Plants affected
All leafy brassicas, including cabbage, cauliflower, broccoli, calabrese, Brussels sprout and kale.
Symptoms
White-winged insects, 1.5mm long, fly up in clouds from the
underside of brassica leaves when disturbed. Flat, oval, whitish-green
scale-like nymphs are attached to the lower leaf surface.
Black sooty moulds and stickiness often develop on the upper
leaf surface below where whitefly are feeding.
Cause
Cabbage whitefly is present on its host plants throughout the year and overwinters as adult insects. The adults lay eggs on the lower leaf surface from which hatch the scale-like nymphs. Both adults and nymphs suck sap and excrete a sugary substance (honeydew) that allows the growth of sooty moulds. Brassicas are tolerant of the pest and even heavy infestations have little impact on plant growth, but sooty mould can be a problem, especially on Brussels sprout buttons.
Although similar to glasshouse whitefly, cabbage whitefly is a different species and does not attack plants other than brassicas. Similarly glasshouse whitefly will not infest brassicas.
Control
Brassica leaves are waxy and they can be difficult to wet with a spray. Fortunately, because whitefly causes little real damage, infestations can be tolerated. If whitefly are a problem, use bifenthrin (Scotts Bug Clear Gun, Bayer Sprayday Greenfly Killer Plus or Doff All-In-One Garden Pest Killer). Only one application of this insecticide is permitted on brassicas during the growing season. Plant oils (Vitax Organic 2 in 1 Pest and Disease Control, Scotts Bug Clear for Fruit & Veg), fatty acids (Bayer Organic Pest Control, Doff Greenfly and Blackfly Killer), plant extracts (Growing Success Fruit & Veg Bug Killer, Growing Success Whitefly Killer, Agralan Whitefly Killer) and pyrethrum (Py Spray Garden Insect Killer concentrate, Scotts Bug Clear Gun for Fruit & Veg) are organic pesticides that can be sprayed onto the lower leaf surfaces. Several applications of these mild insecticides will be necessary to reduce a heavy infestation.