Memoire Online - Alternative à la résistance aux pyréthrinoà¯des au Burkina Faso: évaluation des activités insecticides et répulsives-irritantes d'huiles essentielles de plantes locales seules et en combinaison sur les populations de anopheles gambiae et de aedes aegypti (diptera: culicidae)

Dengue vector control strategies are mostly based on chemicals use against Aedes aegypti populations. The current study aimed at investigating the insecticidal effects of essential oils (EOs) obtained from five plant species, Cymbopogon citrates (D. C.) Stapf. (Poaceae), Cymbopogon nardus (Linn.) Rendle (Poaceae), Eucalyptus camaldulensis Linn. (Myrtaceae), Lippia multiflora Moldenke (Verbenaceae), and Ocimum americanum Linn. Lamiaceae, and combinations of Cymbopogon nardus and Ocimum americanum on Ae. aegypti populations from Bobo-Dioulasso. For this purpose, adults of the susceptible and field strains of Ae. aegypti were tested in WHO tubes with EO alone and binary combinations of O. americanum (OA) and C. nardus (CN; scored from C1 to C9). The extraction of the essential oils was done by hydrodistillation, and their components were determined by GC/MS. Among the 5 EOs tested, L. multiflora essential oil was the most efficient, with KDT50 values below 60 min on all Ae. aegypti strains tested, and also with a rate of mortality up to 100 and 85% for Bora Bora and Bobo-Dioulasso strains, respectively. This efficacy may be due to its major compounds which are with major compounds as â-caryophyllene, p-cymene, thymol acetate, and 1.8 cineol. Interestingly, on all strains, C8 combination showed a synergistic effect, while showed an additive effect. These combinations exhibit a rate of mortality varying from 80 to 100%. Their toxicity would be due to the major compounds and the putative combined effects of some major and minor compounds. More importanly, L. multiflora EO and combinations of C. nardus and O. americanum EO, may be used as alternatives against pyrethroid resistant of Ae. aegypti.

Key words: resistance, essential oil, combination, major compound, rate of mortality

Arboviral diseases have been recognized as a global threat during the last 20 years (Mayer et al. 2017). According to WHO (2021), approximately 3.9 billion people worldwide are at risk of arboviral diseases. These diseases are caused by viruses (arboviruses) usually transmitted, under natural conditions, from vertebrate to vertebrate, by hematophagous insect vector (Aubry and Gauzère 2020) such as

Ae. aegypti mosquitoes. Zika, chikungunya, yellow fever, and dengue are arboviruses whose main vector is Ae. aegypti (Cuervo-Parra et al. 2016; Souza-neto et al. 2019). According to WHO (2020), among the arboviroses, dengue is the most prevalent in the subtropics and tropics areas. Thus, it is a major international public health problem (Guzman et al. 2010). In recent years, it has been detected in several

African cities including Ouagadougou and Bobo-Dioulasso, the major cities of Burkina Faso (Sessions et al. 2013, Ministère de la Santé du Burkina Faso 2017).

Vector control remains the most effective measure to control the transmission of this arboviral diseases (Bhatt et al. 2015). The use of insecticides is the main strategy for controlling Ae. aegypti (WHO 2012) when a dengue epidemic is detected in a city (Achee et al. 2015). Long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS) remain the main vector control methods (WHO 2012).

However, in many countries, resistance to insecticides has been observed in natural populations of disease vectors, and this resistance is increasing. Indeed, previous studies in Burkina Faso reported multiple resistance to chemical insecticides commonly used against Ae. aegypti populations (Sombié et al. 2019), particularly in the Ae. aegypti population from Bobo-Dioulasso (Ouattara et al. 2019). Recently, Namountougou et al. (2020) highlighted this resistance in Ae. aegypti populations in Ouagadougou and Bobo-Dioulasso according to metabolic activities such as nonspecific esterase, cytochrome P450, and glutathione S-transferase activities.

To manage concern related to increasing chemical resistance, which has become an obstacle to fight vector-borne diseases in general and dengue specifically, the use of natural products derived from plants with proven insecticidal efficiency is strongly recommended as an alternative. Among the many natural products, EOs and their constituents have received considerable attention in the search for new pesticides exhibiting insecticidal properties (Tripathi et al. 2009). Plant products are used worldwide to protect people against blood-sucking arthropods, and many studies have reported the adulticidal effects of essential oils (Zoubiri and Baaliouamer 2014). These natural compounds are biodegradable in nature and environmentally safe (Regnault-roger et al. 2012) and generally exhibit low toxicity to mammals (Isman 2000). Moreover, traditional medicine is largely based on plants (herbs or shrubs) and is available at low cost in most tropical areas (de Boer et al. 2010).

EOs have several valuable properties. First, they easily penetrate the cuticle of insects, which increases their bioavailability (N'Guessan et al. 2007). This property reduces mosquito knockdown time. Regarding the weak toxicity of most EOs against vectors, strategies based on their combination may improve their efficiency. Recent studies on Ae. aegypti populations from Ouagadougou (Burkina Faso) showed low toxicity of 3 of the 5 EOs tested in the present study, namely Ocimum americanum, Cymbopogon nardus, Eucalyptus camaldulensis (Yaméogo et al. 2021). According to Chansang et al. (2018), combination of EO-Permethrin shown a synergist effect on Aedes populations from Thailand.

To the best of our knowledge, even if studies have been carried out on the toxicity of EOs in Burkina Faso, studies on their combinations on Ae. aegypti populations are limited. In this study, our objective was to evaluate the insecticidal properties of EOs extracted from five local plants, namely L. multifiora, C. nardus, O. americanum, C. citratus, and E. camaldulensis, and binary combinations of 2 of them, namely O. americanum and C. nardus, against adults of Ae. aegypti populations from western Burkina Faso.

The first step consisted in collecting the aerial parts of the following species: L. multifiora, C. nardus, C. citratus, E. camaldulensis, and O. americanum. The collection was conducted at the botanic garden of the Institut de Recherche en Sciences Appliquées et Technologies

(IRSAT), Ouagadougou. The extraction of EOs was achieved through hydrodistillation using Clevenger apparatus. A dark glass bottle was used for storage at a temperature of 4°C, according to the works done by Drabo et al. (2017). Indeed, the combinations of C. nardus and O. americanum EOs tests were carried out, after having done the bioassays with the individual EOs, at the `Institut de Recherche en Sciences de la Santé/Direction Régionale de l'Ouest' (IRSS/DRO).

Essential oils obtained from the leaves of L. multifiora, C. citratus, C. nardus, E. camaldulensis, and O. americanum were analyzed by gas chromatography (GC) with flame ionization detection (GC- FID) using an Agilent 6890N GC instrument which is FID equipped, along with a DB-5 narrow bore column. Experimental procedures are detailed in Drabo et al. (2017) and Balboné et al. (2022).

For the GC/MS analysis, a GC HP 6890 connected to an MSD HP 5972 (Hewlett Packard, Palo Alto, CA) with a Zebron ZB-5MS capillary column (30 m long, ID 0.25 mm and film thickness 0.25 mm; Agilent) was used. Experimental procedures are detailed in Drabo et al. (2017) and Balboné et al. (2022).

The collection of pupae and larvae of the Ae. aegypti field strain was carried out from June to October 2021, in Bobo-Dioulasso, West Burkina Faso. Bobo-Dioulasso (11°10'37.7"N, 4°17'52.4"W), the economic capital of Burkina Faso, is located in the western part of the country. The pupae and larvae were reared in the insectarium located in the `Institut de Recherches en Sciences de la Santé, Direction Régionale de l'Ouest' (IRSS/DRO) in Bobo-Dioulasso. Larvae were fed with tetraMin Baby Fish Food (Tetrawerke, Melle, Germany). Adult mosquitoes, which emerged from pupae of the field strain, were placed in cages and fed with a 10% sugar solution. Female mosquitoes of first-generation field strain and second-generation Ae. aegypti `Bora Bora' strain maintained at the insectarium, as a reference strain, were used for susceptibility tests. Mosquitoes were reared under conditions of temperature 27 #177; 2°C, relative humidity 70 #177; 5%, 12 h of light and 12 h of dark.

Susceptibility tests were performed with WHO tubes according to standard procedures (WHO 2017). Whattman papers were used for impregnation according to the protocol adopted by N'Guessan et al. (2007). For this purpose, four rectangular papers (size 12 cm x 15 cm) were impregnated with 2 ml of a given concentration of an essential oil/combination of EOs diluted in acetone. For combination tests, 10 binary combinations varying from C1 to C9 were prepared. Thus, 1) CN 10%: OA 90%; 2) CN 20%: OA 80%; 3) CN 30%: OA 70%; 4) CN 40%: OA 60%; 5) CN 50%: OA 50%; 6) CN 60%: OA 40%; 7) CN 70%: OA 30%; 8) CN 80%: OA 20%; and 9) CN 90%: OA 10% corresponding to C1, , C3, C4, C5, C6, C7, C8, and C9, respectively, led us to carry out the combination tests.

Preliminary bioassay tests were used to select a range of EO concentrations for the current tests. The concentrations chosen were: 0.1, 0.5, and 1% for each EO or combination of EOs. Acetone was used as a negative control and permethrin 0.75% was used as a positive control. The test conditions were: temperature 25°C (#177; 2°C) and humidity 70-80%. Diagnostic concentrations were derived from the susceptible Ae. aegypti `Bora Bora' strain.

The data obtained were analyzed using the statistical software XLSTAT version 2015.1.01. By probity analysis, the knockdown times (KDT₅₀and KDT₉₅), lethal concentrations _(LC50and LC₉₉), and 95% confidence limits (95% CL) were calculated to compare the toxicity of the EOs and combinations against the adult mosquitoes tested. The relationship between KDT, mortality, and concentrations was evaluated by probity regression. The LC50, LC99, and mortality values were significantly different between the EOs and combinations (P < 0.05) if their confidence intervals did not overlap. Corrections were not necessary as no mortality was found in the negative controls. According to WHO (2017), the diagnostic concentration corresponds to twice the LC₉₉on susceptible strain.

Determination of EO interactions in the combinations was done using fractional inhibitory concentration (FIC) indices. The calculation and interpretation procedures have been described in Schelz et al. (2006) and Bassolé and Juliani (2012).

FIC indices were obtained by using the following formula: FIC = FIC_A+ FIC_Band FIC_A= MICA(A+B)/MICA and FIC_B= MICB(A+B)/ MIC_Bwhere MIC_Aand MIC_Bare the minimum inhibitory concentrations that kill adult mosquitoes for EOs A and B, respectively, MICA(A+B) ^and_MICB(A+B)are the minimum inhibitory concentrations of A and B, respectively in the combination. Also, FIC_Aand FIC_Bare the fractional inhibitory concentrations of A and B, respectively. The effects were classified as additive, synergistic, indifferent, and antagonistic as follows: 1) Synergistic if FIC < 0.5; 2) additive, if 0.5 ~ FIC ~ 1; 3) indifferent if 1 ~ FIC ~ 4; or 4) antagonistic if FIC > 4.

Data on the chemical analysis of the five EO samples are given in Table 1. The compounds of all these EOs were dominated by monoterpenes. The EO of C. citratus was composed of neral (44.7%) and geranial (55.2%). As for C. nardus EO, it was characterized by six compounds, dominated in majority by citronellal (41.7%), ge-raniol (20.8%), and â-elemene (11%). Sixteen compounds were found in E. camaldulensis EO with 1.8-cineole (59.55%) as major compound. L. multifiora EO consisted 16 compounds including â-caryophyllene (20.1%), p-cymene (14.6%), thymol (12.0%) acetate, and 1,8-cineole (11.6%) as the major compounds. Twenty-six compounds were found in O. americanum EO with 1,8-cineole (31.22%) and camphor (12.73%) the major compounds.

KDT values and mortalities were depending of the EO or combination. Also, they varied in connection with the treatment concentration and the Ae. aegypti strain tested.

Indeed, the lowest KDT values and the highest mortalities were obtained on the susceptible strain Bora Bora. All the EOs tested in the present study showed more or less high insecticidal properties on the both strains of Ae. aegypti. Lippia multifiora EO was the most toxic to all Ae. aegypti strains with a _KDT95value of 113.5 min and a mortality rate of 85% at the 1% concentration (Table 2) on Ae. aegypti of Bobo-Dioulasso. Also, this EO gave a proportion of 0.79 and 1.25% for LC₅₀and _LC99,respectively (Table 3), on Ae. aegypti from Bobo-Dioulasso.

As for E. camaldulensis EO, it was the least toxic with a mortality of 0.7% at the 1% concentration on Ae. aegypti from Bobo-Dioulasso (Table 2).

At the 1% concentration, C. nardus and O. americanum EO showed mortalities of 12.75 and 2.3%, LC₅₀values of 2.94 and 3.1%, respectively, on Ae. aegypti from Bobo-Dioulasso.

At the 1% concentration, there were no significant differences between the KDT obtained with the EOs on both strains of Ae. aegypti tested, except for those of C. nardus and L. multifiora.

Overall, all combinations showed low KDT values (Table 4). Combinations C8 (CN 80%; OA 20%), (CN 20%; OA 80%), and C9 (CN 90%; OA 10%) with LC₅₀values below 1%, whereas those of C4 (CN 40%; OA 60%) and C6 (CN 60%; OA 40%) were below 2%. The LC₅₀values and diagnostic concentrations obtained with these combinations were significantly lower than those obtained with the two EOs taken individually and with permethrin (Table 3). These combinations also had significantly higher mortalities than the individual EOs and permethrin (Tables 2 and 4).

The synergistic effect was obtained on both strains with the C8 combination, and the additive effect was found with the and C9 combinations (Tables 5 and 6). No antagonistic effect was observed but some combinations showed no effect.

Vector control strategies have mainly consisted of the use of chemicals against mosquitoes. The frequent use of chemicals has raised many concerns in the environment and human public health (Cuervo-Parra et al. 2016) and caused increasing resistance in most mosquito populations (Mahanta et al. 2019) and particularly Ae. aegypti population of Bobo-Dioulasso (Namountougou et al. 2020); (Badolo et al. 2019). Regarding this situation, plants extracts seem to be an alternative because they contain large number of bioactive, effective, biodegradable compounds and have been used currently for mosquito control (Boonyuan et al. 2014; Yaméogo et al. 2021). From the present knowledge, these are the first time that works on toxicity of combinations of whole EOs on Ae. aegypti populations were investigated.

Here, our current study showed the variable adulticidal effects of EOs of C. nardus, C. citratus, O. americanum, L. multifiora, and E. camadulensis on susceptible Bora Bora and resistant Aedes aegypti strain collected in Bobo-Dioulasso. These results partially confirm the work of Yaméogo et al. (2021) who showed the toxicity of six EOs including C. nardus, E. camaldulensis, Lippia multifiora, and O. americanum on Ae. aegypti populations collected in Ouagadougou. They also confirm the works done by Drabo et al. (2017) who had shown the adulticidal properties of C. citratus and O. americanum on Bemisia tabaci populations confirming their broad-spectrum efficiency against most of insects.

Interestingly, the EO of L. multifiora was the most toxic on both strains of Ae. aegypti. This toxicity of L. multifiora EO could be explained by the insecticidal effect of the most dominant compounds which are thymol acetate, p-cymene and â-caryophyllene. Indeed, previous studies reported that â-caryophyllene exhibits insecticidal properties (Almadiy 2020) and toxicity of Eucalyptus tereticornis EO could be explained by the presence of the other component as p-cymene (Bossou et al. 2013) found as a major component in L. multifiora. Similarly, lethal toxicity of the main components of Ocimum kilimandscharicum (such as camphor, 1.8-cineole and â-caryophyllene) EOs had been reported against two insect pests (Rhyzopertha dominica and Sitophilus zeamais; Bekele and Hassanali 2001).

Table 1. Chemical composition of Lippia multiflora, Cymbopogon nardus, Ocimum americanum, Cymbopogon citratus, and Eucalyptus camaldulensis essential oils

Retention Compounds indices	Cymbopogon citratus(%)	Cymbopogon nardus (%)	Eucalyptus camaldulensis(%)	Lippia multi- flora (%)	Ocimum americanum(%)
á-Thujene 925	--	--	--	1.6	0.13
á-Pinene 933	--	--	9.17	0.9	6.87
Camphene 945	--	--	0.19	--	2.77
Sabinene 975	--	--	--	--	--
â-Pinene 981	--	--	2.49	--	0.65
Myrcene 992	--	--	0.15	--	3.71
2-Carene 999	--	--	--	--	2.04
á-Phellandrene 1,003	--	--	0.32	3.1	0.39
á-Terpinene 1,017	--	--	--	1.41	0.40
p-Cymene 1,027	--	--	4.73	14.6	0.21
Limonene 1,028	--	--	8.76	--	4.66
1,8-Cineole 1,034	--	--	59.55	11.6	31.22
â-Ocimene 1,048	--	--	--	9.4	--
?-Terpinene 1,061	--	--	1.39	--	1.13
Terpinolene 1,081	--	--	0.21	--	0.65
Camphor 1,124	--	--	--	--	12.73
Citronellal 1,158	--	41.7	--	--	--
Citronellol 1,230	--	8.0	--	--	--
Borneol 1,156	--	--	0.46	--	0.13
4-Terpineol 1,168	--	--	1.14	--	0.92
á-Terpineol 1,177	--	--	2.65	--	2.08
Neral 1,242	44.7	--		--
Piperitone 1,252	--	--	0.09	--	2.44
Geraniol 1,253	--	20.8	--	--	--
Geranial 1,268	55.2	--	--	--	--
Bornyl acetate 1,269	--	--	--	--	3.96
Thymol 1,288	--	--	--	8.3	--
Thymol acetate 1,355	--	--	--	12.0	--
â-Elemene 1,372	--	11.0	--	--	--
á-Copaene 1,390	--	3.7	--	--	--
â-Caryophyllene 1,415	--	--	--	20.1	3.55
Trans-á-Bergamotene 1,433	--	--	--	--	5.32
á-Humulene 1,442	--	--	--	4.8	0.25
allo-Aromadendrene 1,446	--	--	1.40	--	--
á-Amorphene 1,485	--	--	--	1.4	--
Germacrene D 1,495	--	--	--	1.5	0.24
(Z, E)-á-farnesène 1,497	--	--	--	--	3.64
B-himachalene 1,499	--	--	--	--	--
Hedycaryol 1,520	--	7.4	--	--	--
Elemol 1,545	--	--	--	2.1	1.6
â-Eudesmol 1,650	--	--	--	2.5	--
Caryophyllene oxyde 1,581	--	--	--	4.5	0.08
Hydrocarbon monoterpenes	-	--	24.89	33.01	23.61
Oxygenated monoterpenes	99.9	77.9	66.96	36.5	53.48
Hydrocarbon sesquiterpenes	--	14.7	1.4	25.8	13.03
Oxygenated sesquiterpenes	--	--	0.34	4.5	1.68
Total	99.9	92.6	93.59	99.81	91.8

Other previous studies reported that the insecticidal properties of an additive effect between 1.8-cineole and sabinene on Sitophilus

(Koul et al. 2008). These compounds were the same of those found If the pre-exposure of mosquitoes to PBO can restore totally or

in O. americanum EO tested here and for which we obtained the partially the susceptibility to pyrethroids (Hien et al. 2021), there

weak ef!cacy. According to Ntonga et al. (2017), 1.8-cineole is re is no doubt that the toxicity of EOs could be improved by binary

ported to contribute strongly to the toxicity of Callistemon citrinus combinations of certain EOs.

oil against adult females of Anopheles coluzzii. On the contrary, E. In the current study, this improvement was outlined through

camaldulensis EO, rich in 1,8-cineole was the least toxic against the KDT, LC₅₀values and rate of mortality obtained with certain

adult females of Ae. aegypti. This low toxicity could be due prob combinations which were higher than those of the EOs tested alone

ably to the absence of sabinene among the main compounds of E. and positive control. It is suggested that combinations reduce the

camaldulensis EO. Indeed, Liu et al. (2020) reported that there was time required to paralyze mosquitoes. Interestingly, the combination

Table 2. KDT50, _KDT95and mortalities from essential oils tested on Aedes aegypti susceptible (Bora bora) and field (Bobo-Dioulasso) strains

			Aedes aegypti `Bora bora'		Aedes aegypti `Bobo-Dioulasso'
Treatments	Concentrations	KDT50 (Min)	KDT95 (Min)	Mortality (%)	KDT50 (Min)	KDT95 (Min)	Mortality (%)
C. citratus	0.1	193.3	291.1	0	221.2	311.2	0
	0.5	192.6	290.2	4.04	228.1	298.5	0
	1	140.6	230.4	8.26	142.5	244.8	0.9
C. nardus	0.1	88.6	134	3.1	179	278	0.98
	0.5	35.6	53.8	9.86	88.7	143.9	5
	1	24	33.7	20.34	35.6	57.2	12.75
E. camaldulensis	0.1	211.3	299.1	0	234.1	343.9	0
	0.5	178.1	285.7	3.3	209.4	297.2	0.1
	1	156.5	267.5	7.14	190.3	287.2	0.7
L. multiflora	0.1	107	178.7	12.35	113.5	182.8	4.1
	0.5	40.9	72	29.33	43	75	16.02
	1	-3.9	7.8	100	20.5	32.8	85
O. americanum	0.1	131.57	194.82	2.08	151.7	264.3	0
	0.5	128.1	188.4	5.56	145.3	247.5	0.2
	1	125.7	210.6	9.14	133.3	249	2.3
Permethrin	0.75	16.6	29.1	98.8	144	188.7	10.1

Table 3. Lethal concentrations at 50 and 99% (LÇ and LÇ) and diagnostic concentrations of all essential oils, combinations, and per-methrin of O. americanum and C. nardus tested with Aedes aegypti

Treatments	Aedes aegypti Bora Bora		Aedes aegypti Bobo-Dioulasso		Diagnostic concentration	Diagnostic concentration	Diagnostic concentration
Treatments	LC50 (%) (CL95)	LC99 (%) (CL95)	LC50 (%) (CL95)	LC99 (%) (CL95)	(%)	(mg/ml)	(mg/cm²)
C. citratus	2.7	4.9	--	--	9.88	98.8	1.10
	(-)	(-)
C. nardus	2.24	4.3	2.94	5.9	8.6	86	0.95
	(1.6-2.7)	(2.7-4.9)	(1.4-3.8)	(5.7-8.7)
E. camaldulensis	3.58	7.81	--	--	15.62	156.2	1.73
	(3.3-4.2)	(-)
L. multiflora	0.57	1.03	0.79	1,25	2.06	20.6	0.23
	(0.5-0.6)	(0.9-1.2)	(0.7-0.8)	(1.1-1.4)
O. americanum	2.7	4.9	3.1	5.3	9.8	98	1.09
	(-)	(3.4-5.3)	(-)	(-)
C1(CN 10%:OA	3.63	6.41	3.9	6.8	12.72	127.2	1.41
90%)	(-)	(-)	(-)	(-)
(CN 20%:OA	0.77	1.28	0.84	1.297	2.56	25.6	0.28
80%)	(0.7-0.8)	(1.2-1.4)	(0.8-0.9)	(1.2-1.4)
C3(CN 30%:OA	2.683	4.9	2.6	4.5	8.4	84	0.93
70%)	(1.46-5)	(3.2-5.9)	(-)	(-)
C4(CN 40%:OA	1.49	2.66	1.98	3.9	5.06	50.6	0.56
60%)	(1.2-2.2)	(2-4.6)	(1.5-2.3)	(2.4-4.6)
C5(CN 50%:OA	4.02	9.697	3.83	8.3	19.4	194	2.16
50%)	(-)	(-)	(-)	(-)
C6(CN 60%:OA	1.79	3.27	1.68	2.92	6.54	65.4	2.16
40%)	(1.4-3.5)	(2.3-7.7)	1.3-3.3	(2.1-3.1)
C7(CN 70%:OA	1.7	2.94	3.47	6.1	5.88	58.8	0.65
30%)	(1.3-1.9)	(1.9-4.4)	(-)	(-)
C8(CN 80%:OA	0.6	1.08	0.7	1.2	2.16	21.6	0.24
10%)	(0.5-0.6)	(1-1.2)	0,6-0.8	(1.1-1.3)
C9 (CN 90%:OA	0.97	2.29	0.99	2.56	4.58	45.8	0.51
10%)	(0.8-1.1)	(1.9-3)	(0.9-1.3)	(2-3.5)
Permethrin	0.75	1.69	2.2	3.5	3.38	33.8	0.37
	(0.6-1.1)	(1.6-1.8)	(2.1-2.6)	(2.3-5.8)

of C8 (CN 80%: OA 20%) and (CN 20%: OA 80%) improved the toxicity on the Aedes populations. This would be due to the simultaneous toxic effect of the compounds in the combined EOs. It could

be thought that C. nardus EO needed a certain amount of the major compounds of O. americanum EO. Conversely, O. americanum EO needed some constituents of C. nardus EO to perform. Studies had

Table 4. KDT₅₀, KDT₉₅, and mortalities of binary combinations of C. nardus (CN) and O. americanum (OA) tested on Aedes aegypti susceptible (Bora bora) and field (Bobo-Dioulasso) strains

Table 5. The effects of interaction of essential oil combinations from Cymbopogon nardus (CN) and Ocimum americanum (OA) on Aedes aegypti adults of the susceptible strain (Bora Bora) and the type of interaction (n = 150 adults)

Combination of essential oil (%) Codes Mortality at LC₅₀(%) FICCn FICO.a FIC Effect

FIC = FIC_A+ FIC_B; Additive: 0,5 ~ FIC ~ 1; Synergistic: FIC < 0,5; Antagonistic: FIC > 4; No effect: 1 ~ FIC ~ 4.

shown that the addition of a little citronellal enhanced the effect of the citral-myrcene mixture against Ae. aegypti. Nevertheless, mosquito host-seeking behavior was not significantly reduced by citron-ellal alone (Hao et al. 2008).

Interestingly, only C8 combination provides synergistic effect on Aedes resistant populations. Synergistic effect obtained by this combination could also be explained by the simultaneous action of the major compounds. Also, this synergistic effect could be due to the appearance of a new components that does not exist in the individual EOs (Muturi et al. 2017; Wangrawa et al. 2022). Previous studies had shown that some essential oils or their compounds could interact to create a synergistic effect at some ratio and an antagonistic effect at other ratios (Intirach et al. 2012; Pavela 2015). Further studies will evaluate the combination 80:20 ratio on a large wild Aedes populations to appreciate the phenotypic data. As well, component of this mixture will determine. It is necessary before evaluating this combination in semifields.

Indeed, combinations of 1.8-cineole and citronellal as well as several minor compounds and citronellal gave synergistic effects on Spodoptera littoralis larvae (Pavela 2014).

Table 6. The effects of interaction of essential oil combinations from Cymbopogon nardus (CN) and Ocimum americanum (OA) on Aedes aegypti adults of the field strain (Bobo-Dioulasso) and the type of interaction (n = 150 adults)

Combination of essential oil (%)	Codes	Mortality at LC₅₀(%)	FICOa	FICCn	FIC	Effect
CN 0%:OA 100%	OA	3.1	--	--	--	--
CN 10%:OA 90%	C1	3.9	1.25	1.32	2.57	No effect
CN 20%:OA 80%		0.84	0.27	0.28	0.55	Additive
CN 30%:OA 70%	C3	2.6	0.83	0.88	1.71	No effect
CN 40%:OA 60%	C4	2.18	0.7	0.74	1.44	No effect
CN 50%:OA 50%	C5	3.83	1.23	1.3	2.53	No effect
CN 60%:OA 40%	C6	1.68	0.54	0.57	1.11	No effect
CN 70%:OA 30%	C7	3.47	1.11	1.18	2.29	No effect
CN 80%:OA 20%	C8	0.7	0.22	0.24	0.46	Synergistic
CN 90%:OA 10%	C9	1.05	0.34	0.36	0.7	Additive

FIC = FIC_A+ FIC_B; Additive: 0.5 ~ FIC ~ 1; Synergistic: FIC < 0.5; Antagonistic: FIC > 4; No effect: 1 ~ FIC ~ 4.

According to Hyldgaard et al. (2012), each compound of an EO had a specific mode of action but these compounds could interact with compounds of another EO in a combination to give other modes of action. Thus, the combination of compounds with different modes of action could increase the total lethal actions in mixtures (Sarma et al. 2019).

This study opens new perspectives of valorization of EOs. According to Gnankiné and Bassolé (2017), microencapsulation technologies may improve the duration of action of EOs in terms of application taking into consideration the fact that the EOs are volatiles.

Alternative à la résistance aux pyréthrinoà¯des au Burkina Faso: évaluation des activités insecticides et répulsives-irritantes d'huiles essentielles de plantes locales seules et en combinaison sur les populations de anopheles gambiae et de aedes aegypti (diptera: culicidae)

Abstract