Isolation and Identification of Larvicidal Constituents from Cnidium monnieri and Magnolia denudata against Four Mosquito Species and Their Potential Mode of Action

Abstract

The yellow fever mosquito, Aedes aegypti and the Asian tiger mosquito, Aedes albopictus, the malaria vector mosquito, Anopheles sinensis, and the northern house mosquito, Culex pipiens pallens mosquitoes transmit dengue fever, malaria, and West Nile virus diseases, respectively. More than 2.5 billion people are at risk of dengue infection over 100 countries worldwide, and there may be 50?100 million dengue infections every year, including 22,000 deaths annually, mostly among children. A recent study calculated that 3.97 billion people are at risk of dengue infection in 128 countries worldwide. From 1999 to 2010, 37,088 cases of human West Nile virus disease (including 16,196 neuroinvasive disease cases) were reported in the United States, resulting in 1,549 deaths. With global warming and increased international travel, a number of mosquitoes are distinctly increasing in incidence with a high occurrence of dengue fever all over the globe. Widespread insecticide resistance has been a major obstacle in the cost-effective integrated mosquito management program. There is a pressing need for the development of selective alternatives for the control of mosquitoes, with novel target sites to establish a rational management strategy and tactics because vaccines for malaria or dengue are not yet available. The aim of the study was to assess the contact toxicity of two coumarins from the fruits of Cnidium monnieri and 11 structurally related coumarins, the 17 constituents from Magnolia denudata seed hydrodistillation (MD-SHD), and honokiol, linoleic acid and palmitic acid from the seeds M. denudata to third instar larvae from insecticide-susceptible Cx. p. pallens (KS-CP strain) and Ae. aegypti as well as a wild Cx. p. pallens (YS-CP colony), Ae. albopictus and An. sinensis, using a direct-contact mortality bioassay. Quantitative structure-activity relationships (QSAR) of the test compounds are also discussed. In addition, the possible mode of larvicidal action of the constituents was elucidated using histologic and biochemical methods. In this study, a direct-contact mortality bioassay was used to identify the larvicidal constituents of C. monnieri fruit. The larvicidal principles were identified as the linear furanocoumarin imperatorin and the simple coumarin osthole. Imperatorin (LC50, 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 Cx. p. pallens and Ae. aegypti larvae, respectively. Overall, all the compounds were less toxic than the larvicide 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. QSAR indicates that chemical structure and alkoxy substitution and length of the alkoxyl side chain at C8 position are essential for imparting toxicity. The molecular weight, hydrophobic parameter, and molecular refraction are negatively related to the observed coumarin toxicities. An assessment is made of the contact toxicity assess of the 17 constituents from MD-SHD to third instar larvae from insecticide-susceptible Cx. p. pallens and Ae. aegypti as well as wild Ae. albopictus and An. sinensis. Results were compared with those of the four conventional insecticides fenitrothion, fenthion, temephos, and deltamethrin. The efficacy of four liquid formulations (10, 20, 30 and 50 mg L?1 liquids) containing the seed hydrodistillation was compared with that of the commercial larvicide temephos 200 g L?1 emulsifiable concentrate (EC) because the larvicide has low toxicity to mammals and aquatic organisms and is less persistent in the environment. 2, 4-Di-tert-butylphenol was the most toxic constituent (LC50, 1.98?3.90 mg L?1), followed by linoleic acid (7.19?10.49 mg L?1) towards four mosquito species larvae. High toxicity was also produced by nerolidol, (?)-limonene, α-terpinene and γ-terpinene (LC50, 9.84?36.42 mg L?1). The toxicity of these compounds was virtually identical towards four mosquito species larvae, even though An. sinensis larvae were resistant to deltamethrin and temephos. The MS-SHD 50 mg L?1 liquid resulted in 92?100% control towards four mosquito species larvae while commercial temephos 200 g L?1 emulsifiable concentrate was almost ineffective towards An. sinensis larvae (30% mortality). In M. denudata seed, the larvicidal principles were identified as the lignin honokiol and the fatty acids linoleic acid, and palmitic acid. Honokiol (LC50, 6.32, 6.51, 6.13 and 7.37 mg L?1) was the most toxic compound against susceptible Cx. p. pallens, Ae. aegypti, Ae. albopictus, and An. sinensis larvae, respectively. The lignin compound was 1?1.2 and 5?14.8 times more toxic than linoleic acid and palmitic acid, respectively. Overall, all the compounds were less toxic than either fenthion or temephos. The toxic effect of honokiol alone or in combination with linoleic acid and palmitic acid (1:1, 1:2, 1:3, 2:1, and 3:1 ratios) to third instar larvae from insecticide-susceptible and -resistant Ae. albopictus and An. sinensis was evaluated using a direct-contact mortality bioassay. Binary mixture of honokiol and linoleic acid (2:1 ratio) was significantly more toxic against insecticide-susceptible Ae. albopictus (LC50, 2.11 mg L?1) and resistant An. sinensis (2.19 mg L?1) than either honokiol or linoleic acid alone. The binary mixture of honokiol and linoleic acid (2:1 ratio) merit further study as potential larvicides for the control of insecticide-resistant mosquito populations. In acetylcholinesterase (AChE) inhibition assay, no potent inhibition was observed (IC50 >1 × 10-3) in isolated and identified compounds imperatorin osthole from C. monnieri fruits. However, the selected two linear furanocoumarins (osthole, 7-ethoxycoumarin), two simple coumarins (imperatorin, isoimperatorin), one angular furanocoumarins (angelicin) were caused decrease in cAMP levels, indicating that the mechanism of insecticidal action of these coumarins might be due to interference with the octopaminergic system. Honokiol and linoleic acid, with good AChE inhibitory activity against third instar larvae of Ae. aegypti have been sought out from the seed of M. denudata, and have been proved to have potential for development of natural insecticides. In histopathological study, osthole and imperatorin were shown, compound penetration and delocalized or demolished of internal cell organelles of larval body. Transmission electron microscope (TEM) pictures clearly indicated that damaged organelles after 24 hours of LC50 treatment. C. monnieri fruit- and M. denudata seed-derived materials merit further study as potential mosquito larvicides for the control of insecticide-resistant mosquito populations in light of global efforts to reduce the level of highly toxic synthetic insecticides in the aquatic environment.