Spatial distribution pattern of pea aphid, Acyrthosiphon pisum and predatory ladybirds Coccinella septempunctata and Hippodamia variegata in alfalfa fields of Hamedan

The distribution pattern of pest population is one of the factors not only effects on sampling program and data analysis method, but also can be used to measure the density of pests and their natural enemies. Thus, the spatial distribution pattern of the pea aphid, Acyrthosiphon pisum Harris (Hem.: Aphididae) and its two major predators, Hippodamia variegata Goeze (Col.: Coccinellidae) and Coccinella septempunctata (Linnaeus, 1758) (Col.: Coccinellidae) was investigated by different dispersion indices through 2012 and 2013 growing seasons. Dispersion pattern was determined by using Taylor's power law, Iwao's patchiness regression method and variance to mean ration test. Obtained results showed an aggregated distribution pattern of pea aphid and lady beetles. Based on R2 and p-value of regression analysis, Iwao's patchiness regression model provided a slightly more adequate description of variance/mean relationships than Taylor's power law. Among the species studied, the pea aphid adults and H. variegata showed the highest and C. septempunctata showed the smallest coefficients of Taylor's power law respectively. These results provide a reliable basis to develop efficient sampling plans for estimating aphid and their natural enemies populations