Repellency of Carvacrol, Thymol, and Their Acetates against Imported Fire Ants

SIMPLE SUMMARY: Imported fire ants are significant pests of urban, agricultural, and medical importance, causing USD billions of annual losses in the United States. Synthetic insecticides are commonly used in their management. The potential adverse effects of synthetic insecticides highlight the need to develop natural-product-based alternatives for fire ant control. Repellants are useful in managing fire ants; for example, repellants can be used to prevent fire ants from invading sensitive areas, such as electrical equipment, nursing homes, and hospitals. In particular, plant-derived natural repellants may provide a safer and more environmentally friendly alternative. This study demonstrates the repellent effects of the plant-essential-oil-derived compounds carvacrol, thymol, and their acetate derivatives against imported fire ants. Carvacrol, a GRAS compound (Generally Recognized As Safe) was the most potent repellent against red, black, and imported fire ants with minimum repellent effective doses of 0.98 µg/g, 7.80 µg/g, and 0.98 µg/g, respectively, followed by thymol, carvacrol acetate, and thymol acetate. Thymol red essential oil containing carvacrol and thymol also showed repellency. These results indicated that carvacrol and thymol as well as essential oils with high contents of carvacrol and/or thymol are potentially useful in managing imported fire ants. ABSTRACT: In the United States, imported fire ants are commonly referred to as red imported fire ants (Solenopsis invicta Buren), black imported fire ants (S. richteri Forel), and hybrid imported fire ants (S. invicta × S. richteri). They are significant pests, and their control heavily relies on synthetic insecticides. The extensive use of insecticides has led to public concern about their potential negative effects on human health and the well-being of wildlife and the environment. As an alternative, plant-derived natural compounds, particularly essential oils (EOs) and their main constituents, show promise as safe and environmentally friendly products for controlling fire ants. Repellants are useful in managing fire ants, and plant-derived natural repellants may serve as a safer and more environmentally friendly option. This study investigates the repellency of EO-derived compounds carvacrol, thymol, and their acetates against imported fire ant workers. The results revealed that carvacrol, a GRAS compound (Generally Recognized As Safe), was the most potent repellent against S. invicta, S. richteri, and their hybrid, with minimum repellent effective doses (MREDs) of 0.98 µg/g, 7.80 µg/g, and 0.98 µg/g, respectively. Thymol also exhibited strong repellency, with MREDs of 31.25 µg/g, 31.25 µg/g, and 7.8 µg/g, respectively. Furthermore, thyme-red essential oil, characterized by a thymol chemotype containing 48.8% thymol and 5.1% carvacrol, was found to effectively repel the hybrid ants with an MRED of 15.6 µg/g. In contrast, thyme essential oil, characterized by a linalool chemotype lacking thymol and carvacrol, did not exhibit any repellent effect, even at the highest tested dose of 125 µg/g. This study provides the first evidence of the potent repellency of carvacrol and thymol against imported fire ant workers, indicating their potential as promising repellents for fire ant control.