Larvicidal activity of Cnidium monnieri fruit coumarins and structurally related compounds against insecticide-susceptible and insecticide-resistant Culex pipiens pallens and Aedes aegypti.

An assessment was made of the toxicity of imperatorin and osthole identified in Cnidium monnieri fruit, 11 related compounds and five insecticides to larvae from insecticide-susceptible Culex pipiens pallens (KS-CP strain) and Aedes aegypti and wild C.p. pallens (YS-CP colony) using a direct-contact mortality bioassay. Results were compared with those of the conventional larvicide temephos. Imperatorin (LC(50) = 3.14 and 2.88 mg L(-1) ) was 1.9-, 3.7- and 4.2-fold and 2.4-, 4.5- and 4.6-fold more toxic than isopimpinellin, isoimperatorin and osthole against susceptible C. p. pallens and A. aegypti larvae respectively. Overall, all of the compounds were less toxic than temephos (0.011 and 0.019 mg L(-1) ). The toxicity of these compounds was virtually identical against larvae from the two Culex strains, even though YS-CP larvae were resistant to fenthion (resistance ratio RR = 390), deltamethrin (RR = 164), cyfluthrin (RR = 14) and temephos (RR = 14). This finding indicates that the coumarins and the insecticides do not share a common mode of action. The structure-activity relationship indicates that the chemical structure and alkoxy substitution and length of the alkoxyl side chain at the C8 position are essential for imparting toxicity. The C. monnieri fruit-derived coumarins and the related coumarins described merit further study as potential insecticides or lead molecules for the control of insecticide-resistant mosquito populations.