1. Introduction
The two-spotted spider mite (TSSM),
Tetranychusurticae Koch (Acarina: Tetranychidae) represents a highly cosmopolitan and polyphagous pest worldwide in protected and open field conditions [
1]. It predominantly prevails in exhaustive high-yield cropping systems, resulting in a serious economic yield loss (50–100%) in severe infestation conditions [
2,
3]. TSSM in the motile stage normally sucks the sap from the lower leaves’ epidermis, which invokes yellowing and discoloration [
2,
4,
5]. Moreover, severe TSSM invasion induces secondary infestation by fungi, bacteria, and viruses that frequently cause considerable extra injury; additionally, TSSM occurrences induce an allergic syndrome in greenhouse employees [
6]. The control of TSSM is particularly problematic due to their short life cycle, potentially explosive population, and their ability to rapidly develop resistance to more than 80 miticides (acaricides) with a few applications [
1,
7,
8]. Consequently, the extension of the chemical acaricide utilized in TSSM control can comprise the commercialization of agricultural systems and induces harmful impacts to the environment and human health, as well as non-target organisms [
9]. In addition, the ecological crisis attributable to the application of acaricides has been an issue of concern in modern decades. It has been estimated that nearly 2.5 million tons of pesticides are utilized in agricultural production yearly, and the global injury evoked by pesticides achieves
$100 billion yearly. It is, therefore, hoped that natural oils can offer alternative options to synthesis acaricides and contribute to pesticide resistance [
10,
11].
Natural-based insecticides have been introduced as prospective choices for arthropod management, since they represent a possible supply of bioactive secondary metabolites that have been perceived by the public as comparatively harmless and cause fewer threats to the environment, with negligible effects on human health [
12,
13]. Furthermore, natural insecticides typically have a mixture of numerous active molecules that exert diverse modes of action, and hence, are possibly capable of efficiently averting the appearance of resistant pest races [
14]. Studies have demonstrated that natural oils (essential and fixed oils) are safe, precise in action, eco-friendly, and potentially appropriate for utilizing within integrated pest management programs all over the world within the organic system [
12,
15,
16]. There have been a few studies on fixed oils (FO) as insecticides, for example, Raghavendra et al. [
17], with an in vitro study, evaluated the effect of natural fixed oil against TSSM based on the percentage of mortality and the percentage of reduction in egglaying. They indicated that neem oil at 3% can be used to control TSSM.
Essential oils (EO) have obtained a lot of awareness as practical bioactive products, principally in pesticide terms [
15,
18,
19]. The acaricidal action of EO is largely unknown, owing to the complexity of the bioactive substances [
20]. The acaricidal activity of EO in direct contact has been tested and fumigant trials have been conducted [
21,
22,
23]. Some studies have attempted to determine the mode of action of EO and their constituents on arthropods [
24,
25]. The acaricidal properties of EOs may result from more than one mode of action due to the diversity of terpenes and terpenoids or other secondary metabolites that were neuro-insecticides or were species-dependent regarding efficacy; those that showed synergistic efficacy when used in combination, with an octopaminergic system, can mediate the insecticidal activity [
21,
26]
Eggplant (
Solanum melongena L.) is among the top 10 most consumed vegetables worldwide. It is cultivated on over 2 million ha, with a production of about 33 million tons. China represents the world’s top eggplant cultivator, accounting for over half of global acreage, followed by India with about one-quarter of the global production; Indonesia, Egypt, Turkey, and Iraq are the other chief eggplant producing countries [
27]. Eggplant is an important source of phenolic, antioxidant, and anti-microbial substances, which offer hepatoprotection and cardio-protection, as well as dietary fiber, vitamins, and ions, especially iron; the nutrients that it supplies to the diets of the poor are principally imperative throughout history when other vegetables were in little provide [
28]. As for the effect of natural oils on plant growth and productivity, to our knowledge, there are very few reports, i.e., El-Tanany et al. [
12], who revealed that EO might be used as biostimulants, which improved plant growth and yield.
Although the acaricidal properties of many essential or fixed oils have been studied against TSSM, not much work has been done on lavender (Lavandula angustifolia L.), jasmine (Jasminum grandiflorum L.), and mustard (Brassica juncea L.) oil against TSSM; additionally, from the current survey, there are no reports on the effect of natural oils on eggplant development and productivity. Therefore, the current study aimed to evaluate the acaricidal activity of EO or FO against TSSM, and their potential as a bio-rational alternative to control that pest under greenhouse conditions, and tested their impacts on eggplant productivity.
2. Materials and Methods
A randomized complete design with three replication of four treatments was done during the 2019/2020 season in a controlled greenhouse of the National Organic Agriculture Center, Unaizah (26.085478 °N 43.9768123 °E), Saudi Arabia (SA). The experimental four treatments consisted of jasmine (Jasminum grandiflorum L.) essential oil (JEO, 2.5 mL/L); lavender (Lavandula angustifolia L.) essential oil (LEO, 2.5 mL/L), mustard (Brassica juncea L.) fixed oil (MFO, 5 mL/L), and water as a control. The selected concentrations of the natural oils were based on the preliminary experiment in the lab. and the greenhouse conditions depended on the acaricidal activity and enhancement of plant dry mass accumulation.
2.1. Experimental Layouts and Planting Procedure
Before planting, the greenhouse soil was plugged and then divided into eight ridges, each 7 m long and 70 cm apart. Compost (organic fertilizer) as added at 5 ton/ha. The eggplant seedlings were transplanted 50 cm apart, on 1 September 2019, under a drip irrigation system. All agricultural practices were done following the Ministry of Environment, Water and Agriculture, SA, recommendation. The mineral fertilizers that were used during the experimental period were commercial compound fertilizers (© Neutral, macronutrient with micronutrients, Nabat El-Ardh Company, Riyadh, Saudi Arabia) certified in the organic system in SA (©Neutral 4-12-5; from transplanting up to one month; ©Neutral 7-5-4, throughout the vegetative growth period; and ©Neutral 5-5-14, throughout the flowering and fruiting stage) within the fertigation system weekly at recommendation doses.
Once the TSSM infestation percentage naturally reached 50% (1 December 2019), the plants were divided into four groups after the pre-count of the mobile phases of TSSM. Blocks of plants were separated from each other to prevent plant-touching and mites from moving between blocks, and they were surrounded with cloth barricades. The treatments were sprayed until dripping after adding a surfactant at 1%. Foliar applying was repeated, i.e., the first one at 1day post 50% infestation (DPI) and the second at 10 DPI. The entire plants were carefully enclosed by spraying oils, and care was taken to preserve a distance of approximately 30 cm between the nozzle and the plant shoots.
2.2. Sampling Dates and Data Recorded
The TSSM (mobile phases) were counted on the 3rd–5th upper leaves of eggplant in the laboratory using a stereo-binocular microscope, two times one week after each spraying, which recorded the number of motile phases of TSSM, after which the corrected efficacy percentage and reduction percentage were determined. The corrected efficiency percentage was calculated according to Henderson and Tilton’s [
29] equations:
where:
The reduction% were calculated by the formula:
Three weeks after the second spraying, plant samples were collected for determination, growth parameter (plant height, number of branches per plant, number of leaves, and branches/plant, in addition to the leaf area and relative leaf dry mass of the 3rd–5th upper leaves), photosynthetic pigment concentration, ion percentage, as well as phenolic and ascorbic acid concentration.
All harvested fruits from each plot were used for determination of the yield and its components (fruit number/plant, fruit dimension ‘length and diameter in cm,’ and total fruit yield/plant). Representative samples of eggplant fruits were arbitrarily acquired from the treatments at the fourth picking to assess the fruit quality attributes (protein percentage, total soluble phenol, ascorbic acid concentration, total acidity percentage, and total soluble solid content). Additionally, oven-dried (at 70 °C) powdered eggplant fruits were used for the estimation of the ion percentage in the fruit (N, P, and K%).
Leaf photosynthetic pigment concentration was assessed via the technique of Lichtenthaler and Buschmann [
30] and the optical density of the pigment solution was recorded by using spectrophotometry; the concentrations were expressed as mg/g leaf FW.
For ion estimation [
31], dried plant samples (leaves or fruits) were ground to a fine powder, then mineralized by a mixture of sulfuric and nitric acids. Shoot N, P, and K% were determined in a digestible solution. Nitrogen was determined using the micro-Kjeldahl scheme. The colorimetric technique using a UV/VIS spectrophotometer was applied to assess P; finally, a Flamephotometer estimated K.
Total soluble phenols were determined in the methanolic extract; 0.1 mL methanolic extract was mixed with 2.5 mL Folin–Ciocalteu reagent 10%. The mixtures were neutralized by 10% sodium bicarbonate, and optical density was recorded at 765 nm [
32]. In preparation of the methanolic extract, an aliquot of frozen plant materials (leaves or fruits) was homogenized in methanol, after which it was centrifuged at 5000 rpm for 20 min.
Ascorbic acid was estimated using the 2, 6-Dichloroindophenol titrimetric technique following the Association of Official Analytical Chemists [
33].
Total Acidity was determined following the protocol presented in AOAC [
33]. A total of 10 mL of eggplant juice is put into a 100 ml measuring flask, diluted to a tera mark. For the filtered sample, 20 mL of obtained filtrate was taken and inserted into an Erlenmeyer flask. Then, two drops of phenolphthalein were added to the sample and titrated with 0.1 N NaOH until turning pink. The calculation of the total acid was done by the following formula: Total acid = b/a, where a= amount of NaOH 0.1 N for titration (mL) and b = 10 mL of material. Total soluble solids of eggplant were estimated via a manual refractometer at 20 °C, and results were reported as Brix [
34].
The data were statistically analyzed through two-way analysis of variance (ANOVA), at a 95% confidence level, using CoHort Software, 2008 statistical package (CoHort software, 2006; release, New York, NY, USA). The means were compared by Fisher’s least significant test (LSD). The statistical significance was considered as * p≤ 0.05, ** p≤ 0.01; *** p≤ 0.001, and ns—not significant. Additionally, Duncan’s Multiple Range Test (DMRT) at p ≤ 0.05 was selected to establish the significance of differences among treatments. The values in the tables are the means± standard error (SE)