Repellent and insecticidal activities of secondary metabolites from Praxelis clematidea against Solenopsis invicta (Hymenoptera: Formicidae).

Praxelis clematidea is a high-risk invasive species with potent insecticidal and repellent properties. This study isolated and identified its bioactive constituents against the red imported fire ant, Solenopsis invicta. Through bioassay-guided fractionation using a digging-based repellency assay, an active oily mixture (I2-[3]) was obtained, with a minimum repellent effective dose (MRED) of 0.0125 mg/g. Notably, caryophyllene oxide, a major component (22.3%), showed stronger repellency (MRED = 0.00156 mg/g) than DEET (MRED = 0.00625 mg/g). Friedelin (MRED = 0.0625 mg/g), stigmasterol, and 4',5,6,7-tetramethoxyflavone (each MRED = 0.125 mg/g), from moderately active fractions, exhibited only moderate activity. Caryophyllene oxide was selected for further evaluation. In behavioral bioassays, it significantly reduced S. invicta nesting activity for at least 15 days, reduced foraging ants within 1 h, and prolonged food transport time throughout the experiment. Moreover, in field tests, its repellency was comparable to DEET. In addition to its repellent effects, caryophyllene oxide also showed rapid contact toxicity (LT₅₀ = 1.19 h at 0.1 mg/cm2), albeit slightly less potent than DEET (LT₅₀ = 0.558 h). Enzyme assays revealed that caryophyllene oxide exhibited concentration- and time-dependent effects: at low concentrations, acetylcholinesterase (AChE) and carboxylesterase (CarE) showed initial activation followed by inhibition, unlike glutathione S-transferase (GST), which remained consistently activated; at high concentrations, both were rapidly inhibited, with GST inhibition likely due to exceeded detoxification capacity. Overall, these results indicate that P. clematidea contains active compounds against S. invicta, with caryophyllene oxide emerging as a promising dual-action (repellent and contact insecticidal) agent for S. invicta management.