Abstract
BackgroundThe 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.).MethodsThird-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.ResultsFractionation 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.ConclusionBoth 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.
Highlights
The edible and medicinal leguminous plant Cassia tora L. (Fabaceae) is known to possess insecticidal properties against a wide range of plant-feeding insects
Chemical analysis LC-QtoF-MS analysis revealed that anthraquinones dominated the seed extract of C. tora; four of which were identified as aurantio-obtusin, obtusiolin, chrysoobtusin and obtusin (Fig. 1, Table 1)
Fractions 3 and 4 were ~ two- to fourfold less potent than the crude extract and the positive control azadirachtin, as indicated by their LD50 values (5 ppm, 95% CI: 0.004–0.006 for fraction 3; 7 ppm, 95% CI: 0.005–0.01 for fraction 4) and exposure time (Fig. 2)
Summary
The edible and medicinal leguminous plant Cassia tora L. (Fabaceae) is known to possess insecticidal properties against a wide range of plant-feeding insects. 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.). Mosquito control has become a global health priority, owing to their vectoring role of pathogens of many diseases such as malaria, dengue, yellow fever, Zika, West Nile, affecting a significant proportion of the world population. The plant Azadirachta indica (Meliaceae), commonly referred to as neem, is a classic botanical whose insecticidal activity of the crude or derived products have been evaluated against many insects including vectors of medical importance [3, 6, 8]. C. tora which produces large quantities of fruits has an unexplored potential for control of insect pests such as medical disease vectors. We aimed to assess the potential of the crude extract, its solvent fractions and two major anthraquinones against larvae of the malaria vector Anopheles gambiae (s.s.)
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