Seasonal and circadian dynamics of Ageratum conyzoides essential oil and its application in tick control

The aim of this study was to investigate the seasonal and circadian dynamics of the essential oil (EO) of Ageratum conyzoides (purple-inflorescence chemotype) and its relationship with acaricidal activity against Rhipicephalus microplus . Seasonal collections were performed monthly at 6 a.m. throughout 2023, while circadian profiles were obtained at 6, 9, and 12 a.m., and 3 and 6 p.m. in April (rainy season) and October (dry season). The EO was extracted by hydrodistillation, characterized by GC–MS, and evaluated using the larval immersion test. Yields ranged from 0.03 to 0.24% (v/w), with lower and more stable values in the rainy season and higher values in the dry season. A total of 96 constituents were identified, representing 89.62% of the oil composition, with predominance of sesquiterpenes. Major compounds included caryophyllene oxide, spathulenol, humulene epoxide II, α-cadinol, α-santalene, β-elemene, and p-cymene. Acaricidal activity varied markedly between seasons. Rainy-season samples showed low larval mortality across all concentrations, preventing LC₅₀ determination. In contrast, dry-season samples exhibited consistent activity, with LC₅₀ = 130 µg/mL. This enhanced activity was associated with a higher proportion of oxygenated sesquiterpenes, particularly spathulenol. Circadian variation had a secondary effect; however, early morning collections (6 a.m.) during the dry season showed greater chemical stability and improved biological response. These findings indicate that seasonal variation significantly influences the acaricidal efficacy of A. conyzoides EO, and that collection during dry periods and early morning may improve reproducibility and effectiveness for tick control applications. • Seasonal variation modulates the chemical profile of A. conyzoides oil. • Oil shows a sesquiterpene chemotype, enriched in oxygenated forms in dry season. • Spathulenol and humulene epoxide II are markers of acaricidal activity. • Acaricidal activity against R. microplus occurs only in dry-season oil. • Morning dry-season collections optimize bioprospecting and bioassays.