Citronella oil is derived from cultivated lemon grasses and can come in many forms such as lotions, oils, candles, soaps and perfumes. Citronella oil is an essential oil that comes in two types, Java and Ceylon. Citronella is a natural insect repellent that has been registered for use in the USA since 1948, however in Europe it was made illegal to sell it as an active insecticide ingredient in 2006 with out scientific data to back up the claim.
Most studies show that Citronella is mildly effective and best used on the skin in the form of lotions.
One study compared citronella candles, citronella incense, plain candles, and no candles. On average, during a 5 minute period, subjects received 6.2 bites next to citronella candles, 8.2 bites next to citronella incense, 8.2 bites next to plain candles, and 10.8 bites with no protection at all. So the citronella candles and incense reduced bites by 42.3% and 24.2% respectively, not particularly effective protection. The citronella incense was actually only as effective as plain candles.
Another study compared citronella, linalool and geraniol essential oil candles placed at a distance of only 1m from mosquito traps. The citronella candles reduced the mosquitoes by 35% and sand flies by 15%, the linalool candles reduced mosquitoes by 65% and sand flies by 48%, the geraniol candles reduced mosquitoes by 81% and sand flies by 70%. So geraniol candles were more than twice as effective as the citronella candles. The same study found the essential oil candles dropped in their effectiveness significantly when the distance between the subject and candle was increased to 2m and 3m - so you need to stay close to the candles for them to work.
What is Geraniol? Geraniol is actually the primary ingredient of rose oil, palmarosa oil, and the Java type of Citronella oil. Geranium, lemon, and many other essential oils also contain small quantities of Geraniol.
So citronella has been found to be not as effective as geraniol (which is infact an active ingredient of citronella). Which ever essential oil you choose it's important to re-apply often for the repellent to work well. The effectiveness of citronella may be enhanced by combining other essential oils into your homemade lotion.
What is Linalool? Linalool is a naturally occurring terpene alcohol chemical found in many flowers and spice plants. Linalool can be found in mints and scented herbs, laurels, cinnamon, rosewood, citrus fruits, and also birch trees.
An alternative to a citronella candle is a citronella torch which can be a useful tool for repelling mosquitoes and other insects and is popular for backyard uses such as at parties, barbecues and other events, but will most likely have to be used in large quantities to be very effective. The scent can be easily blown away on anything but the most windless days.
Citronella Oil Safety
The USA has reported no toxicity with Citronella oil in over 60 years of usage as an insecticide. There are some safety concerns with methyl eugenol, an ingredient in Ceylon type Citronella oil.
With the danger of serious diseases transmitted by mosquitoes, such as the Dengue fever, West Nile virus, Malaria and most recently the Zika virus, people are always looking for natural methods to effectively repel mosquitoes. Citronella can be used as an alternative to the chemical DEET, which is a common ingredient in many commercial insect repellents, however it may not be as effective over time or for prolonged exposure.