Essential Oil Applications, Leptospermum|

Synthetic pesticides are the cornerstone of vector-borne disease control, but alternatives are urgently needed to tackle the growing problem of insecticide resistance and concerns over environmental safety. Leptospermum scoparium J.R. Forst and G. Forst (manuka) essential oil and its four fractions were analyzed for chemical composition and toxicity against Aedes aegypti larvae. The use of bio-based amylose-N-1-hexadecylammonium chloride inclusion complexes (Hex-Am) as an emulsifier for Lscoparium essential oil was also investigated. Fraction 1 was inactive, fractions 2 (LC50 = 12.24 ppm) and 3 (LC50 = 20.58 ppm) were more toxic than the whole essential oil (LC50 = 47.97 ppm), and fraction 4 (LC50 = 35.87 ppm) had similar toxicity as the whole essential oil. Twenty-one chemical constituents were detected in Lscoparium essential oil compared to 16, 5, 19 and 25 chemical constituents in fractions, 1, 2, 3 and 4 respectively. The two most dominant chemical constituents were calamenene (17.78%) and leptospermone (11.86%) for Lscoparium essential oil, calamenene (37.73%) and ledene (10.37%) for fraction 1, leptospermone (56.6%) and isoleptospermone (19.73) for fraction 2, cubenol (24.30%) and caryophyllene oxide (12.38%) for fraction 3, and γ-gurjunene (21.62%) and isoleptospermone (7.88%) for fraction 4. Alpha-pinene, ledene, and aromandendrene were 2–7 times less toxic than the whole essential suggesting that the toxicity of Lscoparium essential oil was either due to other chemical constituents that were not tested or due synergist interactions among chemical constituents. Leptospermum scoparium essential oil-Hex-Am emulsion (LC50 = 29.62) was more toxic than the whole essential oil. These findings suggest that Lscoparium essential oil is a promising source of mosquito larvicide and that Hex-Am is an excellent emulsifier for Lscoparium essential oil for use as a larvicide.

Muturi, E. J., et al. (2020). “Leptospermum scoparium essential oil is a promising source of mosquito larvicide and its toxicity is enhanced by a biobased emulsifier.” PloS one 15(2): e0229076.

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