Cabbage whitefly can be difficult to control, particularly on allotments where there are likely to be affected plants on nearby plots that will be a source of re-infestation. Fortunately, cabbage whitefly only infests outer leaves and usually causes little real damage to parts of the plant that are consumed. Therefore infestations can usually be tolerated. The immature stages in the life cycle are not very susceptible to insecticides and so several applications may be needed to reduce a heavy infestation. Complete eradication is neither feasible nor necessary as it is only heavy infestations that are likely to cause problems with sooty mould. Brassica leaves are waxy and this makes them difficult to wet with pesticide sprays. Kale is the plant most affected as this insect occurs on young leaves that are going to be eaten. On cabbage, sprouts, cauliflower and broccoli, the edible parts are little affected.
Cabbage whitefly is not attacked by the biological control, Encarsia formosa parasite sold for use against glasshouse whitefly (Trialeurodes vaporariorum) but there are some other Encarsia species that occur naturally in Britain that develop as larvae in cabbage whitefly nymphs. They are not commercially available but are sometimes sufficiently numerous to be effective at keeping cabbage whitefly at a low level. The use of relatively persistent pesticides, such as thicloprid, deltamethrin or lambda-cyhalothrin, will be harmful to these parasites and may allow a resurgence in the whitefly population that is able to reproduce without its numbers being reduced by these natural enemies.
Short persistence organic pesticides can be sprayed onto the lower leaf surfaces. These will have limited effects on natural enemies and have a one day harvest interval. Products include:
- Plant oils (e.g. Vitax Organic Pest and Disease Control Concentrate, Bug Clear for Fruit & Veg, Agralan Whitefly Killer)
- Fatty acids (e.g. Bayer Organic Bug Free, Doff Greenfly and Blackfly Killer, Doff Universal Bug Killer)
- Natural pyrethrum/pyrethrins (e.g. Py Spray Garden Insect Killer concentrate, Bug Clear Gun for Fruit & Veg, Defenders Bug Killer, Growing Success Fruit & Veg Bug Killer, Pyrol Bug & Larvae Killer)
Several applications of these mild contact insecticides will be necessary to reduce a heavy infestation
Synthetic insecticides. These products have longer persistence and should only be used if a damaging infestation of cabbage whitefly has developed. There are restrictions on the number of applications that can be made to food-plants and a longer harvest interval. The label instructions must be followed carefully when applying these products to food plants.
- Deltamethrin (e.g. Bayer Provado Ultimate Fruit & Vegetable Bug Killer)
- Lambda-cyhalothrin (e.g. Westland Resolva Bug Killer)
No more than two applications of these insecticides are permitted on brassicas during the growing season (but up to three for deltamethrin on cauliflower, broccoli, kale and Chinese cabbage). There is a seven-day harvest interval for these two insecticides.
- Thiocloprid (e.g. Bayer Provado Ultimate Bug Killer Ready to Use) is limited to one application during the growing season and has a 14 day harvest interval
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)