The use of repellent substances against blood-sucking parasites probably evolved thousands of years ago and is also known from the animal kingdom. Capuchins anoint their coat with millipedes or leaves of lemontrees and pepperplants, and the Corsican blue tits bind lavender, rockrose, and Corsican yarrow into their nests to keep parasites away from their offspring. The crested auklet, a small seabird, changes its body odors during the breeding season, and emits higher amount of certain aldehydes in order to become less attractive to blood-sucking parasites.
The first recorded use of repellents by humans goes back to 425 BC: Egyptians burned down castor oil, and slept under rudimentary bednets. Romans rubbed a mixture of oil, vinegar, and manna onto their body; additionally they burned certain herbs like laurel, oregano, or black cumin. The native Americans used parsnip infusions to treat their skin, or they put fringed sagewort on campfire coals to repel mosquitoes. The use of sludge also created a certain barrier between blood-sucker and skin. The burning of plant materials is still widely used to repel mosquitoes, especially in the rural tropics. Smoke production reduces humidity, thereby increases the risk of desiccation for the mosquito. It also masks human kairomones that are important for host-finding (particularly CO2 from exhaled breath).
The first modern skin repellents consisted of a mixture of essential oils, like citronella and camphor, however, the provided bite protection was rather poor. In the 1940s, three chemical substances with satisfying repellent properties became available: dimethylphtalate (DMP), Indalone, and Rutgers 612. The US military finally initiated a large substance screening of more than 20.000 candidate compounds. This research was motivated by the enormous impact of vector-borne diseases on the troops in endemic areas. In 1953, diethyltoluamide or Deet was discovered: the “gold standard” amongst available insect repellents until today. Deet is the major compound in a variety of commercial products, and is used by more than 200 million people every year.
Within the past decades, both the number of available repellent substances and the lineup of commercial anti-mosquito products have enlarged, and are no longer limited to formulations to be applied to the skin. The market offers candles, torches, and other essential oil evaporators, electric insecticide evaporators, or coils for outdoor use. Furthermore, one can purchase impregnated textiles, insecticide treated bednets, and even sound emitting devices, that claim to deter mosquitoes by certain frequencies – the latter never showed any repelling effect in a variety of laboratory or field tests. Citronella candles or evaporators are also ineffective, although the essential oil itself bears some repellent properties. Due to the high vapor pressure these effects are shortlived which is a disadvantage of the most essential oils. In case of using a repellent based on essential oils, one has to keep in mind that the skin needs to be treated frequently. They should not be used in tropical areas.
The American Environmental Protection Agency (EPA) recommends skin formulations that contain one of the following active ingredients: Deet, Picaridin, IR3535, para-menthan3,8-diol, catnip oil, oil from lemon eucalyptus, citronella, and undecanone.
Mode of Action
Little is known about the specific mode of action of insect repellents. A widely used definition proposes that a repellent is a chemical stimulus which can be perceived by the olfactory system of the target organism, and which causes a movement away from its source. Such an avoidance behavior cannot always be observed in practice. Repellency is usually mediated through the contact between the target insect and the treated surface; hence, a repellent is rather a substance which provides bite prevention.
References: Debboun, M., Frances, S.P. und Strickman, D. (2007): Insect Repellents. Past, Principle and Uses. CRC Press, Boca Raton
Rose, A. & Kröckel, U. (2009): Prävention vektoriell übertragener Krankheiten, in: Moderne Reisemedizin, Gentner Verlag, Stuttgart
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