Potential use of essential oils from Origanum vulgare and Syzygium aromaticum to control Tetranychus urticae Koch (Acari: Tetranychidae) on two host plant species

Abstract

Plant extracts and plant essential oils are considered alternatives to synthetic chemicals having toxic effects on insects and mites. Acaricidal, repellent, and oviposition effects of commercially available essential oils of Origanum vulgare L. (Lamiaceae) and Syzygium aromaticum (L.) (Myrtaceae) were investigated in this study on Tetranychus urticae, one of the main pests in agriculture, on two host plant species, using leaf disc bioassays. O. vulgare essential oil showed higher toxicity to T. urticae protonymphs and adults inhabiting both bean and tomato leaves than S. aromaticum essential oil. The LC₅₀ values of O. vulgare essential oil were found to be 1.676 and 2.052 µl L⁻¹ air for the bean populations in protonymphs and adult females, and 1.877 and 3.076 µl L⁻¹ air for the tomato populations, respectively. S. aromaticum essential oil in the concentrations of 5% had the highest repellent effect on the bean population of T. urticae after 1, 24, and 48 hours, resulting in 61.22%, 40.81%, and 18% repellence, respectively. The mortality rates of adult females of T. urticae treated with either O. vulgare or S. aromaticum essential oil increased with the increasing concentration and time on both host plants. Both essential oils caused a decrease in egg number and larvae hatching in both bean and tomato populations of T. urticae. 

Introduction

T. urticae is one of the main pests in agricultural fields all over the world. It has a wide host range and feeds on more than 1100 host plant species (Santamaria et al. 2020), mainly on vegetables, fruit trees, and many industrial and ornamental plants. The direct damage caused by larvae, nymphs, and adults by sucking the plant sap can go up to the drying of the plant during periods of high population. The control of this pest is largely based on the use of insecticides/acaricides both in Türkiye and in the world (Çağatay et al. 2018; Rincon et al. 2019). However, repeated applications and overdose of these chemicals, and the high reproductive potential of the pest cause resistance to pesticides in a relatively short period. It has been reported to develop resistance to over 95 active compounds to date (Mota-Sanchez and Wise 2022). As a result, there is a need for both environmentally- and human-friendly, more selective, and cost-effective control methods, which would delay T. urticae resistance and be an alternative to synthetic compounds. For this purpose, researchers have started to evaluate the pesticide effects of plant extracts and plant essential oils as alternatives to synthetic chemicals and their use in pest management strategies in cases of resistance (Basaid et al. 2020).

Since essential oils are produced naturally by plants, they have several advantages over synthetic chemicals: They have no negative effects on the environment and human health, they do not permanently remain in the environment or have a short residual effect and low residue retention (Isman 2006; Ebadollahi et al. 2014), and complex mixtures of many active substances with different action mechanisms delay pest resistance (Pavela and Benelli 2016). Plant essential oils can act as repellents, antifeedants, and growth regulation on insects and mites and can have lethal and toxic effects on them due to the components they contain (Isman 2006; Pavela and Benelli 2016; Lengai et al. 2020). Their toxic effects are usually achieved via contact, ingestion, or inhalation (Tripathi et al. 2009).  Numerous studies have demonstrated that essential oils have insecticidal/acaricidal activity against spider mites, including T. urticae (Lee et al. 1997; Miresmailli et al. 2006; Bozhüyük et al. 2020). Lamiaceae, Asteraceae, Myrtaceae, and Apiaceae are the main plant families whose essential oils or plant extracts have been studied against T. urticae (Rincon et al. 2019). Examples of plants that some species of Mentha (Labiatae), Chrysanthemum (Asteraceae), Haplophyllum (Rutaceae) (Attia et al. 2012), and Rosmarinus officinalis (Lamiaceae) (Miresmailli et al. 2006) have contact, Lavandula angustifolia, Satureja hortensis, and some Teucrium species (Lamiaceae) (Ebadollahi et al. 2014; Ebadollahi et al. 2015) have contact and fumigant effects, and Solanum lycopersicum L. (Solanaceae) (Antonious and Snyder 2006) have repellent effects on T. urticae. Many studies have shown that Origanum vulgare L. (Lamiaceae), known as oregano, and Syzygium aromaticum (L.) (Myrtaceae) Merr. & L.M. Perry, known as clove, were effective in the control of T. urticae (Choi et al. 2004; Çalmaşur et al. 2006; Eldoksch et al. 2009; Roh et al. 2011; Hussein et al. 2013; Beynaghi et al. 2015; Tabet et al. 2018; Yeşilayer and Aslan 2018).

Several factors can potentially change the chemical composition and toxicity of essential oils. Some of these are the quality of the essential oil, method of extraction, plant parts used, phenological age of the plant, its geological location, and refining methods/techniques (Hussein et al. 2013). Considering that more data and information are needed on this subject, we applied market-sold essential oils of O. vulgare and S. aromaticum on two different host plants to study if they can control T. urticae populations. Acaricidal and repellent effects of these essential oils in different concentrations were investigated in T. urticae, as well as their fertility and mortality rates.

Materials And Methods

Tetranychus urticae and host plants

T. urticae culture was obtained from Ankara University, Faculty of Agriculture, Department of Plant Protection, Ankara, Türkiye. Two separate cultures of spider mites were grown on tomato (Solanum lycopersicum) and bean (Phaseolus vulgaris) plants. Bean and tomato plants were cultivated in plastic pots (13 cm diameter x 11 cm height) containing 350 g of soil. Irrigation was performed every 2 days to ensure plant growth. When the plants were fully leafed, i.e., two weeks after germination of beans and one month after germination of tomato, plants were infested with mites, which were then allowed to adapt and reproduce on host plants for about 2 months. Clean bean and tomato plants for laboratory experiments and plants with mite culture were grown in separate climatized rooms at 26 ± 2 °C, 55%–60% humidity, and a photoperiod of 16 h light: 8 h dark.

Essential oils

In the experiments, two essential oils were purchased from the Aksuvital Shiffa Home (Istanbul, Türkiye) (www.shiffahome.com.tr). The essential oils were distilled from the whole plants of Origanum vulgare or the flowers and buds of Syzygium aromaticum, according to the information obtained from the company. The chemical compound analysis of the essential oils is shown in Table 1.

GC-MS analysis

The component analysis of the essential oils was determined by using the GC-MS device in the Medicinal Plants Research Laboratory of the Batı Akdeniz Agricultural Research Institute, Antalya, Türkiye. Essential oil samples were diluted at 1:100 with hexane and components were analyzed using a GC/GC-MS (Gas chromatography (Agilent 7890A)-mass detector (Agilent 5975C)) device and a capillary column (HP InnowaxCapillary; 60.0 m x 0.25 mm x 0.25 μm). Helium was used as the carrier gas (at a flow rate of 0.8 ml/min), and the samples were injected into the device as 1 μl with a split ratio of 40:1. The temperature of the injector was kept at 250°C, and the column temperature program was set at 60°C (10 minutes), from 60°C to 220°C at 4°C/minute and 220°C (10 minutes). The total analysis time was 60 minutes. The mass ranged from (m/z) 35 to 450 and electron bombardment ionization of 70 eV was used for the mass detector, and the data of the WILEY and OIL ADAMS libraries were used to identify the components of the essential oils. The FID detector determined component percentages of the essential oils, and the component identification was performed by the MS detector (Özek et al. 2010). 

Leaf disc bioassays

Effects of the essential oils on adult females and protonymphs of Tetranychus urticae

The filter paper diffusion bioassay was conducted to determine the acaricidal effects of the tested essential oils on T. urticae adult females (one-to-three-day-old) and protonymphs (0-to 24-hour-old) (Choi et al. 2004). In the experiments, leaf discs of 2.0 cm in diameter were cut from clean bean or tomato leaves and placed on Petri dishes (60 mm diameter × 15 mm height). On each leaf disc, 30 adult females and/or 30 protonymphs were placed under a stereomicroscope using a fine hairy brush and leaf discs containing mites were subsequently placed on a wet cotton pad placed on the bottom of a Petri dish. Essential oils in various concentrations were applied directly to the Whatman No. 1 filter paper (1 cm width × 3 cm length) using an automatic pipette. Then, the papers were adhered to the inner surface of Petri dishes to avoid direct contact with mites, and the Petri dishes were sealed with Parafilm (Amcor Flexibles, Zurich, Switzerland) to prevent escaping of mites. The Petri dishes were kept under a controlled atmosphere and alive/dead counts were made after 24 hours. Mites that did not move when touched with a fine brush were considered dead. The concentrations of essential oils used in the experiment were determined in preliminary studies, and 96% ethanol was used to prepare the concentrations. On the bean population, O. vulgare essential oil was used in 1, 2, 4, and 8 µl L⁻¹ air concentrations, and S. aromaticum essential oil was used in 8, 16, 24, 32, and 40 µl L⁻¹ air concentrations. On the tomato population, O. vulgare essential oil was applied in 1, 2, 4, 8, and 16 µl L⁻¹ air concentrations, while S. aromaticum essential oil was applied in 8, 16, 24, 32, 40, 80, and 160 µl L⁻¹ air concentrations. Control solutions consisted of 99.5% dipropylene glycol (Tekkim Kimya Ltd. Şti., Bursa-TR) and 96% ethanol (Tekkim Kimya Ltd. Şti., Bursa-TR) mixed in a 1:1 ratio. Since both essential oils were purchased ready-made, dipropylene glycol was present as a solvent in their contents. Triplicates were provided for each concentration and control. 

Determination of repellent activity

This bioassay was carried out with some modifications to the method of Nerio et al. (2009) to determine the repellent effect of the tested essential oils on T. urticae adult females (one-to-three-day-old). To this end, plastic Petri dishes (60 mm diameter × 15 mm height), a glass spray bottle, and bean and tomato plants were used in the experiments. The two highest concentrations of O. vulgare and S. aromaticum essential oils were used for the bean and tomato populations. These concentrations for both essential oils were 2.5% and 5% in the bean population, while it was 1% and 2.5% in the tomato population. The experiments were carried out in two runs. In the first application, 10.35 µl O. vulgare essential oil was sprayed on half of a bean/tomato leaf cut with a diameter of 2.0 cm, and 10.35 µl of 96% ethanol and dipropylene glycol mixture (1:1 ratio) was applied on the other half (control) using a glass spray bottle. The leaves were then left to dry for half an hour. In the second application, 10.35 µl of S. aromaticum essential oil was sprayed on half of a bean/tomato leaf cut at the same rate and 10.35 µl of ethanol and dipropylene glycol mixture was sprayed on the other half (control). After drying, 10 adult female mites were placed on each leaf and transferred to a Petri dish with a wet cotton pad at the bottom. The lid was closed and surrounded with parafilm. Mites were counted on both halves of a leaf disc after 1, 24, and 48 hours. Repellency (%) was calculated according to the formula provided below. The experiments were carried out in 10 replications and repeated 3 times. Spraying was done at a distance of 10 cm throughout the experiment.

% Repellency = (C-T/N) × 100 (Mozafferi et al. 2013)

Where T = number of mites (treatment), C = number of mites (control), N = number of mites (total)

Effects of the essential oils on the survival and reproduction of Tetranychus urticae

The leaf disc spray method used by Laborda et al. (2013) was used to evaluate the potential effects of both essential oils on the survival and reproduction of T. urticae. Preliminary studies were conducted to determine essential oil concentrations by diluting the oil with ethanol. These concentrations were as follows: 0.05%, 0.1%, 1%, 2.5%, and 5% (v/v) for the bean population and 0.125%, 0.25%, 0.5%, 1%, and 2.5% (v/v) for the tomato population. Ten replicates were made for each concentration. A mixture of ethanol and dipropylene glycol was used as a control. In this experiment, 20.70 µl of each concentration was sprayed on 2.0 cm-leaf discs of either bean or tomato and, after drying, each leaf disc was placed in a Petri dish with a wet cotton pad. Five adult females of T. urticae were placed on the leaves using a fine brush. The mortality rates (%) were recorded after 1, 4, 8, 24, 72, and 120 hours, and the number of eggs laid was recorded on the 1^st, 3^rd, and 5^th day. The number of hatched larvae was also scored. The mortality rates (%) were calculated according to the Abbott formula (Abbott 1925). During this period, no phytotoxicity was observed on the leaves, and their moisture was constantly controlled.

Statistical Analyses

LC₅₀ and LC₉₀ values, 95% fiducial limits, and slopes of essential oils against adult females and protonymphs of T. urticae were calculated using the PoloPlus program (Version 2.0) (LeOra Software 1987). The averages of corrected mortality (%) of adult females transformed with the Abbott formula at 1, 4, 8, 24, 72, and 120 hours, the total number of eggs laid by adult females on the first, third, and fifth day, and the average number of larvae hatched were subjected to the analysis of variance (ANOVA) (P < 0.05). Differences between means were evaluated according to the Tukey multiple comparison test using the JMP 13 statistical software. The mean numbers of female mites present on the treated and untreated (control) halves on tomato or bean leaf discs after 1, 24, and 48 hours were compared using the t-test (P < 0.05).

Results

Essential oil analysis

The analysis results of the components of O. vulgare and S. aromaticum essential oils used in the study were shown in Table 1. A total of 16 compounds were identified in O. vulgare essential oil. The compounds above 1% were carvacrol (54.17%), dipropylene glycol (30.07%), linalool (3.86%), para-Cymene (2.59%), gamma-Terpinene (2.04%), Thymol (1.33%), and beta-Bisabolene ( 1.13%) (Table 1; Fig. 1). Similarly, 5 components were found in S. aromaticum essential oil. These were eugenol (65.31%), dipropylene glycol (25.44%), beta-Caryophyllene (6.58%), alpha-Humulene (1.80%) and Caryophyllene oxide (0.70%), respectively (Table 1; Fig. 2).

Table 1 Essential oils and their components

Rt=Retention Time; MC=Mean Composition (% Area); MM=Molecular Mass; m/z= Mass to Charge Ratio; KI=Kovats Retention Indices; Ri=Retention Indices (Ri¹=Relative to Standard Mixture of n-alkanes in the Same Sample’s Analytical Conditions; Ri²=from literature)

Effects of the essential oils on adult females and protonymphs of Tetranychus urticae

Table 2 shows the LC₅₀ and LC₉₀ values obtained after 24 h of exposure to O. vulgare and S. aromaticum essential oils on adult females and protonymphs of T. urticae in the bean and tomato populations. These essential oils caused toxicity by showing a fumigant effect on adult females and protonymphs of T. urticae. For the bean population, LC₅₀ values of O. vulgare essential oil applied to protonymphs and adult females were 1.676 (1.471-1.884) and 2.052 (1.825-2.293) µl L⁻¹ air, respectively, and the difference between protonymphs and adult females was not significant according to the fiducial limits. LC₅₀ values of S. aromaticum essential oil were 16.576 (15.169-17.767) and 17.456 (15.457-19.299) µl L⁻¹ air for protonymphs and adult females, respectively, and the difference was not significant according to the fiducial limits, as is the case with O. vulgare essential oil (Table 2). For the tomato population, the obtained LC₅₀ values of O. vulgare essential oil applied to protonymphs and adult females were 1.877 (1.621-2.138) and 3.076 (2.572-3.646) µl L⁻¹ air, respectively, while the recorded LC₅₀ values of S. aromaticum essential oil were 22.375 (20.763-23.995) and 29.601 (27.218-32.196) µl L⁻¹ air (Table 2). O. vulgare essential oil showed the highest fumigant toxicity to protonymphs in the bean population with a concentration of 1.676 µl L⁻¹ air. The difference in toxicity between protonymphs and adult females in the tomato population was statistically significant for both essential oils applied. S. aromaticum essential oil showed lower toxicity on protonymphs and adult females than O. vulgare essential oil on both bean and tomato populations (Table 2). T. urticae protonymphs in both populations treated with either essential oil were found to be more sensitive than adult females. In this experiment, the mortality rate in the control was less than 1%. 

Table 2 Toxicity of Origanum vulgare and Syzygium aromaticum essential oils against protonymphs and adult females of the bean and tomato populations of Tetranychus urticae after 24 hours

^anumber of mites; ^bstandard error; ^cfiducial limits; ^ddegrees of freedom

Determination of repellent activity

The results of repellency data for O. vulgare and S. aromaticum essential oils are shown in Table 3. In the bean population of T. urticae at tested concentrations, O. vulgare essential oil showed a repellent effect of 51.83%, 31.77%, and 5.33% at 5% concentration, while the same oil showed a repellent effect of 40.20%, 16%, and -7.3%, at the concentration of 2.5%, after 1, 24, and 48 hours, respectively. In the same population, the repellent effects of S. aromaticum essential oil at 5% concentration were 61.22%, 40.81%, and 18%, and was 58.90%, 38.35%, and 15.82% at 2.5% concentration, after 1, 24, and 48 hours, respectively. In the bean population, both oils provided different percentages of repellency to female mites of T. urticae for 48 hours, which decreased with time.  S. aromaticum essential oil showed a higher repellent activity on the bean population, as compared to O. vulgare essential oil at both concentrations (Table 3). In the tomato population, repellence activity of O. vulgare essential oil was again measured after 1, 24, and 48 hours and it was 0%, -20.83%, and -15.95% at 2.5% concentration, and 10.34%, -10.41%, and -3.80% at 1% concentration, respectively. In the same population, the repellent effects of S. aromaticum essential oil at 2.5% concentration were -27.27%, -47.47%, and -41.09%, respectively, while it was -37.27%, -38.00%, and -32.98% at 1% concentration, after same time intervals, respectively (Table 3). Except for the repellent activity obtained after 1 hour at 1% concentration of O. vulgare essential oil in the tomato population, both essential oils did not show any repellent effect in either concentration in contrast to the bean population (Table 3). 

Table 3 Repellent effect (%) (1, 24, and 48 hours after treatment) of Origanum vulgare and Syzygium aromaticum essential oils on adult females of Tetranychus urticae in tomato and bean populations

Means with ‘*’ are significantly different between treated and untreated by t-test (mean ± SE, P < 0.05); SE means the standard error. 

Effects of the essential oils on survival and reproduction of Tetranychus urticae

The mean mortality rates (%) of two essential oils at different concentrations in both T. urticae populations after 1, 4, 8, 24, 72, and 120 hours are shown in Tables 4 and 5. In the bean population of T. urticae, mortality rates after the application of both essential oils increased as the applied concentration and time increased (Table 4). O. vulgare essential oil caused a death rate of 31.66% after 1 hour at 5% concentration in the bean population, while mortality rates gradually increased to 100% mortality at 72 hours. Mortality rates at the end of 120 hours were statistically significant between the lowest (0.05%) and the highest concentration (5%) applied and ranged from 19.60% to 100%, being significantly higher compared to the control (df= 5, 54; F=1341.208; P<0001) (Table 4). In the control group, although no mortality was observed in the first 24 hours, 4.90% and 8.10% mortality rates were observed after 72 and 120 hours, respectively. When S. aromaticum essential oil was applied to the bean population at the same concentrations, higher mortality rates were observed after one hour at concentrations of 0.10%, 1%, 2.5%, and 5% compared to O. vulgare essential oil (Table 4). Moreover, after 120 hours at the same concentrations, mortality rates varied from 17.60% to 100% and were significantly higher for S. aromaticum essential oil compared to control (df= 5, 54; F=1152.72; P<0001) (Table 4). The mean number of eggs laid on leaf discs on the 1^st, 3^rd, and 5^th day in both populations of T. urticae after the application of essential oils and the mean number of larvae hatched are given in Table 6. The toxicity of O. vulgare and S. aromaticum essential oils also resulted in a reduction in the number of F₁ progeny (Table 6). The egg-laying function of T. urticae decreased with increasing concentrations of both essential oils. While there was no death on the first day at 0.1%, 1%, and 2.5% concentrations of O. vulgare in the bean population (Table 4), approximately 2.15, 3.69, and 5.05 times fewer eggs laid on the 5^th day in the same group, compared to the control. The progeny was inhibited due to 100% deaths on the 3^rd and 5^th days at the concentration of 5% (Table 6). At 5% and 2.5% concentrations of S. aromaticum essential oil in the same population, the difference between the number of eggs laid by adult females on the 1^st, 3^rd, and 5^th day was not statistically significant, but the difference was significant compared to the control (Table 6). As compared with the lowest concentration (0.1%), the number of larvae emerging in control was statistically significant and on average approximately 2.55 and 2.09 times lower in O. vulgare and S. aromaticum, respectively (Table 6).

In the tomato population of T. urticae, mortality rates for both essential oils also increased over time depending on the concentration (Table 5). O. vulgare essential oil caused death rates in the first 1 hour at 2.5% and 1% concentrations, and 100% mortality was observed after 120 hours at these concentrations. Additionally, the mortality rates at the end of 120 hours were significantly different at the lowest (0.125%) and the highest concentration (2.5%), being 65.60% and 100% (df=5, 54; F=1295.99; P<0001), respectively. The rates also showed statistical significance for S. aromaticum essential oil (df=5, 54; F=1409.549; P<0001), and the values between 34.80% and 100% were recorded (Table 5). When O. vulgare essential oil was applied to the tomato population of mites at concentrations as low as 0.125%, 0.25%, and 0.5%, the mortality rates in the first 8 hours were not statistically significant, as compared to the control. From the 24^th hour, significantly different mortality rates were recorded at the 0.125% concentration, compared to the control (Table 5). When S. aromaticum essential oil was applied to the tomato population at low concentrations (0.125% and 0.25%), mortality rates in the first 4 hours were not significantly different, compared to control, while the percentage mortality at 0.25% concentration was seen after 8 hours (Table 5). While no mortality rate was observed in the first 24 hours in the control group for both essential oils in the tomato population, a mortality rate close to 10% was observed at the end of 120 hours. Considering the tomato population, statistically, significant differences between control and any concentration of O. vulgare and S. aromaticum essential oils on the 1^st, 3^rd, and 5^th day were recorded. The number of eggs laid by T. urticae was approximately 1.60 and 1.68 times less at the concentration of 0.25% on the 5^th day of O. vulgare and S. aromaticum, respectively (Table 6). No larvae emerged at the highest concentration (2.5%) for both essential oils and a 1.80- and 1.84-fold decrease were observed in the number of larvae at the lowest concentration (0.25%) of O. vulgare and S. aromaticum, respectively, compared with control treatment.

Table 4 Mean mortality rates of Origanum vulgare and Syzygium aromaticum essential oils on the bean population of Tetranychus urticae adult females after 1, 4, 8, 24, 72, and 120 hours

Means in columns followed by the same letters are not significantly different (Tukey-Kramer Test, P > 0.05) df=degrees of freedom

Table 5 Mean mortality rates of Origanum vulgare and Syzygium aromaticum essential oils on the tomato population of Tetranychus urticae adult females after 1, 4, 8, 24, 72, and 120 hours

Means in columns followed by the same letters are not significantly different (Tukey-Kramer Test, P > 0. 05) df=degrees of freedom

Table 6 Means the number of eggs laid by Tetranychus urticae females after the 1^st, 3^rd, and 5^th day and the number of emerged larvae in the bean and tomato populations

Means in columns followed by the same letters are not significantly different (Tukey-Kramer Test, P > 0.05) df=degrees of freedom

Discussion

The present study demonstrated that essential oils from O. vulgare and S. aromaticum had an acaricidal effect on adult females and protonymphs of T. urticae on bean and tomato plants, although characteristics of this effect varied. Fumigant toxicity and repellent effect of both oils were higher in the bean population of T.urticae. In the tomato population, however, earlier death and a decrease in the number of eggs laid were observed at 2.5% concentrations whereas the same effect was achieved with double that amount in the bean population. These results suggest that O. vulgare and S. aromaticum essential oils can potentially be used in the control strategies against T. urticae.

The biological activities of plant essential oils are related to the monoterpenes and phenols in their contents (Amizadeh et al. 2013). Consistent with previous studies (Singh et al. 2012; Koç et al. 2013; Kheradmand et al. 2015; Imtara et al. 2021), the main component of O. vulgare essential oil used in this study was carvacrol (54.17), while the main component of S. aromaticum essential oil was eugenol (65.31) (Table 1). Notably, different substances have also been shown as the main component in O. vulgare essential oil in previous studies. For example, Onaran et al. (2014) found thymol (50.41%) as the main component followed by carvacrol (12.96%) in the chemical composition of O. vulgare essential oil, while Şahin et al. (2004) found caryophyllene and spathulenol to be major components. Generally, speaking the higher the percentage of carvacrol in the essential oil, the higher the acaricidal or insecticidal effect will be (Koç et al. 2013). In the present study, O. vulgare essential oil showed very high toxicity to protonymphs and adult females of T. urticae in both bean and tomato populations within 24 h. Specifically, 100% mortality was observed at 5% concentration in the bean population and 2.5% in the tomato population. Consistent with our study, Miresmailli et al. (2006) found that some individual compounds that were not toxic to mites feeding on bean plants were relatively toxic to mites feeding on tomato plants. This may explain the results we obtained in this study. Pavela et al. (2016) reported that the chemical composition of essential oils has a significant effect on fumigation toxicity in all developmental stages of T. urticae. In similar studies, natural monoterpenes such as linalool, carvone, and menthol (Badawy et al. 2010),  and in addition to those carvacrol and thymol (Pavela et al. 2016) have a potent fumigant effect against all stages of T. urticae.  Therefore, the toxic effects of O. vulgare essential oil reported in this study may be the result of the synergistic effect of several compounds. Eugenol is the main compound responsible for antioxidant, antifungal, antiviral, and insecticidal activities of clove, making it a medically important drug. The percentage of eugenol in S. aromaticum essential oil used in this study was 65.31% and it was found to be less effective against protonymphs and adults of T. urticae than O. vulgare in both host plants. Similarly, Eldoksch et al. (2009) reported that the phenol compound eugenol has an acaricidal effect on T. urticae and this effect may be due to a phenolic function that can increase the acaricidal properties of terpenes. This was supported by Hussein et al. (2013) who found S. aromaticum (syn. Eugenia caryophyllata) essential oil to be less toxic than Triticum vulgare and Eucalyptus globulus essential oils in adult females of T. urticae. Conversely, Kheradmand et al. (2015) reported a lower LC₅₀ value (6.13 µl L⁻¹ air) of S. aromaticum essential oil in T. urticae adults on bean leaf discs after 24 hours compared to our study (LC₅₀ = 17.456 µl L⁻¹ air).

The Lamiaceae family has great potential in pest management strategies due to having both insecticide and acaricide properties (Ebadollahi et al. 2020). The Origanum genus from this family has been investigated for these effects extensively. For example, O. vulgare essential oil has been demonstrated to have a very high acaricidal effect on adult females of T. urticae at a concentration of 8.52 mg L^-1 air (Mahmoud et al. 2019), while its hydroethanolic extract caused 75% of mortality in T. urticae adult females after 24 h in another report (Tabet et al. 2018). The essential oil vapors of O. vulgare at a concentration of 2 µl L⁻¹ air caused a mortality rate of 95% in adults and nymphs of T. urticae in 120 hours (Çalmaşur et al. 2006), whereas its vegetable oil achieved a 100% mortality rate at 19 × 10⁻³ µl L⁻¹ air concentrations after 24 hours on T. urticae adults (Choi et al. 2004). In another study, essential oil from Origanum onites containing 68.23% carvacrol showed a dose-dependent acaricidal effect against Tetranychus cinnabarinus (Sertkaya et al. 2010). Compared with the reports focusing on the toxicity of plants in the Myrtaceae family on T. urticae suggest a broad range of results (Rincon et al. 2019). In a previous study, S. aromaticum essential oil (0.1% concentration) caused a 41.3% mortality rate in T. urticae adult females by leaf disc residue dipping method (Roh et al. 2011). In another study, Lee et al. (1997) reported that carvacrol, thymol, and eugenol from monoterpenoids exhibited 100% mortality at 10.000 ppm after 24 hours in T. urticae. Similar to our study, S. aromaticum (1.5% concentration) showed a mortality rate of 53.33, 63.33, 73.33 and 98.54% in Oligonychus coffeae Nietner (Acari: Tetranychidae) after 24, 48, 72 and 120 hours, respectively (Barua et al. 2015).

In this study, S. aromaticum essential oil at 2.5% and 5% concentrations showed a stronger repellent effect on adult females of T. urticae compared to O. vulgare in the bean population for ≈ 48 hours after the application. However, the opposite was observed in tomato plants, i.e., the same concentrations of the essential oil (1% and 2.5%) attracted adult females of T. urticae toward tomato plants. Hence, the repellent effects differed depending on the host plant and the type of essential oil applied. This finding is noteworthy, as, to the best of our knowledge, the present study appears to be the first study to compare the repellent effects of two different essential oils on two different host plants in T. urticae. It has been reported that phenols such as eugenol have a stronger repellent potential than monoterpenes such as p-cymene found in Origanum species (Koul et al. 2008). Our results are compatible with this line of information. Conversely, in a repellency trial on different Origanum species, O. vulgare essential oil showed a repellent effect of 70.3% at the concentration of 0.5% and 95.4% at the concentration of 4% after 48 hours (Yeşilayer and Aslan 2018). Carvacrol and thymol showed both toxic and repellent activity on T. urticae (Tak and Isman 2017). Kheradmand et al. (2015) reached similar results in experiments with essential oils of S. aromaticum, which had a strong potential repellent effect on T. urticae adults. In the present study, the observed attractive effect of S. aromaticum essential oil applied onto a tomato leaf may be due to the structure of the tomato plant or that eugenol, which has shown to have an attractant effect on some pests such as flies (Reis et al. 2016) or, to other minor compounds present in the essential oil. Repellents lose their effects in a short time as they contain volatile compounds (Nerio et al. 2010). The high repellent effect of S. aromaticum oil on the bean plant in a short time is very important in terms of dispersing the T. urticae population, preventing egg-laying, and thus reducing the F₁ generation and eventually preventing the damage, it will cause to the plant. Similar to our study, rosemary oil has been shown to have a repellent effect on T. urticae for 6 h, after which the effect declined gradually (Ebadollahi et al. 2015).

When the two essential oils were applied to the tomato population, no repellent effect was observed on the tomato plant, however, there was still a decrease in the number of eggs laid by the females; resulting in a decreased number of larvae. It could be argued that some substances synthesized by tomato plants can reduce the nutritional and reproductive performance of T. urticae and/or affect the toxicity of essential oils to T. urticae. It is still unclear which volatiles or other compounds synthesized by the tomato plant can cause such an effect; however, a possible explanation for this might be the presence of physical and chemical barriers (Santamaria et al. 2020) as a resistance mechanism against T. urticae in wild tomato genotypes, although decreased or absent in cultivated tomatoes. Moreover, a tomato plant is inherently more hostile to T. urticae as the presence of glandular hair may direct mites to fall into a trap, and some toxic substances can cause direct poisoning (Gotoh et al. 1993).  It is thought that the differences observed in toxicity and repellent effects on mites as well as in their reproduction in tomato and bean populations recorded in our study may be related to plant resistance and/or individual and mixture differences between the components in both oils. However, much more detailed studies are needed to prove this hypothesis.

To the best of our knowledge, there is no study showing the oviposition-inhibiting effect of O. vulgare and S. aromaticum essential oils on two different populations of T. urticae. In a recent report, the effect of a sublethal dose of S. aromaticum essential oil (LC₂₅) on the demographic parameters of T. urticae has been studied and the total number of eggs laid in the trial was reduced by about half as compared to the control (Beynaghi et al. 2015). Biological activities of plant essential oils are mainly related to monoterpenes and phenols (Amizadeh et al. 2013). In this study, it was observed that O. vulgare and S. aromaticum essential oils significantly inhibited the number of eggs laid in both host plants and thus the development of F₁ progeny. It is thought that this may be due to the separate or combined synergistic activity of carvacrol, para-Cymene, gamma-Terpinene, and eugenol compounds, which are the major compounds of O. vulgare and S. aromaticum essential oils. Roh et al. (2011) reached similar results in experiments with Thymus vulgaris and O. vulgare essential oils, which are rich in thymol and carvacrol, and identified these as potential antifeedants and oviposition inhibitors to T. urticae. Similarly, Origanum majorana and Origanum compactum strongly inhibited oviposition behavior by more than 80% in T. urticae (Pavela et al. 2016), and Origanum onites essential oil inhibited the same behavior of Planococcus citri females by 63.7%, and Rosmarinus officinalis and Majorana hortensis oils caused a significant reduction in total numbers of eggs laid in T. urticae over the 10 days at all concentrations tested (0.125%-2%) (Amer et al. 2001). It has been reported that essential oils containing carvacrol, thymol, and anethole can be recommended as reproduction-deterring fumigants for the control of greenhouse pest T. cinnabarinus (Erler and Tunç 2005). Santamaria et al. (2020) showed that 7-epizingiberene, a sesquiterpene found in a tomato plant, resulted in reduced mite fecundity and affected population density. 

Bean and tomato plants are both economically important species and are among the plants most susceptible to T. urticae in both open fields and greenhouse cultivation. According to the results of this study, essential oils obtained from O. vulgare, and S. aromaticum showed toxicity to T. urticae, and significantly reduced oviposition and hence F₁ progeny in both host plants. In addition, considering the repellent effects of S. aromaticum essential oil in the bean population, these results suggest that O. vulgare and S. aromaticum essential oils can both be used as a control measure against T. urticae, especially in controlled environments such as in greenhouses. In the present study, we did not study synergistic or antagonistic relationships between secondary metabolites present in essential oils or the mechanisms of action of essential oils. Along with research on these matters, further studies are required to evaluate the effects and cost-effectiveness of these essential oils, especially in commercial greenhouses.

Declarations

Acknowledgements 

Thanks to Bursa Uludag University, Agriculture Faculty, Plant Protection Department, and Entomology Division for their technical support during the study. 

Author Contribution Statement 

Hilal Susurluk: Conceptualization, Methodology, Investigation, Formal analysis, Writing - review & editing. 

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