Insecticidal Effect of Solenostemma argel Extracts Against Culex pipiens

Of the various plant extracts from 10 plant species tested against larvae of Culex pipiens in the laboratory, only extracts from Solenostemma argel exhibited larvicidal activity. A chloroform leaf extract of S. argel exhibited relatively high activity with a lethal concentration causing 50% mortality (LC₅₀) of 15.89 ppm, while chloroform and ethyl acetate extracts of S. argel fruits were 19.70 and 19.52 ppm, respectively. The chloroform fruit extract at 10 ppm reduced the hatchability of Cx. pipiens eggs by 20%, whereas the chloroform leaf extract was found to be less effective (5% reduction). At 10 ppm, adult emergence was reduced by 84% and 75% for chloroform and ethyl acetate extracts of fruits, respectively. Metamorphosis of larvae exposed to chloroform fruit extract (10 ppm) was extended to 15 days, as compared to 10 days for control larvae. It took 12 days at 1 ppm, and 15 days at 6 and 10 ppm for chloroform fruit extract-treated embryos to develop into adult mosquito while it took 10 days in the control treatment. However, 100% toxicity was observed in the embryos of zebrafish, Danio rerio, treated with the ethyl acetate fruit extracts (LC₅₀ of 20 ppm and LC₁₀₀ of 40 ppm) and chloroform leaf extract (LC₅₀ of 30 ppm and LC₁₀₀ of 60 ppm). These findings emphasize the need to further isolate the bioactive molecules in S. argel crude extracts that may still be mosquitocidal but produce no, or minimal, adverse effects on nontarget organisms such as zebrafish.