Screening of terpenes and phenylpropanoids against Amblyomma sculptum nymphs: Non-spatial and spatial repellency

Repellents are important tools for preventing bites from arthropod vectors of pathogens, including the tick Amblyomma sculptum, a vector of the etiological agent of Brazilian Spotted Fever. Evidence from the scientific literature indicates that essential oils (EOs) and their constituent compounds, including terpenes and phenylpropanoids, have potential as tick repellents. Thus, the aim of this study was to evaluate the non-spatial (contact) and spatial (olfactory) repellent activity of botanical compounds (= natural compounds) commonly found in EOs against unfed A. sculptum nymphs, and to compare their repellent efficacy with that of DEET, the gold-standard arthropod repellent. Initially, Petri dish assays (contact, non-spatial repellency) were conducted using the following botanical compounds: (i) monoterpenes - carvone, citral, β-citronellol, linalool, p-cymene, α-pinene, and thymol; (ii) sesquiterpenes - α-bisabolol, caryophyllene, and nerolidol; and (iii) phenylpropanoids - cinnamyl acetate, trans-anethole, and (E)-cinnamaldehyde; at concentrations of 1% and 5% (w/v). DEET was tested at a concentration of 15%. All compounds were commercially obtained from Sigma-Aldrich. Subsequently, the compounds that showed the highest efficacy in the contact repellency assay (nerolidol and α-bisabolol: > 85%) were evaluated for olfactory (spatial) repellency using a Y-olfactometer. It was observed that, in the contact test, the monoterpenes at the 1% concentration did not exceed 80% of repellency, with linalool reaching the higher rate (69%). At the 5% concentration, thymol, citral, and β-citronellol showed over 80% of repellency, although none reached the established cutoff value (85% of repellency). Among the sesquiterpenes, caryophyllene exhibited the best repellency mean at 1% (70.7%) without statistical significance, while α-bisabolol (92.5%) and nerolidol (89.2%) surpassed 85% repellency at 5%, with significant results. Only α-bisabolol outperformed DEET (81%), and nerolidol was statistically equivalent. Among the phenylpropanoids, trans-anethole stood out at 1% (mean 76%), and cinnamyl acetate showed the best performance at 5% (mean 81.5%), but none reached 85%. In the spatial repellency test, DEET achieved 75.9% repellency; α-bisabolol showed no repellent effect, and nerolidol exhibited significant spatial repellency at both concentrations. Thus, it can be concluded that, among all the botanical compounds tested, α-bisabolol and, particularly, nerolidol were the most promising candidates for the development of tick bite repellents.