A common pest on all brassica crops in the summer and early autumn are the dense clusters of cabbage whitefly that infest Brussels sprouts and kale as well as cabbages. There can be so many of them that if you brush against the plants you may well see the insects taking flight in small clouds. The adult flies readily overwinter in mild years and can be active all year round. Where the infestation is large enough, you may well find black sooty mould developing on the foliage as well. This is caused by fungus growing on the sugary sap excreted from the whitely onto the leaves underneath – and in some cases this can form a thick, sticky residue to build up.
At times like this, it is often a case of gardeners trying to come up with a quick fix to the problem. That really best takes the form of washing the insects from the plants or wiping them off using your thumb and forefinger. And the black sooty mould can be regularly rinsed off too. But it is probably better to try and avoid the problem in the first place with some good growing and an understanding of the whitefly lifecycle.
How to prevent cabbage whitefly
In common with many other plant bests, whitefly are looking for a tasty snack and an easy source of food. The more nitrogen that you give plants, in the form of fertiliser or manure, the softer the leaf growth will be. And the softer the leaf growth, the easier it is to feed on for the insect pests such as cabbage whitefly. Although it is often recommended to give brassicas and other leafy crops plenty plenty of manure and even blood, fish and bonemeal, it is better to apply feeds with that contain potash to balance the effects of nitrogen.
Balanced fertilisers such as Growmore do contain some potash, but even this can encourage leafy plants to grow soft in my experience. I tend to use pelleted chicken manure or Vitax Q4 if absolutely needed, but most of the time I rely on the nitrogen in my garden compost and balance it with a small amount of well-weathered wood ashes or sulphate of potash. This is given a couple of times during the growing season.
The cabbage whitefly lifecycle
Adult cabbage whiteflies lay their eggs on the host plants, which hatch into shield-like ‘sales’ or immature nymphs. Keep a look out for these and pick them off as they are easier to deal with than the flying adults. Remember that the eggs and adults may be overwintering on your neighbours’ brassicas even if you only grow vulnerable crops in the summer. Eggs are also laid on ornamental cabbage relatives and some wildflowers that can then re-infest your crop plants.
Where the pest becomes a persistent problem, organic pesticides are available based on plant oils, fatty acids or pyrethrum. Always follow the application instructions carefully.
Cabbage whitefly are a different species to the greenhouse whitely.