Acaricidal activity of hydroethanolic formulations of thymol against Rhipicephalus sanguineus (Acari: Ixodidae) and Dermacentor nitens (Acari: Ixodidae) larvae

Larvicidal and repellent effects of essential oils on the brown dog tick (Rhipicephalus sanguineus Sensu lato) with description of new larval repellent activity test method

The aim of this research was to investigate the larvicidal and repellent effects of essential oils (EOs) obtained from two Lamiaceae plant species, Origanum minutiflorum O. Schwarz & P.H. Davis and Dorystoechas hastata Boiss. & Heldr. ex Bentham, both endemic to Turkey, on Rhipicephalus sanguineus s.l. Latreille (Acari: Ixodidae). The study also introduces a new test method that can be used to assess the repellent effects against ticks. Both plant EOs exhibited the highest larvicidal activity against brown dog tick larvae after 24 h and LC50 and LC90 values were determined as 0.101% and 0.125% for O. minutiflorum essential oil and 0.937% and 2.1% for D. hastata essential oil, respectively. In this study, we have described a detailed protocol for a novel larval repellent activity test (LRAT) for essential oils and extracts, using simple equipment. The advantages and limitations of LRAT, when compared to other tests commonly used to determine repellent effect against ticks, are also included in this study. The LRAT was developed with modifications of the larval immersion test (LIT) and proves to be a highly efficient and easily observable method. It can be used to test any active substance that may be toxic to humans and animals. According to the LRAT, at the end of 3 h, O. minutiflorum essential oil showed a high repellent effect, varying between 84.14% and 100% at 1% concentration. This result was not statistically different from the DEET, the positive control. When comparing the larvicidal and repellent activities, O. minutiflorum essential oil was found to be more effective than D. hastata essential oil.

Controlling Argas arboreus and A. persicus (Acari: Argasidae) by Adiantum capillus-veneris L. extracts with phytochemical analysis

Argas species are parasites associated mostly with birds. Their infestations of the host may cause blood loss, resulting in anemia and finally death. Egypt loses millions of tons annually from birds because of these parasites. In addition, they can transmit pathogens to animals and humans. The acaricidal effects of the ethanolic and methanolic extracts of Adiantum capillus-veneris at different concentrations (1-4%) against semi-fed adults of Argas arboreus and A. persicus were investigated during 30 days after treatments. Mobility and mortality, acaricide efficacy, and the concentration that kills 50% of specimens (LC50) were estimated. The percentage of dead adults of both Argas species appeared during 6 days considerably until 30 days was significantly increased after treatment of either ethanol or methanol extracts of Adiantum at 1-4%, versus control groups. Ethanolic extracts (100% mortality) were more effective than methanolic ones (90% mortality) for both Argas species. Argas arboreus (80% efficacy and 5.9% LC50) was more resistant than A. persicus (100% efficacy and 4.1% LC50). Generally, males were more resistant than females. The chemical profile (by gas chromatography-mass spectrometry analysis) for the ethanolic extract of Ad. capillus-veneris at 4% (the most effective extract) was provided for the first time, which showed that the major group was sugars and sugar alcohols, and the main components were thymol-ß-d-glucopyranoside, D-(-)-Tagatofuranose, D-Arabinose, D-Galactose, D-(-)-Fructofuranose and Anthracene, 1-methyl. The efficiency of all these components was discussed. Based on the findings, bioactive compounds present in Ad. capillus-veneris have the potential to be applied as substitutes for synthetic acaricides and a biological control agent in the management of A. arboreus and A. persicus ticks.

Role of antioxidant activity of essential oils in their acaricidal activities against Rhipicephalus annulatus

Essential oils of Origanum majorana and Satureja thymbra as well as carvacrol are natural products that are known to have potent antioxidant activities. The current study was designed to investigate the role of the antioxidant properties of these natural products in their acaricidal activities against Rhipicephalus annulatus larvae. The synergistic and/or antagonistic effects of the addition of vitamins E and C and hydrogen peroxide (H2O2) to these natural products were also evaluated. Larval packet tests were used to evaluate the acaricidal activities against the larvae of R. annulatus. The antioxidant effectiveness of these products was determined by a DPPH (2,2-diphenyl-1-picrylhydrazyl) free radical scavenging assay. The addition of vitamin E at 100 mg/mL to O. majorana and S. thymbra decreased the concentrations required to achieve the death of half of the larvae (LC50) to 0.44 and 0.47%, respectively. The combination of O. majorana and S. thymbra attained the LC50 at 1.54% which was decreased to 0.69% after addition of vitamin E. Also, the addition of vitamin E to carvacrol reduced the LC50 to 0.27%. The total antioxidant activity of these natural products increased significantly in presence of vitamin E. The addition of H2O2 inhibited the acaricidal activity of all tested materials, especially at low concentrations. All treatments induced an increase in lipid peroxidation, whereas carvacrol-treated larvae revealed the lowest values for the superoxide dismutase. Glutathione peroxidase and catalase activity decreased in larvae treated with S. thymbra combined with vitamin E. In conclusion, the addition of vitamins E and C increased the acaricidal activities of the tested compounds, whereas the addition of H2O2 decreased these activities. The antioxidant activities of essential oils and their active components may play an important role in mediating their acaricidal activities.

Acaricidal activity of essential oils of Cinnamomum zeylanicum and Eremanthus erythropappus, major compounds and cinnamyl acetate in Rhipicephalus microplus

This study aimed to chemically characterize the essential oils (EOs) of Cinnamomum zeylanicum (cinnamon) and Eremanthus erythropappus (candeia) and evaluate their acaricidal activity, together with that of their major compounds and cinnamyl acetate derivative, against Rhipicephalus microplus. Essential oil compounds were identified through gas chromatography. The larval packet test (LPT) at concentrations ranging from 0.31 to 10.0 mg/mL and the adult immersion test (AIT) at concentrations between 2.5 and 60.0 mg/mL were performed. (E)-cinnamaldehyde and α-bisabolol were the major compounds in cinnamon (86.93%) and candeia (78.41%) EOs, respectively. In the LPT, the EOs of cinnamon and candeia and the compounds (E)-cinnamaldehyde, α-bisabolol and cinnamyl acetate resulted in 100% mortality at concentrations of 2.5, 2.5, 5.0, 10.0 and 10.0 mg/mL respectively. In the AIT, percentage control values > 95% were observed for cinnamon and candeia EOs, (E)-cinnamaldehyde and α-bisabolol at the concentrations of 5.0, 60.0, 20.0, and 20.0 mg/mL, respectively, whereas cinnamyl acetate showed low activity. We conclude that EOs and their compounds showed high acaricidal activity, whereas the acetylated derivative of (E)-cinnamaldehyde presented less acaricidal activity on R. microplus engorged females.

Thymol and eugenol microemulsion for Rhiphicephalus sanguineus sensu lato control: Formulation development, field efficacy, and safety on dogs

The present study aimed to develop a microemulsion formulation containing thymol and eugenol for field control of Rhipicephalus sanguineus sensu lato on dogs, as well to evaluate its safety and the physical characteristics of the formulation. The microemulsion using thymol and eugenol (5.0 + 5.0 mg/mL) had as vehicles water, propylene glycol, polysorbate 80 and canola oil. On the next day the preparation (formulation freshly prepared) and after 24 months, the size of the microemulsion droplets, polydispersion index (PdI), organoleptic properties (color, viscosity), and presence of precipitate in the microemulsion were evaluated. For the field assay, on day -1, 10 English Cocker Spaniel dogs were experimentally infested with 200 larvae, 100 nymphs and 30 adults of R. sanguineus s.l. On day 0, after tick counts, the animals were divided into two groups: treated with the freshly prepared microemulsion (10 mL/kg), and control, which received the vehicle (10 mL/kg). Tick counts on dogs were performed daily for three more days. Engorged females were recovered from the dogs and their biological and reproductive parameters were monitored. The dogs' clinical parameters (temperature, mucosa color, and general physical condition) were evaluated daily. In addition, blood samples were collected before infestation to verify hematological (packed cell volume) and biochemical parameters (total serum protein, albumin, globulins, creatinine, urea, alanine transaminase, aspartate aminotransferase, and alkaline phosphatase). Freshly prepared and 24-month aged microemulsions had droplets with mean sizes of 30.94 nm and 27.93 nm, and PdI values of 0.214 and 0.161, respectively. In addition, no difference in the organoleptic properties and no precipitation formation were observed, indicating physical stability. Treatment with the microemulsion resulted in reduction of larvae (p < 0.05) parasitizing the dogs on day 1 while the number of nymphs and adults was not reduced (P> 0.05). In the evaluation of the reproductive biology of engorged females, the larval hatchability (%) was compromised (p < 0.05), and the microemulsion had control rate of 85.5 %. The microemulsion and its vehicles did not change the clinical, hematological and biochemical parameters of the dogs. We concluded that the microemulsion was efficient against R. sanguineus s.l. by reducing the number of larvae and affecting the reproductive parameters of engorged females, safe for dogs, and stable (physical stability) during a two-year interval.

Toxicity, repellency and anti-cholinesterase activities of thymol-eucalyptus combinations against phenotypically resistant Rhipicephalus annulatus ticks

The current study was designed to compare the acaricidal activity of pure thymol (T) crystals, eucalyptus oil (E), and their combinations (T + E) at concentrations ranging from 0.625 to 10% against phenotypically deltamethrin-resistant Rhipicephalus annulatus ticks. The adulticidal, larvicidal, ovicidal, and repellent activities of the tested compounds and their possible mechanism of action were studied. Thymol showed 93.3% adulticidal activity at 10% concentration, whereas at 2.5% it achieved 100% larvicidal activity. Eucalyptus oil had a significant effect on tick adults at concentrations > 5%, and at 10% concentration it showed 96.7% adulticidal activity and stopped the egg hatching. On deltamethrin-resistant larvae, the median lethal concentration (LD₅₀) was 0.61, 7.5, and 0.61% for T, E, and T + E combinations, respectively, and repellent activities were 85.7, 62.5, and 100% at the 10% concentration used. Combinations of T + E did not induce additional effects as adulticidal and larvicidal in a comparison with T treatments. The tick's acetylcholinesterase significantly decreased as a result of all treatments including deltamethrin, whereas lipid peroxidation significantly increased compared with the control untreated ticks. The glutathione content significantly decreased as a result of all treatments except for treatments with 10% thymol and its combination with eucalyptus oil. Moreover, 10% eucalyptus oil significantly decreased the tick's glutathione peroxidase and catalase activities reflecting the attenuation of antioxidant defense in the ticks. In conclusion, thymol and eucalyptus oil showed excellent activities against deltamethrin-resistant R. annulatus isolates. Their action may be mediated via acetylcholinesterase inhibition. The effect of thymol-eucalyptus combinations recorded higher activities as an ovicidal and as a repellent.

Evaluation of Eugenol and (E)-Cinnamaldehyde Insecticidal Activity Against Larvae and Pupae of Musca domestica (Diptera: Muscidae)

Musca domestica L., 1758, is an important mechanical vector of several pathogens for humans and livestock, making it essential to study new alternatives of more efficient and safer control for this dipteran. This study evaluated the toxicity of the phenylpropanoids eugenol and (E)-cinnamaldehyde on its life stages. A contact test with 10 repetitions (n = 10) was performed for each substance concentration on each post-embryonic immature life stage. Both substances presented insecticidal activity on the immature life stages of the dipteran, and secondary effects on development caused by sublethal concentrations. Larvicidal activity was shown from the 1.25 mg/ml concentration by eugenol and from 2.5 mg/ml by (E)-cinnamaldehyde, and both substances had a 100% larval treatment efficacy (LTE) from the 5mg/ml concentration. For pupal treatment, (E)-cinnamaldehyde differed from the control from the 10 mg/ml concentration (P < 0.05), and both phenylpropanoids caused malformation in adults from 10 mg/ml. The highest pupal treatment efficacy (PTE) was obtained from the 30 mg/ml concentration, 67.2% for (E)-cinnamaldehyde, and 32% for eugenol. The products tested in this study showed high larvicidal potential, and both presented pupicidal effects and caused malformation in adults from treated pupae.

Thymol: Effects on reproductive biology and Gene's organ morphology in Rhipicephalus sanguineus sensu lato engorged females (Acari: Ixodidae)

Thymol is a monoterpene with proven acaricidal activity on different tick species and life stages. The objective of this work was to evaluate the effect of thymol on the reproductive biology of engorged females of Rhipicephalus sanguineus sensu lato and the morphological changes caused in the cells of the tubular and accessory glands of Gené's organ. The females were exposed to thymol by immersion. Seven groups were established: group I (distilled water), group II (30% ethanol), group III (thymol 1.25 mg/mL), group IV (thymol 2.5 mg/mL), group V (thymol 5.0 mg/mL), group VI (thymol 10.0 mg/mL), and group VII (thymol 20.0 mg/mL), with 20 replicates for each treatment. The ticks were kept in a BOD incubator at 27 ± 1 °C and RH > 80 ± 10%. Ten females from each group were evaluated daily until death, and the remaining ten were kept in the incubator under the same conditions for five days and then dissected for the removal of Gené's organ to note possible damage to cell morphology by scanning electron microscopy (SEM) and hematoxylin-eosin (HE) techniques. Thymol demonstrated 63% and 98% efficacy in groups VI (10.0 mg/mL) and VII (20.0 mg/mL), respectively. In these groups, the cells of the tubular and accessory glands of Gené's organ showed signs of damage: irregular eosin staining, rupture and deformation of the cellular limit, presence of fragmented nuclei, changes in cytoplasmic homogeneity and areas with deformation (folds) in the basal region.

Thymol action on cells and tissues of the synganglia and salivary glands of Rhipicephalus sanguineus sensu lato females (Acari: Ixodidae)

Thymol is a monoterpene present in plants of the families Lamiaceae, Verbenaceae and Apiaceae. Despite its proven acaricidal activity, little is known about the mechanism of action of thymol in ticks. Thus, the aim of this study was to perform a morpho-histochemical analysis of the synganglion and salivary glands of partially engorged females of the brown dog tick, Rhipicephalus sanguineus sensu lato (s.l.), exposed to thymol at different concentrations. Five groups were established: Control Group I (distilled water), Control Group II (ethanol 30%), Group III (thymol 1.25 mg/mL), Group IV (thymol 2.5 mg/mL) and Group V (thymol 5.0 mg/mL). The females were exposed to the treatments by the immersion method and subsequently kept in a climatic chamber (27 ± 1 °C and relative humidity 80 ± 10%) for five days. After this period, the synganglion and salivary glands were removed, and the hematoxylin/eosin morphological technique was applied. The von Kossa staining method with counterstaining neutral red was performed on the salivary glands. The results showed that females exposed to thymol had damaged synganglia, with pyknotic nuclei and vacuoles in the cortex and subperineurial regions, as well as rupture of the neural lamellae. The salivary glands showed type I acini with a dilated lumen. Cells with extremely vacuolated cytoplasm and fragmented nuclei were observed in type II and III acini. Type II acini of the females exposed to thymol revealed different calcium staining when compared to the Control Groups I and II. We therefore conclude that the salivary glands and synganglion are subject to changes in morphology and calcium levels when exposed to thymol at concentrations of 1.25, 2.5 and 5.0 mg/mL, demonstrating that this monoterpene has acaricidal potential on partially engorged females of R. sanguineus (s.l.).

Activity of the extract of Acmella oleracea on immature stages of Amblyomma sculptum (Acari: Ixodidae)

This study evaluated the acaricidal activity of the methanol extract of Acmella oleracea with 0.187% of spilanthol against immature stages of Amblyomma sculptum. The packet test was used to evaluate the extract's activity on unengorged larvae and nymphs, testing concentrations of 0.4 to 50 mg/mL for larvae and 12.5 to 200.0 mg/mL for nymphs. For the engorged stages, the immersion test was used, at concentrations of 0.4 to 50 mg/mL for larvae and 12.5 to 200.0 mg/mL for nymphs. The methanol extract caused 100% mortality of the unengorged larvae and nymphs starting at concentrations of 12.5 and 200.0 mg/mL, respectively. For engorged larvae and nymphs, the mortality was 100% starting from concentrations of 12.5 and 150.0 mg/mL, respectively. The LC₅₀ for unengorged larvae was 3.2 mg/mL, while for engorged larvae it was 6.6 mg/mL. For unengorged nymphs, the LC₅₀ was 38.5 mg/mL, but it was not possible to calculate the corresponding value for engorged nymphs because the data did not fit the probit model. These results demonstrate that the methanol extract of A. oleracea has acaricidal activity against different immature stages of A. sculptum.

Synergism of thymol, carvacrol and eugenol in larvae of the cattle tick, Rhipicephalus microplus, and brown dog tick, Rhipicephalus sanguineus

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 LC₅₀ 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.

Acaricidal activity of thymol against larvae of Rhipicephalus microplus (Acari: Ixodidae) under semi-natural conditions

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.

Entomopathogenic nematodes associated with essential oil of Lippia sidoides for control of Rhipicephalus microplus (Acari: Ixodidae)

In vitro effect of the association of the entomopathogenic nematodes Heterorhabditis bacteriophora HP88 and Heterorhabditis indica LPP1 with the essential oil of Lippia sidoides on engorged females of Rhipicephalus microplus was assessed. Engorged females with homogeneous weights (p >0.05) were divided into six groups of ten ticks each (each female = an experimental unit). In the treated groups, the ticks were exposed to the oil (40 μl/ml=4%) and nematodes (300 entomopathogenic nematodes (EPNs/tick) separately and also to the nematodes together with the oil, while in the control group, the females were immersed in Tween 80 (3%). All the treatments caused a significant reduction in the egg mass weight (p <0.05). The hatching percentage of larvae was reduced (p <0.05) in the groups treated with H. bacteriophora HP88, H. indica LPP1, and H. bacteriophora HP88 associated with the oil, but in the group treated only with the oil, the hatching of percentage of larvae rate was similar to that of the control group (p >0.05). The percentage of control in the group exposed only to the oil was 50.7%, while in the other treated groups, the percentage of control was greater than 95 %, reaching 100 %in the group treated with H. indica LPP1 associated with the oil. The results of the L. sidoides essential oil in the concentrations tested in association with both H. bacteriophora HP88 and H. indica LPP1 indicate that these combinations can be used to control R. microplus.