BIOASSAY GUIDED FRACTIONATION OF SPHAERANTHUS INDICUS EXTRACT AGAINST MOSQUITO VECTORS

Phytochemicals with mosquitocidal potential are now recognized as potent alternative insecticides. This study aims at exploring the mosquitocidal potential of the extract of the medicinal plant Sphaeranthus indicus. The soxhlet extracts of flowers, leaves and aerial parts of S. indicus with four solvents viz., hexane, chloroform, ethyl acetate, and methanol were screened against the larvae of the vector mosquitoes Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus. The commercially available synthetic insecticide temephos was used as positive control. The LC₅₀ values for the hexane extract of the flowers, leaves and aerial parts were 75.62, 48.22, 70.23 mg/L; 18.61, 53.34, 33.04 mg/L and 191.9, 71.58, 116.21 mg/L respectively for A. aegypti, A. stephensi and C. quinquefasciatus. Bioassay-guided fractionation was carried out for the hexane extract of the leaves of S. indicus using a silica gel column with eluents hexane followed by 5% and 10% ethyl acetate in hexane. Fractions were examined by thin layer chromatography (TLC) using silica gel, 60 F₂₅₄ plates using hexane-ethyl acetate (95:5), pooled the similar fractions and screened for mosquito larvicidal activity. The bioassay-guided fractionation showed that the fractions F3-F7 eluted with hexane showed 100% mosquito larvicidal activity against all the three species of mosquitoes at 10 mg/L. The FT-IR spectrum of this fraction revealed the presence of long-chain alkanes. Phytochemical analysis showed the presence of terpenes. Hence the active fraction consists of long-chain alkanes and terpenes. S. indicus extract may be developed as a botanical insecticide for mosquito larval control.