This study aimed at assessing the combined effect of thymol, carvacrol and (E)-cinnamaldehyde on Amblyomma sculptum and Dermacentor nitens larvae. The effects resulting from treatments were evaluated by means of the modified larval packet test. In order to determine the LC50, components of essential oils, the monoterpenes thymol, carvacrol and phenylpropanoid (E)-cinnamaldehyde were individually tested at different concentrations. After determining the LC50, each essential oil component was separately evaluated and then combined with another substance at a 1:1 proportion at the LC50 concentration and at 1/2 and 1/4 of the LC50. For A. sculptum, the lowest LC50 value was obtained for (E)-cinnamaldehyde (1.40 mg/ml), followed by thymol (2.04 mg/ml) and carvacrol (3.49 mg/ml). The same order of effectiveness was observed for D. nitens, with values of 1.68, 2.17 and 3.33 mg/ml, respectively. In the evaluation of component associations of essential oils against A. sculptum larvae, only the combinations between carvacrol and thymol (LC50) and carvacrol and (E)-cinnamaldehyde (1/4 LC50) presented a moderate synergetic effect. In turn, for D. nitens larvae, the combinations between thymol and carvacrol (LC50 and 1/2 LC50) presented a synergetic effect, while the others presented an additive or antagonistic effect. Therefore, it can be concluded that the combination of thymol and carvacrol (LC50) has a moderate synergetic effect against A. sculptum larvae, while thymol, combined with carvacrol (LC50 and 1/2 LC50), has a synergetic effect against D. nitens larvae.

The aim of this study was to evaluate the acaricidal activity of eugenol, with different solubilizations and concentrations, on Rhipicephalus microplus and Dermacentor nitens larvae and to determine the lethal time. The study consisted of four experiments, and the mortality was assessed using the larval packet test with adaptations. The mortality observed in the first experiment was 100 % for all the groups treated with eugenol solubilized in different solvents. In the second, the hydroethanolic formulation of eugenol was used, and the mortality rates for R. microplus and D. nitens was 100 % starting from the concentration of 5.0 μl/ml. In the third experiment, the mortality was 100 % for larvae of both R. microplus and D. nitens after 1 h of contact. And in the fourth experiment, the mortality was above 90 % and statistically similar (p > 0.05) for the four methods the test evaluated.

The effects of combinations of the monoterpenes thymol and carvacrol and the phenylpropanoid eugenol in larvae of Rhipicephalus microplus (Canestrini, 1888) (Acari: Ixodidae) and Rhipicephalus sanguineus sensu lato (s.l.) (Acari: Ixodidae) were assessed by the larval packet test. The CompuSyn program was used to make qualitative assessments of the effects (synergistic, additive and antagonistic) of the associations. The effects of all combinations tested against R. microplus larvae were synergistic, with combination indices (CIs) <0.70. When tested against R. sanguineus, eight of the mixtures showed a synergistic effect (CI < 0.70); only the carvacrol + thymol mixture at LC presented a moderate synergistic effect, with CIs between 0.70-0.90. This study is the first to determine the effects of the interactions of these substances in the control of these two tick species. The combinations of carvacrol + thymol, carvacrol + eugenol and thymol + eugenol have synergistic effects in R. microplus and R. sanguineus s.l. larvae.

The objective of this study was to assess the acaricidal activity of carvacrol, thymol, eugenol, and (E)-cinnamaldehyde on unengorged larvae and nymphs of Amblyomma cajennense and Rhipicephalus sanguineus, using the modified larval packet test. Carvacrol, eugenol, and (E)-cinnamaldehyde were tested at concentrations of 2.5, 5.0, 10.0, 15.0, and 20.0 μl/ml, while thymol was tested at concentrations of 2.5, 5.0, 10.0, 15.0, and 20.0 mg/ml, in all cases with 10 repetitions per treatment. For the A. cajennense larvae, mortality rates caused by carvacrol, thymol, eugenol, and (E)-cinnamaldehyde at the lowest concentration were 45.0, 62.7, 10.2, and 81.6%, respectively, reached 100% at the concentration of 5.0 μl/ml for carvacrol and (E)-cinnamaldehyde and 5.0 mg/ml for thymol, while this mortality was observed at 15.0 μl/ml for eugenol. For the nymphs of this species, carvacrol and thymol caused 100% mortality starting at a concentration of 5.0 μl/ml and 10.0 mg/ml, respectively, while eugenol caused 100% mortality at 20.0 μl/ml and the mortality caused by (E)-cinnamaldehyde did not exceed 64%. In the tests with R. sanguineus larvae, the lowest concentration of carvacrol and (E)-cinnamaldehyde resulted in 100% mortality, while this percentage was observed starting at 10.0 μl/ml for eugenol. For nymphs, carvacrol and thymol at the smallest concentration caused 100% lethality, unlike the results for eugenol and (E)-cinnamaldehyde, where 100% mortality was only observed starting at the concentration of 10.0 μl/ml. The results obtained indicate that the tested substances have acaricidal activity on unengorged larvae and nymphs of A. cajennense and R. sanguineus.

The repellent activity of eugenol was evaluated on Rhipicephalus microplus and Dermacentor nitens larvae in a bioassay based on the vertical migration of these tick larvae during the host-seek phase. Solutions were used at concentrations of 10, 20, 30, 40, and 50 μl/ml and a control only with the solvent ethanol 50 GL. The larvae's reaction of climbing the rods of filter paper was observed each hour for 12 h and once more at 24 h. The repellence was calculated as a percentage through visual estimation, from the tendency of the larvae to climb the rods when the top part was impregnated with the eugenol solutions. For R. microplus larvae, the four highest concentrations produced repellence levels were greater than 80 % for up to 9 h. The repellence rates after 10 h for the concentration of 10 μl/ml was lower than 2 % (p > 0.05); and after 24 h, the values observed for the concentrations of 40 and 50 μl/ml were 77 and 80 % (p < 0.05). For the D. nitens larvae, the repellence was greater than 80 % for up to 5 h at the concentrations of 40 and 50 μl/ml; and after 9 h, the repellence fell for all treated groups were significantly different than the control were found only at the three highest concentrations. Therefore, eugenol has repellent activity on the larval stage of these two tick species, with the R. microplus larvae being more sensitive.