Abstract: 【Objective】 The objective of this study is to study the degradation dynamics of abamectin in cucumber and soil, and to formulate a scientific basis for the reasonable dose and the pre-harvest interval on controlling cucumber root-knot nematode with hole application. 【Method】 Degradation dynamics experiments were conducted by application abamectin once on cucumber and soil with hole application at 5.62 g·m^-2(1.5 times recommended dosage) after transplanting cucumber seedlings. Degradation dynamics samples were collected and determined at random from each plot at different time intervals i.e, 0 day (2 h after application), 1, 3, 5, 7, 10, 14, 21, 28, and 45 days after the abamectin application. Two doses were set in terminal residue experiment. Abamectin was applied once at 3.75 g·m^-2 (low concentration, recommended dosage) and 5.62 g·m^-2(high concentration, 1.5 times recommended dosage) after transplanting period of cucumber seedlings. Terminal residue samples were collected and determination of abamectin during the harvesting periods. The dispersive solid phase extraction-high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was used to determine the abamectin residue in cucumber and soil. The homogenized cucumber samples, plant samples or soil samples (10 g) were firstly weight into a polypropylene centrifuge tube. After adding 4 g Na₂SO₄, and 1 g NaCl, the sample was extracted by 10 mL acetonitrile. Secondly, the extraction solution was sonicated and centrifuged for several minutes. Then 2 mL of the clarified supernatant was transferred into a clean centrifuge tube containing 50 mg of C₁₈ and 50 mg PSA dispersive solid phase extraction (DSPE) sorbents for purification. Finally, the solution filtered through a 0.22 μm membrane prior to LC-MS/MS analysis. 【Result】The established method for determination of abamectin in cucumber and soil with DSPE combined LC-MS/MS was rapidly and easy of operation. The recoveries of abamectin in cucumber and soil were 78%-101% with RSD between 1.9% and 9.4% with spiking at 10 μg·kg^-1, 50 μg·kg^-1 and 100 μg·kg^-1. The limit of quantitation (LOQ) of this method was 10μg·kg^-1, which could meet the requirement of max residue limits (MRLs) of abamectin. In degradation dynamics experiments, abamectin was not detected in all degradation plant samples collected from in Beijing and Harbin, which suggested that it is not a systemic pesticide. However, abamectin was detected in all soil samples in degradation experiment. The degradation dynamic equations of abamectin in soil were fitted the first-order reaction dynamic equations, and the half-lives of degradation of abamectin were estimated at 7.9 d and 18.7 d in soil. The residue period of abmectin was longer with hole application compared to that with spray application in soil. Abmectin residue was detected in three soil samples collected from Beijing in 2012 with concentration of 10 μg·kg^-1, 30 μg·kg^-1 and 170 μg·kg^-1, respectively. And similarly, abmectin residues were found only in two soil samples collected from Harbin with concentration of 10 μg·kg^-1. 【Conclusion】The abamectin sustained release granules (1%) applied with hole application at 3.75 g·m^-2 can be considered safe in cucumber planting.

Key words: abamectin, high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS), dispersive solid phase extraction, degradation, residue, hole application