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Question: Mosquitoes are becoming more of a concern, as the incidence of diseases such as West Nile and Zika viruses become more prevalent. People are searching for natural remedies, and one of the most popular is the citronella plant. Does this plant really keep mosquitoes away? And do citronella oil or candles work better than the plant? — Russel Mullin
Robert Pavlis: Citronella grass (Pelargonium citrosum) is sold in many nurseries, and is usually displayed along with a big sign promoting its ability to ward off mosquitoes. It’s no wonder people buy them and plant them in their gardens or in a planter on the patio. All of this promotion is a big myth; this so-called mosquito plant is not the same plant that’s used to make citronella oil.
Citronella oil is actually extracted from various species of lemongrass in the genus Cymbopogon. The citronella plant itself gives off very small amounts of chemicals. The levels are so low that they have no effect on mosquitoes. Studies have clearly shown that even if you sit right beside the plant, it will not reduce the number of mosquito bites you get. Neither the imposter plant, nor its oil, repel mosquitoes. In fact, mosquitoes seem to be attracted to the plant and enjoy sitting on it.
A study at the University of Guelph tested citronella candles. In a five-minute period, subjects received 6, 8, and 11 bites for citronella candles, regular candles, and no candles, respectively. Citronella candles were marginally better than regular candles and reduced bites by half, compared to using nothing. So they work a bit, but I don’t consider one bite a minute as satisfactory. The idea that citronella candles keep mosquitoes away is a myth. The amount of oil in candles is extremely small, making them only slightly more effective than regular candles, and neither work well.
Citronella oil is a natural product, but that in itself doesn’t mean it’s safe to use. How toxic is citronella oil? One way to measure toxicity is to measure the LD50 , the lethal dose that kills half of a given group of test subjects (in this case, the LD50 on rabbit skin). Citronella oil has a value of 4,700 mg/Kg, and DEET is 4,280 mg/Kg. Both are considered safe by the USDA, but should not be used on the delicate skin of babies and children.
Do fragrant plants keep mosquitoes out of your garden? There are lots of claims that plants such as catnip, bee balm, marigolds, lemon balm, lavender, geraniums, thyme, wormwood, rosemary and various mints repel mosquitoes in the garden. The plants themselves may not always give off enough scent to deter mosquitoes. The one possible exception is the eucalyptus tree, which seems to have a measurable effect when deterring mosquitoes in particular. Tearing and crushing leaves and stems of these plants releases an odor that may repel some insects, including mosquitoes. An even better remedy includes making extracts of some of these herbs. More information on choosing the best mosquito-repellant plants to grow can be found at Grow Safe, Natural Mosquito Repellant.
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