Global Monitoring and Human Health Program Identifying High Mosquito- Producing Rice Fields in California

Key Investigator: Byron L. Wood

Worldwide, over 140 million hectares are devoted to the cultivation of rice. Much of this area is found in countries of the tropics and subtropics where malaria still constitutes a serious human health problem. Flooded rice fields provide ideal breeding habitat for a number of potential mosquito vectors of malaria. One of these vectors, *Anopheles freeborni*, lays its eggs in rice fields in northern and central California (Fig. 1) . However, not all rice fields produce high numbers of mosquitoes. The goal of this research was to evaluate the use of remote sensing and geographic information system (GIS) technologies to identify the high mosquito-producing fields. This research serves as a model that can be applied to other parts of the world where malaria and rice production coincide.

The results of this research demonstrated that remote sensing and GIS technologies can be used to identify high mosquito-producing rice fields with greater than 90% accuracy. These high-producing fields could be identified two months prior to peak anopheline production using three different approaches. In the first approach, Landsat Thematic Mapper (TM) data were used to distinguish between rice fields based on the rate of vegetation development. Rice fields with rapid early season canopy development supported higher anopheline larval populations than fields that developed later. One method of characterizing canopy development was to use the Normalized Difference Vegetation Index (NDVI), which is a remote sensing indicator of the presence and condition of green vegetation. High and low anopheline-producing fields were distinguished by combining the remote sensing NDVI values with GIS measurements of distance between rice fields and cattle pastures (potential bloodmeal sources). Fields with high NDVI values located near cattle pastures were found to produce high anopheline populations (Fig. 2) . Finally, high and low anopheline-producing fields could be distinguished based on the landscape composition surrounding individual rice fields (Fig. 3) . Anopheline production was positively associated with landscapes that contained a mixture of cattle pastures and orchards (mosquito resting sites), and negatively associated with landscapes dominated by rice and other annual crops. These approaches can be used to direct mosquito control measures in California, or, with modification, in other rice-growing regions of the world where malaria transmission remains a human health problem.

COLLABORATORS: University of California, Davis; California State University, Fresno; Uniformed Services University of the Health Sciences; Centro de Investigacion de Paludismo, Mexico; University of Texas Health Sciences Center, Houston.

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