Abstract: The essential oils extracted of six plants have been tested on adults of Anopheles gambiae (Diptera: Culicidae) in view to determine their insecticide effect. It is about Ocimum canum, Laggera pterodonta, Plectrancthus glandulosus, Eucalyptus camaldulensis, Hyptis spicigera and Pittosporum viridiflorum. These oils have been diluted in the hexane to different concentrations (5, 10, 15, 20, 30, 40 and 50 mg of active matter by m2 of screen) in order to expose anopheles in the OMS-cone. The introduction of 20 anopheleses in cones permits to note that alone two plants Hyptis spicigera and Pittosporum viridiflorum presented a weak insecticide activity. Except P. viridiflorum, the five other plants presented of lethals concentrations 50 (LC50) after 24 h of exhibition. Thus, we record for H. spicigera and efficient concentration Laggera pterodonta to 50 mg m‾2. On the other hand Ocimum canum is efficient to the concentration 40 mg m‾2. Lethal concentrations 50 (LC50) of O. canum and Plectrancthus glandulosus are, respectively 11.95 and 12.5 mg m‾2 after 24 h of exhibition. These essential oils also possess the best lethal h 50 (LH50) with lengths of exhibition of 6 h 36 min 16 sec and 8 h 34 min 12 sec, respectively for O. canum and of P. glandulosus.
INTRODUCTION
Bugs, especially mosquitoes are responsible of several parasitic illnesses at the such man the malaria, the yellow fever, the dengue and filariosis 8. The malaria in Africa contributes to a decrease of fruitfulness that affects the raw interior product heavily (Fekam, 2004). In Africa to the South of the Sahara, the malaria represents among the main reasons of morbidity and mortality for the man (Skinner and Johnson, 1980). Of synthesis products and derivative several types have been used without success against these vector agents (Njan Nlôga, 1994). In most cases, these products, not only present limits, but mislead the development of the resistance phenomenon at mosquitoes, giving back so the anti-vectorial chemical struggle little efficient. The program of struggle against mosquitoes only aims to reduce the density of the adult population for one very short time (El Hag et al., 1999, 2001). Pyrethrinoïdes have been used extensively to protect populations against stings of mosquitoes because of their repulsive effect and insecticide (Njan Nlôga, 1994). Substances of plant origin in general, the essential oils in individuals constitute this fact, a promising means of struggle against the vector bugs of illnesses (Prajapati et al., 2005; Strauss et al., 1968). Lees essential oils are recognized of a general manner as source natural of struggle against bugs (Gbolade et al., 2000; Adebayo et al., 1999). Generally, the essential oils are no poisonous for the man and the other mammals (Tripathi et al., 2002; Tedonkeng et al., 2002). The present research permits to put in evidence the insecticide effects of six oils essential of local plants, on adults of Anopheles gambiae.
MATERIALS AND METHODS
Study Area
The study is carried out in Ngaoundere (Cameroon), the headquarters of the
Adamaoua province. It is a transition zone between the forested south and the
sudan savannas of the north. The region is situated between latitudes 6°20
and 7°40 North and between longitudes 11° and 15° East. Its altitude
varies between 900 m and 1500 m. Rainfall stretches over seven months with an
average of 1500 mm. Average annual temperature varies between 23 and 25°C
(Stranss et al., 1968; Tripathi et al., 2004).
Materials
The plant material is constituted of six plants Ocimum canum, Laggera
pterodonta, Plectrancthus glandulosus, Eucalyptus camadulensis,
Hyptis spicigera and Pittosporum viridiflorum on the introverted
information basis close to the local populations for their efficiency in the
struggle against mosquitoes. Leaves of these plants have been picked and have
been dried to the shade during one week. Essential oil extractions have been
achieved by the hydrodistillator of Clevenger type. The introverted oils in
small bottles, tightly closed to the parafilm, are kept to the refrigerator
to a controlled temperature to 4°C until the day of their utilization.
The used bugs are mosquitoes of the species Anopheles gambiae. These bugs are raised to the Laboratory of Biology of the University of Ngaoundere from a stock of eggs coming of the OCEAC to Yaounde (Cameroon).
Method
Eggs are soaked in the restrained source water in trays of 40 cm of diameter
that are units of mosquito production. Every unit of production receives some
pinches of nourishing gunpowder (shrimps+cookies 5 g) (Skinner and Johnson,
1980). This fasting is introduced every morning to 8 h and this counterpart
5 days. The water of trays is renewed every two day. The renewal of the water
of source permits to maintain its quality and to avoid its pollution of the
fact of the daily introduction of the nourishing gunpowder. Nymphs are withdrawn
then of trays and are transferred in the transparent glasses. Glasses also containing
the water of source are placed inside of cages made of canvas of volume 8000
cm3 screen. In every cage, is placed a box of kneaded containing
the sugary juice made to basis of sucrose 10%, serving of food for the adult
mosquitoes.
Every essential oil is diluted to 7 different concentrations in the Hexane. Concentrations go to 5, 10, 15, 20, 30, 40 and 50 mg m-2. Screens filled of the different oils to the precise concentrations, are fixed then on the basis of the cone-OMS with the help of transparent sticky paper.
In every cone and for all concentrations, 20 anopheleses have been introduced. The effect Knock down has been observed after 3 min of exhibition and the device is maintained until 24 h with observations every 2 h. The number of mosquitoes killed is noted to calculate the death rate. The lethal concentration 50 (CL50) has been gotten after 24 h of treatment. The experience has been repeated for every oil and for every concentration, 5 times.
The regression of the logarithm of the dose as well as the transformation of mortality rates in probit permitted to determine the CL50 (Tedonkeng et al., 2002). The second regression is achieved according to hours to get the HL50.
RESULTS
Efficiency of the Essential Oils
Oil essential of L. pterodonta possesses a more efficient insecticide
effect on adults of A gambiae. The maximal mortality of mosquitoes, 100%
are observed to the concentration 50 mg m-2 after 24 h of exhibition
(Table 1). The insecticide effect increases with the concentration
of the essential oil used. It comes out again that mortality grows with the
concentration. The insecticide activity of this oil begins from the concentration
20 mg m-2. The Table 1 watch that concentrations
5, 10 and 15 mg m-2 present a weak activity beyond the 18th h.
Oil essential of H. spicigera has a progressive insecticide effect but limited on adults of A. gambiae. So, no concentration didn't provoke the death of all mosquitoes. Concentrations 5, 10 and 15 mg m-2 have insecticide effect from the 12th h of exhibition (Table 2). After 24 h of exhibition, the concentration 50 mg m-2 induced 70% of mortality. The concentration 20 mg m-2 has an insecticide efficiency splices the 22nd and the 24th h. The efficient dose was not therefore reached, even to the highest concentration of 50 mg m-2 used.
Of the Table 3, it comes out again that the death rate also grows with the concentration and the time of exhibition. With oil essential of O. canum, concentrations 40 and 50 mg m-2 misled one death rate to 100% at the end of 24 h.
| Table 1: | Mortality of Anopheles gambiae adults according to the length of exhibition to the different concentrations of essential oil of Laggera pterodonta |
| Table 2: | Mortality of Anopheles gambiae adults according to the length of exhibition to the different concentrations of essential oil of Hyptis spicigera |
| Table 3: | Mortality of Anopheles gambiae adults according to the length of exhibition to the different concentrations of essential oil of Ocimum canum |
What translates a more elevated efficiency of this essential oil with regard to L. pterodonta. Concentrations 5, 10 and 15 mg m-2 of oil essential of O. canum could not have provoked a mortality of 50% after 24 h of exhibition. The efficient concentration of O. canum on adults of A. gambiae is 40 mg m-2.
Of the Table 4, it comes out again that the death rate also grows with the concentration and the time of exhibition. Concentrations 5, 10 and 15 mg m-2 of oil essential P. glandulosus become efficient from the 18th h. At the 24th h, the concentration 50 mg m-2 misled 100% of mortality at adults of A. gambiae. What wants to say that the concentration efficient of P. glandulosus is 50 mg m-2.
Oil essential of E. camaldulensis proved out to be efficient as the one of O. canum. Concentrations 5, 10 and 15 mg m-2 begin their insecticide effect from the 12th h (Table 5). One notes that the death rate grows with the concentration. On the other hand no concentration could have provoked one death rate to 100%. To the biggest concentration of 50 mg m-2, 90% of expositions mosquitoes died.
Essential oil of P. viridiflorum, possesses a weak toxicity not only, but has an extremely retarded insecticide effect (Table 6). It takes out again the face 6 that, some either concentration understood between 5 and 15 mg m-2, essential oil of P. viridiflorum remains inactive on adults of A. gambiae. No concentration could not have misled a mortality to 50% after 24 h of exhibition. Only to the biggest concentration of 50 mg m-2, 45% of mortality have been observed after 24 h.
| Table 4: | Mortality of Anopheles gambiae adults according to the length of exhibition to the different doses of essential oil of Plectrancthus glandulosus |
| Table 5: | Mortality of Anopheles gambiae adults according to the length of exhibition to the different concentrations of essential oil of Eucalyptus camaldulensis |
| Table 6: | Mortality of Anopheles gambiae adults according to the length of exhibition to the different concentrations of essential oil of the Pittosporum viridiflorum |
| Table 7: | Different CL50 of essential oil used on adults of Anopheles gambiae after 24 h of exhibition |
| Values on the same column followed by the same letter are not significant different at 5% level; *** Very highly significant (p = 0.001); ** Very significant (p = 0.01); *Significant (p = 0.05) | |
| Table 8: | Different HL50 of essential oil used on adults of Anopheles gambiae after 24 h of exhibition |
| *** Very highly significant (p = 0.001); ** Very significant (p = 0.01) | |
Assessment of the CL50 (Lethal Concentration 50)
The determination of the essential oil concentrations that provokes 50%
of mortality at adults of A. gambiae has been done from the right of
regression of the essential oil concentrations according to the death rate expressed
in values of probit.
Table 7 represent the equation of regression, the coefficient of interrelationship, the degree of liberty and the CL50 gotten for the different essential oils below.
The analysis of the different results watch that, essential oil of oh. canum; P. glandulosuses were revealed, respectively more efficient with the CL50 of 11.95 and 12.5 mg m-2, follow-up of E. camaldulensis and of L. pterodonta with the CL50 of 16.76 and 17.06 mg m-2, respectively.
Assessment of the HL50 (Lethal Hour 50)
The determination of hours to leave of which the concentration of induced
essential oil 50% of mortality at A. gambiae has been done. This
parameter has been gotten from the right of the logarithm of hours according
to the death rate expressed in value of probit.
It takes out again the Table 2 that O. canum possesses a HL50 weak, follow-up of P. glandulosus and of L. pterodonta (Table 8). One notes that it exists an interrelationship between the efficiency of an essential oil (CL50) and its lethal hour (HL50) at certain plants. For example, essential oil of O. canum that is more efficient also has a HL50 weak (6 h 36 min 36 sec). It is some in the same way for oil essential of P. glandulosus.
Of general manner, one noted an interrelationship between the efficiency of every essential oil (HL50) and its lethal hour 50 (HL50). The essential oils that presented a weak efficiency also presented a HL50 elevated.
Of all used essential oils, the one of P. viridiflorum was revealed least efficient with regard to others, because its CL50 doesn't have can be gotten that to leave 71.79 mg m-2 and its HL50 very elevated, is valued to 101 h 23 min 24 sec.
In sum, the poisonous effects of every plant on adults of A. gambiae depend on the plant, concentration and the length of exhibition.
DISCUSSION
Essential oil from tested plants possesses some real insecticide properties on adults of A. gambiae. Essential oil of O. canum revealed the same mortality at lower concentrations. The lethal concentration of 50 (CL50) was obtained at 11.95 mg m-2 after 24 h of treatment. An insecticide activity of essential oil of O. canum on adults of A. stephensi, Aedes aegypti and Culex quinquefasciatus, with the more elevated concentrations (75, 82.4 and 115.3 mg of active matter in a volume of 0.50 mL of a volatile solution of acetone) were reported by Rodhain and Perez (1985) and Rao et al. (2000). This exhibition dragged in every case, one death rate of 95% after 1 h of exhibition. The difference between these results and Present results, may be attributed to differences in mosquitoes species as well as the selected concentrations used during manipulations.
Results from other authors (Rodhain and Perez, 1985; Chittihunsa and Samngamnim, 1999; Keita et al., 2002) suggested that essential oil of O. canum exercises a toxicity and possesses an anorexigen property on other bugs species.
Essential oil of was less efficient than those of O. canum, P. gladulosus and L. pterodonta due to the fact that 24 h were not enough to provoke 100% mortality for E. camaldulensis compared to others. Mark et al. (2002) studying the repulsive effect and the remanence of essential oils of 38 plants found that Eucalyptus oil was the only product able maintain a long term remanence from more than 6 months.
The ongoing survey on the remanence of these different essential oils of the local plants, will permit the comparison of insecticide effects with regard to the currently used chemicals such as Lambdcyalothrin. This product at a concentration of 15 mg m-2 was reported to have a remanence of 7 months closely with a reduction of 69% on adults of A. moucheti (Njan Nlôga, 1994). At Kumbantonie in Tanzania, Magesa et al. (1991) observed reductions of 70.2% of the aggressive densities, 93% of the daily rate of inoculation for adults populations of A.. gambiae and A. funestus, for a concentration of 30 mg m-2. Other tests achieved in Tanzania by (Njunwa et al., 1991) indicated that field application of Lambdacyalothrin at 30 mg m-2 and 10 mg m-2 would provoke a mortality of 100% of adult population of A. gambiae for six months. The efficiency of these essential oils can be assigned to the organic solvent used or certain substances present in plants. From other results (Njunwa et al., 1991; Keita et al., 2002; Jirovetz et al., 2002), monoterpeneses hydrocarboneses are very poisonous molecules to mosquitoes. Results by Khan et al. (1969); Tripahi et al. (2004) have revealed that the presence of thiamine and vitamin B1 in Eucalyptus oil may be responsible for the insecticide properties of this plant.
Contrarily to information received from peasants, laboratory tests have revealed that essential oil from H. spicigera is less efficient on adults population than others. Nevertheless, the higher concentration used could confirm its efficiency. Works done by Jirovetz et al. (2002) and Ngassom et al. (2002) on the chemical composition of essential oils of plants sampled in Cameroon showed that H. spicigera contains a weak proportion of 13.36% of sesquiterpenes oxygenated and 43.11% of monoterpene hydrocarbon. On the other hand, essential oils of Annona senegalensis fairly rich in monoterpene hydrocarbon (32.95%) and in oxygenated sesquiterpenes (31.12%), as well as Lippia rugosa with 19.76% of monoterpene hydrocarbon and 71.66% of oxygenated monoterpene were revealed to be more efficient than H. spicigera. The very low efficiency of H. spicigera, well rich in monoterpene hydrocarbon would be the origin of its weak toxicity on mosquitoes.