Indira S. Weerasinghe’s research while affiliated with Medical Research Institute of Sri Lanka and other places

The insecticidal activity of essential oil of Mentha piperita L. emend. Huds. against local mosquitoes as disease vectors was recognized and found to be due to the presence of menthol, which is the major aroma compound of the oil. The minor compounds of the oil, i.e. menthone, beta-caryophyllene, menthyl acetate, limonene, alpha-pinene and pulegone, showed either less or no activity against the mosquitoes tested. L-Menthol derivatives were synthesized and their knockdown effect and mortality were evaluated against local mosquitoes of Culex quinquefasciatus Say, Aedes aegypti L. and Anopheles tessellatus Theobald as disease vectors. This is the first report of mosquitocidal activity of menthol and its derivatives against Cx. quinquefasciatus, Ae. aegypti and An. tessellatus. Derivative synthesis followed by structure-activity relationship studies identified several derivatives, i.e. menthyl chloroacetate, menthyl dichloroacetate, menthyl cinnamate, menthone glyceryl acetal, thymol, alpha-terpineol and mugetanol, with enhanced mosquitocidal activity against Cx. quinquefasciatus, Ae. aegypti and An. tessellatus relative to the parent compound L-menthone. In ester derivatives of L-menthol the optimum activity is dependent on the size and shape of the ester group and the presence of chlorine atoms in the ester group. In structurally related derivatives of L-menthol the optimum activity is dependent on the aromaticity, the degree of unsaturation, the position of the hydroxy group and the type of functional group.

Insecticidal properties of essential oils of leaf and bark of Ceylon Cinnamomum zeylanicum Blume (Lauraceae), Cymbopogon nardus Rendle (Gramineae) and Cymbopogon citratus DC. (Gramineae) and their 10 compounds were investigated against Musca domestica (housefy). Cinnamomum zeylanicum bark and Cymbopogon citratus oils showed good knock down and mortality against M. domestica (LD50 1.37 and 1.71 μg/insect, respectively) than C. zeylanicum leaf (LD50 2.75 μg/insect) and C. nardus (LD50 3.10 μ/insect) oils. Cinnamaldehyde (65%) was a major constituent of C. zeylanicum bark oil, eugenol (77%) in the C. zeylanicum leaf oil, geraniol (24.8%) in the oil of C. nardus and citral (78.8%) in C. citratus leaf oil. Cinnamaldehyde, eugenol, eugenyl acetate, geraniol and geranyl acetate were more active against M. domestica than their corresponding essential oils, while cinnamyl acetate, citral, citronellal, citronellol and α-terpineol were less active.

Mosquito control properties of essential oils of leaf and bark of Ceylon Cinnamomum zeylanicum Blume (Lauraceae) and their eight compounds were investigated against Culex quinquefasciatus, Anopheles tessellatus and Aedes aegypti. Cinnamomum zeylanicum bark oil showed good knock-down and mortality against A. tessellatus (LD50 0.33 μg/mL) and C. quinquefasciatus (LD 0.66 μg/mL) than leaf oil (LD 1.03 and 2.1 μg/mL). Cinnamaldehyde was a major constituent of the bark oil and eugenol in the leaf oil. Cinnamaldehyde and eugenol both were more active against C. quinquefasciatus, A. tessellatus and A. aegypti than the bark and leaf oil. Mosquitocidal activity of cinnamyl acetate against three mosquito species was comptarable to that of the Cinnamomum bark oil, whereas eugenyl acetate was effective on A. tessellatus and C. quinquefasciatus. The other compounds showed less or no activity against mosquitoes tested.