Repellent activity of the botanical compounds thymol, carvacrol, 2 nootkatone, and eugenol against Amblyomma sculptum nymphs

Abstract

This study evaluated the repellent activity of the botanical compounds thymol, carvacrol, nootkatone, and eugenol (5%), as well as the combination of 2.5% nootkatone + 2.5% eugenol, against Amblyomma sculptum nymphs under laboratory and field conditions. In contact bioassays, carvacrol and nootkatone showed the highest mean repellency rates (94.5% and 93.7%), followed by thymol and eugenol (90.2% and 87.2%). The combination (nootkatone + eugenol) resulted in 92.8% repellency, with 100% efficacy in some evaluation periods. The repellency of 7% DEET (positive control) was 82.2%. Nootkatone 5% and the combination (nootkatone + eugenol) were also tested in a Y-tube olfactometer, showing repellency rates of 86.1% and 72.2%, respectively, both higher than 7% DEET (69.4%). In field trials, volunteers wore treated socks and walked through an area naturally infested with nymphs. A significant reduction (p < 0.05) in tick counts was observed in the treated groups (about five unfed nymphs) compared to the control (about 40 unfed nymphs). Cumulative efficacy exceeded 85% in both treatments. In the cost simulation, the estimated cost of a 100 mL formulation containing 5% nootkatone was USD 50.8, while the combination (nootkatone + eugenol) presented a cost USD 28.6, representing a 44% reduction with no loss of efficacy in field conditions. These results indicate that all compounds tested showed repellent activity in the laboratory. Nootkatone has high repellent activity, and its combination with eugenol is a promising and more economically viable alternative for tick repellency.