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Culex quinquefasciatus is a common domestic mosquito that is widespread in many areas of Thailand and serves as a southeastern vector of Japanese encephalitis. The present study investigated the acute toxicity of crude extracts and alkaloid compounds of Piper retrofractum (Piperales: Piperaceae) in Cx. quinquefasciatus third instar larvae. P. retrofractum was sequentially extracted using hexane, dichloromethane, ethyl acetate, and methanol, and the crude extracts were tested on mosquito larvae. Detoxification and neuroenzymes were analyzed to establish the mode of action. Acute toxicity was assessed on Poecilia reticulata (Cyprinodontiformes: Poeciliidae) to determine the possibility of toxicity in a nontarget species. Our results showed crude hexane extract had the highest toxicity in Cx. quinquefasciatus (0.9 ppm). Piperine and piperanine, which are alkaloid compounds from the crude hexane extract, showed LC50 values of 0.27 and 2.97 ppm, respectively, after 24 h of exposure. All the crude extracts showed low toxicity in P. reticulata compared with that in the mosquito larvae. The carboxylesterase, glutathione-S-transferase, and acetylcholinesterase activities in Cx. quinquefasciatus were reduced after treatment with all the extracts and the two alkaloid compounds. Thus, P. retrofractum shows larvicidal effects against Cx. quinquefasciatus and low toxicity for nontarget species. Thus, P. retrofractum could be a choice for controlling Cx. quinquefasciatus.

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Background: Mosquitoes are vectors for diseases damaging human health and thus, there is an urgent need for insecticidal compounds to control their population. The objective of this study was to determined the efficiency from trans-cinnamic acid isolated from Alpinia galanga (Zingiberales: Zingiberaceae) for control of Aedes aegypti (Dipetera: Culicidae), Anopheles dirus B (Dipetera: Culicidae) and Culex quinquefasciatus (Dipetera: Culicidae). Methods: Alpinia galanga (Zingiberales:Zingiberaceae) was extracted by soaking in a sequence of solvents (hexane, dichloromethane, ethyl acetate and methanol), and the isolated trans-cinnamic was separated by preparative thin layer chromatography. All crude extracts and isolated trans-cinnamic were evaluated for their control and affect on detoxification enzyme activities of the third-instar larvae of each mosquito species in laboratory conditions. Results: Our results showed that the hexane crude extract had the best control efficiency in all species, particularly in Cx. quinquefasciatus. The trans-cinnamic acid, isolated compound from hexane crude extract showed as active ingredient against third-instar larvae of each mosquito species. Mortality in this case may result from the inhibition of carboxylesterase. Conclusion: These results indicated that A. galanga which had trans-cinnamic acid as active ingredient compound could represent a promising naturally occurring control agent for all three mosquito species. However, this research consider as an initial prospective study, the other side effect on nontarget species need to concerned before used as commercial product.