Dissipation Patterns of Insecticide Sulfoxaflor in Spinach and Korean Cabbage

Introduction

Spinach and cabbage are popular vegetables grown and consumed worldwide. They have high content of minerals and vitamins and are consumed as boiled, salad, pickles, cooked curries, fermented (“Kimchi”) and dehydrated vegetable (Butnariu & Butu, 2015) and cover the global estimated production of 26.68 and 71.26 million tons respectively; of which Korea reportedly produces these vegetables 0.089 and 2.5 million tons, accordingly (FAOSTAT, 2018). However, production of these important vegetables is constrained by several insect pests, with the major ones being aphids, diamond black moth, root flies, bugs, loopers, leaf miners, cutworms and beetles. In a desperate move to save their crop from insect damage, farmers have made application of pesticides an imperative routine (Gupta et al., 2015).

The consequence of the extensive use of pesticides has not only led to many insects developing resistance to most of the commonly used pesticides but a health hazard to humans as well (Mengistie et al., 2017). Therefore, pesticides in newer groups of insecticides like neonicotinoids and sulfoximines are reported to be effective against a broad range of insect pests as they do not exhibit cross-resistance (Nugent et al., 2015). Sulfoxaflor (Table 1), a notable sulfoximine, has particularly been showed to have greater efficacy against a majority of the insect pests, including those displaying crossresistance to many of the insecticides currently in use (Sparks et al., 2013).

Table 1.

Chemical structure and physico-chemical properties of sulfoxaflor

The potency of sulfoxaflor against a broad spectrum of insects such as hoppers, whiteflies and aphids is through its action as a nicotinic acetylcholine receptor (nAChR) competitive modulator (Sparks et al., 2013; Watson et al., 2017). Pesticide use in crop protection often results in residues of parent compounds and/or degradation metabolites. In particular, sulfoxaflor is known to degrade to two metabolites in plants: X11719474 and X11721061, though both of which have been showed to have low mammalian toxicity (FAO and WHO, 2011) The pesticides applied are either absorbed and persist in plants or they reach the soil environment which acts as a sink for further degradation and/ or distribution to other environmental compartments (Arias-Estévez et al., 2008). Residual portions of pesticides parent compounds and/or their degraded metabolites may thus be hazardous to the consumers, and as a consequence, regulation of food commodities treated with pesticides is paramount (Hrouzková et al., 2013).

World over, food commodities treated with pesticides are highly regulated using measurement of pesticide residue levels (Hrouzková et al., 2013; Tsakiris et al., 2015). The maximum residue level (MRL) is the tolerable residue level in any commodity set by authorities in a particular country or regions as a means to minimize pesticides intake for good consumer health (Singh & Singh, 2014; Sabarwal et al., 2018). The MRL for a particular pesticide on a given agricultural product is either set by a country/region or the Codex Alimentarius values are used. In Korea, the Ministry of Food and Drug Services (MFDS) sets MRL values for pesticides on different crops based on their toxicological effects and consumption levels. According to the MFDS, the MRLs for sulfoxaflor are 3.0 mg/kg and 0.3 mg/kg in spinach and Korean cabbage respectively, whereas the European Union Commission, MRLs for sulfoxaflor in Spinach and cabbage are 6.0 mg/kg and 2.0 mg/kg respectively (SANTE, 2018). Once applied to crops, pesticide concentration reduces with time as the pesticide dissipates; and since pesticides are applied following regiments like 21-14; 14-7; 10-3 or 7-0 days before harvest, determining a regiment that results in residues below the pesticide MRL is vital. Due to differences in crop morphology and chemical molecular structure, pesticides retention and spatial concentrations tend to differ across different plants (Buchholz and Trapp, 2016). Pesticide residue analysis is routinely carried out on vegetables and other minor crops for establishing MRLs of pesticides in Korea in previous studies (Lee et al., 2012; Son et al., 2012; Jeon et al., 2015).

Various methods have been reported for analysis of sulfoxaflor residues in agricultural commodities. Primarysecondary amine (PSA) and ultra-performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) was used for detection of sulfoxaflor in vegetables, fruits, and soil (Xu et al., 2012). Tian et al., (2016) determined residues of sulfoxaflor in animal-based food using dispersivesolid-phase extraction (d-SPE), a multi -plug filtration cleanup method and UPLC–MS/MS. Several earlier attempts made successful for analysis of pesticides in minor crops in Korea by using modification of QuEChERS techniques (Lee et al., 2012; Kim et al., 2017). In this study, sulfoxaflor and its metabolites were determined using QuEChERS extraction and clean-up with primary-secondary amine (PSA) followed by LC-MS/MS analysis.

For safety purposes therefore, establishing application regiments that result in minimal pre-harvest residues of either pesticide parent compounds and/or the metabolites is vital for consumer safety. Therefore, the current study is aimed to establish a safe pre-harvest day of sulfoxaflor application by maintaining the residue level below the MRL.

Materials and methods

LC-MS/MS Instrumental conditions

The LC-MS/MS analysis system consisted of a Shimadzu LC-MS 8030 with Nexera UFLC coupled to triple quadrupole tandem mass (Shimadzu Scientific Inst. Inc., Japan) equipped with electron spray ionization (ESI) source. A Capcell PAK C18 MG 100 Å 3 μm column (150 × 3.0 mm, Shiseido Co., Japan) was used for separating analytes injected in at 10 μL. A mobile phase comprising of a binary solvent system of 0.1% formic acid in methanol (A) and 0.1% formic acid in water (B) was used in a gradient mode with a flow rate of 0.3 mL/min. The mobile phase starting composition was 1% A (0 – 3 min), ramped to 37% A (3 – 6 min), held at 37% A (6 – 10 min), decreased to 1% A (10 – 13 min), then was maintained at 1% A (13 – 15 min).

The mass spectrometry (MS) conditions were: capillary voltage, 2.0 kV; desolvation temperature 250^oC; collision gas (Ar), 230 kPa; nebulizing gas flow 3.0 L/min; drying gas flow 15.0 L/min; electrospray mode was. The MS measurements were performed in positive electrospray ionization (ESI +) mode with multiple reaction monitoring (MRM) and details of MRM parameters are noted in Table 2. Data acquisition/processing and instrumental control were done using LabSolutions LCMS Version 5.4 for LCMS-8030 software.

Table 2.

Multiple reaction mode parameters to determine sulfoxaflor and its metabolites in spinach and Korean cabbage

Results and discussion

Analysis method validation

Specificity and Linearity

The specificity of the method was assessed by spiking blank samples of spinach and Korean cabbage with sulfoxaflor or its metabolites (X11719474 and X11721061) and comparing with results of samples that did not contain any of the target compounds. The chromatograms of sulfoxaflor (Fig. 1), metabolite X11719474 (Fig. 2) and metabolite X11721061 (Fig. 3) in solvent, fortified and blank samples exhibited absence of interfering peaks at the target compound retention times. Matrix-matched calibration curves linearity was evaluated using a seven level concentration ranging from 0.1 – 10.0 ng/L for both sulfoxaflor and metabolites (Sulfoxaflor, X11719474 and X11721061). Adequate linearity with coefficients of determination R² ≥ 0.9995 was achieved for all tested analytes.

Fig. 1.

Chromatograms of sulfoxaflor in spinach (A) and Korean cabbage (B), (I) matrix-matched standard (1.0 mg/kg); (II) blanks (control); (III) fortified samples (1.0 mg/kg); and (IV) field-incurred samples (7-0 days).

Fig. 2.

Chromatograms of X11719474 in spinach (A) and Korean cabbage (B), (I) matrix-matched standard (1.0 mg/kg); (II) blanks (control); (III) fortified samples (1.0 mg/kg); and (IV) field-incurred samples (7-0 days).

Fig. 3.

Chromatograms of X11721061 in spinach (A) and Korean cabbage (B), (I) matrix-matched standard (1.0 mg/kg); (II) blanks (control); (III) fortified samples (1.0 mg/kg); and (IV) field-incurred samples (7-0 days).

Recovery

The recovery of the analytes (Sulfoxaflor, X11719474 and X11721061) were determined in triplicate to evaluate extraction efficiency. The recovery experiment on both spinach and Korean cabbage consisted of two fortification levels (0.2 and 1.0 mg/kg) for both sulfoxaflor and the two metabolites (X11719474 and X11721061) (Table 3). In all cases, good recoveries of analytes were registered and ranged from 89.0% to 107.9% with standard deviation values of 0.7 – 4.2%. These results are consistent with specified acceptable range of 70 – 120% implying good precision of analysis method used in this study.

Table 3.

Recoveries of sulfoxaflor and metabolites (X11719474 and X11721061) in Korean cabbage and spinach at fortification levels of 0.2 and 1.0 mg/kg

Sulfoxaflor residues analysis

Residues of Sulfoxaflor in spinach and Korean cabbage sprayed two times at different pre-harvest days were determined after conducting recovery tests. Table 4 shows residual levels of sulfoxaflor and its metabolites in spinach and Korean cabbage samples. Maximum residues were recorded in samples sprayed at 7-0 days before harvest (2.97 mg/kg and 1.19 mg/kg in spinach and Korean cabbage, respectively) and 10-3 days (2.16 mg/kg and 0.74 mg/kg in spinach and Korean cabbage, respectively). These residual levels were above the MRLs (2.0 mg/kg) for spinach, and 0.3 mg/kg in Korean cabbage. Overall, residual levels of sulfoxaflor substantially reduced with time following these pre-harvest application regimes (Kim et al., 2017). Morphologically, Korean cabbage has high water mass which is expected to offer more dilution to the pesticide and hence resulting in generally lower concentrations. These results are consistent with early research results (Hanafi et al., 2010; Lee et al., 2012; Bletsou et al., 2013) on the effects of plant shape on pesticide residues. The implication of this is that at the same application rates, sulfoxaflor would dissipate faster in Korean cabbage than spinach.

Table 4.

Residues of Sulfoxaflor in Korean cabbage and spinach aggregated as per the different pre-harvest application regimes

Dissipation of sulfoxaflor in spinach and Korean cabbage

The dissipation of sulfoxaflor in spinach and Korean cabbage was of exponential nature and corresponded to a first order kinetics.

Initial concentration of sulfoxaflor in spinach (2.97 mg/ kg), was higher than that in Korean cabbage (1.17 mg/kg), despite the similarity in application rate (0.07 kg a.i. /ha). This may be due to differences in their physiology and growth habits. Furthermore, Korean cabbage is expected to offer more dilution effect to the pesticide as compared to spinach owing to more water held in its thick tissue mass. It is noted here that the half-lives of sulfoxaflor in the two crops, that is Korean cabbage (3.54 days) and spinach (3.83 days) closely correlates with previous studies (Table 5). Previous studies report the biological half-life of sulfoxaflor in cucumber to be in the range of 1.6-2.9 days (Chen et al., 2016) and 1.5 days in greenhouse lettuce (Kim et al., 2017). The concentration of the pesticides at any given time could be determined using the dissipation equations C = 3.3571e^−0.181t for spinach, and C = 1.2432e^−0.196t for Korean cabbage (Fig. 4).

Table 5.

Biological half-lives of sulfoxaflor in spinach and Korean cabbage

Fig. 4.

Dissipation patterns of sulfoxaflor in spinach (A) and Korean cabbage (B).

Conclusions

In summary, the residue patterns of sulfoxaflor varied in spinach and Korean cabbage under different pre-harvest application regiments. Comparatively, the residual amount of sulfoxaflor were low in Korean cabbage over the pre-harvest period investigated. Due to the high moisture in the fleshy tissue of Korean cabbage that offers a higher dilution effect of the pesticide residue in plants. Residues of sulfoxaflor in spinach at 10-3 days (2.16 mg/kg) and 7-0 days (2.97 mg/kg) before harvest and in Korean cabbage 10-3 days (0.74 mg/ kg) and 7-0 days (1.19 mg/kg) before harvest were above the MRL. Thus, application of sulfoxaflor on spinach or Korean cabbage at 10-3 and 7-0 days before harvest is not safe for consumers. Therefore, the application regimes of 21-14 days and 14-7 days before harvest will be recommended. With regard to terminal residues attributed dissipation and biological half-lives of sulfoxaflor in spinach (3.83 days) and Korean cabbage (3.54 days). The dissipation rate equations for sulfoxaflor in spinach and Korean cabbage can be used to compute pre-harvest residues of sulfoxaflor in either of these crops at any point time and hence ensure safe application.

Acknowledgments

This research was supported by a grant (00-18-8-056300) from Ministry of Food and Drug Safety in 2018.

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