Anopheles gambiae (Giles, 1902) : quelques aspects de son écologie, influence de trois insecticides chimiques et effet insecticide des huiles essentielles de trois plantes locales

Abstract

In Cameroon, malaria is the leading cause of mortality and morbidity, especially among children under 5 years. In the absence of a vaccine, vector control is the only known form of collective protection. It is confronted to the resistance of Anopheles to chemical insecticides conventionally used. The aim of this present work is to study some aspects of the biology of An. Gambiae in order to know the behaviour of this vector vis-à-vis the chemical and biological insecticides. We studied the production of larvae and pupae of An. gambiae s.l. in the absence and presence of cypermethrin in nature at Nkolbisson. In the laboratory, we studied the influence of some abiotic factors as well as that of cypermethrin, dimethoate and methyl-parathion on the larvae of An. gambiae s.s. and the insecticide effect of essential oils of Aframomum melegueta, Callistemon citrinus and Monodora myristica, three local plants on larvae and adults of An. gambiae S.S. Harvests were made in 3 sites to determine the monthly and seasonal productions of individuals aquatic stages of An. gambiae s.l. Nkolbisson in 2007 and 2008. We compared the productions of individuals of aquatic stages of the untreated field to those of the field treated with cypermethrin. In the laboratory, from larvae of the first stage one day old, we studied the influence of some abiotic factors as well as those three chemical insecticides on the development of An. gambiae ss. We proceeded to the tests of essential oils of Aframomum melegueta, Callistemon citrinus and of Monodora myristica on the larvae of each stage and the females of An. gambiae s. s. to search their larvicidal and adulticide effects. The study of productions of larvae and pupae in 2007 and 2008 at Nkolbisson shows that individuals of aquatic stages of An. gambiae s.l. are present during 8 months of each year which are: March, April, May, June, July, September, October and November. Productions of larvae and pupae is null for 4 months each year which are: January, February, August and December. The peaks of larvae and pupae productions are observed in June and October of each year. The peak in June is slightly lower (approximately 1,000 individuals) than in October (over 1100 individuals) in untreated fields. In the field treated with cypermethrin, peaks are low (about 300 individuals). The appearance of the aquatic stages of An. gambiae s.l. is correlated with rainfall in the short rainy season (March to June) and the long rainy season (September to November). The productions of individuals of aquatic stages were higher during the short rainy season (on average 2500 individuals in untreated fields with insecticides and 550 individuals in treated field cypermethrin) during the long rainy season (averaging 2000 individuals in untreated fields and 460 individuals in the fields treated with cypermethrin). The study of the influence of some abiotic factors on the larvae of A. gambiae s.s. led us to notice that for a food ration of between 20 mg and 60 mg per 100 larvae, for larval densities between 50 and 150 per 100 ml of water, for a shelter water depth between 0.5 and 10 cm and for a shelter water pH between 5 and 10.5, the development of An. gambiae s.s. is harmonious. This harmonious development results in a development time of larvae and pupae less than 11 days, less than 20 % mortality, a male wing length larger than 3.10 mm and female wing length greater than 3.20 mm, a number of eggs laid per female greater than 100 eggs with a hatching rate higher than 95 %. The study of the influence of chemical insecticides on larvae of An. gambiae s.s. has led us to observe that, for a cypermethrin concentration of less than 0.0250 g / l, a dimethoate concentration of less than 200 g / l and a methyl parathion concentration of less than 0.12 g / l, the development of An. gambiae s.s. is harmonious. From the above mentioned concentrations, the harmonious development of An. gambiae s.s. is significantly disrupted. In the absence of chemical insecticides, the development of An. gambiae s.s. is harmonious with a larvae and pupae mortality rate of less than or equal to 2.50 %. Between the 1st and the 6th generation, the larvicidal effect of cypermethrin and methyl parathion on individuals aquatic stages of An. gambiae s.s. does not decrease significantly. From the 7th generation, the "tolerance" of An. gambiae s.s. to cypermethrin and methyl-parathion appears and the mortality rate decreases significantly. From the 8th generation, the "tolerance" An. gambiae s.s. to dimethoate appears, the larvae and pupae mortality records movements comparable to those of cypermethrin and methyl parathion; however, the larvae and pupae mortality rate remain high. Seeds of M. Myristica (1.20 %) are approximately 2 times richer in essential oil than the leaves of C. citrinus (0.75 %) and more than 6 times richer in essential oil than the seeds of A. melegueta (0.19 %). Essential oils from A. melegueta and C. citrinus are rich in 1,8-cineole which is an oxygenated monoterpene and that of M. Myristica is rich in α-phellandrene which is a hydrocarbon monotepène. The essential oil of A. melegueta causes total mortality of 1st larvae 1st of An. gambiae s.s. after 120 min. and the fourth larval stage after 180 min.; the LC95 of this oil is 27.38 ppm for the first stage larvae of and 41.42 ppm for the fourth larval stage. The essential oil of C. citrinus causes total mortality of the first stage larvae of An. gambiae s.s. after 140 min. and the fourth larval stage after 200 min.; the LC95 of this oil is 36.57 ppm for the first stage larvae of and 62.80 ppm for the fourth larval stage. The essential oil of M. myristica causes total mortality of the first stage larvae of An. gambiae s.s. after 120 min. and the fourth larval stage after 180 min.; the LC95 of this oil is 27.48 ppm for the first stage larvae of and 41.39 ppm for the fourth larval stage. The essential oil of A. melegueta causes total mortality of females of An. gambiae s.s. at a concentration of 250 ppm; the LC95 is of 169.06 ppm. The essential oil of C. citrinus causes total mortality of female of An. gambiae s.s. at a concentration of 200 ppm; the LC95 is of 131.62 ppm. The essential oil of M. myristica causes total mortality of female of An. gambiae s.s. at a concentration of 250 ppm; the LC95