This time of the year, it is difficult to go anywhere outside without the familiar high-pitched buzz of mosquitoes becoming a part of the routine. Aside from the noise, it is often the resulting red bumps and incessant itchiness that are most frustrating.
Plants, their extracts, and their essential oils have been used for centuries to fight aggressive mosquitoes responsible for malaria, dengue, sleeping sickness but also insects acting as vectors for many other diseases.
The repellent and insecticidal potential of plant materials have been exploited for thousands of years by man, most simply by burning or hanging bruised plants in houses.
Sometimes, the crushed leaves or plant oil is applied on the body to ward off mosquitoes. The leaves are good mosquito repellents because of their smell and toxicity to insects.
Plant-based repellents have been used for generations in traditional practice as a personal protection measure against mosquitoes. Recently, commercial repellent products containing plant-based ingredients have gained increasing popularity, after being perceived as “safe” in comparison to synthetic repellents.
Expert’s comparison of Ocimum gratissimum (efinrin in Yoruba or Basil Leaves) and Ocimum basilicum with other plant repellents in the 2012 edition of Bulletin Environmental Pharmacology and Life Sciences indicated they were comparable to Azadirachta indica, Citrus lemon, and Jatropha curcas in preventing mosquito bites.
In fact, application of oils from O. gratissimum and O. basilicum in liquid paraffin, researchers at the Obafemi Awolowo University, Ile-Ife said demonstrated pronounced bite protection greater than 70 per cent for two to three hours at all concentrations tested.
In the 2000 edition of the Journal of Tropical Medicinal Plants, the researchers said while complete protection was evident following the application of at least 15 per cent of O. gratissimum oil solution, its repellence is comparable to that of a commercial repellent wipe containing synthetic repellents.
Moreover, a study in the 2010 publication of Journal of Applied Bioscience listed 10 plants used by the indigenous people of Ibadan against Anopheles stephensi. It involved Egunyomi A, Gbadamosi I.T., and Osiname K.O, from the Department of Botany and Microbiology, University of Ibadan.
These are Azadirachta indica (neem leaves), Cymbopogon citratus (Lemongrass leaves), Ocimum gratissimum (Basil leaves), Ageratum conyzoides (Goatweed leaves), Annona squamosa (Sweetsop leaves) and Hyptis suaveolens (Bush tea leaves).
Others were Tridax procumbens (Coat buttons leaves), Citrus sinensis (Orange fruit peels), Lantana camara (Wild sage leaves) and Solanum nigrum (Black nightshade leaves.
The study, which compared the repellent activities of these plants reported that seven out of the 10 plants (C. citratus, A.squamosa, L. camara, C. sinensis S. nigrum, T.procumbens and A. Indica) showed promising repellent activity against A. Stephensi. Low repellency was observed in O. gratissimum, A. conyzoides and H. suaveolens.
The extracts of C. citratus and L. camara showed very high repellency while the methanol extract of H. suaeveolens were inactive against the mosquitoes.
Also, complete protection was observed within 30minutes of application of hexane and methanol extracts of C. citratus and L. camara.
The mosquito repellency of different extract was measured on the basis of the number of mosquitoes that fed within a specified time (minute).
Given that the toxicity tests of the plants also confirm their safety in administration, they suggested that crude drugs such as ointments and oils could be prepared from these plants for topical application as mosquito repellents.
In addition, they suggested that the plants can be used alone or combined for effective protection against mosquitoes, including the control of its breeding.
But in 2016, expert’s review of mosquito repellents plants used traditionally in the African region put this at 64 in the journal, Experimental parasitology. Citrus sinensis, Eucalyptus, Lantana camara, Ocimum gratissimum and Lippia javanica the most commonly used in all the study regions.
Used since the 1940s, lemon eucalyptus oil is one of the more well-known natural repellents. The Centres for Disease Control and Prevention (CDC) have approved eucalyptus oil as an effective ingredient in mosquito repellent.
A recent study showed that a mixture of 32 per cent lemon eucalyptus oil provided more than 95 per cent protection against mosquitoes for three hours.
Although neem oil is advertised as a natural alternative, there are mixed results about its effectiveness. A recent study about the effectiveness of neem oil in Ethiopia found that it offered more than 70 per cent protection for three hours. Also, neem oil is not approved as a topical repellent because it can cause skin irritation.