The seasonal fluctuation of Brevicoryne brassicae) L.) population and its parasitoied, Diaeretiella rapae McIntosh

The cabbage aphid Brevicoryne brassicae (L.) is one of the important pests of the Brassicae napus. Some natural enemies such as the parasitoid wasp Diaeretiella rapae are active on this pest. The seasonal population changes of the cabbage aphid, was studied on canola by weekly sampling.The aim of this study was to determine peak of cabbage aphid and its seasonal fluctuation and the percent parasitism for ovulated of efficiency of D. rapae. Samples were taken weekly from 20 middle leaves of the plant at the rosette stage and 20 stems at the later stages. The number of adults and nymphs of the aphid were counting the aphids they recorded in prepared tables and the results indicated the maximum number of aphid host was observed in 16 April 2011 and the average population was 259 ± 13/65 aphids (total nymphs and adults) per stem when the weather warmed and reduced pest population the humidity reduced in vegetation of plants. The parasitism activity of D. rapae on this aphid started from early February and average parasitism was 14/18± 2/98 % and maximum 41/33 ± 3/53% that observed in 7 may 2011.