Mosquitoes find citronella only mildly repellent.
Of the thousands of species of mosquitoes in the world, the only one that matters is the one in your backyard. New chemicals have taken the place of DDT, which was sprayed throughout towns in the 1950s, but survivors still show up at dusk on their bloodthirsty mission. The search continues for a natural alternative to repel the little vampires. Does this Spark an idea?
The Problem
By 2011, 176 varieties of mosquito made their homes -- and raised their broods -- in the United States, according to the American Mosquito Control Association. In addition to presenting a nuisance to campers, farmers and picnickers, mosquitoes pose more serious problems as vectors, or carriers, of malaria, yellow fever, dengue, St. Louis encephalitis, La Crosse encephalitis and West Nile virus. Mosquitoes also vector eastern and western equine encephalitis, diseases that cause high mortality rates in horses and other mammals as well as death or neurological damage in humans.
Repellents
Head-on battles with pyrethrin or malathion kill only the mosquitoes contacted by the sprays. Screens and tight-fitting doors and windows, landscapes with plenty of sunshine, free movement of air and elimination of standing water all help discourage mosquito swarms. In the search for plants that repel mosquitoes, several have been identified, including Cymbopogon nardus, a variety of tropical lemongrass; Melissa officinalis, or lemon balm; Thymus citriodorus, or lemon thyme; and Pelargonium citrosum, a variety of an African geranium, Pelargonium graveolens. All contain citronella, a chemical used in the manufacture of oil used in burners and candles.
Effectiveness
The most effective repellent is a chemical -- N,N-diethyl-meta-toluamide, or DEET. Developed by the U.S. Army, the chemical has been in use since 1946 and was reapproved for human use by the U.S. Environmental Protection Agency in 1998. Citronella-containing plants are much less effective than DEET in repelling mosquitoes. Citronella used for repellent oil comes from citronella grass (Cymbopogon nardus). Lemon balm, lemon thyme and the citrosa geranium have decreasing percentages of citronella, making them unlikely to repel mosquitoes, no matter how many plants sit along the walk or on the deck.
Mosquito Plant
The citrosa geranium, known as the mosquito plant, contains only 0.09 percent citronella, making it one of the least effective natural mosquito remedies. When a passerby bruises it or crushes a leaf to rub on her skin, the plant yields some of its citronella and scent, providing between 30 to 40 percent of the repellent quality of DEET. A study at the University of Guelph was even less enthusiastic about the citrosa plant, often touted as a cross between Pelargonium graveolens and Cymbopogon nardus. The study found no trace of citronella-grass genetics in the citrosa plant. Lemon balm is 38 percent citronella, and crushed leaves of lemon thyme, which produce 62 percent of the repellency of DEET, make better mosquito plants, says Iowa State University entomologist Donald Lewis, quoting an article in "Organic Gardening" magazine.
Tags: Cymbopogon nardus, lemon thyme, citronella making, citrosa geranium, citrosa plant, Lemon balm