Dissipation pattern of insecticides in spring onions and residue assessment in onion bulbs and soil at harvest

A field study was conducted to investigate the dissipation behaviour of seven insecticides, viz., fipronil, tolfenpyrad, spinetoram, isocycloseram, spinosad, profenofos and deltamethrin in spring onion and to assess their residue levels in onion bulbs and soil at harvest. The results revealed that the initial residues of fipronil, deltamethrin, tolfenpyrad, spinetoram, isocycloseram, spinosad and profenofos were 2.3, 0.4, 2.8, 0.9, 1.6, 2.6 and 9.9 mg kg−1, respectively. The persistence of the insecticides varied considerably, with half-life values of 3.4, 9.3, 1.5, 2.2, 3.4, 0.8 and 3.8 days, respectively. A pre-harvest interval (PHI) of 10 days is proposed for all the insecticides except for tolfenpyrad. Residue analysis of mature onion bulbs collected at harvest (45 days after the second spraying) revealed no quantifiable residues of any of the tested insecticides. In soil samples collected at harvest, only fipronil residues were detected (0.06 ± 0.002 mg kg−1), while residues of the remaining insecticides were below the limit of quantification of 0.01 mg kg−1. The findings indicate effective dissipation of the tested insecticides under field conditions. However, as the residues of fipronil were detected in soil at harvest and those of tolfenpyrad persisted till 20 days after application, use of these two pesticides may not be recommended, as these may pose risk for environment and human health, respectively.