Abstract:
BACKGROUND : The edible and medicinal leguminous plant Cassia tora L. (Fabaceae) is known to possess insecticidal
properties against a wide range of plant-feeding insects. However, the bioactivity of extracts of this plant and their
constituents against vectors of medical importance has been largely unexplored. We investigated the mosquito larvicidal
activity of the seed extract and its major anthraquinones against larvae of the African malaria vector Anopheles gambiae (s.s.).
METHODS : Third-fourth instar larval mortality was observed after 24, 48, 72 and 96 h of exposure to varying doses of the
extracts, and two anthraquinones isolates identified using liquid chromatography- quadrupole time of flight mass
spectrometry (LC-QtoF-MS). The mosquito larval mortality was evaluated relative to the natural insecticide azadirachtin.
RESULTS : Fractionation of the crude extract decreased mosquito larvicidal activity, however, larvicidal activity increased with
increasing dose of the treatment and exposure time. The known anthraquinones aurantio-obtusin and obtusin were
identified as key larvicidal compounds. Aurantio-obtusin and obtusin, exhibited similar toxicity to larvae of A. gambiae (s.s.)
with LD50 values of 10 and 10.2 ppm, respectively. However, the two anthraquinones were four- and ~ six-fold less potent
than that of the crude seed extract and azadirachtin, which had comparable LD50 values of 2.5 and 1.7 ppm, respectively.
CONCLUSION : Both aurantio-obtusin and obtusin showed mosquito larvicidal activity which were comparable to their
respective fractions although they were less potent relative to the crude extract and azadirachtin. Further studies need to be
conducted on C. tora for its exploitation as a potential eco-friendly tool in mosquito larval source reduction.
