Why Are Tiny White Insects Flying Off My Brassicas?

If you brush against your broccoli or cauliflower and a small cloud of white, moth-like insects rises from the underside of the leaves, you have brassica whitefly (Aleyrodes proletella). This is a distinct species from the glasshouse whitefly and is a persistent outdoor pest of brassicas, particularly from late summer through autumn and winter. It feeds on plant sap, deposits sticky honeydew and can significantly weaken plants in large numbers.

How to identify brassica whitefly

Adults are tiny (about 1.5mm), white-winged and fly in small clouds when disturbed. The immature nymphs are flat, scale-like and white, found on the leaf underside. Both adults and nymphs feed by sucking sap from the leaves. Heavily infested leaves develop sticky honeydew deposits, which quickly become colonised by black sooty mould — giving leaves a grimy, blackened appearance. Leaf yellowing and premature drop can follow in very severe infestations.

Managing whitefly organically

Strong water jets directed at the leaf undersides physically dislodge adults and nymphs. Do this repeatedly every few days to prevent a population explosion. Yellow sticky traps hung near the plants catch adults and help monitor population levels. Insecticidal soap or pyrethrum-based sprays applied to the leaf underside at dusk kill nymphs and adults on contact, but repeat applications are needed as eggs are unaffected. Encourage natural predators — parasitic wasps in particular attack brassica whitefly nymphs.

Why infestations peak in autumn

Brassica whitefly populations build through summer and reach their peak from August to October. Overwintering kale and Brussels sprouts are commonly the most heavily affected plants. Removing old, heavily colonised leaves and composting brassica debris at the end of the season reduces the population going into winter. Rotation prevents the pest from building large persistent populations in the same spot.