Effects Of Insect Growth Regulator Pyriproxyfen On Dragonfly Nymphs As Predators Of Anopheles Mosquitoes At Mahanga, Vihiga County, Kenya

ABSTRACT

Malaria in sub-Saharan Africa is transmitted mainly by Anopheles gambiae Complex mosquitoes. One way of controlling these vectors is by targeting their aquatic stages, which is anticipated to cause significant reduction in adult vectors, hence in malaria transmission. Use of insect growth regulator, Pyriproxyfen, is one potential way of controlling malaria vectors. This study set out to determine the nymphocidal activity of Pyriproxyfen on non-target aquatic dragonfly nymphs during its application to control malaria vectors in western Kenya highlands. In this study, validation of dragonfly nymphs as predators of malaria vectors was done and impact of Pyriproxyfen on these nymphs was determined in Mahanga Village of Vihiga County in western Kenya highlands. One hundred dragonfly nymphs were exposed to third instar larvae of A. gambiae to determine their predation efficiency by counting the number of larvae remaining after predation. Eighty 5th instar dragonfly nymphs were exposed to Pyriproxyfen (Sumilarv0.5 G) at concentrations of 0.01ppm, 0.05ppm, 0.1ppm, and filtered tap water as control. The experiment was replicated four times and repeated in ten rounds in the laboratory. Observations were made at 24 hour intervals and data collected on mortality of dragonfly nymphs. One gram of Pyriproxyfen was applied in the 10 randomly selected mosquito breeding habitats at Mahanga once every month. Control experiments in An. gambiae breeding habitats were done at Muluhoro study site located 10Km away from Mahanga to avoid contamination by Pyriproxyfen (Sumilarv 0.5G). Percentage predation of 95% was obtained in 24 hours of exposure of An. gambiae larvae indicating that the dragonfly nymphs are efficient predators of A. gambiae. The insect growth regulator Pyriproxyfen (Sumilarv 0.5G) had nymphocidal activity on the dragonfly nymphs at a concentration of 0.05ppm and 0.1ppm in laboratory assays. Abundance of dragonfly nymphs in An. gambiae breeding habitat was determined by comparing the abundance of dragonfly nymphs in the intervention sites at Mahanga and non intervention site at Muluhoro. The results indicated that the dragonfly nymphs were present in both sites over the 11month period. Analysis using Generalized Estimation Equations (GEE) showed that the abundance of dragonfly nymphs was significantly different (p0.05). The findings of this study have shown that Pyriproxyfen (Sumilarv 0.5G) had insignificant nymphocidal activity on dragonfly nymph when used as a larvicide at lower concentrations of 0.05ppm (p>0.05). The study recommends that dragonfly nymphs should be included in mosquito control programs as they are predators of mosquito larvae. Additionally, Pyriproxyfen should not be used at higher dosages of more than 0.05ppm as it affects non target dragonfly nymphs.