What it is
The cabbage aphid (Brevicoryne brassicae) is a small (1.5 to 2.5 mm) grey-green, sap-sucking insect covered in a powdery grey-white wax that makes colonies look dusty or mouldy. It feeds in dense colonies on new growth, leaf undersides, and inside heads, causing leaf curling, yellowing, and stunting. It excretes honeydew that grows black sooty mould and can transmit brassica viruses. It only attacks brassicas and can damage up to 90% of a crop.
Symptoms to look for
- Dense colonies of small grey-green aphids with a powdery, dusty wax coating
- Colonies on new growth, leaf undersides, and inside the developing head
- Curled, yellowed, distorted leaves and stunted growth
- Sticky honeydew on leaves with black sooty mould growing on it
- Reduced quality and marketability; virus symptoms where aphids spread viruses
When it spreads
Cool, dry weather, which lets numbers grow rapidly; warm dry spells also favour them. Continuous brassica crops and cruciferous weeds sustain populations. Ants that farm aphids for honeydew protect them from predators, and the waxy coating shields them from contact sprays.
Treatment
Field practices
Rotate away from brassicas, remove and destroy crop residues and old stalks after harvest to cut overwintering, control cruciferous weeds, encourage natural enemies with flowering border plants, manage ants, and inspect new growth and head interiors regularly to catch colonies early.
Organic options
Spray insecticidal soap or neem oil directly onto colonies (the wax needs thorough coverage and often repeat sprays every 5 to 7 days). A strong water jet dislodges colonies. Encourage ladybirds, lacewings, and parasitic wasps, and control ants that protect the aphids. Remove and destroy heavily infested leaves and old stalks.
Active ingredients
- insecticidal soap
- neem oil (azadirachtin)
Chemical options
If numbers are high, use a systemic aphicide such as spirotetramat, or a selective product like flonicamid or pymetrozine, which spare many beneficial insects. Add a wetter/spreader because the waxy coating repels sprays. Rotate chemical groups since cabbage aphid develops resistance, and avoid broad-spectrum sprays that kill its natural enemies.
Active ingredients
- spirotetramat
- flonicamid
- pymetrozine
Safety
Cabbage is eaten and aphids hide inside heads, so choose products registered for cabbage and strictly follow the pre-harvest interval. Avoid broad-spectrum sprays that harm bees and natural enemies; spray in the evening if needed. Wear gloves and a mask, keep children and animals away until dry, and wash heads well before eating.
Preventing it next season
Scout new growth and head interiors from early in the crop, use row covers on young plants, keep cruciferous weeds down, remove crop debris promptly, rotate crops, protect natural enemies, and manage ants. Acting while colonies are small is far easier than after they build up under the wax.