Repellent effect of oregano essential oil and carvacrol analogs against imported fire ants

Imported fire ants are highly destructive invasive pests that damage agriculture and ecosystems. Their control largely depends on synthetic pesticides, which raise concerns about environmental contamination and resistance. This highlights the need for natural product-based alternatives.

Oregano essential oil from Origanum vulgare exhibited potent repellency against red imported fire ants (Solenopsis invicta) and hybrid imported fire ants (S. invicta × S. richteri), with a minimum repellent effective dose (MRED) of 15.6 and 7.8 μg g^-1, respectively. Gas chromatography-mass spectrometry analysis identified carvacrol (63.9%) as the predominant monoterpenoid component. The high concentration and strong activity of carvacrol (with an MRED of 0.98 μg g^-1 against both red and hybrid imported fire ants) suggest that it is the primary contributor to the oil's repellent effect. To further investigate structure-activity relationships, 21 carvacrol analogs (compounds 1-21), obtained commercially or via synthesis, were evaluated for their repellency. Ten compounds (1, 7, 12-18, and 20) showed significant activity, with MRED values ranging from 7.8 to 15.6 μg g^-1. Analysis indicated that the presence of a free hydroxy or amino group on a 1,2,4-trisubstituted benzene ring is essential for repellent efficacy. Molecular docking with the pheromone-binding protein Gp-9 from hybrid imported fire ants revealed that hydrogen bonding with Tyr17 or Gln16 likely mediates repellent action.

This study demonstrates the potential of oregano essential oil-particularly its high carvacrol content-and related structural analogs as promising natural repellents for the management of imported fire ants. © 2025 Society of Chemical Industry.