Leptospermum scoparium essential oil is a promising source of mosquito larvicide and its toxicity is enhanced by a biobased emulsifier

Authors: Muturi, Ephantus; Selling, Gordon; Doll, Kenneth - Ken; Hay, William; Ramirez, Jose

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2020
Publication Date: 2/20/2020
Citation: Muturi, E.J., Selling, G.W., Doll, K.M., Hay, W.T., Ramirez, J.L. 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. https://doi.org/10.1371/journal.pone.0229076.
DOI: https://doi.org/10.1371/journal.pone.0229076

Interpretive Summary: Synthetic pesticides are widely used to control mosquitoes and other insects that transmit diseases, but alternatives are urgently needed to tackle the growing problem of insecticide resistance and concerns over environmental safety. This study evaluated manuka essential oil as an environmentally friendly larvicide for mosquito control. Results show that manuka essential oil and some of its fractions are highly toxic to mosquito larvae. In addition, the use of a starch-based emulsifier enhanced the toxicity of manuka essential oil against mosquito larvae. These findings suggest that manuka essential oil can be harnessed as a source of insecticide for mosquito control and that starch-based emulsifiers can be used to enhance the toxicity and improve the solubility of manuka essential oil for use as an insecticide.

Technical Abstract: 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 L. scoparium 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 L. scoparium 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 L. scoparium 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 '-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 L. scoparium 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 L. scoparium essential oil is a promising source of mosquito larvicide and that Hex-Am is an excellent emulsifier for L. scoparium essential oil for use as a larvicide.