The use of pesticides is one of the essential measures of modem agricultural practices in protecting the crops from different pests. On the other hand the residues resulting from the misuse of pesticides on vegetables and fruits is a major concern in many countries as well as in Pakistan. However, the hazards to health can be minimized to a great extent if these residues are kept below their prescribed safe level (Maximum residue limit). A field survey was conducted in the major growing areas of fruit and vegetables in Peshawar regarding the use of insecticides by local farmers on these crops. On the basis of questionnaires, 105 (70 vegetable and 35 fruit) farmers were interviewed concerning the types of insecticides used, the frequency and doses of application, and the time of insecticide application prior to harvest. Fifteen insecticides belonging to different groups were found to be commonly used on fruits and vegetables by the respondent farmers. Of these, the most frequently used insecticide was cypermethrin followed by methamidophos and chlorpyriphos.
In order to assess the residue levels of insecticides in fresh fruit and vegetables, samples were collected at harvest from the fields/orchards of the farmers at about 100 different sites in Peshawar. The analysis of insecticides was limited to the treatment history obtained from the vegetable and fruit grower. The samples were extracted in ethyl acetate. The extracts were cleaned up through Florisil column with ethyl acetate as eluent. Four HPTLC techniques namely, (i) o-toluidine + KI (ii) aluminum oxide G incorporated with silver nitrate (iii) enzyme inhibition with cow liver extract and β- naphthyl acetate substrate and (iv) enzyme inhibition with horse blood serum and acetylthiocholine iodide substrate were used for residue analysis. Six hundred and eight samples of vegetables and fruits were analysed using these HPTLC methods. The most commonly detected residues were those of methamidophos (9.8% of 608 samples), cypermethrin (8.5%), endosulfan (4.9%), chlorpyriphos (4.4%), trichlorfon (3.3%), methidathion and methomyl (2.8%), dimethoate (2.6%) and λ-cyhalothrin (1.8%) depending on the type of insecticide being used by the grower. Of all analysed fruit and vegetable samples (608), 250 samples (41%) contained detectable residues. Of these, 13.8% had residues that exceeded Codex maximum residue limits (MRLs). For individual crops, contaminated samples ranged from 10 to 100 % of the number of samples analysed.
Six supervised trials were conducted on three vegetables, viz. tomato, okra and cauliflower and three fruit crops, viz. guava, apple and peach during 2000-02 to study residue dissipation rates of eight insecticides after application to these crops under agro climatic conditions of NWFP. The insecticides selected for field experiments were five organophosphorus insecticides (methamidophos, dimethoate, methidathion, trichlorfon and chlorpyrifos); and one each of pyrethroids (cypermethrin), carbamates (methomyl) and organochlorines (endosulfan). Experiments on okra, cauliflower, tomato, and peach were conducted at the farm of the Agricultural Research Institute, Tarnab, Peshawar, while experiments on guava and apple were conducted on farmers' orchards in Kohat and Swat, respectively.
For each vegetable, triplicate (2 kg per field replicate) samples were randomly collected from supervised trials at 0 (1 hr after spray), 1, 3, 5, 7, 10 and 15 days after insecticide application. The treated fruits of guava, apple and peach were randomly sampled in triplicates (2 kg per field replicate) at intervals of 0, 3, 7, 14, and 21 days after treatment. The samples were extracted in ethyl acetate and were cleaned up through Florisil column with ethyl acetate as eluent. The cleaned up extracts were analysed for residues with both HPTLC and HPLC methods. Recoveries of the 8 insecticides from 6 crops fortified at 0.1-3.0 mg kg-1 were 59-103 %.
The number of days for residues to drop below tolerance limits was highly dependent on the magnitude of the initial residue. Large variations in initial residues were observed between years, between insecticides and between crop types. Organophosphate residues were found to have half-lives of 9.0 to 12.0 days for methamidophos, 6.8 to 8.8 days for dimethoate, 13.3 to 26.0 days for trichlorfon, 3.6 to 14.8 days for chlorpyriphos and 7.5 to 15.2 days for methidathion. The half-life of cypermethrin was 6.0 to 18.5 days, for methomyl it was 1.5 to 3.4 days and for endosulfan it was 3.7 to 4.6 days. Based on the dissipation pattern of residues from different insecticides in relation to their respective prescribed maximum residue limits, withholding periods of 11-13 days for cypermethrin and of 2-3 days for methomyl for tomato; 10-15 days for cypermethrin and 3-4 days for endosulfan for okra; and 9-12 and 7-9 days for cauliflower after treatment with methamidophos and cypermethrin were suggested. Withholding periods of 8-10 and 23- 26 days were suggested for peaches after treatment with chlorpyriphos and trichlorfon. Apples treated with methidathion and chlorpyriphos, exhibited withholding periods of 12-15 and 7-14 days respectively. The withholding periods were respectively 13-14 and 7-8 days for guava after treatment with trichlorfon and dimethoate.