Seasonal Population Fluctuations of the Diamondback Moth, Plutella xylostella (L.) (Lep.: Plutellidae) in Cauliflower Fields of the South of Tehran

The diamondback moth، plutella xylostella (Linnaeus، 1758) (Lep.: Plutellidae) is the most destructive insect pest of crucifer plants throughout the world. In order to study its seasonal population fluctuation، sampling was conducted in the cauliflower fields of south of Tehran from late may until October 2011. One hectare cauliflower field in the middle of a large field was randomly selected in Jahan-abad، Kahrizak، Shokr-abad and Palayin regions. 600 m2 fields in Collage of Agricultural Sciences، Shahed University، Tehran، Iran was selected، this field was not treated with insecticides. Each plant was presumed as a sampling unit and sample size was determined as 20 host plants. Sampling was carried out every 10 days and all eggs; larvae and pupa on the host plant were recorded. The peak of egg population (42. 30 eggs per plant) was observed in all regions at October 25th. The highest density larval and pupal density were recorded 6. 68 (larvae per plant) and 4. 92 (pupa per plant)، respectively in all regions at October 25th. The highest density of total stages was recorded in all regions at October 25th (53. 90 insect per plant) and lowest density of total stages was seen at June 19th (4. 86 insect per plant). The results showed that number of larval and pupal stages based on units density، between all experimental regions and Shahed university station had significant difference.