Coates, Ph.D., chair of the entomology Department of Iowa State University led the effort to test catnip’s ability to repel mosquitoes. They presented their findings last August in Chicago at the 222nd national meeting of the American Chemical Society, the world’s largest scientific society. These are the same researchers who, in 1999, reported that catnip essential oil effectively repels cockroaches.
Why catnip repels mosquitoes is still a mystery, says Peterson. "It might simply be acting as an irritant or they don’t like the smell. But nobody really knows why insect repellents work."
The mosquito study used the Aedes aegypti mosquito, which can carry the yellow fever virus from one host to another and is found in most parts of the country. Peterson stated, however, that catnip should work against all types of mosquitoes. Yellow fever has been essentially wiped out in the United States by vaccines and mosquito control programs. The last reported outbreak in here was in 1905. According to the Centers for Disease Control, yellow fever still does occur in Africa and South America and is occasionally reported among unvaccinated travelers returning from these regions.
More disturbing to Missouri residents at this time are current reports of increasing incidence of West Nile virus among birds in the state. First detected in the United States in New York in 1999, the virus has since spread along the Eastern seaboard and into the Midwest. You can get the virus if you are bitten by an infected mosquito. The mosquito becomes infected by feeding on an infected bird or animal. While most people who are infected with the virus don’t have any symptoms, people over 50, those with an illness that suppresses the immune system or those on medications that suppress the immune system are at higher risk of developing severe illness. West Nile virus is expected to hit all fifty states within 2 years. There is no vaccine for the virus.
In the laboratory, Peterson put groups of 20 mosquitoes in a two-foot glass tube, half of which was treated with nepetalactone, a biologically active characteristic constituent of catnip. After 10 minutes, only an average of 20 percent – about four mosquitoes – remained on the side of the tube treated with a high dose (1.0%) of the oil. In the low dose test (0.1%) an average of 25% – five mosquitoes – stayed on the treated side.
In the laboratory, repellency is measured on a scale ranging from +100 percent, considered highly repellent, to -100 percent, considered a strong attractant. A compound with a +100 percent repellency rating would repel all mosquitoes, while -100 percent would attract them all. A rating of zero means half of the insects would stay on the treated side and half on the untreated side. In Peterson’s tests, catnip ranged from +49 percent to +59 percent at high dose (1.0 %), and +39 percent to +53 percent at low doses (0.1 %).
When the same tests were conducted using DEET (diethyl-meta-toluamide), approximately 40 to 45% – eight to nine mosquitoes – remained on the treated side. A ten-fold higher concentration of DEET was required to obtain results similar to those of the Catnip. Most commercial insect repellents contain from about 5 percent to 25 percent DEET. Presumably, much less Catnip oil would be needed in a formulation to have the same level of repellency as a DEET-based repellent.
There are concerns about the potential toxic effects of DEET, especially when used by children. Children who absorb high amounts of DEET through insect repellents have developed seizures, slurred speech, hypotension and bradycardia.
Adults who ingested too much DEET have developed seizures, hypotension and coma. It has been reported that when repellents with high concentrations of DEET are applied over several days, they can accumulate in the skin and cause an acute encephalopathy. In addition, many products containing DEET are flammable, as are the aerosol canisters which are frequently used for packaging. However, many of these products do not have warning labels. Users of DEET products are (or should be) cautioned to not use the product on the skin, and to wash the skin thoroughly after coming indoors. DEET sprayed on clothes remains efficacious (and available for absorption through the skin) for up to two weeks. Women who are pregnant should certainly avoid DEET as complications of its use have yet to be studied.
As one might expect, a patent application for the use of nepetalactone as an insect repellent has been submitted by the Iowa State Research Foundation. Surely commercial products using extracted or even synthesized nepetalactone will follow close on the heels of studies intended to evaluate the inevitable side effects and safety of using only one of the over 200 compounds present in the oil. Herbalists, of course will prefer to continue to utilize the whole plant or essential oil.
Rob Pappas of the Essential Oil University of New Albany, Indiana says, "To effectively repel mosquitoes you need specifically the nepetalactone type" of catnip oil. "This specific chemotype has been found to be the only really effective oil for repelling mosquitoes." My own experience with catnip oil of this chemotype is that mosquitoes, bees and other flying insects not only do not bother me but give me a wide berth as I work among the plants and flowers in our garden.
Catnip oil has a deep rich golden color and a sweet pastoral scent reminiscent of newly mown grass or hay. It blends well with other mints and should be a part of the insect repellent blend of anyone looking for maximum protection with a chemical-free product. Recent reports from those who use catnip as a repellent emphasize that cats respond to the user as if they were a big catnip plant. The addition of very aromatic oils, such as peppermint seem not to deter cats from the expected behavior. If you are not a cat lover you might wish, therefore, to use catnip oil with some caution.
References:
American Chemical Society. Catnip Repels Mosquitoes More Effectively than DEET. August 28, 2001.
Benson, Linda. Some Insect Repellents May Offer Children More Harm than Good. Dermatology Times. December, 1999.
Grieve, Mrs. M. A Modern Herbal. Dover Publications, New York. 1971.
Hopman, Ellen Evert. A Druid’s Herbal. Destiny Books, Rochester, Vermont. 1995
Johns Hopkins News-letter. A New Way to get rid of Roaches?Newsbriefs, September 9, 1999.
New York Department of Health. General Information on West Nile Virus. June 2002.
Tenney, Louise. Today’s Herbal Health, 3rd Edition. Woodland Books, Provo, Utah. 1992.
West, Kim. Preliminary Research Suggests Catnip Effective as Mosquito Repellent. Herbalgram 54, 2002.
This article, with minor updates, also appears in the Winter, 2003 – 2004 issue of the Aromatherapy Journal, published by the National Association for Holistic Aromatherapy (NAHA).