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Mosquitoes and Repellents:
Research of articles By Roy Linker

From the Annals of Internal Medicine, 1 June 1998. By Mark S. Fradin, MD 128:931-940.

This paper is intended to provide the clinician with the detailed and scientific information needed to advise patients who seek safe and effective ways of preventing mosquito bites.

N,N-diethyl-3-methylbenzamide (DEET) is the most effective, and best studied, insect repellent currently on the market. This substance has a remarkable safety profile after 40 years of worldwide use, but toxic reactions can occur (usually when the product is misused). When DEET-based repellents are applied in combination with permethrin-treated clothing, protection against bites of nearly 100% can be achieved. Plant-based repellents are generally less effective than DEET-based products. Ultrasonic devices, outdoor bug "zappers," and bat houses are not effective against mosquitoes. Highly sensitive persons may want to take oral antihistamines to minimize cutaneous reactions to mosquito bites.

Only female mosquitoes bite. Male mosquitoes feed primarily on flower nectar, whereas female mosquitoes require a blood meal to produce eggs. They usually feed every 3 to 4 days; in a single feeding, a female mosquito typically consumes more than its own weight in blood. Certain species of mosquitoes prefer to feed at twilight or nighttime; others bite mostly during the day.

Insect Repellents

Despite the obvious desirability of finding an effective oral mosquito repellent, no such agent has been identified. Thus, the search for the perfect topical insect repellent continues. This ideal agent would repel multiple species of biting arthropods, remain effective for at least 8 hours, cause no irritation to the skin or mucous membranes, cause no systemic toxicity, be resistant to abrasion and rub-off, and be greaseless and odorless. No available insect repellent meets all of these criteria.

How To Choose and Apply DEET Repellents

For casual use, a high concentration of DEET is not needed. Products with 10% to 35% DEET will provide adequate protection under most conditions. The American Academy of Pediatrics recommends that repellents used on children contain no more than 10% DEET. Products with a DEET concentration of more than 50% are probably best reserved for circumstances in which insect biting pressures are intense and in which other factors, such as high temperature and humidity, may promote rapid loss of repellent from the skin surface.

Repellents may be applied directly to the skin or to clothing, window screens, mesh insect nets, tents, or sleeping bags. Persons who are particularly concerned about potential toxicity from DEET may limit application of the repellent to their clothes. If DEET-treated garments are stored in a plastic bag between wearings, the repellent effect can last for many weeks. 

Repellents containing DEET must be carefully applied because they can damage plastics (such as watch crystals and eyeglasses frames), rayon, spandex, other synthetic fabrics, leather, and painted or varnished surfaces. DEET does not damage natural fibers, such as cotton or wool, and has no effect on nylon.

Until 1989, the standard-issue insect repellent of the U.S. military consisted of 75% DEET in an alcohol base. Complaints about the aesthetic feel of this product and concerns about potential toxicity under long-term daily use led to U.S. Army-sponsored studies to produce new formulations. The 3M Company (St. Paul, Minnesota) developed a slow-release, polymer-based product containing 35% DEET; this has become the repellent provided to all U.S. military personnel. This product is available to the general public exclusively through the Amway Corporation (New York, New York) under the brand name HourGuard. If lower-strength formulations of extended-release DEET are desired, Minnetonka Brands (Eden Prairie, Minnesota) offers products containing 6.5% and 10% DEET. As a general rule, higher concentrations of DEET provide longer-lasting protection. Unfortunately, no guidelines are available to help consumers decide what concentration of DEET is appropriate for their specific needs. The number of variables that affect a repellent's effectiveness precludes assigning an "insect repellent factor" to individual products.

Avon (New York, New York) Skin-So-Soft bath oil received considerable media attention several years ago when some consumers reported it to be effective as a mosquito repellent. When tested under laboratory conditions against Aedes aegypti mosquitoes, this product's effective half-life was 30 minutes. Against Aedes albopictus, Skin-So-Soft oil provided 40 minutes of protection from bites, a duration 10 times less than that of 12.5% DEET. It has been proposed that the limited mosquito repellent effect of Skin-So-Soft oil could be caused by its fragrance or the presence of diisopropyl adipate and benzophenone in the formulation, both of which have some repellent activity. Avon now markets products under the Skin-So-Soft label that contain an EPA-recognized repellent

Plant-Derived Repellents

Thousands of plants have been tested as potential sources of insect repellents. None of the plant-derived chemicals tested to date demonstrate the broad effectiveness and duration of DEET, but a few show repellent activity. Plants whose essential oils have been reported to have repellent activity include citronella, cedar, verbena, pennyroyal, geranium, lavender, pine, cajeput, cinnamon, rosemary, basil, thyme, allspice, garlic, and peppermint. Unlike synthetic insect repellents, plant-derived repellents have been relatively poorly studied. When tested, most of these essential oils tended to give short-lasting protection, usually less than 2 hours.

Citronella is the active ingredient most commonly found in "natural" or "herbal" insect repellents marketed in the United States. It is registered with the EPA as an insect repellent. Citronella oil has a lemony scent and was originally extracted from the grass plant Cymbopogon nardus. Limited data are available from studies that directly compared the efficacy of citronella-based products with that of DEET-based products. In one study, 0.01 µmol of DEET per L of air was sufficient to prevent 90% of mosquitoes from landing on their targets; a 1000-fold higher concentration of citronellol (one of the active chemicals in citronella oil) was required to achieve a similar effect.

Studies show that citronella can be an effective repellent, but it provides shorter complete protection time than most DEET-based products. Frequent reapplication of the repellent can partially compensate for this. The manufacturer of Natrapel (Tender Corp., Littleton, New Hampshire) has laboratory data showing that their 10% lotion reduced mosquito bites by 84% during a 4-minute test period. In contrast, 14% DEET reduced biting by 96% in the same test period. Buzz Away (Quantum, Inc., Eugene, Oregon) with 5% citronella oil provided an average protection time of 1.9 hours against Aedes aegypti. In field testing, Buzz Away Oil provided an average of 88% repellency during a 2-hour exposure. In general, the repellency of Buzz Away was greatest within the first 40 minutes after application and decreased over the remainder of the test period.

Citronella candles have been promoted as an effective way to repel mosquitoes in the backyard. One study compared the ability of commercially available 3% citronella candles, 5% citronella incense, and plain candles to prevent bites by Aedes mosquitoes under field conditions. Persons near the citronella candles had 42% fewer bites than controls, who had no protection (a statistically significant difference). However, burning ordinary candles reduced the number of bites by 23%. The efficacy of citronella incense and plain candles did not differ. The ability of plain candles to decrease biting may result from their action as a decoy source of warmth, moisture, and carbon dioxide.

The citrosa plant (Pelargonium citrosum 'van Leenii') has been marketed as being able to repel mosquitoes through the continuous release of citronella oils. Unfortunately, when tested, these plants offer no protection against bites.


Pyrethrum is a powerful, rapidly acting insecticide, originally derived from the crushed and dried flowers of the daisy Chrysanthemum cinerariifolium. Permethrin is a human-made synthetic pyrethroid. It does not repel insects but works as a contact insecticide, causing nervous system toxicity that leads to the death or "knockdown" (out of the air) of the insect. The chemical is effective against mosquitoes, flies, ticks, and chiggers. Permethrin has low toxicity in mammals, is poorly absorbed by the skin, and is rapidly inactivated by ester hydrolysis.

Permethrin should be applied directly to clothing or other fabrics (such as tent walls or mosquito nets), not to skin. The spray form is nonstaining, nearly odorless, and resistant to degradation by heat or sun and maintains its potency for at least 2 weeks, even through several launderings. The combination of permethrin-treated clothing and skin application of a DEET-based repellent creates a formidable barrier against mosquito bites. In a field trial conducted in Alaska, persons wearing permethrin-treated uniforms and a polymer-based 35% DEET product had more than 99.9% protection (1 bite/h) over 8 hours, even under conditions of intense biting pressures; unprotected persons received an average of 1188 bites/h.

Permethrin-based insecticide sprays can be apply to clothing. Spray each side of the fabric (outdoors) for 30 to 45 seconds, just enough to moisten it. Allow the garment to dry for 2 to 4 hours before wearing it.

Relief from Mosquito Bites

Several strategies exist for relieving the itch of mosquito bites. Topical corticosteroids can reduce the erythema, itching, and induration. Topical diphenhydramine and caine-containing derivatives should be avoided because of concerns about inducing allergic contact sensitivity. Oral antihistamines can be effective in reducing the symptoms of mosquito bites.


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