Why Are Clouds of Whitefly Rising from My Brassicas?

When you brush against a Brussels sprout plant and clouds of tiny white insects rise up, you are dealing with cabbage whitefly — Aleyrodes proletella, a species specific to brassica crops and different from the glasshouse whitefly that attacks tomatoes and cucumbers. Cabbage whitefly is a persistent and common pest of outdoor brassicas throughout summer and autumn. Heavy infestations cause significant leaf distortion, sticky honeydew deposits, and the development of sooty mould on the leaves below.

Identifying cabbage whitefly

Adults are tiny — about 1.5 mm — white-winged insects that fly up in clouds when disturbed. The immature stages (nymphs and pupae) are flat, scale-like, and found on the undersides of the leaves — they are greenish-white and slightly waxy in appearance. Both adults and immature stages feed by sucking sap from the leaf undersides, excreting sticky honeydew as they do so. This honeydew coats the leaves below and quickly becomes colonised by black sooty mould fungi, which give the plant a dirty, unhealthy appearance.

Impact on the crop

Light infestations cause minimal harm to established plants. Heavy infestations weaken plants, particularly young or newly transplanted ones, and the honeydew and sooty mould reduce photosynthesis. The honeydew also makes harvested sprouts unpleasant to handle. In very severe cases, young plants can be seriously checked in their growth. Cabbage whitefly overwinters in the UK and northern Europe, sheltering among old brassica leaves, and populations build through summer.

Control measures

Physically removing heavily infested leaves and disposing of them is a useful first step. A strong jet of water directed at the undersides of leaves dislodges adults and nymphs. Insecticidal soap or plant-derived pyrethrin sprays can be applied to the undersides of leaves and repeated weekly — coverage of the leaf undersides is critical as this is where the insects feed. Encouraging natural predators — particularly parasitic wasps — by growing flowering plants nearby helps reduce populations over the longer term.

Garden hygiene to reduce overwintering populations

After harvesting brassica crops, remove all old stems and leaves promptly. Cabbage whitefly overwinters as adults on old brassica debris, and removing this removes a major part of the overwintering population. Avoid leaving old brassica stumps in the ground over winter. Crop rotation reduces the buildup of any overwintering whitefly that persists in the soil or at the soil surface.