Discussion
This study attempted to record knowledge about plant species used as repellents against insects/mosquitoes in 6 localities of Cameroon to identify promising candidate plants that might be formulated as insect repellents. The results achieved indicated 16 plant species used by the people of Lolodorf, Bipindi, Dimako and their surroundings, and Londji, Kon-Yambetta and Babete of Cameroon to drive off insects, especially mosquitoes.
Most of the interviewed informants were men (86.8%) against 13.2% of women, similar to the pattern previously reported by Cheikhyoussef et al. [
16], Bekalo et al. [
17], and Okello and Segawa [
18]. Besides, a meta-analysis on the continental level recently indicated significant differences in the knowledge of men and women in Africa when it comes to ethnobotanical knowledge. Additionally, women in rural Africa frequently collect plants for firewood purpose as one of the domestic activities they are responsible for [
19].
The records showed that the local inhabitants (78.6%) had knowledge about the mosquito-repellent property of plants and their use. Most of the plants recorded (50%) were reported to be burnt to produce smokes inside the house, aligning with the claims of Pålsson and Jaenson [
20] that indicated that burning plants might be effective in repelling insect. Similar results were equally obtained in Ethiopia and Tanzania, supporting this approach across communities as consistent to drive away mosquitoes [
21,
22]. Biran et al, [
23] equally reported mixed evidence especially for using firewood smoke to protect against mosquito bite. Despite the scarcity of data on how repellent smokes or their constituents act, the repellent activity of burned plants appears to be due to the release of specific volatile compounds, either already present in the fresh/dried plant or created during the combustion process [
23,
24]. Such compounds like ß-ocimene have been previously showed to exert mosquito repellent activity [
24]. Moreover, the smoke could act by disguising human kairomone cues targeted by insects and disrupting the convection currents essential for mosquito host location [
22,
23].
Smashed plant samples for topical application (31.2%) and hung plants leaves inside the house (18.8%) were also recorded during our study as important modes of administration. These modes of repellent plants administration have been exploited for thousands of years by man, and are still in wide use throughout the developing countries [
4]. Plants have been used for centuries in the form of crude fumigants where they are burnt to drive away mosquitoes and later on as formulations that are applied to the skin or clothes as firstly recorded in writings by ancient Greek [
25], Roman [
26], and Indian scholars [
27].
Among the repellent plants species recorded in this survey, the main parts used were reported to be the leaf (52.9%) and the bark (17.6%). These findings are consistent with those of Zorloni et al. [
28] who reported plants leaves and bark as predominantly used for tick control in West Ethiopia. The prominent application of leaves might be owing to the ready availability of their active constituents that are more likely volatile compounds. Indeed, plants that are commonly used for repellent properties are mostly those that contain essential oils, and when crushed or brushed against, leaves release strong odours of which some are pleasant, and some not so pleasant to insects.
The traditional use of plants or plant products against biting insects is a common practice in Africa [
29]. Their insect-repellent effect observed may be attributed to their chemical composition [
30]. Various active constituents with insects’ repellent activity have been previously reported from some of the checked plants species. Apart from
Glossocalyx longicuspis and
Greenwayodendron suaveolens for which very little has been reported as repellent properties, the other listed species or related species have previously been reported to display mosquito/insect repellent or insecticidal activities. Within this scope, the insecticidal activity of the bark extract of
Erythrophleum ivorense was previously reported in the Ashanti region of Ghana [
31]. Besides
, Erythrophleum ivorense is resistant to fungi, dry wood borers and termites [
32]. This denotes repellency/insecticidal properties that might be explained by the presence of pharmacologically active alkaloids in the bark and seed such as cassaine, cassaidine and erythrophleguine. However, it should be noted that high doses of the bark extract are extremely strong, rapid-acting cardiac poison in warm-blooded animals causing shortness of breath, seizures and cardiac arrest in a few minutes. Furthermore, the seeds are reported to be more toxic due to a strong haemolytic saponin which acts synergistically with the alkaloids [
32]. Fresh bark of this plant was reported to be burnt by Mbamda (Bafia) people to repel in-house mosquitoes. Given the presence of toxic alkaloids in the bark, the resulting smokes are highly likely to be equally poisonous to insects and human, stressing the fact that it should be used with caution or simply discontinued.
Canarium schweinfurthii (Burseraceae), commonly known as African elemi or canarium, is a species of large tree native to tropical Africa
. The African elemi tree is one of several sources of the economically useful oleoresin known elemi. In West Africa this resin is traditionally burned for fumigating dwellings and mixed with oil for body paint [
33], and might likely exhibit insect repellent activity.
C. schweinfurthii was the more cited by informants (FC > 32 when recorded) and was also previously reported to have insecticidal activity against
Callosobruchus maculates, a major pest of cowpeas, green gram and lentils [
34]. The authors assumed that this activity might be due to the presence of saponnins that have otherwise been found to affect the respiratory system of insect and to cause emetic effect by their detergent action. In addition, tannic acid found in this plant species was reported by David [
35] to act as toxin and feeding deterrent to insects.
Chromolaena odorata (Compositae) is a tropical and subtropical species of flowering shrub in the sunflower family. It is native to North America, from Florida and Texas to Mexico and the Caribbean [
36], and has been introduced to South America, tropical Asia, West Africa, and parts of Australia [
37,
38]. It is sometimes grown as a medicinal and ornamental plant. It is used as a traditional medicine in Indonesia, Thailand, Malaysia and parts of Africa. In traditional medicine of Thailand, the plant is used for the treatment of wounds, rashes, diabetes, and as insect repellent. It has antifungal and antibacterial properties. It has previously been reported to have insecticidal properties against adult stage of
Periplaneta americana, an omnivorous and opportunistic feeder which is the largest common species of pest cockroach especially in the tropics and subtropics [
39]. Moreover, a survey undertaken in the Ashanti Region of Ghana revealed the use of
C. odorata leaf as repellent with insecticidal properties [
31].
C. odorata leaf and root have been shown to contain alkaloids, phenols, flavonoids, saponins, cardenolides, anthraquinones and tannins [
40] that might elicit the insecticidal activity. Furthermore, this plant contains carcinogenic pyrrolizidine alkaloids that are toxic to cattle and can also cause allergic reactions [
38,
41]. Previous findings also indicate that this class of alkaloids (retronecine type alkaloids) can elicit insecticidal activity [
42]. This activity could also be linked to constituents such as flavonoids which are a class of phenolic compounds occurring naturally in this plant [
43] and that have anti-feeding and attracting deterrent properties, thereby exerting toxic effects to insects, fungi, bacteria, nematodes and weeds [
40].
Citrus limon, otherwise called lemon, is a small tree of the Rutaceae family that originated in Asia and is now grown commercially worldwide in tropical, semi-tropical, and warm temperate countries for the fruit, which is used fresh and in beverages and cooking, and is also used as a preservative due to its anti-oxidant properties. Lemon oil, obtained from the peel, is used as a wood cleaner and polish, and as a non-toxic pesticide. Traditional medicinal uses for the fruit, peels, and oil obtained from the seeds include treating fever and colic, and as an astringent and diuretic [
44]. The topical repellent property of extracts from peels of
C. limon against mosquitoes was previously reported by Effiom et al. [
45]. In ancient medicine, Lemon citrus (
C. limon) has long been used as natural insect repellent. Moreover,
C. limon essential oil showed to be effective against mosquito larva, and to be also repellent against malaria vector,
Anopheles stephensi in laboratory animal and human [
46]. Kazembe and Chaibva [
47] also showed that the whole extract of fruit peel and volatile oils had mosquito repellency against
Aedes aegypti. On the specific compositional scale, citronellol, the most prominent component of
C. limon essential oil, and linalool have been shown to be the main active ingredients of lemon in the distillate, and have also been identified among the main active ingredients of other botanical repellents such as citrosa and eucalyptus [
46].
Cymbopogon (Poaceae), better known as lemongrass, is a genus of Asian, African, Australian, and Tropical Island plants in the grass family [
48,
49]. Some species (particularly
C. citratus) are commonly cultivated as culinary and medicinal herbs because of their scent, resembling that of lemons (
Citrus limon). Common names include lemon grass, lemongrass, barbed wire grass, silky heads, citronella grass, fever grass, amongst many others. Research has shown that lemongrass oil has antifungal properties [
50], but also the ability to repel some insects, such as mosquitoes. However, its oil is commonly used as a kind of trap to attract honey bees, working conveniently as honeybee's attractant pheromones (Beekeeping/Guide to Essential Oils). Essential oils from lemon grass principally contain geraniol and citronellol that are antiseptics, hence their use in household disinfectants and soaps.
Cymbopogon spp. have a long history of use to repel mosquitoes and are as effective as the chemical insect repellent N,N-diethyl-meta- toluamide (DEET) [
51].
C. nardus cultivation was introduced by Religious nuns at the Monastery Saint-Benoit in Mbouda-Cameroon and is used as mosquito repellent. Historically, it was used by the Indian Army to repel mosquitoes at the beginning of the 20
th century [
52]. Essential oils and extracts from
C. nardus and globally from the
Citronella genus are commonly used as ingredients of plant-based mosquito repellents in commercial preparations [
53]. Moreover, attempts to scientifically demonstrate the repellent efficacy of Citronella grass essential oils and formulations against mosquitoes and arthropods have been previously undertaken [
53,
54]. Trongtokit et al. [
55] also demonstrated that 100% essential oil of Citronella protects against
Aedes aegypti,
Culex quinquefasciatus and
Anopheles dirus and related this activity to the presence of citronellol and related compounds.
Saccharum officinarum (Poaceae) or sugarcane, is a large, strong-growing species of grass in the genus
Saccharum. It is cultivated in tropical and subtropical countries worldwide for the production of sugar and other products.
S. officinarum has been shown to act as repellent, toxicant, and anti-feeding against a number of Coleoptera that attack stored food crops. The insecticidal property of
S. officinarum bagasse-based lignin may be due to the presence of phenolic and alcoholic compounds [
56]. Also, the efficacy of application of the stem juice of this plant on the skin as traditional personal protection method was evaluated against mosquito bites and general nuisance in Bolifamba, a rural setting of the Mount Cameroon area. The field evaluation of the skin application of the juice showed significant protection against mosquito bites though, less than the commonly used commercial diethyltoluamide [
5].
Ocimum gratissimum, (Lamiaceae) commonly called clove basil, African basil is a native species of Africa, southern Asia, and the Bismarck Archipelago [
57].
O. gratissimum is a common culinary herb in West Africa. Its essential oil contains eugenol and shows some evidence of antibacterial activity [
58,
59]. This compound has otherwise showed anti-insect activity, therefore supporting the use of the plant as insect repellent. The authors alluded that this activity of eugenol was dependent on the structure of its phenolic hydroxyl [
60]. The essential oil has potential for use as a food preservative [
61]. An ethnobotanical investigation carried out in Bamenda, Cameroon reported
O. gratissimum to be used as insect repellent [
62]. This mosquito repellent plant is cultivated around houses for such purpose [
63]. Also, people from many parts of Tanzania burn this plant to release smokes or hang it in houses to drive mosquitoes away [
64]. Moreover, many other reports describe the insect repellent and insecticidal properties of
O. gratissimum in field and laboratory trials, particularly against the main vectors of malaria and lymphatic filariasis [
63,
65‐
67]. As asserted above, other studies also suggested that the insecticidal activities might be due to the presence of eugenol found in
O. gratissimum [
68,
69].
Musa paradisiaca L. and
Musa sapientum L. (Musaceae) are mainly grown in the tropical and subtropical countries and are widely used for their nutritional values all over the world. The fruits as well as the other parts of the plant are used to treat different diseases in human in traditional medicines [
70]. Our records indicated that
M. paradisiaca and
Musa sp. are used as insect repellents. This information was corroborated by previous reports showing that the dried leaf and stem of
M. paradisiaca are burnt by the Ayta people of Porac, Pampanga province, Philippines, to drive off insects especially mosquitoes. In addition, laboratory investigations revealed the remarkable mosquitocidal activity of the petroleum ether root extract of this plant against
Ae. aegypti,
An. stephensi and
Cx. Quinquefasciatus [
71]. As well, 10% (w/v) concentration of
M. paradisiaca leaf extract showed repellent protection of
Pterygota excelsa wood against termite,
Odontotermes obesus [
72,
73]. Also, this extract exerted insecticidal activity against malaria vector
Anopheles stephensi with 90% lethal effect after 24 h exposure [
74]. Besides, the phytochemical studies of many parts of
M. paradisiaca and
M. sapientum, including leaf, fruit, peeled fruit, fruit pulp, fruit peel, flower, bracts, and scape previously revealed the presence of many chemical classes of components such as anthocyanins, Catecholamines, tryptophan, indole compounds, pectin, flavonoids and related compounds (Leucocyanidin, quercetin and its 3-Ogalactoside, 3-O-glucoside, and 3-O-rhamnosyl glucoside), tannins, Acyl steryl glycosides such as sitoindoside-I, sitoindoside-II, sitoindoside-III, sitoindoside-IV and steryl glycosides such as sitosterol gentiobioside, sitosterol myo-inosityl- β-D-glucoside, triterpenes such as cyclomusalenol, cyclomusalenone, 24- methylenecycloartanol, stigmast-7-methylenecycloartanol, stigmast -7-en-3-ol, lanosterol and β -amyrin [
70,
75]. Among all these chemical classes, there are some that might elicit biological activities sustaining the repellent feature of
Musa sp. For example, compounds such as anthocyanins have been suspected to act in a vast array of plant/animal interactions, including attraction of pollinators and frugivores, as well as the repellence of herbivores and parasites [
76]. Also, saponins that derive from a sugar moiety glycosidically linked to a hydrophobic aglycone which may be a triterpene or a steroid have been shown to trigger plant resistance against insects [
77]. The mechanisms underlying the action of these compounds against insects are based on their various biological properties. They have membrane-permeabilising, haemolytic, antioxidant, anti-inflammatory, immunostimulant and anticarcinogenic activities, and can affect feed intake, growth and reproduction in animals, and can be used as fungicides, molluscicides and pesticides, as well as against some bacteria and viruses. They can also increase mortality levels, lower food intake, weight reduction, retardation and disturbances in development, and decrease reproduction in insects [
78].
The main hypotheses are therefore that saponins could either make the food less attractive to eat (repellent/deterrent activity), bear digestive problems, causing moulting defects or having toxic effects on insects. Another class of compounds such as flavonoids are small molecular secondary metabolites synthesized by plants with various biological activities. Due to their physical and biochemical properties, they are capable of participating in interactions with other organisms. Both flavonoids and isoflavonoids protect plants against insect pests by influencing their behavior, growth and development [
79,
80]. In this line, Naringenin, hesperetin-7-O-rutinoside and quercetin-3-O-rutinoside were reported to stimulate oviposition in swallowtail butterfly Papilio on young leaves of citrus plants [
81]. Also, Chrysin, Kampferol, and 3,7- dimethylether quercetin were found to exert remarkable repellent action against house flies (
Musca domestica) [
82]. Another interesting example is the tannins that were assessed as repellents in other studies. Tannins have a strong deleterious effect on phytophagous insects and affect the insect growth and development by binding to the proteins, reduce nutrient absorption efficiency, and cause midgut lesions [
83].
Elaeis guineensis (Arecaceae) commonly called African oil palm or macaw-fat is the principal source of palm oil. It is native to west and southwest Africa, specifically the area between Angola and the Gambia. An innovative phytodrug (API-PALU) was recently formulated from the crude alkaloids extract of this plant and is extensively used in West Africa to treat malaria (http://www.africanews.com/2016/06/24/beninese-wins-100000-for-innovation-of-anti-malaria-drug/). The innovator recently won many prizes including the 1
st IPA 2016 prize (http://www.africanews.com/2016/06/24/beninese-wins-100000-for-innovation-of-anti-malaria-drug/).
E. guineensis was one of the most cited species recorded in our survey and has been reported in Africa as reducing mosquito biting activity when used as repellent [
84]. In fact, smokes from burned infructescences of
E. guineensis were reported to reduce the numbers of mosquitoes indoors at night. Also, a field experiment using these smokes showed 69% repellent activity [
20]. As well,
E. guineensis palm nut (lotions and creams) and oil were shown to reduce significantly the number of bites by
Simulium damnosum and
Anopheles gambiae [
85,
86]. Moreover, flowers of
E. guineensis were recorded in this study to be the primary ingredient to combine with the leaves of
Chromolaena odorata, Saccharum officinarum, Musa paradisiaca, Musa sp., and the bark of
Cylicodiscus gabunensis to repel insects and mosquitoes. The phytochemical composition of the leaf and flower of
E. guineensis could justify the repellent activity elicited by this plant. Indeed, a recent study recently revealed the presence of phenolic compounds, flavonoids, tannins, coumarins, alkaloids, saponins, terpenoids and steroids, and carbohydrates in the leaf of
E. guineensis [
87]. Another previous study identified p-methoxyallylbenzene (estragole) as the predominant (~95%) constituent of essential oils from both male and female flowers of
E. guineensis [
88]. Many of the secondary metabolites classes identified in the leaf have already been reported as having deleterious effect on insects [
40,
42,
43,
56,
77,
79‐
81]. From recent studies, it was demonstrated that the main component of
E. guineensis flowers essential oils (estragole) has strong repellent effect to three of the major grain pest insect species,
viz. Rhyzopertha dominica, Sitophilus zeamais, and
Tribolium confusum. Interestingly, estragole and (E)-anethole (a related phenolic compound also found in essential oils) showed a strong synergistic co-repellent effect against
R. dominica [
89]. These latter findings further emphasize the potential of components of single and/or mixed plants to act synergistically to repel insects.
Cylicodiscus gabunensis (Leguminosae) is a large tree, common in the rainforests of West and Central Africa. The stem bark is used to treat jaundice and malaria among other diseases. It contains triterpene saponins, cylicodiscus acid, a dihydroxy-pentacyclic triterpene carboxylic acid and cyclodione, a dimeric diterpene [
90]. There are however, no references in the literature describing the repellent application of the stem bark of this plant despite its strong odour [
91]. Nevertheless,
C. gabunensis wood is very resistant to worm and insect attack [
92]. However, the strong odour exhaled by
C. gabunensis stem bark is likely elicited by its terpenoids content, and might otherwise justify its repellent effect against insects.
Our study also reported
Piper umbellatum (Piperaceae) to be used as mosquitoes/insects repellent. It is commonly known as English cow-foot leaf (Sierra Leone),
Fula-Pulaar (Guinea); Poponidagui
(Sierra Leone). It is an upright shrub to 2 m high in moist shady places occurring from Guinea to Cameroon, and widespread throughout the tropics where it is used as condiment, and also considered a fetish as well as a medicine to treat a vast array of ailments including but not limited to pain, arthritis, rheumatism, fever, diarrhoea, dysentery, venereal diseases, hemorrhoids [
93].
Previous studies have reported the repellent and insecticidal activities of the essential oil of
P. umbellatum when investigated against grain storage pest insects, bean weevil (
Callosbruchus maculatus) and rice weevil (
Sitophilus oryzae), suggesting its suitability for insect pest control [
31,
94]. As well, when
P. umbellatum fresh leaves are crushed and rubbed on the skin there is an effective though transient (1-2 h) repellent effect, particularly against mosquitos. The active agent was found to be the essential oil consisting of aldehydes, ketones, and phenols in addition to the principal constituents,
viz. cadinene, caryophyllene, and phallandrene. The repellent effect may be prolonged by mixing the crushed leaves with oils or glycerin-alcohol [
94,
95]. On another hand, extracts from closely related species
Piper nigrum,
Piper guineense, and
Piper tuberculatum contain isobutyl amides, and other plant secondary metabolites that act as neurotoxins and showed repellent activity against insects [
96].
Premna angolensis (Lamiaceae) has also been reported to have repellency potential [
97]. The various
Premna species are well known for their medicinal properties and their further phytochemical investigations resulted in the isolation of secondary metabolites including iridoids and their glycosides, diterpenoids, sesquiterpenoids, triterpenoids, flavonoids, isoflavones, lignans, xanthones and other classes of compounds [
97] that might be involved in the repellency properties. Many plants from Lamiaceae family have been found to be effective against a variety of mosquito vectors. For instance, the crude aqueous, chloroform and methanol extracts and essential oils from the leaves of
P. latifolia showed mosquito larvicidal efficacy against the fourth instar larvae of
Aedes albopictus Skuse (Diptera: Culicidae) [
98].
P. angolensis and
P. quadrifolia leaves are burned and used as fumigant in the attics of cereals against pests. Besides, their essential oils displayed insecticidal and repellent effects against
Sitotroga cerealella, an insect pest of rice stocks [
99].