Evaluation of population fluctuation of predacious species of syrphid flies (Dip.: Syrphidae) on Cabbage aphid Brevicoryne brassicae in rapeseed fields

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The population fluctuation of predacious species of syrphid flies was studied in rapeseed fields of Kermanshah province in 2011-12. Malais traps were used for collecting syrphid flies and larvae were collected and reared in laboratory. For sampling cabbage aphid, number of infested foliage as well as rate of infestation were assessed on five randomly selected foliage belonging to 10 plants. The surveys were done both in sprayed and none-sprayed fields. Overall 11 syrphid species were collected and identified in these fields. Eupeodes corollae was regarded as predominant species. This study indicated that the syrphid flies spent two generations from April to July. The study showed that Brevicoryne brassicae had only one peak at the same duration. The peak of aphid population was occurred just after the first peak of syrphid population. At this time syrphid flies were observed as larvae and egg stages in the field. In sprayed field populations of both syrphid flies and cabbage aphid reduced following spraying and increased again after a while, but the population level was not as high as that of the none-sprayed field.

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