Population density and spatial distribution patterns of Tetranychus urticae (acari, tetranychidae) and its predator Stethorus gilvifrons (coleoptera: coccinellidae) on different agricultural crops

Abstract

Population density and spatial distribution pattern of Tetranychus urticae Koch and its predator Stethorus gilvifrons Mulsant on different agricultural crops including cucumber (var. Superdaminus) and five bean varieties including Phaseulus vulgaris var. Talash, P. lunatus var. Sadaf, P. calcaratus var. Goli, P. calcaratus var. Sun-ray and Vigna sinensis var. Parastoo were determined in the Tehran area, Iran, during 2005. The mean population density of the adults and total life stages of T. urticae on cucumber (150.71 mites per leaf) was significantly more than bean crops. Among bean varieties, the higher and lower density of the pest population was observed on Sun-ray (59.37 mites per leaf) and Parastoo (4.73 mites per leaf), respectively. Spatial distribution pattern of T. urticae was determined on different crops using Morisita’s index and Taylor’s power law regression method and spatial distribution of S. gilvifrons was measured using variance to mean ratio. The slope of Taylor’s regression for T. urticae was 1.60, 1.89, 2.05 and 1.35 on Talash, Goli, Sadaf and Sun-ray, respectively. Morisita’s index for T. urticae was significantly greater than one for all crops (Iä>1). The variance to mean ratio for S. gilvifrons was calculated 1.64, 1.90 1.32 and 11.54, on Talash, Sun-ray, Parastoo and cucumber, respectively. The results indicated that the spatial distribution pattern of T. urticae was aggregated and this pattern for S. gilvifrons in most cases was aggregated and in a few cases was random. Linear regression between density of T. urticae (independent variable) and density of S. gilvifrons (dependent variable) was used for determining the reaction of S. gilvifrons to the population density of T. urticae. The reaction of S. gilvifrons to density of T. urticae on Sun-ray variety was density independent and on other crops was density dependent. It could be concluded that different crops can influence the population density of T.urticae and spatial distribution pattern of S. gilvifrons. Spatial distribution parameters can be used to promote sampling programs and exact estimation of the population density of an organism.