Bioassay-Guided Isolation of Active Phytochemicals Against Tuta absoluta (Meyrick) from Turraea floribunda and Caesalpinia welwitschiana
Ehawa, Flaure Rosette Essoung
Mohamed, Samira Abuelgasim
Chhabra, Sumesh Chander
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Tuta absoluta Meyrick is now one of the most harmful insect pests of Solanaceae in various parts of the world. Synthetic pesticides are the most used, current control method, but are associated with several problems including the development of resistance and negative ecological effects. These led to the search for more eco-friendly methods of controlling the pest, such as a search for phytochemicals that show subtle anti-pest properties. In the present study, the effects of the constituents of methanolic extracts associated with Turraea floribunda and T. nilotica leaves (Meliaceae) and those of Caesalpinia welwitschiana and C. bonduc roots (Fabaceae-Caesalpiniaceae) were evaluated on second instar larvae of T. absoluta. The extract from T. floribunda leaves was the most active (LD50 = 587.0 ng/μl), followed by C. welwitschiana (LD50 = 779.1 ng/μl). Bioassay-guided isolation of active compounds from these active extracts, using column chromatographic and preparative HPLC, led to the identification of twenty-two compounds. Their structures were established using spectroscopic techniques, including MS, 1 and 2D-NMR, and also by comparison with reported data. The methanolic leaf extract of T. floribunda yielded ten compounds (β-sistosterol, stigmasterol, ursolic acid, betulinic acid, a mixture of β-sistosterol-3-O-β-D-glucopyranoside and stigmastreol-3-O-β-D-gluco pyranoside, fridelin, lupeol, 11-epi-21-hydroxytoonacilide and 11β, 12α-diacetoxycedrelone). The methanolic roots extract of C. welwitschiana afforded twelve constituents, including apigenin, luteolin, afzelin, quercitrin, epiafzelechin-3-O-gallate, Kaempferol 3-O-α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranoside, Kaempferol, dipteryxic acid, neocaesalpin L, rutin, methyl gallate, and galic acid. Some of the isolates were tested on T. absoluta eggs. They showed varying levels of ovicidal effect, with quercitrin being the most active constituent at 81%. The results of the study showed potential of the phytochemicals of these plants in the management of T. absoluta.