Abstract
The population fluctuation of overwintering larvae of Rice stem borer, Chilo suppressalis Walker (Lepidoptera: Pyralidae) was studied on the three rice varieties including, Fajr, Khazar and Nemat in paddy fields of northern Amol in Mazandaran province. The results indicated that the population fluctuation was significantly different on the three rice varieties at statistical level 5%; the highest density was obtained on variety Khazar and the lowest one on Nemat. The results of several samplings in different dates of autumn indicated that there was not any significant difference between the sampling dates. The mean comparison of reciprocal effect "sampling date × variety" on the population density of overwintering larvae indicated that there was no significant different. In attention to the results of this project, firstly the variety Nemat can be a resistant variety because of low density of overwintering larvae on it. Second, overwintering larvae have little replacement to other hosts after finding the suitable and pleasant overwintering shelters, because of any vacillation on population density in different sampling dates. In addition to the population fluctuation of rice stem borer, many natural enemies of overwintering larvae were studied in Amol region. Several predator beetles of the families Carabidae and Staphylinidae, and also many dipteran and hymenopteran parasitoids of the families Braconidae, Ichneumonidae, Sarcophagidae and Phoridae were identified as the efficient factors on decreasing of population density of overwintering larvae.

Key words: Population fluctuation, Overwintering larvae, Chilo suppressalis, Paddy field, Mazandaran, Predators, Parasitoids