Within and between field variability of residue data and sampling implications

Information on the variability of residues in individual fruits and vegetab les is required to estimate the acute dietary exposure to pesticides. The d istribution of residues in apples, kiwi fruits, potatoes and butter beans w as studied in field experiments representing commercial farming practice. N o correlation was found between the residue concentration or surface residu e and the mass of apples. The relative frequency distributions of residues in crop units were continuous and skew positive. The log-normal transformat ion did not result in a normal distribution in any of the trials. Consequen tly, 299, 120 and 59 random samples should be analysed to estimate 99th, 97 .5th and 95th percentile of the residues at 95% confidence level. The distr ibution of residues is not significantly influenced by the mean residue, pr e-harvest interval, chemical and physical properties of the active ingredie nt, formulation of pesticide or, on top fruit, the foliar application metho d. However, the residue distribution is likely to be influenced by the size , shape and density of the plants, and mode of application of pesticides. T he variability factor should be defined as the ratio of the highest value a t a specified percentile of residues occurring in unit crops and the popula tion mean. Generic variability factors may be determined for various groups of commodities. Variability factors of 5 and 9 are recommended for medium size fruits, and potato following granular application of pesticides, respe ctively.