The larvicidal activity of 13 essential oils, 14 major monoterpenoids and 9 N-methyl carbamates based on these monoterpenoids was tested according to the recommendations of the WHO against fourth instar larvae of Culex pipiens L. The results of the larvicidal assays after 24 hr revealed a high potential for the essential oils, such as cinnamon, chenopodium and eucalyptus. Of the monoterpenoids, chlorothymol was found to be the most effective followed by thymol, carvacrol and cinnamaldehyde. Conversion of the most active monoterpenoids into their N-methyl carbamate derivatives resulted in higher larvicidal activities than those of the corresponding monoterpenoids. The use of piperonyl butoxide (PBO) to improve the efficacy of the most active monoterpenoids resulted in higher larvicidal activity. Also, all N-methyl carbamate derivatives were synergized by PBO. Carvacrol derivative, which was strongly synergized by PBO, was comparable with the standard synthetic insecticide malathion.
