Effects of organic and conventional fertilization on the occurrence of Brevicoryne brassicae L. and its natural enemies in white cabbage

Recent years have witnessed a rapid growth in organic farming in Lithuania. We aimed to compare the occurrence of Brevicoryne brassicae L., its primary parasitoid Diaeretiella rapae (McIntosh) and predators from Coccinellidae and Cecidomyiidae families in non-fertilized, organically (with cow manure) and synthetically fertilized white cabbage crops. Occurrence of B. brassicae was higher on organically fertilized plants. Parasitoid D. rapae had the strongest effect in reducing the aphid population, as other predators occurred only in small densities. Quantity of parasitized aphids by D. rapae was higher on manure fertilized plants. In non-fertilized cabbage plants seasonal parasitization per cent of B. brassicae by D. rapae increased along with the expected lower aphid pressure compared with fertilized plants and reached on average 28.5%. D. rapae reduced the population of cabbage aphid by on average 15.5% in manure fertilized and −12.9% in synthetically fertilized plants. The regression showed a positive relationship between the increase of the number of B. brassicae and activity of D. rapae. No coccinellids were found in aphid colonies on synthetically fertilized plants. Different fertilization did not have any significant effect on the incidence of Aphidoletes aphidimyza Rond. In conclusion, organic fertilization increased the population of cabbage aphid more than its natural enemies and the abundance of natural enemies was too small to provide effective reduction of this pest.