Effects of Thiamethoxam and Fenvalerate Residue Levels on Light-Stable Isotopes of Leafy Vegetables

Abstract

Accurate identification of the rational and standardized use of pesticides is important for the sustainable development of agriculture while maintaining a high quality. The insecticides thiamethoxam and fenvalerate and the vegetables spinach, cabbage, and lettuce were used here as study objects. Descriptive analysis and primary reaction kinetic equations were used to analyze the changes in metabolic residues of the two insecticides after different numbers of application in three vegetables. The effects of pesticide residue levels on the δ¹³C, δ¹⁵N, δ²H, and δ¹⁸O values of vegetables were analyzed by one-way analysis of variance and correlation analysis. Partial least squares discriminant analysis (PLS-DA) was applied to build discrimination models of the vegetables with different pesticide residues based on stable isotopes. The results showed that the first degradation residues of thiamethoxam and fenvalerate in spinach, cabbage, and lettuce conformed to primary reaction kinetic equations, but the degradation half-lives were long, and accumulation occurred in the second application. The differences in the four stable isotope ratios in the control group of the three vegetables were statistically significant, and two-thirds of the stable isotope ratios in the three vegetables with different numbers of pesticide applications were significantly different. The δ¹³C and δ¹⁵N values of spinach, the δ¹³C, δ¹⁵N, and δ²H values of cabbage, and the δ¹³C, δ¹⁵N, δ²H, and δ¹⁸O values of lettuce were significantly correlated with different residues of thiamethoxam and/or fenvalerate applications. The control groups of the three vegetables, spinach-thiamethoxam-first, spinach-thiamethoxam-second, cabbage-thiamethoxam-second, cabbage-fenvalerate-first, and lettuce-thiamethoxam-first, were fully identified by PLS-DA models, while the identification models of other vegetables containing pesticide residues still need to be further improved. The results provide technical support for identifying the rational use of pesticides in vegetables and provide a reference method for guaranteeing the authenticity of green and organic vegetables.

Keywords: PLS-DA; cabbage; fenvalerate; lettuce; light-stable isotopes; relationship; spinach; thiamethoxam.

Figure 1

Structural formulas of thiamethoxam and fenvalerate.

Figure 2

Degradation curve of thiamethoxam and fenvalerate for the first (a) and the second (b) applications in spinach, cabbage, and lettuce.

Figure 3

Distribution of stable isotopes in the three vegetable control groups. “×” represents mean values; “-” represents median values; different letters within a row indicate a significant difference for each region (p < 0.05).

Figure 4

PLS-DA score plots (a) and VIPs (b) for spinach, cabbage, and lettuce with different pesticide residue levels.