Mode of Action for Natural Products Isolated From Essential Oils of Two Trees Is Different From Available Mosquito Adulticides

Insecticidal properties of natural products may present alternatives to the use of synthetic molecule pesticides that are of diminishing effectiveness due to resistance. Three compounds, thymoquinone, nootkatone, and carvacrol, components of Alaska yellow cedar, Chamaecyyparis nootkatensis (D. Don) Spach, and incense cedar, Calocedrus decurrens (Torr.), essential oils, have been shown to have biological activity against a variety of mosquito and tick species. Although these components act as both repellents and insecticides, how they function in either capacity is unknown. Their use as mosquito control insecticides would be greatly increased if their mode of action is not the same as that of currently used commercial products. This study compared the lethal dosages for nootkatone, carvacrol, and thymoquinone by using colony strains of Anopheles gambiae Giles with known mutations at three different target sites. The altered target sites evaluated were the sodium channel para-locus mutation (L1014 F KDR) that confers permethrin resistance, the ACE-1 gene that confers organophosphate and carbamate resistance, and a gamma-aminobutyric acid receptor mutation of the Rdl locus conferring dieldrin resistance. Significant increases in lethal dose were not observed in any of the mosquito strains for any of the compounds tested compared with the doses required of chemicals with known modes of action at the mutated sites. Although the mode of action was not determined, this screening study indicates that none of these compounds interact at the target sites represented in the test mosquito strains. These compounds represent a different mode of action than existing chemicals currently used in mosquito control.