Background
Whilst it is well known that rice production in Afiica increases the production of
Anopheles gambiae mosquitoes, most studies have investigated this in irrigation schemes.
Here I examine the colonisation of 'swamp' rice fields by An. gambiae mosquitoes in
The Gambia and examine some of the factors responsible for the presence and absence of
these vectors in field and semi-field conditions. This work is of relevance to large-scale
larval control programmes that have identified rice fields as a major source of malaria
vectors in The Gambia.
Methods
Larval and adult mosquito surveys were carried out in rice fields near Tamba Koto
village in rural Gambia from June to January, 2006, a period that included the wet season,
the period of most malaria transmission. Three transects each 500m long and 200m apart
situated on the edge of the River Gambia floodplains were routinely surveyed. Larval
sampling using area sampler and dippers was done at regular intervals along each
transect. Adult sampling with emergence traps placed over water. I compared three
different water treatments that were commonly found under field conditions in Gambian
rice fields: the presence of algae, cow dung and urea. The number of larvae and pupae
were cotmted daily for 14 days in 16 artificial breeding sites of plastic bowls filled with
tap water, each with a different treatment and an untreated control bowl. This trial was
repeated four times over 12 weeks period.
Results
Three hundred and seventy-five (375) anopheline larvae were caught during the larval
survey in the field, with 349 larvae (93%) collected within the first 350m fi-om the
landward edge of the paddy fields. Out of the An. gambiae complex collected, 36 (45%)
were An. arabiensis, 23 (29%) were An. gambiae sensu stricto and 21 (26%) were An.
melas