Cabbage whitefly ( latin name - Aleyrodes proletella ) is a nuisance when it attacks brassicas in the vegetable garden, particularly brussel sprouts. But cabbage whitefly can be a problem on all leafy brassicas, including cabbage, cauliflower, broccoli, calabrese, brussel sprouts and kale.
The cabbage whitefly is different from the whitefly found in greenhouses. Cabbage whitefly only attacks brassicas.
The cabbage whitefly lay eggs on the underside of the brassica leaves. The eggs hatch into tiny, colourless nymphs which suck the sap from the underside of the leaf.
The whiteflies hatch out and are noticeable as clouds of small, flying, white flies when the leaves are disturbed.
The whiteflies & nymphs suck the sap from the leaves and excrete honeydew which is a sugary substance. This sugary excretion encourages black moulds to grow. So the leaves of the brassicas develop sooty spots under them. It is these sooty moulds which cause the problem, particularly on brussel sprout buttons. They can make the sprouts unusable when there is a severe attack. It is not so important in cabbages, when any sooty outside leaves can be peeled off before use.
Apart from the sooty moulds which appear on the brassicas as a consequence of the cabbage whitefly, the plants do not seem to be weakened by the cabbage whitefly.