Insecticidal activity of different extracts from Scrophularia canina L. against Culex pipiens molestus Forskal (Diptera, Culicidae)

The insecticidal activity of different extracts (aqueous, methyl alcohol, hexane and petroleum ether) of the aerial part of Scrophularia canina against the second and fourth-instar larvae and adult females of Culex pipiens molestus was investigated. The larvicidal activity of all the extracts was tested in the dose range from 7.8 to 1000 ppm. The highest toxicity was exhibited by the petroleum ether extract against second-instar larvae (48 h LC(50) = 23.5 ppm) and by the hexane extract against fourth-instar larvae (48 h LC(50) = 23.6 ppm). Methyl alcohol and aqueous extracts did not show any larvicidal activity. Sub-lethal doses of petroleum ether and hexane extracts induced increasing mortalities during 13 days after treatment but did not affect the duration of larval and pupal stages. In persistence tests, the hexane extract retained a satisfactory larvicidal activity after a 10-day period of test solutions incubation. Topical treatment of adult females with doses from 0.25 to 8 mu g per mg of female body weight of different extracts showed a similar toxicity for the hexane (24 h LD(50) = 1.7 mu g mg(-1)) and petroleum ether (24 h LD50 = 1.8 mu g mg(-1)) extracts which were significantly more toxic than methyl alcohol extract (4.2 mu g mg(-1)). Aqueous extract did not induce adult mortality. The marked mosquitocidal activity of petroleum ether and hexane extracts of the aerial part of S. canina against different life stages of C. pipiens molestus is promising to develop effective alternatives to synthetic insecticides.