This study evaluated the larvicidal activity of ethanol, acetone, hexane, and petroleum ether extracts of the leaf, stem, and root parts of P. hysterophorus against the 4th instar larvae of An. arabiensis. The toxic effect of P. hysterophorus against the 4th instar larvae of An. arabiensis depends on the solvent type, test concentration of the extract and plant parts. Petroleum ether and hexane extracts of the stem and root of P. hysterophorus resulted in relatively high larvicidal activity against 4th instar larvae of An. arabiensis. High larval mortality (≥ 93%) was achieved with petroleum ether root extracts at ≥ 160 ppm concentrations. Lower concentrations of petroleum ether root extracts (40 ppm and 120 ppm) also caused appreciably high (≥ 80%) larval mortalities (Fig. 3A) with dose dependent efficacy in toxicity. The LC90 value of Petroleum ether root extracts (LC90 = 105.5 ppm, Table 1) could be optimized to relatively low concentration formulations for application under field conditions. Hexane root extracts also produced promising efficacy (85.0% and 96.7%) but at relatively higher concentrations (LC90 = 338.3 ppm; Table 1). The high larvicidal activity of petroleum ether and hexane extracts of the root parts of this plant could be due to the presence of toxic phytochemicals such as saponins, phytosterols, phenols, flavonoids, and tannins, alkaloids and phenolic compounds detected in P. hysterophorus plant parts [9, 26–29].
The acetone (LC50 = 478 ppm, LC90 = 1358.2), hexane (LC50 = 368 ppm, LC90 = 809.2 ppm) and petroleum ether (LC50 = 393.1, LC90 = 1352.6 ppm) extracts of P. hysterophorus leaf part were moderately effective against 4th instar larval of An.arabiensis at higher concentrations and need further evaluation for use as mosquito larvicides under field conditions. In consistent with this result, acetone extracts of Solanum trilobatum leaf against 2nd instar larvae of Culex quinquefasciatus (LC50 = 265.7 ppm, LC90 = 558.3 ppm) and Aedes aegypti (LC50 = 301.1 ppm, LC90 = 582.3 ppm) were moderately effective [30]. The methanol extract of Melaleuca cajuputi leaf against Ae. aegypti (LC50 = 183.4 ppm and LC90 = 1000 ppm) and Ae. albopictus (LC50 = 191.8 ppm LC90 = 1000 ppm ) [31], and methanol extract of the Erythrina indica leaf against the larvae of An. stephensi (LC50 = 69.43, LC90 = 75.13), Ae. aegypti (LC50 = 91.4 ppm, LC90 = 125.5), and C. quinquefasciatus (LC50 = 134.3, LC90 = 167.14) were moderately effective [32]. In addition, petroleum ether extract of Cassia fistula and Nicotiana tabacum leaf caused a relatively high mortality of 3rd instar larvae of C. quinquefasciatus with a respective LC50 and LC90 of (203.5 ppm, 542.8 ppm) and (223.9 ppm, 645.5 ppm) [33]. The Ethyl acetate and methanol extract of Sterculia quinqueloba leaf against late 3rd instar larvae of An.gambiae resulted in LC50 and LC90 of (178.9 µg/ml, 526.3 µg/ml) and (3762.4 µg/ml, 63762.4 µg/ml) respectively after 24 hours of exposure time [34]. In contradict with this study, Kumar et al., [20] reported that acetone, hexane, petroleum ether and diethyl ether extract of P. hysterophorus leaf were not effective against 3rd and 4th instar larvae of Ae.aegypti. This variation could be attributed to the difference in the type of mosquito species involved in the experiment.
Petroleum ether, hexane and acetone extracts of P. hysterophorus stem showed a moderate larvicidal activity against the 4th larval instar of An.arabiensis with a respective LC50 and LC90 values of (196.3 ppm, 701.1 ppm), (112.4 ppm,331.9 ppm) and (259.4 ppm,1814.6) which also need a further field evaluation and screening. Similarly, hexane extract of Achyranthes aspera, Cassia occidentalis, Catharanthus roseus, Lantana camara and Xanthium strumarium stem exhibited a 100% mortality against 4th instar larvae of Ae. aegypti with LC50 and LC90 of (68.1 ppm, 115.1 ppm), (149.7 ppm,206.3 ppm), (108.2 ppm,184.2 ppm), (89.6 ppm,125.9 ppm) and (460.9 ppm,1074.0 ppm) respectively [35]. Ethyl acetate extracts of Sterculia quinqueloba stem resulted appreciable larvicidal activity against the 3rd instar larvae of Ae. aegypti (LC50 = 227.3 µg/ml, LC90 = 642.03 µg/ml) and An. gambiae s.s (LC50 = 135.4 µg/ml, LC90 = 313.73 µg/ml) [34]. Likewise, the respective (LC50 and LC90) values of hexane and petroleum ether extract of P. hysterophorus stem were (379.8 mg/L, 1314.4 mg/L) and (438.5 mg/L, 870.6 mg/L) against 3rd larval instar of Ae. aegypti [20].
A number of studies indicated that root of different plant species have a larvicidal potential against different species of mosquitoes [36–38]. In this study, petroleum ether (LC50 = 10.7 ppm, LC90 = 105.5 ppm) and hexane (LC50 = 87.9 ppm, LC90 = 338.3 ppm) extract of P. hysterophorus root caused a significant mortality of 4th instar larvae of An. arabiensis, this makes petroleum ether root a most effective of all extracts tested. Likewise, methanol and ethanol extract of Aristolochia saccata root was effective against Ae. albopictus larvae with a respective LC50 and LC90 of (14.5 ppm, 42.7 ppm) and (17.3 ppm,58.5 ppm) [39]. The same study also indicated that methanol (LC50 = 31.9 ppm, LC90 = 81.1 ppm) and ethanol (LC50 = 19.8 ppm, LC90 = 60.4 ppm) extract of Aristolochia saccata root caused a significant mortality of Cx. quinquefasciatus larvae starting from the lowest concentration. In addition, a significant larval mortality of Cx. quinquefasciatus was recorded because of petroleum ether extract of Solanum xanthocarpum root (LC50 = 41.3 ppm and LC90 = 111.2 ppm) [40]. Furthermore, a high (100% mortality at 200 mg/L within the 8th day of exposure time) larvicidal activity of aqueous-ethanoic extracts of Syzgium guineense root against Ae. albopictus was reported [41]. However, this result is by far superior to the larvicidal effect of petroleum ether and hexane extract of P. hysterophorus root against the 4th instar larvae of Ae.aegypti (LC50 and LC90 of (562.5 mg/L, 1232.1 mg/L) and (432.8 mg/L, 1118.5 mg/L) respectively) [20]. This could be due to the difference in mosquito species used for the experiment.
The phytochemical analysis of P. hysterophorus showed the presence of major secondary plant metabolites such as saponins, tannins, flavonoid, terpenoid and phlobatannins. Similarly, other studies revealed that P. hysterophorus is endowed with various chemical constituents such as, alkaloids, proteins, saponins, tannins, carbohydrate, glycosides, terpenoids, steroids, volatile oils, amino acids, amino sugars, lignans, phenolic compounds, flavonoids, metallic elements, organic acids, terpenoids [21, 26–28, 42].