Why Is There a Cloud of White Flies Under My Cabbage Leaves?

Lifting a cabbage leaf and being met with a cloud of tiny, white-winged insects flying up is the characteristic behaviour of brassica whitefly (Aleyrodes proletella). This is a specific whitefly of brassicas and related plants — distinct from the greenhouse whitefly (Trialeurodes vaporariorum) that affects tomatoes and cucumbers. The brassica whitefly is a widespread outdoor pest in UK gardens, particularly on cabbages, Brussels sprouts, and kale from late summer through winter. Both adults and nymphs suck sap from the leaf underside and produce honeydew, which leads to sooty mould development.

Is brassica whitefly seriously damaging?

On healthy, established cabbages, moderate brassica whitefly populations cause cosmetic sooty mould on leaves and some minor reduction in vigour — they rarely threaten the plant's survival or the edibility of the head. The main practical problem is the nuisance of clouds of whitefly rising from the plants when they are handled, and the black sooty mould deposited on the outer leaves. In high populations on smaller, younger plants, more significant sap removal and leaf damage can occur. The pest is almost impossible to eliminate entirely from an outdoor brassica crop once established.

The lifecycle complicates control

Brassica whitefly overwinters on brassica plants as adults and nymphs, making year-round persistence the norm in gardens with a continuous brassica presence. Nymphs (flat, oval, translucent scale-like stages) are resistant to most contact sprays. Only adults and newly hatched crawlers are susceptible. This means that a single application rarely gives good control — multiple applications are needed to target successive adult generations. Removing and destroying old brassica stumps at the end of the season breaks the overwintering cycle for that site.

Management options

Yellow sticky traps capture significant numbers of adults and reduce populations — useful as a monitoring and suppression tool. Insecticidal soap applied to the leaf underside kills adults and crawlers on contact — three applications at weekly intervals target successive generations. Encouraging natural predators (small parasitic wasps naturally parasitise whitefly nymphs in outdoor conditions) provides ongoing biological suppression. Clear all old brassica material promptly after harvest to remove the overwintering population.