Na-Hyun Lee, Ah-Hyeon Jeong, Namhyun Chung, Hoi-Seon Lee
Haemaphysalis longicornis, an ixodid vector associated with severe fever with thrombocytopenia syndrome, remains an important target for vector-control. Screening of Alpinia oxyphylla fruit essential oil identified nootkatone as the principal constituent associated with nymphicidal activity. Purified nootkatone showed activity against H. longicornis nymphs, with an LC50 of 0.058 μg/cm2. NTN3 and NTN3 plus piperonyl butoxide (PBO) yielded numerically lower LC50 values of 0.034 and 0.026 μg/cm2, respectively; however, because no formal LC50 comparison or interaction analysis was performed, these values were interpreted only as descriptive shifts in apparent potency. To characterize exposure-associated biochemical responses, AChE activity, detoxification-related enzyme activities, PBO-associated LC50 shifts, and molecular docking predictions were examined. In vitro nootkatone exposure reduced acetylcholinesterase (AChE) activity by 39.0-80.9% across the tested concentrations, while LC50 exposure was associated with a 48.0% reduction in AChE activity. These findings indicate an association between nootkatone exposure and reduced AChE activity but do not establish a causal role in mortality. 7-Eethoxycoumarin O-deethylation (ECOD) activity increased by 15.4% after nootkatone exposure, whereas glutathione S-transferase (GST) and α/β-esterase activities were not altered. Because ECOD is a general CYP450-linked proxy assay, this increase indicates altered monooxygenase activity rather than nootkatone-specific detoxification. PBO pre-exposure reduced ECOD activity and was accompanied by numerically lower LC50 estimates, but these parallel responses alone, without formal mediation analysis, do not demonstrate that ECOD inhibition caused the LC50 shift. Nootkatone is a promising vector-control candidate against H. longicornis nymphs, although causal mechanisms, environmental safety, and field applicability require further evaluation.