Status of dengue vector breeding in pineapple plantations and environmental factors affecting their breeding

Abstract

BACKGROUND: Pineapple plantations are believed to have contributed to the transmission of dengue fever in pineapple growing areas in Sri Lanka. Hence, there is a need to introduce effective control methods to reduce dengue vector breeding in leaf axils of pineapple plants, a characteristic larval habitat of dengue vectors. The current study was designed primarily to obtain baseline information required for a much longer study planned to evaluate the use of pyriproxyfen and temephos in leaf axils of pineapple. METHOD: The study was carried out in five pineapple plantations in Medical Health Office area Meerigama in the Gampaha District from December 2014 to June 2015. Ninety plants from the five plantations were selected randomly and the volume of water in each plant and number of leaf axils positive with immature stages of mosquitoes was recorded twice a month. The water contents of leaf axils positive for mosquito larvae were transported to the laboratory and larvae were identified using standard keys. Chemical parameters: pH and temperature were measured in situ in the same samples and a few selected chemical parameters (concentrations of nitrates, phosphates, potassium and total dissolved solids) were measured in the laboratory using standard methods. Climatic data were obtained from the Meteorological Department of Sri Lanka. RESULTS: Eight mosquito species belonging to five genera were identified in leaf axil water of pineapple. Four species of Aedes including Aedes albopictus, the secondary vector of dengue, were identified while the primary vector Aedes aegypti was not recorded during the entire study period. The water volume per plant varied from 2.34 – 25.27 ml and it showed a significant positive correlation with the vector density (r=0.54, p=0.001). The number of rainy days in each month (r=0.76, p=0.000) and the monthly mean rainfall (r=0.53, p=0.001) showed significantly positive correlations with monthly vector density. Although monthly mean pH value showed no correlation with the monthly mean water temperature correlated negatively with the vector density (r= -0.5, p=0.004). None of the chemical factors except the concentration of nitrate (r=-0.45, p=0.016) in leaf water were associated with the vector density. CONCLUSIONS: Ae. albopictus was the most abundant species in all months except in January. The study revealed that the water volume in leaf axils, number of rainy days per month, monthly mean rainfall, water temperature and nitrate concentration of axil water seemed to affect the Ae. albopictus breeding, which are the important data useful in evaluating the use of pyriproxyfen and temephos in leaf axils of pineapple in the future. However, a continuation of the study for another six months to observe seasonal variation within a year is recommended.