Agriculture and the promotion of insect pests: rice cultivation in river floodplains and malaria vectors in The Gambia.

Lamin B S Jarju,Vasilis Louca,Silas Majambere,Ulrike Fillinger,Clare Green,Steven W Lindsay

doi:10.1186/1475-2875-8-170

Lamin B S Jarju, Vasilis Louca + Show 4 more

Open Access

https://doi.org/10.1186/1475-2875-8-170

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Abstract

BackgroundAnthropogenic modification of natural habitats can create conditions in which pest species associated with humans can thrive. In order to mitigate for these changes, it is necessary to determine which aspects of human management are associated with the promotion of those pests. Anopheles gambiae, the main Africa malaria vector, often breeds in rice fields. Here the impact of the ancient practice of 'swamp rice' cultivation, on the floodplains of the Gambia River, on the production of anopheline mosquitoes was investigated.MethodsRoutine surveys were carried out along 500 m transects crossing rice fields from the landward edge of the floodplains to the river during the 2006 rainy season. Aquatic invertebrates were sampled using area samplers and emergence traps and fish sampled using nets. Semi-field experiments were used to investigate whether nutrients used for swamp rice cultivation affected mosquito larval abundance.ResultsAt the beginning of the rainy season rice is grown on the landward edge of the floodplain; the first area to flood with fresh water and one rich in cattle dung. Later, rice plants are transplanted close to the river, the last area to dry out on the floodplain. Nearly all larval and adult stages of malaria vectors were collected 0–100 m from the landward edge of the floodplains, where immature rice plants were grown. These paddies contained stagnant freshwater with high quantities of cattle faeces. Semi-field studies demonstrated that cattle faeces nearly doubled the number of anopheline larvae compared with untreated water.ConclusionSwamp rice cultivation creates ideal breeding sites for malaria vectors. However, only those close to the landward edge harboured vectors. These sites were productive since they were large areas of standing freshwater, rich in nutrients, protected from fish, and situated close to human habitation, where egg-laying mosquitoes from the villages had short distances to fly. The traditional practice of 'swamp rice' cultivation uses different bodies of water on the floodplains to cultivate rice during the rainy season. A consequence of this cultivation is the provizion of ideal conditions for malaria vectors to thrive. As the demand for locally-produced rice grows, increased rice farming will generate great numbers of vectors; emphasizing the need to protect local communities against malaria.

Highlights

Anthropogenic modification of natural habitats can create conditions in which pest species associated with humans can thrive
The rainfall was similar to the mean annual rainfall of 772.8 mm (95% Confidence intervals = 694.9–850.7 mm) for the period 1990–2005
This study demonstrates how humans exploit the changing conditions on the floodplain of the Gambia River to practise the ancient craft of swamp rice cultivation, and how, in turn, the human-made changes are exploited by anopheline mosquitoes resulting in increased malaria transmission

Summary

Introduction

Anthropogenic modification of natural habitats can create conditions in which pest species associated with humans can thrive. The main Africa malaria vector, often breeds in rice fields. It is ironic that the world's huge agro-ecosystems designed to feed the ever increasing human population provide a habitat for a far greater number of insects to exploit and thrive in. Irrigation is associated with the production of vectors that transmit pathogens to humans, including those responsible for malaria [2]. In order to manage vector populations, it is important to know which specific human practices promote these pests. Increases in the number of Anopheles gambiae sensu lato, the major malaria vector, typically correspond with the beginning of rice cultivation, when paddies are first flooded and rice is short [5,8,9,10,11]. Most research on rice and malaria focuses on irrigated rice production [10,11,12,13,14,15,16] and rarely traditional practices [17,18]

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