Mythimna genus moths are some of the most important pests of gramineous plants’ especially cereals, that can substantially reduce crop yields by feeding on leaves and on ears. The larvae, called "armyworm" with gregarious feeding at different larval instars and in favorable conditions can destroy a rice field within a short time. Their damages are seen periodically and in some years in the west of Mazandaran are such that can cause irreparable damage to rice fields. In comprehensive studies on population dynamics of Mythimna species during 1999 - 2001, three species namely, Mythimna unipuncta (Haworth), Mythimna pudorina (Denis & Schiffermuller) and Mythimna vitellina (Hübner) were collected and identified. Among them, M. unipuncta as the dominant species can cause the main damage. Larval and adult population fluctuations showed that M. unipuncta bears three generations per year with the maximum larval activity occurring in July, August and September. The second generation larval activity was the highest which can cause the greatest damage. Developmental period under insectarium conditions was 48.27, 37.48, and 44.44 days for first to the third generations, respectively. Head capsule size measurements indicated that each generation has six larval instars. Cumulative survival is gradually reduced from first larval instar, so that only a few number of larvae can reach adult stage. Obtained data from Robinson light trap showed that 3 flight peaks, in early July, middle of August and September, can be seen that respectively belong to the first, second and third generations of pest.
)