Blackfly and greenfly (aphids) are very common sap sucking pests affecting nearly all types of plants. Colonies are visible around new buds or shoots in spring onwards. Leaves may twist or yellow and become puckered and distorted as they develop. Not really a significant pest of brassicas.
Product solution: Use Provado Ultimate Bug killer ready to use, Provado Ultimate Fruit and Vegetable Bug Killer - ready to use / concentrate, Sprayday Greenfly Killer, Organic Bug Free, Natria Bug Control.
Cultural control: Brassica plant can be netted or protected with fleece until established and to protect against the cabbage white butterfly laying clusters of eggs on the brassica crop.