Field trials were conducted in Guangzhou, Nanning, and Nanjing in two consecutive years to evaluate the terminal residue levels and dissipation trend of pymetrozine and chlorpyrifos in rice ecosystem. Analyses were carried out by high-performance-liquid-chromatography for pymetrozine and gas-chromatography-mass-spectrometry for chlorpyrifos, achieved good linear relationship over range from 0.01 to 5.0 mg·kg-1 for both (r > 0.9998). Average recoveries were 86.0% to 106.0% for pymetrozine, and 79.7% to 102.3% for chlorpyrifos at the spiking levels of 0.01, 0.1 and 1.0 mg·kg-1. Half-lives of pymetrozine in paddy water, paddy soil and rice plant were 0.35-2.81, 2.69-6.95 and 1.22-3.70 days, while that of chlorpyrifos were 0.86-1.88, 3.09-6.86 and 0.58-2.84 days. Final residues of pymetrozine and chlorpyrifos in brown rice ranged from less than 0.6 to 26.0 μg·kg-1 and 14.3 to 191.6 μg·kg-1, respectively. It is recommended that 25% pymetrozine and chlorpyrifos suspension be sprayed twice at the intervals of 10 days with dosages ranging from 375 (maximum recommended dosage) to 562.5 g a.i.·ha-1 (1.5 times of the maximum recommended dosage). The rice can be harvested safely 15 days after the last application of pymetrozine and chlorpyrifos. The research results help ensures the safe application of pymetrozine and chlorpyrifos in rice ecosystem.
Keywords: Pesticides residues; field trial; gas chromatography-mass spectrometry; high performance liquid chromatography.