Mosquitoes are animals most of us would rather do without. When they also carry a disease, such as West Nile virus, the need to protect ourselves from their bites seems even more important. To protect yourself and your family, the first step is to minimize mosquito habitat where you live and play.
Reduce Mosquito Breeding Places:
- Remove, turn over, or cover any container that can hold water, such as tin cans, buckets, ceramic pots, plastic covers, and toys.
- Store unused tires inside a garage or shed, or drill drainage holes in them.
- Change water often (at least once a week) in birdbaths, fountains, wading pools, potted plant trays, and watering troughs.
- Keep water off swimming pool covers. Maintain water quality in the pool. Drain wading pools regularly.
- Make sure roof gutters are draining properly. Clean any debris out once a year.
- Drain (or fill with dirt) unneeded or unwanted pools or puddles.
- Clear obstructions to promote flow of water in catch basins and storm drains
- Fill tree holes with sand or mortar.
- Avoid over irrigation of your lawn.
- Fix leaky outdoor faucets and sprinklers.
Much of the literature about mosquitoes provided by government agencies recommends regular use of a mosquito repellent, most often the chemical DEET. Because of the health and environmental problems associated with DEET, many people are looking for repellents based on other chemicals. A variety of plant-based products have been developed to meet this need.
Are these new products effective? What are the health problems associated with their use? This article has brief answers to those questions.
Oil of citronella is a common ingredient in plant-based mosquito repellents. It is distilled from dried cultivated grasses.1
U.S. Department of Agriculture (USDA) entomologists tested the efficacy of three citronella-based mosquito repellents in 2004. The "protection time" provided by these products varied between 0 and 5 hours, depending on the product and the species of mosquito used in the test.2 An older study by a North Carolina dermatologist and a University of Florida entomologist found protection times of about 15 minutes for two citronella-based products.3
Evaluations of citronella by government agencies have come to different conclusions about its potential hazards. The U.S. Environmental Protection Agency (EPA) noted skin irritation, and that one type of citronella oil caused some allergic reactions. Longer-term toxicology tests were waived.1 The National Pesticide Information Center also notes that citronella can be mildly irritating to the skin and eyes, and it is slightly toxic to aquatic wildlife.4 Canada's Pest Management Regulatory Agency (PRMA) identifies more serious concerns,5 noting that the U.S. Department of Health and Human Service's National Toxicology Program found that a chemical called methyl eugenol, a component of citronella oil, causes liver and stomach tumors, as well as genetic damage.6
EPA has exempted most citronella products from registration requirements as "minimum risk pesticides." The label of the product is required to identify all ingredients.7
Geraniol is an alcohol found in many plant oils, including citronella, lemongrass, and oil of rose.8 It is used as a fragrance in personal care products and detergents and also to flavor drinks, ice cream, and candies.9
In the late 1990s, research at the University of Florida identified geraniol as "the first effective alternative to products containing DEET."10 USDA researchers found that a geraniol-based repellent protected against mosquito bites between two and four hours, depending on the species of mosquito.2
Evaluations of geraniol have identified a few potential health hazards. According to EPA, geraniol causes moderate eye and skin irritation.9 Longer-term toxicology tests were waived. Geraniol has also been shown to be severely irritating to skin, and in one case of occupational exposure it caused allergic contact dermatitis.11
EPA has exempted geraniol products from registration requirements as "minimum risk pesticides." The label of the product is required to identify all ingredients.7
For examples of geraniol-based products, see http://www.bugband.net.
p-Menthane-3,8-diol is found in eucalyptus plants. (For commercial use, it is synthesized chemically.) It is also a major breakdown product of menthol,12 the alcohol in mint oils used as peppermint flavoring.13 p-Menthane-3,8-diol has been registered with EPA as a mosquito repellent since 2000.14
When USDA researchers tested the success of p-menthane-3,8-diol as a mosquito repellent, they found that it provided protection from bites for about 7 hours.2
Evaluations of p-menthane-3,8-diol have identified some health hazards. According to EPA, repellents containing this chemical caused severe eye irritation in rabbits as well as skin allergies in guinea pigs. Follow-up tests with human skin did not demonstrate any allergies, however. Longer-term skin exposure caused microscopic kidney lesions, as well as inflammation and redness of skin.14 Like EPA, Canada's PMRA noted eye irritation caused by this chemical, but also noted one study showing genetic damage.12
EPA classifies p-menthane-3,8-diol as a biochemical pesticide. This means that fewer studies were required as part of the registration process than would be required for a conventional pesticide.14
There are two registered p-menthane-3,8-diol repellents. Both are at least 90 percent unidentified "inert" ingredients.15 Thus it is currently impossible to fully evaluate the potential hazards of these products.
Soybean oil is made from soybeans and is a common ingredient in food.8 Soybeans make up about 90 percent of America's oilseed production.16
When USDA researchers tested the success of a soybean oil repellent, they found that it provided protection from bites between 5 and 8 hours, depending on the species of mosquito they studied.2 Another study found that a soybean-oil based repellent protected against mosquito bites for about 1 1/2 hours.3
Evaluations of soybean oil used as a pesticide have found few hazards. EPA reported that "it is not likely to result in adverse human health effects, based upon available reports and information."8 In a more detailed analysis, Canada's PMRA noted a study showing that blood cholesterol levels increased in laboratory animals fed a diet high in soybean oil. Another study showed that the incidence of breast tumors increased in laboratory animals infected with a tumor virus and fed a high-soybean-oil diet. These effects occurred when over 15 percent of the diet was soybean oil.17
Soybean oil was first registered for use as an insecticide and miticide (a pesticide used to kill mites) in 1959.8 Soybean oil pesticides are now exempt from registration requirements as "minimum risk pesticides," The label of the product is required to identify all ingredients.7
For examples of soybean oil-based mosquito repellents, see Bite Blocker.
Would you rather not use a repellent? Are you spending time in areas where there are so many mosquitoes that repellents aren't enough protection? Consider mesh clothing, which you can purchase from outdoor suppliers. Hats, shirts, pants, socks, and jackets are available.
Because using a mosquito repellent means direct exposure to you, your family, and your pets, NCAP suggests that you choose carefully if you need to use repellents. Consider both the effectiveness of a repellent and its potential health hazards.
- U.S. EPA, Office of Pesticide Programs. 1999. Citronella (Oil of citronella) (021901) fact sheet. https://www3.epa.gov/pesticides/chem_search/reg_actions/registration/fs_PC-021901_01-Nov-99.pdf
- Barnard, D.R. and R. Xue. 2004. Laboratory evaluation of mosquito repellents against Aedes albopictus, Culex nigripalpus, and Ochlerotatus triseriatus (Diptera: Culicidae). J. Med. Entomol. 41(4):726-730.
- Fradin, M.S. and J.F. Day. 2002. Comparative efficacy of insect repellents against mosquito bites. N. Engl. J. Med. 347:13-18.
- National Pesticide Information Center. 2013. Oil of citronella: General fact sheet. http://npic.orst.edu/factsheets/citronellagen.html#symptoms
- Health Canada. Pest Management Regulatory Agency. 2004. Re-evaluation of citronella oil and related active compounds for use as personal insect repellents. http://www.pmra-arla.gc.ca/english/pdf/pacr/pacr2004-36-e.pdf
- U.S. Dept. of Health and Human Services. Public Health Service. National Toxicology Program. 2005. Report on carcinogens, eleventh edition. http://ntp-server.niehs.nih.gov.
- U.S. EPA. 2000. Pesticide registration (PR) notice 2000-6. http://www.epa.gov/PR_Notices/pr2000-6.pdf
- U.S. EPA. Prevention, Pesticides and Toxic Substances. 1993. Reregistration eligibility decision: Flower and vegetable oils. https://archive.epa.gov/pesticides/reregistration/web/pdf/4097fact.pdf
- U.S. EPA. 2004. Geraniol; Exemption from the requirement of a tolerance. Fed. Reg. 69(82):23146-23151, Apr. 28.
- Univ. of Florida. 1999. UF entomologist develops safe, effective alternative to DEET insect repellents. https://news.ufl.edu/archive/1999/06/uf-entomologist-develops-safe-effective-alternative-to-deet-insect-repellents.html
- U.S. National Library of Medicine, National Center for Biotechnology Information. Undated. Compound summary: Geraniol. https://pubchem.ncbi.nlm.nih.gov/compound/Geraniol
- Health Canada. Pest Management Regulatory Agency. 2002. Proposed regulatory decision document: p-Menthane-3,8-diol. http://www.pmra-arla.gc.ca/english/pdf/prdd/prdd2002-02-e.pdf
- Merriam-Webster. 2003. MedlinePlus medical dictionary. [definition for "menthol] http://www2.merriam-webster.com/cgi-bin/mwmednlm?book=Medical&va= menthol
- U.S. EPA. 2000. p-Menthane-3,8-diol (011550) biopesticide registration eligibility document. https://www3.epa.gov/pesticides/chem_search/reg_actions/registration/related_PC-011550_1-May-00.pdf
- Washington State University. 2005. Pesticide information center online. Query for eucalyptus citriodora crystal. http://cru66.cahe.wsu.edu/labels/Labels.php
- United States Department of Agriculture, Economic Research Service. 2019. Soybeans and oil crops: Overview. https://www.ers.usda.gov/topics/crops/soybeans-oil-crops/
- Health Canada. Pest Management Regulatory Agency. 1999. Proposed regulatory decision document: Soybean oil. http://www.pmra-arla.gc.ca/english/pdf/prdd/prdd9902-e.pdf
This article was originally published as:
Cox, Caroline. Plant-based mosquito repellents: Making a careful choice. Journal of Pesticide Reform 25(3):6-7