Situation of the Diamondback Moth (DBM), Plutella xylostella and Its Parasitoids in the Cauliflower Fields of Tehran

Aims: The diamondback moth, Plutella xylostella (L.) (Lep.: Plutellidae) is one of the most important pests of cruciferous plants. Amongst the most important factors to reduce this pest population are parasitoids. The aim of present study was to Identify the parasitoids of the diamondback moth (DMB), computating the density of DBM stages and its parasitoids and their performance in different cauliflower fields of Tehran. Also the objective of this study was to elucidate the reactions of DBM parasitoids to host density. Correlation between different larval densities and parasitism rate of DBM parasitoids were calculated using a recall method in field conditions.
Place and Duration of Study: Place – Department of Plant protection, Faculty of Agricultural Sciences, Shahed university, Tehran, Iran. Duration – June, 2011 to November, 2012.
Methodology: Sampling was carried out every two weeks from late June until early November. From each field 20 plants were selected. All larval instars and pupae on each plant were collected, and were counted and recorded separately for each field. Also, collected DBM larvae and pupae were kept on the host plant under controlled conditions (60 ± 5% RH, 27 ± 2 °C and 14L:10D photoperiod). In order to do recall of larval parasitoids, different densities of 5, 15, 25 and 35 larvae were placed on each plant. For each density five plants were considered as five replication. Analysis of variance (ANOVA) was used to analyze the data on numbers of DBM larvae and pupa in different regions as well as the mean number of parasitoids and parasitism rate. The means were compared at the 5% level of significance using the Duncan studentized range test.
Results: In all studied areas, the larvae, pupae and the total number of larvae and pupae were determined as 3.05, 2.26 and 7.54 (per plant), respectively. The percentage of parasitism by D. anurum, C. plutellae and O. sokolowskii in all regions and total parasitism by these three species were determines obtained as 12.67, 10.29, 13.27 and 36.23% per plant, respectively.
Conclusion: The number of larval and pupal stages based on unit density, was significantly different between all experimental regions and Shahed university station. The highest percentage of parasitism was caused by D. anurum in Tehran. Spraying did not reduce the DBM population in some fields in Tehran, but repeated spraying caused a reduction in the activity of the parasitoids of DBM.