Why Are My Kale Leaves Covered in Black Aphids?

Finding the underside of your kale leaves — or the growing tip of your spinach — smothered in a dense colony of small dark insects is an alarming sight. Aphid colonies on brassicas and spinach can build up with remarkable speed in warm weather, and a plant that was clean one week can be heavily infested the next. The good news is that aphids on established plants are rarely fatal, and there are effective controls that work without harsh chemicals.

Which aphids attack kale and spinach?

The mealy cabbage aphid (Brevicoryne brassicae) is the most damaging on kale — a grey-green or waxy-white aphid that forms dense colonies on the undersides of leaves and around growing points. It reproduces very rapidly and heavy infestations cause leaves to curl, distort and yellow. It is most active from late spring to autumn. Spinach may also be attacked by green peach aphid and occasionally black bean aphid. All cause similar symptoms: sap removal, sticky honeydew, and secondary sooty mould that blocks light from the leaves.

How to control aphid colonies

A strong jet of water directed at aphid colonies knocks large numbers off the plant quickly and effectively. Repeat after two or three days as remaining aphids recolonise. Insecticidal soap spray applied to the undersides of leaves kills aphids on contact and is safe for use on edible crops. Avoid spraying in bright sunlight or when bees are active. For heavy, persistent infestations, neem oil or pyrethrin-based sprays provide stronger knockdown, but both should be used sparingly and in the evening.

Natural predators and prevention

Ladybird adults and larvae, lacewing larvae, and parasitic wasps all feed on aphids. Avoiding broad-spectrum pesticides encourages these natural predators to build up and provide ongoing control. Planting umbellifers (fennel, dill, parsley) nearby attracts these beneficial insects. Kale grown under fine mesh netting to exclude cabbage white butterflies also significantly reduces aphid populations, since the enclosed environment is less accessible. Regularly checking the undersides of leaves from late spring onward catches colonies when they are small and much easier to deal with.

After harvest, the aphids on the leaves

Aphid colonies on kale leaves make the harvest look unappealing but the kale is perfectly edible. Submerge harvested leaves in cold water and swish thoroughly — aphids float off easily. Multiple washes ensure all insects are removed before cooking. This is one reason to pick outer leaves regularly rather than leaving them to build up a large standing colony undisturbed through the whole season.