Population Density and Spatial Distribution Pattern of Thrips tabaci (Thysanoptera: Thripidae) on Different Soybean Varieties

Population density and spatial distribution of Thrips tabaci Lindeman on seven soybean varieties (Williams, Tellar, Sahar, Dpx, L17, Sari and Zane) and one genotype (Ks3494) were studied in Tehran region, during 2007. The highest population density of the thrips per leaf was recorded on Dpx and on Ks3494 (0.810.05 and 0.800.05, respectively). The lowest population density was observed on L17 and Tellar (0.620.04 and 0.640.03, respectively). To estimate the spatial distribution pattern of this pest, data were analyzed through index of dispersion, Lloyd's mean crowding, Morisita's index as well as through regression models (Taylor and Iwao). The index of dispersion and Lloyd's mean crowding indicated an aggregated pattern for the spatial distribution of this insect in all the varieties and the genotype of soybean. Spatial distribution of T. tabaci using Morisita's index was aggregated in most of the sampling dates. In Taylor's model, regression between log S2 and log m was not significant for Dpx varieties (P> 0.05), but the b values of Taylor's power law on Zane, Sahar, Williams and Tellar varieties exceeded 1, indicating aggregated distribution. However, the other varieties had a b value equal to 1, indicating random distribution. Iwao's patchiness regression indicated that the spatial distribution of T. tabaci on Sari, Zane, Williams and Tellar varieties as well as on Ks3494 genotype was aggregated, but on the rest of varieties the pattern was of a random one. It is concluded that soybean varieties affect the population density and spatial distribution of T. tabaci. Spatial distribution parameters of this species can be employed to outline a sampling program as well as to estimate the population density of T. tabaci.