[High proliferation of An. gambiae and An. funestus larvae in irrigated and non-irrigated rice fields in the Western forest region of Côte-d'Ivoire]

Abstract

Entomological surveys were conducted in the forest region of western Côte-d'Ivoire from November 1998 to December 1999 in order to study the ecology of mosquito breeding sites and bioecology of Culicidae. Three following agro-systems were considered: landscaped lowland (R0), the lowland with one growing season of rice per year (R1) and the lowland with two seasonal production of irrigated rice field per year (R2). Two villages were selected by agro-system, and cross-sectional entomological surveys were conducted every six weeks in each village. A total of 5,839 larvae of mosquitoes were collected in the different categories of breeding sites surveyed. From these larvae, 2,199 imagos emerged, giving an emergence rate equal to 37.7%. Nine species belonging to three genera were identified. The genus Anopheles with 1,828 larvae represented 83.1% of the total. Five anophelinae species were identified. An. ziemanni was the most abundant species with 751 larvae (32.5% of the total). An. funestus with 531 larvae represented 24.1% of the total. Larvae from this species were collected in river with raised vegetation and in irrigated rice field at transplanting and cut stages. About 559 larvae of An. gambiae representing 25.4% of the total were collected from artificial breeding sites, particularly in rice fields after the planting stage. Additionally, 38,626 anthropophilic mosquitoes were collected on 936 mannight catches in the study area. Thirty species belonging to seven genera were identified in the three agro-systems. In the villages of the agro-system R0, 2,675 mosquitoes were collected on landing catches. In the villages of the agro-ecosystem R1, 11,311 mosquitoes were collected, which was 4.2 higher than the number collected in the village of agro-system R0 for the same sampling effort. In the village of the agro-system R2, 24,640 mosquitoes were collected, which was 9.2 higher than the number collected in the agro-system R0 and 2.2 higher than the number collected in the agro-system R1. In the villages of agrosystems R0, R1 and R2, An. gambiae represented respectively 72.1%, 56% and 58.3% of anophelinae species collected, whereas An. funestus represented 25.6%, 40% and 31.9% of anophelinae species collected, in the same agrosystems, respectively. These results showed that areas of irrigated rice fields favoured the development of larvae from An. gambiae and An. funestus.