Mosquito larvicidal activity of plant essential oils and essential oil compounds

Abstract

Diseases transmitted by mosquitoes, such as malaria, dengue, dengue hemorrhagic fever, filariasis and Japanese encephalitis, remain as a severe public health problem in many tropical countries. The development of resistance by vector mosquitoes is reducing the effectiveness of current synthetic insecticides for example larvicides in mosquito control. Therefore, natural larvicides based on plant essential could be used as alternative mosquito larvicidal agents. Even though there are previous reports on insecticidal properties of plant essential oils and their compounds, this is the first study on the mosquito larvicidal activity of essential oils of Ceylon Cinnamomum zeylanicum Blume, Cymbopogon nardus Rendle, Cymbopogon citratus DC and Sri Lankan varieties of Eucalyptus microcorys, Eucalyptus grandis and Eucalyptus robusta against local strains of Culex quinquefasciatus Say and Aedes aegypti L. In this study mosquito larvicidal activity of the leaf and bark oils of C. zeylanicum and the leaf oils of C. mardus, C. citratus, E. microcorys, E. grandis and E. robusta grown in Sri Lanka were evaluated and the mosquito larvicidal compounds from these oils were identified. Essential oils of E. microcorys, E. grandis and E. robusta leaves were obtained by hydrodistillation, and ethanol extracts of Eucalyptus were prepared following standard protocols. Constitutional analysis of essential oils were carried out on a gas-liquid chromatography equipped with flame ionization detector. Mosquito larvicidal bio-assay of oils, extracts and compounds was carried out according to WHO guidelines against Cx. quinquefasciatus and Ae. aegypti. Active major compounds of C. zeylanicum, C. nardus, C. citratus, E. microcorys, E. grandis and E. robusta were identified through above bioassay.

Cinnamomum zeylanicum bark oil showed more mosquito larvicidal activity against Cx. quinquefasciatus (LC₅₀ = 25.5 mg I⁻¹) and Ae. aegypti (LC₅₀ = 96.7 mg I⁻¹) than leaf oils of C. zeylanicum, C. nardus, C. citratus and Eucalyptus. Cimamomum zeylanicum bark oil activity is superior to its compounds, cinnamaldehyde (L₅₀ = 30.2 mg 1⁻¹), eugenol (LC₅₀ = 38.1 mg I⁻¹) and other constituents of the oil against Cx. quinquefasciatus. Cinnamomum zeylanicum leaf oil showed more mosquito larvicidal activity (LC₅₀= 34.4 mg I⁻¹) than individual constituents of the oil, except cinnamaldehyde (LC₅₀ = 30.2 mg I’) against Cx. quinquefasciatus. Major constituents of bark and leaf oils; ie. cinnamaldehyde (LC₅₀= 90.1 mg I⁻¹) and eugenol (LC₅₀ = 121.7 mg I⁻¹) showed more mosquito larvicidal activity against Ae. aegypti than their corresponding oils. Leaf oil of C. nardus exhibited good mosquito larvicidal activity for Cx. quinquefasciatus (LC₅₀ = 63.8 mgI⁻¹) and Ae. aegypti (LC₅₀ = 102.9 mg I⁻¹) and activity is lower than its major constituents; citronellol, o-pinene and geraniol. Citronellol is more larvicidal against Cx. guinquefasciatus (LC₅₀ = 48.8 mg I⁻¹) than major constituents of C. nardus oil. Geraniol showed more larvicidal activity against Ae. aegypti (LC₅₀ = 100.5 mg I⁻¹) than citronellol, geranyl acetate, myrcene, «-terpincol and limonene, except a-pinene. Cymbopogon citratus leaf oil showed moderate mosquito larvicidal activity against Cx. quinquefasciatus (LC₅₀ = 64.8 mg I⁻¹) and it had less activity than its major constituents; citral and geraniol. Citral showed more larvicidal activity against Cx. guinquefasciatus (LC₅₀ = 55.3 mg I⁻¹) and Ae. aegypti (LC₅₀ = 99.8 mg I⁻¹) than geraniol, linaly! acetate, linalool and myrcene. Among the Eucalyptus leaf essential oils tested E. robusta oil showed more mosquito larvicidal activity against Cx. quinguefasciatus (LC₅₀ = 44.5 mg I⁻¹) E. grandis against Ae. aegypti (LC₅₀ = 119.0 mg I⁻¹). a-Pinene is a major compound of leaf oils of E. robusta and E. grandis and this showed higher larvicidal activity against both mosquitoes tested in comparison to Eucalyptus oils. There is no considerable mosquito larvicidal activity found in ethanol extracts of E. microcorys, E. grandis and E. robusta leaves. This study indicated that aromaticity of cinnamaldehyde, p-cymene, cinnamy] acetate and eugenol may have an enhancing effect on the mosquito larvicidal activity in above compounds. It appears that the position of the hydroxyl group, i.e. in the aromatic ring, in the cyclohexane ring or in the chain molecule, may influence the larvicidal activity because activity of eugenol is more than geraniol, citronellol, linalool and a-terpineol against Cx. quinquefasciatus. This study also indicated that hydroxyl and aldehyde groups found in aromatic compounds could enhance the mosquito larvicidal activity. It also indicated that aldehyde group in the side chain of an aromatic compound such as cinnamaldehyde can enhance the larvicidal activity. The acetate group in aromatic ring may also increase the larvicidal activity comparison to acetate group in acyclic molecules. For example cinnamyl acetate showed more activity than linalyl acetate and geranyl acetate. Geraniol with less bulky ester group such as acetate tends to decrease the larvicidal activity against Cx. quinquefasciatus. This study reveals that local varieties of bark and leaf oils of C. zeylanicum and leaf oils of C. citratus, C. nardus, E. microcorys, E. grandis and E. robusta could be developed to potential mosquito larvicidal agents against Cx. quinquefasciatus and Ae. aegypti. Several mosquito larvicidal compounds, i.e. cinnamaldehyde, eugenol, cinnamyl acetate, apinene, p-cymene, citral, geraniol and citronellol were identified and these compounds seem to be potential to be developed as mosquito larvicides after toxicological evaluations for vertebrates, including humans, and eco toxicological evaluations and commercial feasibility studies.