Issue Date: October 30, 2006
Once upon a time, I could tolerate mosquitoes, gnats, sweat bees, and other annoying insects. But when Asian tiger mosquitoes invaded the southeastern U.S. in the 1990s and later infiltrated the area where I live in northern Virginia, I quickly lost my tolerance.
These mosquitoes are tenacious rascals not to be taken lightly. Unlike common mosquitoes, tiger mosquitoes are active both day and night, in cool weather and in blazing heat and high humidity. They only need a thimbleful of water for breeding and are unbelievably quick to dodge a hand smack. So at our house, after getting tired of being eaten alive and leery of drenching ourselves all the time with a synthetic insect repellent, we ran out and bought some citronella-scented candles.
On a weekend a couple of months ago, I was sitting around at the tail end of a cookout, waiting for the sun to fade on the horizon, with nothing on my mind except being lazy. By and by, I got nudged back into reality by a few puffs of smoke from the candles burning close by. The almost sickly lemony smell was welcome as it kept the mosquitoes from gnawing on my ankles and allayed my minor concern over the transmission of diseases such as encephalitis or West Nile virus. It finally struck me to do a little research to find out exactly what the active chemicals are in this natural insect repellent.
Citronella oil is one of the important essential oils (volatile compounds that are responsible for the flavors and fragrances associated with the leaves, flowers, seeds, or wood of plants). It's obtained from different species of Cymbopogon grasses that grow wild or are cultivated in tropical regions of Southeast Asia, South America, and the Caribbean. The primary species are C. nardus and C. winterianus, which are called citronella grass. Related Cymbopogon species are known as lemongrass and are used as herbs to add a lemony flavor to foods. Other sources of essential oils include citrus peel, such as orange, lemon, and lime; herbs, such as peppermint and lavender; and trees, such as pine, cedar, and eucalyptus.
Essential oils are widely used in soaps, perfumes, cosmetics, aromatherapy, and food flavorings. The source of their properties typically is monoterpenes, a class of C10 organic compounds produced by both plants and arthropods (insects, spiders, and crustaceans) that contain an isoprene unit (-C=C[CH3]2) on one end and various organic functional groups on the other end. Terpenes in plants help attract pollinators, deter herbivores or destructive insects, and protect against harmful bacteria and fungi. As pheromones in arthropods, they help regulate social behavior and reproduction.
Studies on the chemical composition of citronella oil have found it contains a mix of more than a dozen monoterpenes, with the major components being aldehydes and alcohols. The primary aldehydes are the dienes geranial and neral, respectively known as trans- and cis-citral (3,7-dimethyl-2,6-octadienal), as well as citronellal (3,7-dimethyl-6-octenal), which has only one carbon-carbon double bond. The alcohols are geraniol and citronellol, which are analogs of the aldehydes.
Citronella oil has been used as a fragrance in personal care products for more than 50 years. It's increasingly being used today as the main ingredient, or one of several essential oil ingredients, in insect-repelling products, including candles, sunscreen, pet collars, food packaging, and clothing. The terpenes in the oil are thought to block neural pathways in insects such as mosquitoes and to interfere with their movements and metabolism without killing them. As I watched a burning citronella candle at my cookout, I could see tiger mosquitoes pull up short when they got a whiff of citronella-laced smoke, as if an invisible force field was in the way.
But is citronella itself effective as an insect repellent, or is it just the smoke from the candles—or our imagination? A study in the New England Journal of Medicine (NEJM) concluded that repellents containing up to 10% citronella oil applied to the skin were effective at reducing the frequency of mosquito bites for an average of about 20 minutes. That compares with about five hours of protection for a repellent containing 23.8% DEET (N,N-diethyl-m-toluamide), the most common synthetic active ingredient in insect repellents (N. Engl. J. Med. 2002, 347, 13).
One of the citronella-containing products in the NEJM study was Avon's Skin So Soft moisturizing lotion, which some people claim works well as an insect repellent. I tried it years ago, but I found no noticeable benefit. That was probably because, as the study noted, the amount of citronella in the moisturizer is only 0.05% and it afforded only three minutes of mosquito protection, on average.
The NEJM study pointed out that so far only DEET-based products, which have been in use for nearly 50 years, combine proven low toxicity and adequate protection against mosquitoes and other disease-carrying arthropods, such as ticks. At our house, we think the combination of citronella vapor and smoke from lighted candles also works pretty well to keep the mosquitoes at bay without using any bug juice, as long as you don't mind the occasional puff of smoke in your eye.
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