This study showed that adult and larval mosquito abundance was high in the irrigated fields in the Gyinyase irrigated vegetable farm. Dug-out wells which served as the reservoirs for irrigation water had the highest larval abundance and mosquito productivity, followed by the footprints and then the furrows. Human footprints were seen to be present for a long time and were able to support larval breeding. Most mosquito larval breeding habitats in Africa are a result of anthropogenic environmental studies such as those described here.
Conditions in these breeding habitats are suitable for the survival and development of immature mosquitoes. The study site was sunlit and the irrigation water in the dug-out wells and furrows were clean, conditions which support the breeding of An. gambiae. Most habitats also had algal cover with less grass cover. These explain the high larval abundance, which also translated to a high production of adult mosquitoes. An. gambiae also prefers small, open habitats for oviposition rather than large habitats[12, 21]. An. gambiae complex normally exploits the increased resources of warmer open habitats that tend to produce more algae (the main food source for the An. gambiae complex) than do shaded habitats. Small habitats such as the dug out wells, furrows and the human footprints tend to have warmer temperatures and this shortens larvae-to-pupae development time while also reducing mortality associated with desiccation. The An. gambiae complex may have evolved to exploit these favorable conditions by selecting small and open habitats for oviposition. It has been demonstrated in lowland areas of western Kenya, that habitat size is an important determinant of habitat stability, pupal occurrence, and mosquito abundance.
Just like studies by Munga et al., in western Kenya, mosquito productivity was high in the irrigation water sources at Gyinyase. It was further shown that habitat type affected productivity of adult An. gambiae s.l. It was observed that water in the habitats normally does not dry up because water seeps continuously from the soil below. Thus the immature mosquitoes are able to develop to become adults. The vegetable farmers employ poultry manure on their farms. The manure gets washed into the mosquito breeding waters and contributes to the high algal growth in these habitats. Few predators were found in the habitats suggesting that larval death due to predation might be minimal. These factors also explain the high productivity of mosquitoes in the irrigation scheme.
Rainfall was found to correlate negatively with both adult and larval abundance. The breeding habitats rely on water seeping from the ground and thus, rainfall, especially when heavy, rather washes the larvae and the pupae away leading to low mosquito productivity and larval abundance. Munga et al., also reported that higher amounts of rainfall washed larvae out of habitats, and thus reduced the abundance of mosquito larvae in western Kenya. According to these authors, this led to a reduced number of positive larval habitats during the rainy season.
The results of the present study have implications for the control of larval mosquitoes in the city of Kumasi. Knowledge of An. gambiae larval habitat productivity is important in planning and designing mosquito larval control interventions in such irrigated areas. Since it was observed from the present study that mosquito breeding sites were confined to only the dug-out wells, furrows and human footprints, it makes it easier to apply larval control measures to these breeding sites. This will help reduce the number of adult mosquitoes in the area and thus potentially reduce malaria transmission. However, in most times resources for vector control are limited and thus it is always useful to look for a target to implement control measures. Since it was found that dug-out wells are the most productive of all habitats, this could be targeted for vector control. When larvicides or bacteria formulations are applied to the dug-out wells their effects could be felt in the other irrigation water reservoirs such as the furrows and human foot prints. Besides, since it is water from the dug-out wells that also fills the furrows and the human footprints, targeting the dugout wells will be the best option in a resource limited situation.
The focus group discussions brought to the fore some important issues. The major challenges to community involvement in mosquito larval source reduction activities are in educating people about the sources of the mosquitoes and motivating people to assume responsibility for controlling mosquitoes in and around their homes[29, 30]. These responsibilities are often assumed to be that of the government. In Dakar, Senegal, farmers who operate market garden wells in the city were required by law to have the fish Gambusia sp., which is a mosquito larval predator, in their wells. This fish is supposed to prey on An. arabiensis which breeds in the wells. Failure to observe this results in a fine. Involving the persons using An. gambiae s.l. larval habitats in larval control efforts may lead to a more effective programme for the control of mosquitoes and hence of malaria.