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Article

Residues of the Acaricides Abamectin, Hexythiazox, and Spiromesifen in Eggplant (Solanum melongena L.) Fruits Grown under Field Conditions in Najran, Saudi Arabia

by
Jari S. Algethami
1,2,
Mohsen A. M. Alhamami
1,
Mohamed F. Ramadan
3,4 and
Osama I. Abdallah
5,*
1
Department of Chemistry, College of Science and Arts, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia
2
Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, Najran 11001, Saudi Arabia
3
Central Agricultural Pesticide Laboratory (CAPL), Pesticide Analysis Research Department, Agricultural Research Center (ARC), Giza P.O. Box 12618-Dokki, Egypt
4
Quality Measurements Laboratory, Mecca 24372, Saudi Arabia
5
Central Agricultural Pesticide Laboratory (CAPL), Pesticide Residues and Environmental Pollution Department, Agricultural Research Center (ARC), Giza P.O. Box 12618-Dokki, Egypt
*
Author to whom correspondence should be addressed.
Agriculture 2023, 13(1), 116; https://doi.org/10.3390/agriculture13010116
Submission received: 16 November 2022 / Revised: 11 December 2022 / Accepted: 29 December 2022 / Published: 31 December 2022
(This article belongs to the Section Crop Protection, Diseases, Pests and Weeds)
Review Reports Versions Notes

Abstract

:
The dissipation profiles of the acaricides abamectin, hexythiazox, and spiromesifen in the fruits of eggplant (Solanum melongena L.) grown under field conditions in Najran, Saudi Arabia, were studied. Extraction was performed with acetonitrile, and UPLC-MS/MS was used for quantification. Instead of conventional adsorbents, a 2-fold dilution of the sample extract quickly and efficiently reduced interfering co-extracts and matrix effects. The method was successfully validated according to EU regulations. The limit of quantification was set at 5 µg/kg for hexythiazox and spiromesifen and 20 µg/kg for abamectin. The mean recoveries and relative standard deviations were 88.6–98.7% and 5.2–12.4%, respectively. The method precision was evaluated at the LOQ level for each analyte and ranged from 6.7 to 15.7%, with good trueness (recovery) ranging from 85.7 to 97.2%. The matrix effect ranged from −2.2% to −4.6%, indicating negligible signal suppression. First-order kinetics were used to characterize the dissipation rates of abamectin, hexythiazox, and spiromesifen with half-lives (t1/2) of 2.11–2.42, 2.3–2.73, and 1.31–1.47 days, respectively, using the authorized and double authorized doses. Terminal residues were 0.028–0.331 mg/kg, 0.019–0.592 mg/kg, and 0.044–0.408 mg/kg, respectively, at 3, 7, and 10 days after the second treatment. According to the risk assessment results, the percentage of chronic dietary risk quotient was <100, meaning that abamectin, hexythiazox, and spiromesifen are not considered a risk to human health. The preharvest interval (PHI) should be 7, 7, and 3 days, respectively, if the authorized dose is used, and 10, 10, and 3 days, respectively, if double the authorized dose is used. The current study can be a helpful resource for the responsible and safe use of the tested acaricides on eggplant fruits.

1. Introduction

Pesticides are used to protect and improve the harvest of fruits and vegetables [1]. Due to improper use, pesticide residues in food can be high, which has led governments and international organizations to set maximum residue limits (MRLs) for food. Most farmers grow eggplant (S. melongena L.) for commercial purposes [2] because it is a nutrient-rich vegetable that contains vitamins, proteins, and carbohydrates [3]. It is one of the most important crops in Saudi Arabia, where more than 57 k tons per year are grown on 4101 hectares [4]. Eggplant fruits are attacked by many insects, resulting in significant losses in productivity and quality, forcing farmers to use insecticides to prevent or reduce pest infestations [5]. However, the extensive use of insecticides can lead to consumer exposure to their residues [6,7]. For this reason, many authorized organizations, such as the FAO/WHO (Codex Alimentarius) and the European Union (EU), have established maximum residue limits (MRLs) to control these residues. As climatic conditions, soil types, crop types, spray doses, and pest resistance rates to these insecticides change [8], researchers have had to conduct intensive and continuous studies to monitor the residues of these insecticides. Abamectin, spiromesifen, and hexythiazox are among the most commonly used insecticides to control mites on various crops, including eggplant.
Abamectin is a macrocyclic lactone consisting of a mixture of two homologs of avermectin B1a (80%) and avermectin B1b (20%) [9]. It is a broad-spectrum, systemic insecticide that paralyzes insects and mites by attacking their nervous system and causing the release of C-aminobutyric acid. It has a minimal systemic effect on plants but controls mites and other motile insect stages on fruits and vegetables [10]. The systemic acaricide spiromesifen [2-oxo-3-(2,4,6-trimethylphenyl)-1-oxaspiro [4.4] non-3-en-4-yl 3,3 dimethyl butanoate], which belongs to the class of compounds known as spirocyclic tetronic/tetramic acid derivatives, controls mites in various crops by blocking the enzyme acetyl-coenzyme A carboxylase (ACCase) and preventing the production of lipids [11]. Heythzox[(4RS,-5RS)-5-(4-chlorophenyl)-N-cyclohexyl-4-methyl-2-oxo-1,3-thiazolidine-3 carboxamide] is one of the most widely used non-systemic acaricides and an essential element of integrated pest management as it is used to control mites in various crops [12,13].
The study of the rate of dissipation is one of the most critical factors in determining how pesticides act in the environment. Consumers are concerned about the residues of these pesticides on eggplants in Saudi Arabia, but data on these residues are still poor. Residues of abamectin, spiromesifen, and hexythiazox were determined, at different time intervals, in green beans and peppers [14,15], in chili and okra [16,17], and in bean pods and strawberry fruit [18,19], respectively.
Pesticide spraying residue analysis is critical for estimating human and environmental exposure to pesticides. However, samples must be extracted and purified before residue determination [20]. For abamectin, hexythiazox, and spiromesifen, several methods have been developed for pre-quantitative sample preparation in crops, including liquid–liquid extraction [21,22,23,24,25,26].
A “quick, easy, cheap, effective, rugged and safe” procedure was first proposed by Anastassiades et al., in 2003 and by Lehotay et al., in 2005 and 2006 [27,28,29]. Instead of conventional extraction methods, the procedure was used to identify pesticides in plants with high water content [30]. It has been combined with gas chromatography (GC) in conjugation with electron capture detector (ECD) or mass spectrometry (MS), and also with high-performance liquid chromatography (HPLC) in conjugation with diode array detector (DAD) or tandem mass spectrometry (MS/MS), to determine abamectin, hexythiazox, and spiromesifen in different matrices [14,19,31,32,33,34,35,36].
The aim of this study was to (1) develop and validate a rapid, simple, and reliable analytical procedure based on acetonitrile extraction and LC-MS/MS detection for the simultaneous quantification of abamectin, hexythiazox, and spiromesifen in eggplant samples, (2) evaluate the effectiveness of dilution of the final extracts on the matrix effect instead of using adsorbents, (3) evaluate the rate of dissipation, half-life (t1/2), and final residues of the tested acaricides in eggplant fruits under field conditions, (4) evaluate the risks that may result from dietary exposure, and finally (5) propose a safe waiting period for the tested acaricides when applied to eggplant fruit.

2. Materials and Methods

2.1. Chemicals

The certified reference standards abamectin, hexythiazox, and spiromesifen, with a purity of 97.1%, 99.5%, and 99.4%, respectively, were purchased from Dr. Ehrenstorfer (Augsburg, Germany). Acetonitrile, methanol, and acetic acid, in HPLC grade, and formic acid and ammonium formate, in LC-MS grade, were provided by Fisher Scientific (Loughborough, UK). Anhydrous magnesium sulfate and sodium acetate were supplied by Chem-Lab NV (Zedelgem, Belgium). The ceramic homogenizer was purchased from Agilent Technologies Inc. (Wilmington, DE, USA). Commercial emulsifiable concentrate formulations (EC) of abamectin (1.8%; Actamine®; Indogulf Crop Sciences Ltd., Delhi, India), spiromesifen (24%; Oberon®; Bayer Crop Science, Tokyo, Japan), and wettable powder (WP) hexythiazox (10%, Nissorun®, Nippon Soda Co. LTD, Tokyo, Japan), were purchased from the local market. An Ultra-Clear™ system from Evoqua Water Technologies LLC (Guenzburg, Germany) produced ultrapure water.

2.2. Analytical Procedure

2.2.1. Standard and Reagent Solutions

The standard stock solution of 1000 mg/L and the intermediate solution of 100 mg/L of each analyte were prepared individually in acetonitrile. To generate the solvent calibration curves, working standard mixtures containing the three acaricides were serially diluted in acetonitrile in the concentration range of 1–200 µg/L (eight points). The final eggplant matrix extract obtained using the proposed sample preparation procedure was used to generate the matrix-matched calibration curves in the same series. The standard solutions were stored in the dark at −20 °C.

2.2.2. Sample Preparation

In total, 10 g of the homogenized frozen eggplant sample was weighed into a 50 mL polypropylene centrifuge tube. Then, 10 mL of acetonitrile (1% acetic acid) was added, and the mixture was shaken by hand for 2 min after adding a ceramic homogenizer. Next, 5 g of a salt mixture of sodium acetate and magnesium sulfate (1:4 w/w) was added, and then the tube was shaken by hand for 30 s, followed by centrifugation at 5000 rpm for 3 min. The supernatant was diluted 2-fold with acetonitrile and then filtered through a 0.22 µm syringe filter (Whatman, WA, USA) for LC-MS/MS analysis.

2.2.3. Instrumentation

A Dionex Ultimate 3000 RS UHPLC (Ultra-High Performance Liquid Chromatography) was utilized for chromatographic separation using an Accucore RP-MS C18 column (100 × 2.1 mm, 2.6 µm film thickness, Thermo Fisher Scientific) set at a constant temperature of 40 °C. Mobile phase gradient elution was performed using mixtures of pure water with 10 mM ammonium formate and achieved at a pH of 4 using formic acid (A) and methanol (B). Initial gradient conditions were set at 25% B for 2 min, then B raised to 90% for 3 min (2–5 min), and then held for 5 min (5–10 min). After 10.1 min, the gradient was set to 25% B to restore column equilibrium for 6 min. The mobile phase eluted the analytes tested at a flow rate of 0.3 mL/min. The injection volume was set to 2 µL. For MS/MS quantification and confirmation, the Thermo Fisher Scientific TSQ Altis triple quadrupole mass spectrometer was used in conjugation with the UHPLC (Ultra-High Performance Liquid chromatography) and operated in positive mode with an electrospray ionization source (HESI). Temperatures of 350 °C for de-solvation and 300 °C for the ion source were chosen as optimal values. The pressure of the sheath and aux gases were 40 and 10 Arb, respectively, and the capillary voltage was set to 3.8 kV. The MS/MS parameters of the analytes studied were optimized using a Harvard infusion pump (Harvard Apparatus, South Natick, MA, USA). Data were collected using Trace Finder software (version 4.1).

2.2.4. Effect of Dilution on the Matrix Effect

The effects of matrix dilution on reducing the suppression or enhancement effect of the eluted coextract were evaluated using a comparison of the slopes of the calibration curve in the solvent with that in the eggplant matrix at different dilution factors. The constructed calibration curves ranged in concentration from 10 to 100 mg/kg (four points). The evaluation included five levels of matrix dilution, i.e., 2×, 4×, 6×, 8×, and 10×. Another calibration was constructed in an undiluted matrix.

2.2.5. Method Validation

The method was validated according to the SANTE guideline [37]. The linearity, limit of detection (LOD) and limit of quantification (LOQ), precision, accuracy, and matrix effect were investigated. The linearity range of the studied analytes was determined using the correlation coefficient (R2) and the residuals of the obtained calibration curves at concentrations from 1 to 200 µg/kg. The LOD is the concentration at which a signal-to-noise ratio of 3 was obtained. The lowest spiking concentration with a recovery of 70–120% and a relative standard deviation (RSD) of ≤20% is the LOQ. The method accuracy was evaluated regarding the recovery test, which was assessed at 4 spiking levels of 0.02, 0.1, 1, and 2 mg/kg, with 6 replicates tested for each spiking level. Precision was evaluated at the LOQ for each analyte on 1 day (RSDr, n = 6) and within 3 different days (7 days apart) (RSDR, n = 18). One set of calibration standards was prepared in pure solvent (acetonitrile), and the second set was prepared in the extracts of the blank samples. These calibration curves were used in this study to compare their slopes and evaluate the effects of the matrix on the analyte detection [38,39]. The following equation (Equation (1)) was used to determine the matrix effect (ME):
ME %   = slope ( matrix ) −   slope ( solvent ) slope ( solvent ) × 100

2.3. Field Experiments

Field trials were conducted in July 2022 in Khubash city, Najran region, southern Saudi Arabia (longitude: 44°44′03.6° E, and latitude: 17°33′27.8° N). We sprayed eggplants with abamectin (1.8%, EC), hexythiazox (10%, WP), and spiromesifen (24%, SC) formulations at rates of 9, 50, and 96 g a.i./ha (authorized dose), and 18, 100, and 192 g a.i./ha (double dose) as part of good agricultural practice (GAP). Each treatment was applied in three 10 × 10 m2 plots separated by buffer zones. A knapsack sprayer was used to apply the acaricides, and 1000 L/ha of water was used to dissolve the formulations for application. During the experimental period, the usual weather conditions in this geographical region were monitored, i.e., no rainfall, minimum and maximum temperatures of 25–39 °C, and relative humidity of 14 ± 4%. Eggplant samples were randomly collected before pesticide application and then 2 h (0 days), 1, 3, 5, 7, 10, and 15 days after each application. Once the samples were collected, they were packed in polyethylene bags, sealed in dry ice boxes, and immediately delivered to the laboratory. Before further examination, the samples were crushed, homogenized, labeled, and stored at −20 °C. The dissipation kinetics of the three acaricides were calculated using the first-order kinetic model (Equation (2) and (3)) [40,41,42]:
Ct = C0e−kt
t1/2 = ln 2/k
where Ct is the residue concentration (mg/kg) at time t (days), C0 is the initial residue concentration (mg/kg), and k is the rate constant (day−1).

2.4. Terminal Residues

The terminal residue study was conducted after two applications (15 days apart) of abamectin, hexythiazox, and spiromesifen at the recommended dosages of 9, 50, and 96 g a.i./ha, respectively, and the double dosages of 18, 100, and 192 g a.i./ha, respectively, as indicated on the product labels. Three replicate samples of eggplant were taken at random 3, 7, and 10 days after the last spray.

2.5. Dietary Risk Assessment

The long-term (chronic) dietary exposure risk of the three acaricides in eggplant fruits was calculated using Equation (4) [42]:
NEDI = STMRi × Fi/bw
where NEDI (mg/kg.bw) represents the national estimated daily intake, STMRi (mg/kg) represents the supervised median residue, F (kg/day) represents the average vegetable consumption (0.278 kg) [43], and bw (kg) is the average adult body weight (60 kg) [44].
To assess the risks to human health, dietary exposure was compared with the applicable toxicological reference values for the acceptable daily intake (ADI) (Equation (5)). The cases where the tested acaricide residues are not harmful to the health of adult consumers are characterized using an RQ percentage of less than 100 [42,45,46].
% RQc = (NEDI/ADI) × 100

2.6. Statistical Analysis

The sample field tests’ mean concentration, median value, standard deviation, curve fit, and kinetic rate constants were calculated using Microsoft Excel 2021. To determine the significance between groups, the One-way test ANOVA and Fisher’s least significant difference (LSD) test were performed in Microsoft Office Excel 2021, and a probability value of p ≤ 0.05 was considered significant.

3. Results and Discussions

3.1. LC-MS/MS Optimization

For each analyte, the MS full-scan mode was used to confirm the presence of a precursor ion using a standard solution with a concentration of 1 mg/L. A scan between 100 and 1000 m/z shows the molecular ions of spiromesifen (m/z 371.2), hexythiazox (m/z 353.2), and abamectin (m/z 890.4). The precursor ions of hexythiazox and spiromesifen interacted in the ESI+ mode to generate protonated molecular ions [M + H]+, whereas abamectin generated an ammonium adduct [M + NH4]+. According to the full mass spectra, the three acaricides are easily ionized in the ESI+ mode. After identifying the precursor ions, the ideal collision energy for MRM transitions was determined for each acaricide. According to the MS/MS detection criteria in the SANTE guidelines [37], the most intensely specified optimized two MRM transition ions for each analyte were selected, one with the highest intensity for quantitation and the other for confirmation of the target analytes (Figure S1).
Because of their influence on the quality of chromatographic separation and their availability to improve the ionization of analytes, methanol and acetonitrile are commonly used as the mobile phase in liquid chromatography in conjunction with mass spectrometry. Buffers or modifiers are usually added to the mobile phase to improve peak shape and ionization efficiency [47].
The mobile phase consisting of acetonitrile/water in addition to 5 mM or 10 mM HCOONH4, or methanol/water in addition to 5 mM or 10 mM HCOONH4 was tested with or without the addition of formic acid, which can promote the formation of the protonated ions of hexythiazox and spiromesifen. The addition of ammonium salt is essential for the formation of abamectin adducts.
The use of methanol/water as the mobile phase in addition to 5 mM ammonium formate improved analyte ionization and significantly increased the observed signals (peak area) of hexythiazox and spiromesifen (p < 0.05). At the same time, using acetonitrile led to a significant signal enhancement (p < 0.05) of abamectin. Therefore, we decided to use methanol/water (with 5 mM or 10 mM HCOONH4) for further studies. The selected mobile phase component was further tested when the amount of ammonium formate was increased to 10 mM, in addition to the addition of formic acid to bring the water to pH 4. Increasing the amount of ammonium formate to 10 mM significantly (p < 0.05) increased the sensitivity of abamectin. In contrast, a significant decrease was observed for hexythiazox and spiromesifen. Maintaining the ammonium content at 10 mM, adding formic acid, and adjusting the pH to 4 significantly (p < 0.05) increased the sensitivity to abamectin compared to that consisting of only 5 mM or 10 mM. While the sensitivity of hexythiazox and spiromesifen increased significantly (p < 0.05) compared to the component with only 10 mM, it was still significantly (p < 0.05) lower than the component with only 5 mM. Since the sensitivity of spiromesifen and hexythiazox is high enough to be detected at lower concentrations, we only considered the increase in sensitivity of abamectin. We, therefore, decided to use a mobile phase consisting of methanol: water with 10 mM ammonium formate and formic acid for adjusting to pH 4. The acaricides tested were eluted within 10 min, in addition to 6 min for the re-equilibration step, without interfering peaks. Retention times were 9.75, 9.89, and 10.39 min for hexythiazox, spiromesifen, and abamectin, respectively. Table 1 lists the respective LC-MS/MS parameters.

3.2. Matrix Effect

The eluted coextracts during analyte elution can lead to ion suppression or enhancement in the ion source, which alters the signal sensitivity [38,39,48,49]. Adsorbents such as primary secondary amine (PSA) and C18 are used in the purification step of the final extract [50], which was omitted in this study to reduce the cost and make the method compatible with clean chemistry, which is one of the challenges that researchers are currently looking to address.
Ion suppression was observed for the three acaricides when the fortified raw extract was analyzed directly without cleanup. Therefore, the effect of extract dilution was investigated to evaluate the reduction in signal suppression with decreasing matrix loading using five dilution factors of 2×, 4×, 6×, 8×, and 10×. Hexythiazox and spiromesifen showed negligible signal suppression (≤5.5%) with and without dilution, which is considered a negligible effect. In comparison, abamectin showed a matrix effect of −18.6%, which was reduced to −4.3% with a 2-fold dilution factor, which was found to be sufficient to eliminate most matrix effects and allow quantification with solvent-based standards. Further dilution had no significant effect on ME for hexythiazox and spiromesifen.

3.3. Method Validation

3.3.1. Linearity and Matrix Effects

The linearity of the tested analytes was evaluated in a fortified matrix extract (matrix-matched calibration curves) over a range of 1–200 µg/kg (eight points). The correlation coefficient (R2) for hexythiazox and spiromesifen in the 1–100 g/kg and 5–100 g/kg ranges was 0.9993 and 0.9991, respectively. For abamectin, linearity was achieved with an R2 of 0.9992 throughout a more restricted concentration range of 10–200 g/kg. Less than 12.4% of the relative residuals were present.
Linearity data were used to determine the matrix effect (ME) (Equation (1)). Hexythiazox, spiromesifen, and abamectin had ME values of −4.6%, −3.4%, and −2.2%, respectively (Table 2). The matrix effect was generally less than 4.6% and, thus, can be considered insignificant as this variability is close to the repeatable RSD values. It can be concluded that the co-extracted matrix components were reduced more efficiently as the frequency of a small suppression effect was lower.

3.3.2. LOD and LOQ

The LODs of the target analytes ranged from 0.19 to 1.1 µg/kg. The LOQs of hexythiazox and spiromesifen were set at 5 µg/kg, resulting in acceptable recoveries of 92.4% and 95.8%, respectively, and a relative standard deviation (RSD) of 11.7% and 6.7%, respectively. In comparison, the LOQ of abamectin was set at 20 µg/kg, with mean recoveries and an RSD of 88.6% and 8.4%, respectively, in six replicate analyses (Table 2). LOD and LOQ were well below the maximum residue limits (MRLs) established by the European Union for spiromesifen and spirodiclofen in eggplant.

3.3.3. Method Accuracy and Precision

The accuracy of the method was determined as mean recoveries of abamectin, hexythiazox, and spiromesifen at four spiking levels of 0.02, 0.1, 1, and 2 mg/kg, which were in the range of 88.6–94.8%, 93.3–98.4%, and 93.1–98.7%, respectively, while the corresponding relative standard deviations were in the range of 6.9–11.3%, 6.3–10.7%, and 5.2–12.4%, respectively (Table 3). Sample concentrations that exceeded the top limit of the calibration curve were diluted with blank extracts. These results meet the criteria of the SANTE guideline [37], which states that the relative standard deviation is ≤20% and the percent recovery is 70–120%.
The precision of the method was measured at the LOQ level of each analyte within the day (RSDr) by analyzing 6 replicates and within 3 different days spaced 7 days apart (RSDR) by analyzing 6 replicates per day (n = 18). The results (Table 2) for RSDr and RSDR were in the range of 6.7–11.7% and 10.3–15.7%, respectively, with good trueness demonstrated by recoveries in the range of 88.6–95.8% and 85.7–97.2%, respectively (Table 2), which met the requirements of the SANTE guideline [37].

3.4. Dissipation of Abamectin, Hexythiazox, and Spiromesifen in Eggplant Fruits

Information on pesticide dissipation kinetics in crops is an essential component of current risk and impact assessment procedures since pesticide residues in agricultural foods intended for humans and animals are the primary source of pesticide exposure to humans [51]. Considering the time between pesticide application and harvest and better estimation of the half-life of pesticides in/on plants is critical for determining the fate of pesticides in the environment and establishing pre-harvest intervals, both of which are critical to good agricultural practices.
Residue profiles of the acaricides abamectin, hexythiazox, and spiromesifen were presented using residues in eggplant samples collected at various time points after application (Figure S2). Table 4 shows the dissipation rates as well as other kinetic properties.

3.4.1. Abamectin

The initial residues of abamectin were 0.267 mg/kg and 0.429 mg/kg at the authorized and double doses of 9 and 18 g a.i/ha, respectively. The residue concentrations declined to 0.196 and 0.334 mg/kg, 1 day later, respectively. Within 3 days, 49.7% and 59.9% of the residues had declined, respectively. After 7 days, 78.4% and 85.2% of the residues had declined, respectively. After 10 days, more than 91% of the residues had declined. According to the findings, the dissipation profile followed first-order kinetics. The regression equation for dissipation was Ct = 0.247 e−0.287t (R2 = 0.988). The half-life (t1/2) was 2.4 days using the authorized dose. The regression equation for the double dose was Ct = 406e−0.327t (R2 = 0.985), and the half-life (t1/2) was 2.1 days. Our results are in agreement with those of Abdellseid and Rahman (2014) who reported a half-life (t1/2) of 2.4 days in tomatoes [33]. The half-life (t1/2) of abamectin in green beans, tomatoes, and strawberries is 1, 1.06, and 1.02 days, respectively [14,34,52], which is shorter than the values reported in this study.

3.4.2. Hexythiazox

The initial residues were 0.415 and 0.698 mg/kg after application of 50 and 100 g a.i./ha, respectively. The residues decreased to 0.273 and 0.422 mg/kg by day 1, respectively. About 55% and 57% of the initial residues declined within 3 days, and 82% and 83% of the residues declined within 7 days, respectively. After 10 days, the residues were 0.028 and 0.032 mg/kg, respectively, which was lower than the MRL of 0.1 mg/kg (EU-MRL database). The regression equation for dissipation was Ct = 4007 e–0.254t (R2 = 0.983), and the half-life (t1/2) was 2.7 days using the authorized dose, and Ct = 711 e−0.301t (R2 = 0.978), and the half-life (t1/2) was 2.3 days using the double dose. The results show that first-order kinetics can describe dissipation kinetics. Our obtained results were consistent with those of Majumder et al., (2015) who calculated a half-life (t1/2) of 1.42–2.32 for brinjal sprayed at 25 g a.i./ha and 50 g a.i./ha [22]. El-Hamid et al., 2015 also reported a half-life (t1/2) of 2 days when strawberries were sprayed with 20 g a.i./ha [35]. A similar observation was reported by Abd-Alrahman (2012), who calculated a half-life (t1/2) of 2.70 days in bean pods sprayed at 20 g a.i./ha [18]. In contrast, the half-life (t1/2) in this study was slightly longer than that reported by Roy et al., (2019), who investigated a half-life (t1/2) of 1.43–2.01 days in okra sprayed at 25 and 50 g a.i./ha [32]. In contrast, Saber et al., (2016) reported a longer half-life (t1/2) of 3.43–3.59 days in strawberries sprayed at 10 and 15 g a.i./ha [19].

3.4.3. Spiromesifen

The initial residues were 1588 and 2673 mg/kg after application of 96 and 192 g a.i./ha, respectively. The residues declined to 0.889 and 0.1568 mg/kg on the first day, and then the residues were lower than the MRL of 0.5 mg/kg (EU-MRL database) on the fifth day and thereafter. The residues could be detected for 10 days and reached <0.01 (LOQ) mg/kg after 15 days using the authorized dose or double dose. The residue dissipation was consistent with first-order kinetics: Ct = 1.612e−0.0.533t (R2 = 0.998) using the authorized dose and Ct = 3.021e−0.472t (R2 = 0.990) using the double dose, and the calculated half-lives (t1/2) were 1.3 and 1.5 days, respectively. The current results are comparable to half-lives of 0.93–1.38 days and 1.04–1.34 days in tomatoes when the doses of 150 and 300 g a.i./ha were applied [36], respectively, and 1.6, 1.8, 1.9, and 1.7 days in okra, capsicum, chili, and brinjal [24], and 1.32 and 2.18 days in brinjal sprayed at 96 and 192 g a.i./ha, respectively [53]. In contrast, Few reports investigated longer half-lives (t1/2) in different crops: 5.5–6.2 days, 2.52–2.88 days, and 5.0–8.5 days for apple, eggplant, and leaf tea, respectively, after application at the dose of 96 g a.i./ha [23,25,26,54], and 6 and 6.5 days for tomato when a single dose of 125 g a.i./ha and a double dose of 250 g a.i./ha were applied, respectively [55].
Although hexythiazox is a non-systemic insecticide, its dissipation half-life (t1/2) was similar to that of the systemic abamectin investigated in this study. The octanol–water partition coefficient (Kow) values of abamectin, hexythiazox, and spiromesifen were 4.4, 5.5, and 4.5, respectively, indicating that these compounds are hydrophilic. Since eggplant fruit has a waxy surface, the pesticides applied to it can be adsorbed on its surface, slowing down the degradation process [56]. The results show that spiromesifen degrades faster than other acaricides (t1/2 = 1.31–1.47 days). The difference in residual deposition after treatment may be attributed to the different formulation types and application doses of the active ingredients (g a.i./ha), which result in different residue levels. For abamectin, hexythiazox and spiromesifen, residue dissipation was faster in the first 7 days (78.4–97.8%) and slowed down. Dissipation of pesticides on plants generally occurs due to diffusion, specific plant species and exposure areas, growth dilution, physicochemical properties of the pesticide, and meteorological factors such as rainfall, temperature, and relative humidity at the time of application [57,58,59]. In this study, eggplants reached the final fruit diameter size when the treatments were applied, so a low dilution of pesticide residues was achieved by fruit growth, in addition to the absence of rainfall during the study periods. Therefore, solar radiation (photodegradation) may have initially led to faster dissipation and is considered the main reason for the dissipation of the acaricides tested.

3.5. Terminal Residues

Terminal residues of abamectin, hexythiazox, and spiromesifen in eggplants were determined after two applications (15 days between each application) and are shown in Table S1. It can be seen that the target acaricides in eggplants show decreasing trends when the harvest intervals are extended, although the decrease is not apparent, which could be due to the repeated applications. The terminal residues of abamectin, hexythiazox, and spiromesifen after the last spraying of the authorized dose were 0.028–0.143 mg/kg, 0.019–0.232 mg/kg, and 0.044–0.263 mg/kg, respectively. As for the double dose, the terminal residues at 3, 7, and 10 days after application were 0.047–0.331 mg/kg, 0.045–0.592 mg/kg, and 0.088–0.408 mg/kg, respectively. The residues of abamectin and hexythiazox in tomatoes were found to be lower than the MRL of 0.09 and 0.1 mg/kg (EU-MRL database) at 7 and 10 days when the authorized dose was applied, respectively, while the terminal residues of spiromesifen were lower than 0.5 mg/kg (EU-MRL database) at all the harvesting days when both doses were applied. These data are valuable for proposing safety PHIs for the tested acaricides in eggplant fruit.

3.6. Risk Assessment

Human health risks were determined using a comparison of the dietary exposure to relevant toxicological reference acceptable daily intake (ADI) values. following the World Health Organization (WHO) reviews, the ADI of abamectin, hexythiazox, and spiromesifen was 0.001 mg/kg.bw (JMPR 2015), 0.03 mg/kg.bw (JMPR 2008), and 0.03 mg/kg.bw (JMPR 2016), respectively. When the NEDI was compared with the ADI values of the respective acaricides, the percent chronic exposure risk quotient (RQc) for abamectin, hexythiazox, and spiromesifen were 3.34–58.75%, 0.73–9.2%, and 0.74–6.27%, respectively (Table S2), which were less than 100%, indicating that the chronic exposure risks were acceptably low. However, for abamectin and hexythiazox, the highest residue levels in the eggplants were above or near the established MRL of 0.09 mg/kg and 0.1 mg/kg, respectively, after three days using the authorized dose and after three and seven days using double the authorized dose. Therefore, the application rates and PHI should be strictly followed to ensure that food quality complies with the MRLs. The PHI for hexythiazox, spiromesifen, and abamectin is advised based on the results of the residue levels derived from the dissipation study, terminal residues, and dietary risk assessment. When the authorized dose is used, a minimum harvesting time of 7 days, 7 days, and 3 days, or 10 days, 10 days, and 3 days when the double dose is used, is advised, respectively. Our results provide a valuable reference for Saudi Arabia to develop MRLs for abamectin, hexythiazox, and spiromesifen and to guide proper pesticide use in eggplant fruit.

4. Conclusions

The objective of this study was to develop a dispersive liquid–liquid extraction technique for the simultaneous analysis of eggplant fruit for abamectin, hexythiazox, and spiromesifen residues using UPLC-MS/MS. This technique was shown to be rapid, simple, inexpensive, and reliable. The results show that sample dilution successfully reduced signal suppression. The evaluation of linearity, average recoveries, precision, the limit of quantification (LOQ), and matrix effect as a validation criterion based on the quality control criteria (SANTE guideline 2020) was performed. The validated method was utilized to investigate the dissipation kinetics and residue distribution of abamectin, hexythiazox, and spiromesifen when the commercial formulations were applied to eggplant fruit under field conditions. The first-order kinetic model described dissipation patterns with half-lives (t1/2) of 2.11 to 2.42 days, 2.3 to 2.73 days, and 1.31 to 1.47 days, respectively, indicating that the tested acaricides dissipate relatively rapidly. The terminal residues after two applications of the tested acaricides were examined. The chronic risk assessment results indicate that the tested acaricides in eggplant fruit pose a low dietary intake risk. After applying the double doses two times, the preharvest interval (PHI) should be 10, 10, and 3 days for abamectin, hexythiazox, and spiromesifen, respectively. The results of our study serve as a valuable guide for setting MRLs, establishing rules for the safe application of the three acaricides, and ensuring the safety of agricultural products and thus the health of consumers.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture13010116/s1, Figure S1: Optimizing Rf Lens, breakdown curve at 1.5 mTorr (+), and product scan of the precursor ions of abamectin, hexythiazox and spiromesifen, Figure S2: Dissipation pattern of abamectin (A), hexythiazox (B), and spiromesifen (C) in eggplant under open field condition; Table S1: Terminal residues of the three acaricides in eggplant fruits, Table S2: Chronic dietary risk assessment of the three acaricides in eggplant fruits.

Author Contributions

O.I.A. contributed to the conception and design of the experiment and wrote the manuscript. O.I.A. and M.F.R. collected the real samples and performed the analysis. J.S.A. and M.A.M.A. performed the data analysis and revision of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are thankful to the Deanship of Scientific Research at Najran University for funding this work under the Najran Region Research Program Funding program grant code (NU/NAR/SERC/11/2).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The author declares no conflict of interest.

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Table
Table 1. LC-MS/MS parameters.
Table 1. LC-MS/MS parameters.
CompoundPrecursor Ion (m/z)Ionization ModeProduction (m/z)Collision Energy (v)RF Lens (v)tR (min)
Abamectin890.4[M + NH4]+305.4267510.39
567.31475
Hexythiazox353.2[M + H]+22815619.75
168.12561
Spiromesifen371.2[M + H]+273.110639.89
255.22463
The ions in italic bold are the quantifiers; tR retention time.
Table
Table 2. Calibration parameters, LOD, LOQ, ME (%), and precision of the three acaricides in eggplant fruits.
Table 2. Calibration parameters, LOD, LOQ, ME (%), and precision of the three acaricides in eggplant fruits.
AnalyteLinear Range
(µg/kg)		R2Residual
(%)		LOD
(µg/kg)		LOQ
(µg/kg)		ME (%)Precision
Inra-Day (n = 6)Inter-Days (n = 18)
RSDr (%)R (%)RSDR (%)R (%)
Abamectin10–2000.999212.42.220−4.68.488.614.885.7
Hexythiazox1–1000.99938.900.195−3.411.792.415.795.3
Spiromesifen5–1000.999110.31.35−2.26.795.810.397.2
Table
Table 3. Recovery and RSD of the three acaricides.
Table 3. Recovery and RSD of the three acaricides.
AnalyteRecovery ± RSD (%), (n = 6)
0.02 mg/kg0.10 mg/kg1 mg/kg2 mg/kg
Abamectin88.6 ± 11.392.3 ± 9.694.8 ± 5.491.3 ± 6.9
Hexythiazox94.5 ± 10.796.7 ± 7.693.3 ± 9.198.4 ± 6.3
Spiromesifen93.1 ± 12.496.2 ± 8.698.7 ± 5.295.1 ± 10.2
Table
Table 4. Dissipation kinetics of abamectin, hexythiazox, and spiromesifen in eggplant.
Table 4. Dissipation kinetics of abamectin, hexythiazox, and spiromesifen in eggplant.
AbamectinHexythiazoxSpiromesifen
Dosage (g a.i./ha)Dosage (g a.i./ha)Dosage (g a.i./ha)
9185010096192
Intercept (C0)0.2470.3270.4010.7111.6123.021
K (day−1)0.2870.4070.2540.3010.5330.472
r20.9880.9850.9830.9790.9980.991
t1/2 (days)02.4202.1102.7302.3001.3101.47
K: dissipation rate; r2: correlation coefficient.
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MDPI and ACS Style

Algethami, J.S.; Alhamami, M.A.M.; Ramadan, M.F.; Abdallah, O.I. Residues of the Acaricides Abamectin, Hexythiazox, and Spiromesifen in Eggplant (Solanum melongena L.) Fruits Grown under Field Conditions in Najran, Saudi Arabia. Agriculture 2023, 13, 116. https://doi.org/10.3390/agriculture13010116

AMA Style

Algethami JS, Alhamami MAM, Ramadan MF, Abdallah OI. Residues of the Acaricides Abamectin, Hexythiazox, and Spiromesifen in Eggplant (Solanum melongena L.) Fruits Grown under Field Conditions in Najran, Saudi Arabia. Agriculture. 2023; 13(1):116. https://doi.org/10.3390/agriculture13010116

Chicago/Turabian Style

Algethami, Jari S., Mohsen A. M. Alhamami, Mohamed F. Ramadan, and Osama I. Abdallah. 2023. "Residues of the Acaricides Abamectin, Hexythiazox, and Spiromesifen in Eggplant (Solanum melongena L.) Fruits Grown under Field Conditions in Najran, Saudi Arabia" Agriculture 13, no. 1: 116. https://doi.org/10.3390/agriculture13010116

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