For many years, Plant-based repellents have been as a personal protection measure against blood-sucking mosquitoes. Knowledge on traditional repellent plants obtained through ethnobotanical studies is a valuable resource for the development of new natural products.
Recently, commercial repellent products containing plant-based ingredients have gained increasing popularity among consumers, as these are commonly perceived as “safe” in comparison to long-established synthetic repellents such as icaridin and DEET. Carried out by Marta Ferreira Maia and Sarah J Moore from The National Center for Biotechnology Information (NCBI), this study is a summary of recent information collected on the testing, efficacy and safety of plant-based repellents as well as promising new developments in the field.
Naturally, most plants contain compounds that they use to protect themselves from plant-eating insects. These chemicals fall into several categories, including repellents, feeding deterrents, toxins, and even growth regulators. Although the primary functions of these compounds is defence against harmful insects, many are also effective against mosquitoes and other biting insects. Insects can detect the odours of the chemical using their receptor neurons that are exposed to the external environment. Interestingly, the same odour receptors that respond to DEET also respond to plant repellents.
This repellency of plant material has been exploited for thousands of years by man, most simply by hanging bruised plants in houses, a practice that is still in wide use throughout the developing countries. Plants have also been used for centuries in the form of crude fumigants where plants were burnt to drive away nuisance mosquitoes and later as oil formulations applied to the skin or clothes which was first recorded in writings by ancient Greek, Roman and Indian scholars.
Plant-based repellents are still extensively used in this traditional way throughout rural communities in the tropics because for many of the poorest communities the only means of protection from mosquito bites that are available, and indeed for some of these communities, as in the Europe and North America “natural” smelling repellents are preferred because plants are perceived as a safe and trusted means of mosquito bite prevention.
Oils and extracted from to citronella plants are commonly used as ingredients of plant-based mosquito repellents. Citronella has found its way into many commercial uses through its familiarity, rather than its efficacy. Citronella was originally extracted for use in perfumery, and its name derives from the French citronelle around 1858. It was used by the Indian Army to repel mosquitoes at the beginning of the 20th century and was then registered for commercial use in the USA in 1948.
With concentrations of of 5-10%, citronella is one of the most widely used natural repellents on the market. This is lower than most other commercial repellents but higher concentrations can cause skin sensitivity. However, there are relatively few studies that have been carried out to determine the efficacy of essential oils from citronella as arthropod repellents.
Citronella-based repellents only protect from host-seeking mosquitoes for about two hours although formulation of the repellent is very important. Initially, citronella, which contains citronellal, citronellol, geraniol, citral, α pinene, and limonene, is as effective dose for dose as DEET.
Neem is widely advertised as a natural alternative to DEET, and it has been tested for repellency against range of arthropods of medical importance, with variable results. Several field studies from India have shown very high efficacy of Neem-based preparations, contrasting with findings of intermediate repellency by other researchers. However, these contrasting results may be due to differing methodologies, and the solvents used to carry the repellents. The EPA has not approved Neem for use as a topical insect repellent. It has a low dermal toxicity, but can cause skin irritation, such as dermatitis when used undiluted. Due to the paucity of reliable studies, Neem oil is not recommended as an effective repellent for use by travellers to disease endemic areas, although it may confer some protection against nuisance biting mosquitoes.
Essential oils distilled from members of the culinary herbs and aromatic grasses are commonly used as insect repellents throughout the globe. Many members of these families are used in rural communities through burning or hanging them within homes. In Europe and North America there is a strong history of use of the oils dating back to Ancient times. Almost all of the plants used as repellents are also used for food flavouring or in the perfume industry, which may explain the association with these oils as safer natural alternatives to DEET.
Many commercial repellents contain a number of plant essential oils either for fragrance or as repellents including peppermint, lemongrass, geraniol, pine oil, pennyroyal, cedar oil, thyme oil and patchouli. The most effective of these include thyme oil, geraniol, peppermint oil, cedar oil, patchouli and clove that have been found to repel malaria, filarial and yellow fever vectors for a period of 60-180 mins. Most of these essential oils are highly volatile and this contributes to their poor longevity as mosquito repellents. However, this problem can be addressed by using fixatives or careful formulation to improve their longevity.
Natural oils and emulsions
Several oils have shown repellency against mosquitoes. It is likely that they work in several ways; by reducing short range attractive cues i.e. kairomones, water vapour and temperature, by reducing the evaporation and absorption of repellent actives due to the presence of long-chained fatty molecules, and by containing fatty acids are known to be repellent to mosquitoes at high concentrations.
Bite Blocker, a commercial preparation containing glycerin, lecithin, vanillin, oils of coconut, geranium, and 2% soybean oil can achieve similar repellency to DEET, providing 7.2 hours mean protection time against a dengue vector and nuisance biting mosquitoes in one study, and protection for 1.5 hours, equivalent to that of low concentration DEET in a second study. It would appear that the soybean oil in Bite Blocker helps only contributes to repellency as it is not repellent when evaluated on its own. Other plant-based oils that have shown some repellent efficacy are coconut oil, palm nut oils and andiroba oil.
Although all of these three oils are far less effective than DEET, they may be useful as carriers for other repellent actives as they are cheap and contain unsaturated fatty acids and emulsifiers that improve repellent coverage and slow evaporation of volatile repellent molecules.