Dissipation of Pesticides in Tea Shoots and the Effect of Washing

Dissipation of residues of dicofol, endosulfan and quinalphos in tea shoots was studied in field experiment conducted at Borbhetta Tea Estate, Tocklai, Jorhat. The pesticides dicofol (Kelthane 18.5 EC @ 0..85 kg ai ha⁻¹), endo sulfan (Thiodan 35 EC @ 0.350 kg ai ha⁻¹) and quinalphos (Ekalux 25 EC @ 0.250 kg ai ha⁻¹) were sprayed separately with hand sprayer to tea bushes at active vegetative growth stage (April-May) using 400 L ha⁻¹ of spray fluid. Samples of new shoots (two leaves and a bud) were plucked at periodic interval and analysed by capillary column gas chromatography. Results indicated that residues of all three pesticides declined rapidly with time following first order kinetics with half-life values of 2.7, 2.1 and 1.3 days, respectively, for dicofol, endosulfan and quinalphos. The residues in tea shoots, 7 days after treatment, were 3.06, 2.84, and 0.29 mg kg⁻¹, respectively. The toxic metabolite of quinalphos, quinalphos-oxon was not detected in any of the samples. Traces of dibenzophenone (DBP), an oxidation product of dicofol, were detected in some samples mostly as surface residues on tea shoots. However, endosulfan treated shoots showed the presence of its toxic metabolite endosullan sulphate from 1 day after treatment. The α-endosulfan dissipated faster (t_½ 1.4 days) on tea shoots than the β-isomer (t_½ 2.62 days). Most of the residues of the three pesticides were present on the surface of tea shoots even after 3–7 days. Washing removed 22–49% of the residues from field treated samples of tea shoots.