The Colorado Potato Beetle, Leptinotarsa decemlineata (Say), had been a quarantined pest in Iran until 1984. It was observed in Ardebil region in 1984 for the first time spreading into the Arasbaran region of the East Azerbaijan since 1996. In this study the pest densities, its seasonal variation and its biology were studied through weekly recording of the number of individuals in all stages of development on each of 50 randomly selected plants in 5 fields in Ahar and Varzeghan regions (Arasbaran) of the East Azerbaijan province in year 2000. The overwintered adults were first noticed on potato plants on the 5th of May in Ahar region. Oviposition was initiated from 11th of May. The first generation adults, egg masses, larvae and first generation adults on potato plants were observed on the 31th of May, 28th May, 5th June and 11th July respectively in Ahar and on the 15th June, 16th June, 13th June and 24th July in Varzeghan respectively. In both areas the pest produced two generations per year. The development periods for the first and second generations were estimated to be 48 and 40 days respectively. In Ahar the mean density for the overwintered adults, egg masses and larvae of the first generation were 0.183, 0.62 and 11.68 per plant respectively. The mean density for the first generation adults, egg masses and larvae of the second generation per plant were 0.50, 0.83 and 8.20 respectively. In Varzeghan, the mean density of the overwintered adults, egg masses and larvae of the first generation were 0.20, 0.30 and 9.68 per plant respectively. The mean density for the first generation adults, egg masses and larvae of the second generation per plant were 0.65, 0.58 and 7.20 respectively. The biology and population trend for the pest were closely similar in both study areas.
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