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In this research, (-)-usnic acid and (+)-usnic acid, commonly encountered lichen secondary metabolites, were evaluated for their insecticidal effects against the larvae of Culex pipiens L. (Diptera: Culicidae) under laboratory conditions. Both compounds showed strong larvicidal activity and caused 100% mortality on third-fourth larval stages of the species at 24 h at the doses of 5 and 10 ppm. Bioassays with (-)- and (+)-usnic acids against larvae of C. pipiens revealed that the LC50 values were 0.8 and 0.9 ppm, respectively. The results suggest that lichen compounds could be useful in the search of new insecticides.

The insecticidal activity of essential oils isolated from two species of Origanum (Origanum onites L. and Origanum minutiflorum (O. Schwarz and P.H. Davis) (Labiate)) was investigated against 3rd and 4th instar larvae of the mosquito, Culex pipiens L. (Diptera: Culicidae). The LC50 and LC90 values were 24.8 and 61.3 ppm, and 73.8 and 118.9 ppm, respectively. This activity was attributed to the carvacrol, the major constituent of these oils.

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