Phytochemical and Insecticidal Activity of Some Thyme Plants’ Essential Oils Against Cryptoblabes gnidiella and Scirtothrips mangiferae on Mango Inflorescences

Simple Summary

Mango crops face significant threats from pests such as Cryptoblabes gnidiella and Scirtothrips mangiferae. This study investigated the potential of using essential oils from five thyme plants—Thymus vulgaris, Origanum vulgare, Thymus argenteus, Thymus citriodorus, and Origanum syriacum—as natural insecticides. Essential oils were extracted and analyzed using hydrodistillation, revealing high concentrations of thymol and carvacrol, especially in T. vulgaris and O. vulgare oils. The insecticidal effects of these oils and the compounds isolated from them were tested on the target pests. The results indicated that T. vulgaris and O. vulgare oils were the most effective, with low lethal concentrations (LC₅₀ values) being potent against both pests. This study also measured biochemical changes in pest enzymes at these concentrations. Overall, thymol and carvacrol show promise as natural insecticides, offering an alternative to synthetic options for protecting agricultural crops.

Abstract

Mango fruits are one of the strategic fruit crops in different countries that are attacked by several serious pests such as Cryptoblabes gnidiella and Scirtothrips mangiferae. Natural extracts, especially essential oils, provide several promising insecticide agents to control different insects as an alternative to synthetic insecticides. Using Clevenger-type hydrodistillation, the essential oils of five thyme plants—Thymus vulgaris, Origanum vulgare, Thymus argenteus, Thymus citriodorus, and Origanum syriacum—from Saudi Arabia and Egypt were extracted, and GC/MS analysis was performed. In addition, some chemical parameters of the five species were determined, such as chlorophyll a, chlorophyll b, β-carotene, total antioxidant capacity, total phenols, and total flavonoids. Two compounds, thymol and carvacrol, were identified in T. vulgaris and O. vulgare at ratios of 69.45 and 64.82%, respectively. These major compounds were isolated and identified using ¹H NMR analysis. The insecticidal potentials of the five essential oils and their pure isolated compounds were evaluated on C. gnidiella and S. mangiferae on mango inflorescences. The results showed that T. vulgaris and O. vulgare oils were the most potent against C. gnidiella (LC₅₀, 183.33 and 164.68 ppm, respectively) and S. mangiferae (18.93 and 16.93 ppm, respectively). Thymol and carvacrol had the highest effect on both insects. Furthermore, the effect of thymol and carvacrol at LC₅₀ values on some biochemical parameters of C. gnidiella was determined. Therefore, thymol and carvacrol from Thymus species are promising compounds that could be used as insecticides against the harmful insects C. gnidiella and S. mangiferae on mango inflorescences.

1. Introduction

Around the world, medicinal plants play a significant role in both medicine and the economy. In particular, folk medicine has created a large database that can be used to search for new drugs. Several species in the Lamiaceae family hold unique medical and economic significance due to their diverse content [1]. Because of their pharmacological and biological qualities, Thymus species are widely used as therapeutic herbs. Thyme plant leaves and flowering parts are widely used in traditional medicine as a tonic and in herbal tea, in addition to being antibacterial, carminative, and antitussive [2].

Thymus is a particularly species-rich genus with a very broad geographic distribution due to its diversity and adaptability. The physical characteristics and metabolism of different thyme plants affect their chemical composition. T. vulgaris, O. vulgare, T. argenteus, T. citriodorus, and O. syriacum are the five plants of thyme that exhibit chemical variations that are typified by distinct plant oil compositions, typically in the absence of physical differences. Every plant usually contains two main types of oil; the Thymus genus is known for its phenols and terpenes. Some of the main ingredients are thymol, carvacrol, γ-cymene, γ-terpinene, 1,8-cineole, camphor, linalool, borneol, eugenol, and borneal [3,4,5,6,7].

Recently, researchers have been drawn to thyme plants due to their high production of certain secondary metabolites, including terpenoids, phenylpropanoids, and fatty acid derivatives. The usage of these secondary metabolites in the beauty and medical fields is growing [8,9,10,11]. These volatile compounds present in secondary metabolites include thymol, which has antibacterial, antifungal, and antiseptic properties, and carvacrol, recognized for its significance as a disinfectant and anti-infective. According to the authors of [12], additional compounds include limonene and α-terpineol, present in soaps and cosmetics; 1,8-cineole, used in medicine; and 1-borneol, which naturally repels insects. Certain species of thyme also contain essential oils that have insecticidal, allelopathic, antioxidant, and antibacterial qualities [4].

Mango (Mangifera indica L.), one of the major fruit crops in India and other countries, is known as the king of the fruits, known for its delicious taste and high nutritional value [13]. It is susceptible to attacks from several pests, such as the honeydew moth (HM), Cryptoblabes gnidiella Mill [14], and mango thrips, Scirtothrips mangiferae Priesner [15]. The Mediterranean region is home to the opportunistic C. gnidiella [16]. Worldwide, C. gnidiella has been observed to attack many hosts, including pomegranate, mangoes, grapes, avocado, citrus, and various vegetable crops [16,17]. Recently, Abdel Kareim et al. [14] observed high populations of the pest on mango inflorescences in Egypt, resulting in significant damage, especially when accompanied by mealybugs. According to Harari et al. [18], C. gnidiella can cause indirect damage to clusters by invading wounded berries with fungi, as well as direct damage when the larvae feed on the berries. This insect has caused losses in crop production of up to 30%. Mango thrips (S. mangiferae) have been known to attack mango in Egypt, Greece, Libya, Aden, Israel, and Sudan [15]. In addition to causing feeding damage, thrips play a significant role in the spread of viruses to plants, making them potentially dangerous pests [19]. Naturally occurring secondary metabolites have been promoted for use as insecticides due to growing concerns about the possibly harmful effects of artificial pesticides on humans and the environment. This study aimed to isolate the active principles from essential oils and evaluate their insecticidal activity against mango thrips (S. mangiferae) and honeydew moths (C. gnidiella) under laboratory conditions. Additionally, it assessed certain biochemical parameters, in light of renewed interest in finding natural insecticides or biopesticides derived from plants and the important role essential oils play in this field.

2. Materials and Methods

2.1. Chemicals and Instruments

Materials used: Aluminum sheets with TLC silica gel Merck GF254 precoated plates measuring 20 × 20 cm (Darmstadt, Germany). Petroleum ether (40–60 °C) (PE), ethyl acetate (EA), methylene chloride (MC), and methanol (MeOH) were acquired from LOBA CHEMIE PVT. Ltd. in Mumbai, India. Spray reagent for P-anisaldehyde. NMR analyses were performed in CDCl₃ using a Bruker (400 MHz, AQ 4.089 s, Billerica, MA, USA). A UV–Visible spectrophotometer (model MA9523-SPEKOL 211, ISKRA, Horjul, Slovenia) was used for the spectrophotometric measurements.

2.2. Plant Materials

Egypt (Thymus vulgaris and T. argenteus) and the Kingdom of Saudi Arabia (Origanum vulgare, T. citriodorus, and O. syricum) provided the three Thymus species and two Origanum species seedlings. After two months of growth from seedlings, thyme plants were taken from the greenhouses and identified by the Biotechnology Research Lab, H.R.I., A.R.C., Giza, Egypt, and the Vegetable and Medicinal and Aromatic Plants Research Departments, Dokki, Giza, Egypt [20].

2.3. Essential Oil Isolation

A fresh weight of 1.0 kg from each plant was used to extract essential oil in 4 L of distilled water via hydrodistillation using a Clevenger-type apparatus for 8 h. The obtained oils were dehydrated over anhydrous sodium sulfate and stored at −4 °C in the refrigerator until analysis, following the method by Mostafa et al. [21].

2.4. GC-MS Method

Essential oil analyses were performed using an Agilent 7890A GC (Santa Clara, CA, USA) equipped with a 5975 Inert MS with a triple-axis detector and a DB-5 capillary column (30 m × 0.25 mm × 0.25 μm film thickness). The GC oven temperature was programmed to maintain a temperature of 50 °C for 5 min before increasing to 300 °C at a rate of 7 °C/min. Injector and detector temperatures were 250 °C and 230 °C, respectively. The MS was set at 70 eV ionization energy with a mass–electron (m/z) range of 50–550. The splitless mode was used to inject the sample solution, and helium was used as a carrier gas at a flow rate of 1 mL/min. The mass spectra of compounds were interpreted using spectral matches from the NIST Mass Spectral Library (Standard Reference Data, National Institute of Standards and Technology, Gaithersburg, MD, USA) and Wiley Registry of Mass Spectral Data (John Wiley & Sons, Hoboken, NJ, USA).

2.5. Isolation and Characterization of Active Ingredient

The essential oils of T. vulgaris and O. vulgare (0.2 g) were isolated over silica gel column chromatography using hexane as mobile phase with increasing polarity by ethyl acetate to give three subfractions in both plants Subfraction II (110 and 90 mg) in both plants, respectively, were purified with preparative thin-layer chromatography using aluminum sheet silica gel Merck GF₂₅₄ precoated plates (20 × 20 cm) via an eluent system with 100% methylene chloride to yield compounds (37) and (38) (Rf 0.76, 21 mg, 13 mg), respectively.

Compound (37) was isolated from T. vulgaris as a pale-yellow oily residue (21 mg; Rf 0.76), which gave a purple color upon reacting with an anisaldehyde spray reagent. GC/MS, m/z (rel. int.): 150 (30%) [M, C₁₀H₁₄O]⁺, 135 (100) [M-CH₃]⁺, 115 (18) [M-3CH₃]⁺, 107 (10) [C₇H₇O]⁺, 91 (20) [C₇H₇]⁺. ¹H NMR (400 MHz, CDCl₃, δ_H, ppm, J, Hz): 6.76 (d, J 7.7 Hz, H-3), 7.11 (dd, J 7.7, 2.2 Hz, H-4), 6.54 (d, J 2.2 Hz, H-6), 3.20 (pent, J 6.8 Hz, H-7), 1.25 (d, J 6.8 Hz, 6H-8,9), and 2.26 (s, 3H-10). Compound (38) was isolated from O. vulgare as a pale-yellow oily residue (13 mg; Rf 0.76), which gave a purple color upon reacting with an anisaldehyde spray reagent. GC/MS, m/z (rel. int.): 150 (37%) [M, C₁₀H₁₄O]⁺, 135 (100) [M-CH₃]⁺, 115 (3) [M-3CH₃]⁺, 107 (15) [C₇H₇O]⁺, 91 (20) [C₇H₇]⁺. ¹H NMR (400 MHz, CDCl₃, δ_H, ppm, J, Hz): 6.66 (d, J 1.7 Hz, H-2), 6.72 (dd, J 7.7, 1.7 Hz, H-4), 7.03 (d, J 7.7 Hz, H-5), 2.82 (pent, J 6.9 Hz, H-7), 1.22 (d, J 6.9Hz, 6H-8,9), and 2.21 (s, 3H-10).

2.6. Assay of Some Chemical Composition Parameters

Six phytochemical parameters were assessed for the five thyme plants.

2.6.1. Chlorophyll (a and b) and β-Carotene Determination

The method developed by Nagata and Yamashita [22] was used to measure the contents of β-carotene and chlorophylls a and b (ChlA, ChlB). Approximately 10 mL of 80% acetone was used to grind 0.2 g of each thyme leaf sample, which was then filtered using Whatman No. 1 filter papers. Using a UV spectrophotometer, the filtered extract was placed in a cuvette, and absorbance was measured at 663, 645, 505, and 453 nm. The contents of β-carotene, chlorophyll a, and chlorophyll b were calculated using the following formulas:

• The value of chlorophyll a is 0.999 A₆₆₃ − 0.0989 A₆₄₅
• β-Carotene = (0.216 A₆₆₃ − 1.22 A₆₄₅) − (0.304 A₅₀₅ + 0.452 A₄₅₃)
• Chlorophyll b = −0.328 A₆₆₃ + 1.77 A₆₄₅.

2.6.2. Total Antioxidant Capacity, Total Phenol, and Total Flavonoid Determinations

Approximately 2 g of green leaves were ground in a mortar with 20 mL of 80% methanol to create a methanolic extract before filtering. The total antioxidant potential of extracts from thyme leaves was measured using Prieto et al.’s methodology [23]. In an Eppendorf tube, 1 mL of reagent solution (0.6 M sulfuric acid, 28 mM sodium phosphate, and 4 mM ammonium molybdate) was mixed with 0.1 mL of sample solution. After being sealed, the tubes were incubated for 90 min at 95 °C in a thermal block. The absorbance of each sample’s aqueous solution was measured at 695 nm in comparison to a blank once the samples had cooled to room temperature. Antioxidant capacities were expressed as equivalents of α-tocopherol and ascorbic acid, respectively.

The total phenolic content was measured by using a 100 mL volumetric flask containing 60 mL of distilled water, which was mixed with 1 mL of the sample, blank, or standard (gallic acid) in water. Mix in the 5.0 mL of FC reagent. Add 15 mL of 20% sodium carbonate solution after 1 min. Adjust the volume to 100.0 mL. After about 2 h at about 23 °C at 760 nm, read the color produced [24].

Twenty milliliters of methanol, 1 mL of 5% AlCl₃ (wt/vol), and 2 mL of each sample or reference solution (quercetin in the range of 4.0–12.0 µg/mL) were added. The volume was then increased to 50 mL using methanol at 20 °C. At 425 nm, the absorbances were measured after 30 min according to Woisky and Salatino [25].

2.7. Rearing of Tested Insects

Samples of mango inflorescences were obtained from Kafr Al Baramoun Research Farm, Horticultural Research Institute, Mansoura, Dakahlia, Egypt. We ensured that the samples were free of any pesticides. Small mango seedlings were used to rear mango thrips, S. mangiferae, which were kept in cages (1.0 × 1.0 × 1.0 m) in a glass greenhouse until treatment began. Afterward, the Plant Protection Research Institute’s Taxonomy Department verified the identification of the insects.

For the honeydew moths, C. gnidiella after their emergence, pairs of females and males were placed in plastic boxes (25 × 25 × 10 cm) containing a piece of moistened cotton wool with a 10% honey solution and a piece of inflorescence for oviposition. The eggs were extracted daily using a sensitive hairbrush. When the eggs developed to the larval stage, the resulting larvae were daily provided with fresh flowers of mango until they reached to the tested larval stage [14].

2.8. Bioassay Activity

The spray method was used to assess the toxicity of the essential oils and isolated compounds on mango thrips and honeydew moths under laboratory conditions, based on the method by Ragab et al. [26] and Allam et al. [27] with some modifications. Five serial concentrations were prepared for each treatment using Tween 80 and distilled water (3 replicates/concentration). For thrips, ten healthy thrips nymphs were placed on a mango leaf disk (3 cm in diameter) in a Petri dish for each replicate. One milliliter of the test solution was sprayed onto each treatment. Ten C. gnidiella 2nd instar larvae were deposited in a Petri dish containing an appropriate portion of fresh mango inflorescence and then sprayed with 2 milliliters of the test solution. Only Tween 80 and distilled water were used in the control treatment on both insects. All treatments were kept in an incubator at 25 ± 2 °C and 60 ± 5% RH during the experiment. Abbott stated that mortality was counted and corrected after 24 h for thrips and 72 h for honeydew moth [28]. The LC₅₀ and LC₉₀ values, along with their confidence intervals and the slope of the regression lines, were provided by Finney [29]. Additionally, the Sun equation was used to calculate the toxicity index [30].

Concentration in ppm = Weight in g/Volume in mL × 10⁶

2.9. Biochemical Investigation of C. gnidiella

Enzyme activity measurement in insects is a useful method for understanding a number of biological processes, including stress reactions and insecticide resistance. The active isolated thymol 37 and carvacrol 38 were used to measure enzyme activity. Thirty individuals of 2nd instar C. gnidiella were sprayed with two milliliters of the thymol solution at the LC₅₀ value; distilled water was also used to spray the control treatment. These experiments were replicated three times for each compound. According to Ragab et al. [26,31], the live insects were weighed and frozen in an appropriate tube following a 72 h treatment. The insect enzymes Acid Phosphatase (ACP) [32], Alkaline Phosphatase (ALP) [33], Glutathione S-Transferase (GST) [34], and Acetyl Choline Esterase (AchE) [35] were estimated. All enzyme activities were measured at the Analysis Unit, Plant Protection Research Institute, Agriculture Research Center, Mansoura, Egypt, using a spectrophotometer.

2.10. Statistical Analysis

One-way analysis of variance (ANOVA) was conducted using CoStat software (version 6.400, 798 Lighthouse Ave, PMB 320, Monterey, CA, USA). Fishers’ least significant difference (LSD, 0.05) test was used to calculate significant differences between the means. The LC₅₀ and LC₉₀ values were ascertained via probit analysis.

3. Results and Discussion

3.1. Identification of Compounds (37) and (38)

The major compounds (37) and (38) were isolated as oily substances from the essential oil of T. vulgaris and O. vulgare, respectively.

The mass spectrum of the compounds (37, 38) showed the presence of M⁺ ion at m/z 150 (30%), corresponding to the molecular formula C₁₀H₁₄O. The spectrum also showed an ion base peak at m/z 135 (100%) due to the expulsion of a methyl group. Additionally, the fragment peak at m/z 91 (20%) resulted from the formation of the tropylium ion formula C₇H₇.

The ¹H NMR data for compound (37) (

Table 1. ¹H NMR of compounds 37 and 38 in CDCl₃ (δ) [ppm] (Multiplicity, J [Hz]).

H-Atom	1H (Multiplicity, J) of Compound 37	1H (Multiplicity, J) of Compound 38
2	-	6.66 (d, J 1.7 Hz, 1H)
3	6.76 (d, J 7.7 Hz, 1H)	-
4	7.11 (dd, J 7.7, 2.2 Hz, 1H)	6.72 (dd, J 7.7, 1.7 Hz, 1H)
5	-	7.03 (d, J 7.7 Hz, 1H)
6	6.54 (d, J 2.2 Hz, 1H)	-
7	3.20 (pent, J 6.8 Hz, 1H)	2.82 (pent, J 6.9 Hz, 1H)
8, 9	1.25 (d, J 6.8 Hz, 6H)	1.22 (d, J 6.9 Hz, 6H)
10	2.26 (s, 3H)	2.21 (s, 3H)
-OH	5.23 (brs, 1H)	5.34 (brs, 1H)

, Scheme 1) displayed a proton signal pattern characteristic of a trisubstituted benzene ring moiety with an ABX system at δ_H 7.11 ppm (dd, J = 7.7, 2.2 Hz, H-4), 6.76 (d, J = 7.7 Hz, H-3) and at 6.54 (d, J = 1.8 Hz, H-6). In addition, the observation of an olefinic methyl proton signal at δ_H 2.26 (s, 3H-10) and an isopropyl unit at δ_H 3.20 (pent, J = 6.8 Hz, H-7), 1.25 (d, J = 6.8 Hz, 6H-8,9) confirmed the identification of compound (37) as thymol. A comparison of the spectral data of compound 37 with previously published data revealed that the structure was thymol.

The ¹H NMR spectrum (Table 1, Scheme 2) of compound (38) was similar to that of compound (37), but the olefinic methyl proton signal appeared at δ_H 2.21 (s, 3H-10) and the isopropyl unit at δ_H 2.82 (pent, J = 6.9 Hz, H-7), confirming the identification of compound (38) as carvacrol. This was in agreement with previously published spectral data from the literature [36,37].

3.2. GC/MS Analysis

The five essential oils extracted from the five plants of thyme—T. vulgaris, O. vulgare, T. argenteus, T. citriodorus, and O. syriacum were analyzed via GC/MS to identify their compounds.

Table 2. Chemical constituents of five thyme plants identified using the GC/MS technique.

Compound Name	RT	Mol. Formulae	Mol. Wt.	% of Compounds in Thyme Plants
				1	2	3	4	5	RI
α-Thujene (1)	7.18	C10H16	136	0.10	-	0.07	-	-	910
α-Pinene (2)	7.36	C10H16	136	0.06	2.86	0.06	1.66	3.23	917
Camphene (3)	7.79	C10H16	136	0.07	0.29	0.07	0.06	0.34	932
β-Pinene (4)	8.65	C10H16	136	0.02	-	-	1.08	0.25	963
1-Octen-3-ol (5)	8.81	C8H16O	128	0.10	-	-	0.13	0.17	969
β-Myrcene (6)	9.11	C10H16	136	0.18	-	0.08	0.19	0.14	980
Yamogi alcohol (7)	9.51	C10H18O	154	-	-	-	2.02	0.39	995
α-Terpinene (8)	9.87	C10H16	136	0.09	-	-	0.13	0.82	1007
p-Cymene (9)	10.15	C10H14	134	9.45	-	1.85	1.06	1.16	1016
D-Limonene (10)	10.26	C10H16	136	0.13	-	0.68	0.44	-	1020
Eucalyptol (11)	10.33	C10H18O	154	0.24	8.53	0.77	-	1.88	1022
Santolina alcohol (12)	10.67	C10H18O	154	-	-	-	0.04	-	1033
1-Nonen-3-ol (13)	11.08	C9H18O	142	-	-	-	0.21	-	1046
γ-Terpinene (14)	11.20	C10H16	136	0.82	-	0.81		1.50	1050
trans-4-Thujanol (15)	11.48	C10H18O	154	0.41	1.06	0.56	-	-	1060
Linalool (16)	12.50	C10H18O	154	1.79	-	7.10	1.48	1.19	1092
Carveol (17)	13.19	C10H16O	152	-	-	0.48	-	-	1114
α-Campholenal (18)	13.34	C10H16O	152	-	-	-	0.16	-	1119
(E)-Pinocarveol (19)	13.73	C10H16O	152	-	-	0.15	1.48	0.03	1131
(+)-Camphor (20)	13.93	C10H16O	152	0.40	3.32	0.42	-	0.37	1136
Pinocarvone (21)	14.51	C10H14O	150	-	-	-	2.14	-	1156
(-)-Borneol (22)	14.58	C10H18O	154	1.44	1.18	1.24	-	0.23	1158
(E)-Pinanone (23)	14.83	C10H16O	152	-	-	-	-	0.06	1167
Terpene-4-ol (24)	14.93	C10H18O	154	0.52	-	0.46	0.42	0.35	1170
p-Mentha-1(7),8-dien-2-ol (25)	15.27	C10H16O	152	-	-	0.18	0.80	-	1181
α-Terpineol (26)	15.40	C10H18O	154	0.11	-	0.45	-	-	1185
(-)-Myrtenol (27)	15.54	C10H16O	152		-	-	0.83	0.11	1189
cis-Dihydrocarvone (28)	15.61	C10H16O	152	0.05	-	0.10	-	-	1192
Berbenone (29)	15.97	C10H14O	150	-	0.74	-	-	0.08	1204
Fenchyl acetate (30)	16.18	C12H20O2	196	-	-	0.26	0.13	0.03	1211
Thymol methyl ether (31)	16.61	C11H16O	164	1.31	-	0.67	0.80	0.06	1226
p-Cymene methyl ether (32)	16.88	C11H16O	164	1.01	-	3.70	-	0.22	1236
(-)-Carvone (33)	17.07	C10H14O	150	-	-	0.20	-	-	1242
cis-Geraniol (34)	17.30	C10H18O	154	-	-	0.28	0.47	-	1250
β-Citral (35)	17.79	C10H16O	152	-	-	-	0.18	-	1267
(-)-Bornyl acetate (36)	18.15	C12H20O2	196	-	0.82	1.32	0.21	0.21	1280
Thymol (37)	18.39	C10H14O	150	69.45	13.77	36.87	4.54	13.00	1288
Carvacrol (38)	18.68	C10H14O	150	4.46	64.82	1.24	2.42	2.63	1297
α-Cubebene (39)	19.92	C15H24	204	-	-	0.19	0.12	0.12	1342
α-Copaene (40)	20.67	C15H24	204	0.40	-	1.10	0.38	2.64	1368
Geranyl acetate (41)	20.86	C12H20O2	196	-	-	-	1.00	-	1375
β-Bourbonene (42)	20.93	C15H24	204	0.05	-	0.35	-	0.19	1377
β-Elemene (43)	21.11	C15H24	204	0.04	-	0.55	-	0.51	1383
α-Gurjunene (44)	21.62	C15H24	204	-	-	0.19	0.04	0.26	1401
Caryophyllene (45)	21.89	C15H24	204	2.47	0.82	2.56	2.19	5.10	1412
trans-α-Bergamotene (46)	22.26	C15H24	204	-	-	-	-	0.19	1426
Alloaromadendrene (47)	22.42	C15H24	204	-	-	-	0.02	0.03	1433
α-Muurolene (48)	22.55	C15H24	204	-	-	0.05	0.02	0.05	1437
Humulene (49)	22.80	C15H24	204	0.40	-	0.61	0.27	4.70	1447
cis-Muurola-4(15),5-diene (50)	23.03	C15H24	204	-	-	-	0.15	0.16	1457
Cadina-1(6),4-diene (51)	23.31	C15H24	204	-	-	0.07	-	0.24	1467
γ-Muurolene (52)	23.38	C15H24	204	0.10	-	0.11	0.13	0.38	1471
Germacrene D (53)	23.52	C15H24	204	0.88	-	4.25	2.16	11.64	1477
α-Selinene (54)	23.67	C15H24	204	0.06	-	-	-	0.85	1481
γ-Gurjunene (55)	23.93	C15H24	204	0.23	-	2.29	3.08	3.10	1492
β-Bisabolene (56)	24.17	C15H24	204	0.10	-	0.11	-	0.16	1501
γ-Cadinene (57)	24.37	C15H24	204	0.12	-	2.63	-	1.60	1509
Selina-3,7(11)-diene (58)	24.48	C15H24	204	-	-	-	1.46	-	1513
δ-Cadinene (59)	24.58	C15H24	204	0.60	-	1.38	-	7.07	1517
Kessane (60)	24.75	C15H26O	222	-	-	-	3.43	0.09	1523
Cubebol (61)	24.84	C15H26O	222	0.02	-	1.47	0.67	5.93	1527
α-Cadinene (62)	24.96	C15H24	204	-	-	-	-	0.20	1531
Caryophyllene oxide (63)	25.38	C15H24O	220	-	-	-	4.33	0.24	1556
Palustrol (64)	25.79	C15H26O	222	-	-	0.13	0.10	0.05	1563
Spatulenol (65)	26.02	C15H24O	220	0.09	-	0.45	2.47	0.48	1573
Isoaromadendrene epoxide (66)	26.15	C15H24O	220	0.42	-	0.53	2.01	0.49	1578
Salvial-4(14)-en-1-one (67)	26.42	C15H24O	220	-	-	0.12	-	-	1589
β-Oplopenone (68)	26.79	C15H24O	220	-	-	0.27	0.71	0.18	1604
β-Eudesmol (69)	27.07	C15H26O	222	0.06	-	0.08	-	-	1615
γ-Eudesmol (70)	27.31	C15H26O	222	-	-	-	10.02	0.95	1634
T-Cadinol (71)	27.55	C15H26O	222	0.42	-	0.65	-	3.47	1637
δ-Cedrol (72)	27.64	C15H26O	222	-	-	0.29	-	0.59	1641
T-Muurolol (73)	27.85	C15H26O	222	0.68	0.67	2.36	-	5.46	1650
Pogostole (74)	28.01	C15H26O	222	-	-	-	-	2.57	1656
(-)-Globulol (75)	28.26	C15H26O	222	-	-	0.73	23.06	0.25	1667
(Z)-α.-Atlantone (76)	29.31	C15H22O	218	-	-	1.70	0.11	0.40	1713
α-Cyperone (77)	30.14	C15H22O	218	-	-	-	0.60	0.29	1749
Hexahydrofarnesyl acetone (78)	31.92	C18H36O	268	-	-	-	0.84	2.43	1829
Isopimara-9(11),15-diene (79)	33.58	C20H32	272	-	-	1.37	0.35	0.39	1908
Aristol-1(10)-en-9-yl isovalerate (80)	33.92	C17H36O2	262	-	-	-	0.04	0.13	1925
Phytol (81)	37.35	C20H40O	278	-	-	-	0.50	-	2102
Total %				99.35	98.88	86.67	83.26	92.10	--

R.T.: Retention time; Mol. Formulae: Molecular formulae; Mol. Wt.: Molecular weight; 1: T. vulgaris; 2: O. vulgare; 3: T. argenteus; 4: T. citriodorus; 5: O. syriacum; RI: Retention Index.

displays the identification of 81 compounds from the chromatograms of these species by comparing their mass spectra with those of their analogs reported by the NIST library and Retention Index values (RI). These compounds can be classified mainly into three classes as follows: 37 monoterpenes including (1–4), (6–12), (14–38), and (41); 40 sesquiterpenes including (39–40), (42–78), and (80); two diterpenes (79) and (81); and two acetogenins (5) and (13). Two compounds—thymol (37) with ratios 69.45, 13.77, 36.87, 2.42, and 2.63%—and carvacrol (38)—with ratios 4.46, 64.82, 1.24, 4.54, and 13.00%—were found in high percentages in the five thyme plants. p-Cymene, eucalyptol, and linalool were found with percentages of 9.45, 8.53, and 7.10% in T. vulgaris, O. vulgare, and T. argenteus, respectively. γ-Eudesmol (70) and (-)-globulol (75) were also identified in T. citriodorus with high percentages of 10.02 and 23.06%, respectively. In addition to germacrene D (53), identified at 11.64%, and δ-cadinene (59), identified at 7.07%, they were also identified from O. syriacum. The essential oil of T. vulgaris mainly contains thymol, p-cymene, and carvacrol [38,39]. O. vulgare essential oil contains carvacrol and thymol as major compounds [40]. Another study on T. vulgaris oil from Saudi Arabia indicated the presence of thymol as a major compound (38.71%) when compared with T. vulgaris from Egypt (69.45%) [41]. Our study agreed with Walasek-Janusz et al. [42], indicating that carvacrol was a major compound in O. vulgare essential oil from four countries, albeit in different ratios. These findings are in line with previous research; however, the varying percentages may be attributed to morphological characteristics and metabolism, which influence the chemical composition of the samples.

3.3. Chemical Composition Assessment

The total chlorophyll (a and b), carotenoids, total phenolics, total flavonoids, and total antioxidant contents of the five thyme plants were assessed (

Table 3. Chemical composition of five thyme plants.

Thyme Plants	Chl A mg\100 g	Chl B mg\100 g	Carotene mg\100 g	Total Phenol mg\100 g	Flavonoids mg\100 g	Total Antioxidant mg\100 g
T. vulgaris	0.969 a	0.576 a	86.64 a	0.161 b	0.579 b	28.61 c
O. vulgare	0.890 b	0.485 c	83.26 b	0.295 b	1.034 a	34.04 b
T. argenteus	0.182 d	0.167 e	19.56 e	0.516 a	0.308 b	26.47 d
T. citriodorus	0.247 c	0.230 d	24.37 d	0.151 b	0.295 b	23.86 e
O. syricum	0.251 c	0.535 b	29.37 c	0.677 a	0.393 b	43.36 a
L.S.D. (0.05)	0.004	0.003	0.48	0.214	0.407	0.26

a, b, c, d, and e: Duncan’s letters. The same letter in the same column means non-significant.

). The ChlA, ChlB, and Carotene data indicated that T. vulgaris had significantly higher values at 0.969, 0.576, and 86.64 mg/100 g, respectively. Meanwhile, T. argenteus had significantly lower values of 0.182, 0.167, and 19.56 mg/100 g, respectively. On the other hand, the data on total phenol compounds varied significantly between thyme plants. The highest value was 0.677 mg/100 g for O. syriacum compared with the lowest values 0.151, 0.161, and 0.295 mg/100 g recorded for T. citriodorus, T. vulgaris, and O. vulgare, respectively. Flavonoid compounds varied significantly between thyme plants. The highest value was 1.034 mg/100 g for O. vulgare compared with other species. The total antioxidant content ranged between 43.36 and 23.86 mg/100 g. The highest value was recorded for O. syriacum, whereas the lowest was for T. citriodorus.

As shown in Table 3, all measured chemical parameters had high values in two plants, T. vulgaris and O. vulgare. These chemical parameters, including chlorophyll [43], carotenoids [44], and total flavonoids [45], are significant for environmental health, human well-being, and plant defense mechanisms. Chlorophyll and flavonoids play a crucial role in the biosynthesis of secondary metabolites, which enhance plant defense responses. Furthermore, the two active principles, thymol and carvacrol, have good insecticidal activity against the two insects under study. Thus, this information suggests that plants are safer when used as insecticides.

3.4. Insecticidal Activity

The toxicity of the five essential oils of thyme and two major compounds, thymol and carvacrol, was evaluated against two harmful insects (S. mangiferae and C. gnidiella). The results showed that the essential oils of T. vulgaris and O. vulgare were highly toxic against S. mangiferae (LC₅₀, 18.93 and 16.93 ppm) and C. gnidiella (LC₅₀, 183.32 and 164.68 ppm), respectively, compared with the other oils (

Table 4. Toxicity of thyme essential oils and two major compounds, thymol and carvacrol, against mango thrips (S. mangiferae) after 24 h under laboratory conditions.

Treatment	LC50 (ppm)	Confidence Limit 95% (ppm)		LC90 (ppm)	Confidence Limit 95% (ppm)		Slope ± SE	Toxicity Index (%) at LC50 Value
		Lower	Upper		Lower	Upper
T. vulgaris	18.93	14.21	25.06	88.34	57.77	182.96	1.92 ± 0.30	40.94
O. vulgare	16.93	12.55	22.37	80.62	52.97	165.91	1.89 ± 0.29	45.78
T. argenteus	44.54	33.74	59.52	207.10	133.15	444.29	1.92 ± 0.30	17.40
T. citriodorus	89.60	66.20	119.54	448.01	287.94	969.56	1.83 ± 0.29	8.65
O. syriacum	77.38	57.92	102.98	371.16	239.93	789.53	1.88 ± 0.29	10.02
Thymol 37	7.75	5.63	10.32	39.34	25.54	83.70	1.82 ± 0.29	100
Carvacrol 38	8.45	6.36	11.04	37.03	24.98	71.66	2.00 ± 0.30	91.72

and

Table 5. Toxicity of thyme essential oils and two major compounds, thymol and carvacrol, against honeydew moth (C. gnidiella) after 3 days under laboratory conditions.

Treatment	LC50 (ppm)	Confidence Limit 95% (ppm)		LC90 (ppm)	Confidence Limit 95% (ppm)		Slope ± SE	Toxicity Index (%) at LC50 Value
		Lower	Upper		Lower	Upper
T. vulgaris	183.32	140.87	236.75	729.23	503.18	1339.02	2.14 ± 0.31	46.03
O. vulgare	164.68	121.89	217.35	779.77	514.61	1589.77	1.90 ± 0.30	51.24
T. argenteus	446.55	342.37	588.93	1913.89	1265.64	3847.49	2.03 ± 0.30	18.90
T. citriodorus	565.77	429.97	740.03	2439.76	1637.84	4757.78	2.02 ± 0.30	14.92
O. syriacum	479.07	367.52	619.88	1929.21	1325.72	3573.81	2.12 ± 0.31	17.62
Thymol 37	87.46	66.23	113.94	373.74	253.31	714.83	2.03 ± 0.30	96.49
Carvacrol 38	84.39	62.13	112.14	416.99	270.78	882.68	1.85 ± 0.29	100

). Furthermore, the isolated compounds, thymol and carvacrol, from T. vulgaris and O. vulgare were the most potent toward the two insects, respectively. Thyme is a plant whose essential oil is known for its insecticidal properties. Pavela et al. posited that the phenolic components of thyme essential oil are the main reason for its insecticidal activity [46]. According to Dargahi et al. [47], Thyme plants exhibit potent insecticidal properties against a variety of insects, including Aedes aegypti, Anopheles stephensi, and Staphilus oryzae. The toxicity of both T. vulgaris and O. vulgare could be due to their chemical composition, particularly the presence of thymol (69.45%) and carvacrol (64.84%), respectively [47,48]. In addition, in a previous study, thymol and carvacrol from T. vulgaris had been evaluated for their strong insecticidal activity against Alphitobius diaperinus [49]. It is worth noting that these oils and their active ingredients are being used for the first time as insecticides on the insects under study.

3.5. Effect of Thymol and Carvacrol on C. gnidiella Enzyme Activity

The effect of thymol and carvacrol on the biochemical parameters (ACP, ALP, AchE, and GST) of C. gnidiella was determined (

Table 6. Enzyme activities of C. gnidiella larvae in the presence of thymol and carvacrol at the LC₅₀ value.

Treatments	ACP (U/L) ± SE	ALP (U/L) ± SE	AchE (U/L/min) ± SE	GST (U/L) ± SE
Control	22.30 ± 1.13 a	176.4 ± 3.49 a	316.1 ± 14.09 a	671.8 ± 22.34 a
Thymol 37	15.09 ± 0.56 b	90.7 ± 5.41 b	71.17 ± 6.21 b	319.6 ± 9.68 c
Carvacrol 38	13.88 ± 0.75 b	101.0 ± 3.11 b	83.50 ± 0.29 b	371.1 ± 2.54 b
LSD, 0.05	2.94	14.29	30.78	48.92

LSD: least significant difference; SE: standard error; a, b, and c: Duncan’s letters. The same letter in the same column means non-significant.

). The activity of acid and alkaline phosphatases decreased significantly in the presence of thymol and carvacrol compared with the control (Table 6). The hydrolytic enzymes (ACP and ALP) are responsible for separating phosphate groups from proteins and nucleotides under different conditions [50]. Additionally, the digestion and absorption efficiency of nutrients is affected by the activity of both ACP and ALP enzymes. The decrease in the ALP and ACP enzyme activity led to a reduction in the release of phosphate groups, thereby affecting metabolism and digestion functions [51]. AchE enzyme activity showed a significant decrease in the presence of thymol and carvacrol compared with the control (71.17, 83.50, and 316.1 µg AchBr/min/gm body weight, respectively). Maazoun et al. posited that phenolic compounds inhibited AchE activity and affected the insects’ nervous systems [52]. Furthermore, the detoxifying enzyme activity (GST) significantly decreased compared with the control. GST was considered the main detoxification enzyme responsible for insect resistance [53,54,55]. The inhibitory effect of both thymol and carvacrol on C. gnidiella AchE aligned with their effect on Drosophila melanogaster AchE [56]. The above results indicate that C. gnidiella is susceptible to both compounds (thymol and carvacrol), leading to insect death.

4. Conclusions

Cryptoblabes gnidiella and Scirtothrips mangiferae are serious pests that attack many strategic fruit crops such as mango trees. Using natural extracts as an alternative to synthetic insecticides to control these insects has become an urgent need. The essential oils of five thyme plants—T. vulgaris, O. vulgare, T. argenteus, T. citriodorus, and O. syriacum—were tested against C. gnidiella and S. mangiferae. Two compounds, thymol and carvacrol, were identified in all species at a high ratio, especially T. vulgaris and O. vulgare at 69.45 and 64.82%, respectively. Thymol and carvacrol were found to be the most toxic to both insects. Furthermore, a significant effect of the two compounds on certain biochemical parameters of C. gnidiella, including Acid Phosphatase, Alkaline Phosphatase, Glutathione S-Transferase, and Acetylcholine Esterase enzymes, was observed at LC₅₀ values. Given its wide distribution and bioactive compounds like thymol and carvacrol, thyme shows strong potential as a botanical insecticide for crop protection.