In vitro efficacy of essential oils against for the control of Rhipicephalus (Boophilus) microplus: Ultrastructural analysis of larval alterations using scanning electron microscopy

Rhipicephalus (Boophilus) microplus is an ectoparasite of significant economic impact in livestock, exacerbated by resistance to synthetic acaricides. This study evaluated the acaricidal activity of eight essential oils (EOs) on larval and engorged female stages of R. (B.) microplus and, analyzed the morphological alterations in larvae using Scanning Electron Microscopy (SEM). The stability of the EOs was also tested at different temperatures using the Preliminary Accelerated Stability Test (PAST). EOs from Cinnamomum zeylanicum, Ocimum basilicum, Eucalyptus globulus and Citrus reticulata exhibited efficacy greater than 80 % against larvae at concentrations ≥ 2,5 %. The EO of C. zeylanicum was particularly notable, inhibiting 100 % of oviposition in engorged females at concentrations of 5 % and 2,5 %. The chemical composition of the EOs was determined by gas chromatography coupled with mass spectrometry (GC-MS), identifying -limonene, eucalyptol, camphor, linalool, eugenol, and germacrene-D as major compounds. SEM revealed, in larvae exposed to the EOs of C. reticulata, O. basilicum, E. caryophyllus, and E. globulus, cuticular lesions, loss of integrity, oral deformities, and damage to locomotor appendages. The EO of E. globulus maintained its sensory properties even when subjected to different temperatures over 30 days, demonstrating efficacy of 97,4 % and 71 % against R. microplus larvae when stored at −20°C and 4°C, respectively. The data demonstrated acaricidal property of EO of E. globulus against the larvae and adults of R. microplus.