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 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.

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This is the first study to investigate the activity of thymol on Rhipicephalus microplus larvae under semi-natural conditions. For this purpose, tests were conducted in pots with Brachiaria decumbens seedlings containing cattle tick larvae. Thymol, diluted in ethanol 50° GL, was tested at concentrations of 2.5, 5.0, 10.0, 15.0, and 20.0 mg/mL, along with the control group treated with the solvent alone. Each treatment was composed of five pots (1 pot = a repetition). The experiment was performed in three steps. On the first day, the larvae were applied at the base of the signalgrass. Twenty-four hours later, approximately 25 mL of the solution was applied with thymol on the top of the vegetation in each pot. The survival of the larvae was measured 24 h after application of the solutions. Each pot was analyzed individually, and the grass fillets contained larvae were cut with scissors, placed in Petri dishes, and taken to the laboratory to count the number of living larvae. At the highest concentrations (10, 15, and 20 mg /mL), the number of live larvae declined by more than 95 % in relation to the control group. The lethal concentration 50 % (LC50) and LC90 values were 3.45 and 9.25 mg/ml, respectively. The application of thymol in semi-natural conditions starting concentration of 10 mg/mL significantly reduced the number of living R. microplus larvae.