It’s the time of year when mosquitoes seem to outnumber humans. We often go to great lengths to find the perfect pest repellent, allowing us to remain outdoors and enjoy our favorite summertime activities, whether it’s fishing and camping, swimming and tennis, or a leisurely picnic with friends. With summer’s ever-present mosquito battle in mind, we chose citronella as this week’s crystal.
People are accustomed to seeing citronella as an oil or spray, so it may be hard to believe it could be a crystal. But think about butter, which has a similar chemical structure: At warm temperatures, butter melts. At cool temperatures, butter solidifies.
Compared to butter, the citronella molecule has a really short backbone of single-bonded carbon atoms, with one terminal oxygen atom. (If citronella were a fatty acid, like butter, we could call it “saturated” because the carbons have as many hydrogen atoms attached as possible.) Because the molecule is so small and composed of so many lightweight atoms, it has a low vapor pressure, which means that it evaporates easily when warm. That’s good news for repelling mosquitoes.
According to the Environmental Protection Agency, oil of citronella was initially registered in 1948 as an insect repellant under the name of McKesson’s oil of citronella. It had human applications (as in you could use it on your body, hair, clothing and footwear) to repel adult gnats and mosquitoes. Citronella is a biochemical pesticide with a non-toxic “mode of action.” Some studies have shown it to be effective in deterring lice and the mosquitoes that cause Dengue Fever.
These days, citronella is found in “natural” insect repellents, candles, deodorants and perfumes, astringent skin cleaners, and aromatherapy aimed at addressing nervous fatigue and headaches.
A side effect of the oil is that when directly applied to human skin, it has been known to cause irritation. The warm sensation it creates on the skin has also purported benefits for joint pain.
Want more info on repelling mosquitoes? Check out this Science Nation video about Vanderbilt University researchers working to unleash a more-powerful-than-DEET insect repellent. The EPA also has great resources on mosquito control.
Top photo: An Asian Tiger mosquito. Credit Ary Farajollahi, USDA Forest Service
Middle photo: An aerosol spray canister. USDA researchers Lyle Goodhue and William Sullivan invented the aerosol spray canister, dubbed the “bug bomb,” to dispense insecticides. The design, patented in 1943, is the ancestor of many popular commercial spray products. Pressurized by liquefied gas, which gave it propellant qualities, the small, portable can enabled soldiers to defend against malaria-carrying bugs by spraying inside tents during World War II. Propellants used in these older aerosol cans have since been replaced with environmentally friendly ones. Credit: USDA
Bottom photo: A Culex species mosquito biting a human hand. Credit: Bob Dusek, USGS