The toxicity of imperatorin (1) and osthol (2) identified in Cnidium monnieri seed and four structurally related compounds to third instar larvae of insecticidesusceptible (KS-CP strain) and field-collected (DJ-CP colony) of Culex pipiens pallens was examined. Results were compared with those of to conventional mosquito larvicide, fenitrothion and temephos. Based on 24-h LC50 values, imperatorin was 1.9, 3.7, 4.2, 12.4, and 15.1 times more toxic than isopimpinellin, isoimperatorin, osthole, xanthotoxin, and bergapten against KS-CP larvae, respectively. Overall, these compounds were less toxic than either fenitrothion or temephos. However, these compounds did not differ in toxicity against larvae from the two Culex strains, even though the DJ-CP larvae exhibited high levels of resistance α-cypermethrin, deltamethrin, chlorpyrifos, fenthion, and chlorfenapyr (resistance ratio, 94-1179). This finding indicates that the isolated compounds and the pyrethroid, organophosphorus, and pyrrole insecticides do not share a common mode of action or elicit cross-resistance.

The toxicity of materials derived from dried fruit of Cnidium monnieri against early third instar larvae of Culex pipiens pallens was examined using direct contact mortality bioassay. Results were compared with those of currently used mosquito larvicide, temephos. The bioactive principle was identified as xanthotoxol by spectroscopic analysis. As judged by 24 h LD50 value, xanthotoxol was highly effective (4.40 ppm), although this compound was less toxic than temephos (0.16 ppm). C. monnieri fruit-derived materials, particularly xanthotoxol, merit further study as potential mosquito larvicides for protection from humans and domestic animals from vector-borne diseases and nuisance caused by mosquitoes.