Repellency effect of Pilocarpus spicatus A. St.-Hil essential oil and nanoemulsion against Rhipicephalus microplus larvae

Rhipicephalus microplus, the cattle tick, is a major cause of economic losses in bovine production. Due to the widespread acaricidal resistance to commercially available products, as well as their toxicity and environmental impact, alternative control methods are required. Nanoformulations produced from plant extracts as bioactive substances are very promising as innovative acaricidal agents. Thus, the aim of this study was to evaluate the in vitro repellent activity of Pilocarpus spicatus essential oil and its nanoemulsion against R. microplus, using larval repellent test (RT). The essential oil was extracted by hydrodistillation, using a Clevenger-type apparatus. The nanoemulsion was prepared with 5% essential oil, 5% tween 80, and 90% water, using the phase inversion method (50 mg/mL). Limonene was the major component (46.8%) of the essential oil, as determined by gas chromatography-mass spectrometry (GC/MS) and confirmed by flame ionization detection (GC/FID). According to the RT results, the essential oil had a repellent activity greater than 69%, from concentrations of 3.12 mg/mL (69.81 ± 10%) to 50 mg/mL (98.10 ± 0.6%), whereas the nanoemulsion at 50 mg/mL presented repellent activities of 97.14 ± 1.37% and 97.89 ± 0.52% 6 and 10 h after treatment, respectively. These values regarding to total repellency were very close to those calculated for mortality corrected by Abbott's formula. The phase inversion method preserved the chemical and physical characteristics of the essential oil since both reached an equal repellent effect at the same concentration. Therefore, P. spicatus essential oil and nanoemulsion had excellent repellent activities against R. microplus larvae, demonstrating its potential for future use as an alternative for tick control.

First report of the effect of Ocotea elegans essential oil on Rhipicephalus (Boophilus) microplus

Rhipicephalus (Boophilus) microplus is responsible for reducing animal welfare, causing a drop in productive performance and transmitting hemoparasites. The main strategy of tick control is application of synthetic acaricides. However, parasite resistance to these compounds is a major concern. Therefore, the acaricidal and repellent in vitro effect of the Ocotea elegans essential oil on larvae and adult females of R. (B.) microplus were evaluated. The larval packet test (LPT), larval repellency test (RT) and adult immersion test (AIT) were performed. The essential oil was analyzed by gas chromatography (GC/FID) and the structure of the oil's major constituent (92.2% sesquirosefuran) was elucidated by nuclear magnetic resonance. In the AIT, efficacy higher than 90% was detected from the concentration 25 mg/mL upward. In both LPTs performed after 48 h, only the 100 mg/mL concentration resulted in mortalities above 70%. On the other hand, the essential oil caused an average of 95.8% repellency from 0.78 to 100 mg/mL. The LC₅₀ in the two LPT (48 h) tests were 59.68 and 25.59 mg/mL, respectively. The LC₅₀ and LC₉₀ in the AIT were 4.96 and 17.37 mg/mL, and in the RT they were 0.04 and 1.24 mg/mL respectively. We conclude that the essential oil of O. elegans leaves has a significant acaricidal effect on engorged females and on larval repellency of R. (B.) microplus ticks, and can be a promising alternative for the control of this ectoparasite.