Study of the spatial distribution of a pest and its natural enemies provides better decision tool for integrated pest management. The spatial distribution and seasonal activity of Panonychus ulmi (Koch) and its predator Zetzellia mali (Ewing) were studied in an apple orchard in Khoramdareh (Zanjan Province, Iran) during 2007. The interaction (density dependence) between the prey and its predator was determined. For P. ulmi RV (relative variation) and reliable sample size were calculated with 25

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... percent variation from primary sampling at 18.8% and 59 leaves, respectively. The distribution pattern of both species was analyzed using nine statistical formulae: Taylor's power law, Iwao's patchiness regression, index of dispersion, Morisita's coefficient of dispersion, Lloyd's mean crowding, David and Moore's index, Cole's index of dispersion, Green's index and coefficient of 'K' (coefficient of aggregation). The results indicated that the highest population density of P. ulmi and Z. mali were on 11 August (23.92 per leaf) and 11 September (8.2 per leaf), respectively. The slopes of Taylor's power law and Iwao's patchiness regression methods were 1.82 and 2.18 for prey and 1.39 and 1.92 for predator, respectively. These slopes had significant difference from one, indicating aggregated spatial distribution in prey and predator. The index of dispersion (I D) showed that the spatial distribution of P. ulmi in apple orchards was aggregated but it showed random distribution for Z. mali. The Morisita's coefficient, Lloyd's mean crowding and Green's index showed an aggregated distribution for both species. The regressions between population densities of P. ulmi and Z. mali indicated a density independent reaction of predator to the prey. The effect of temperature and humidity on the prey and predator populations was estimated. Spatial distribution parameters of the prey and predator can be used in integrated pest management programs.