Whitefly and Greenhouses (Horticulturalist Peter Whyte gives advice)

Whitefly is a serious pest of greenhouse crops.  It can severely damage plants and is hard to control.  The adult flies are covered in a protective layer of white wax.  They fly up in clouds from plants when disturbed.  They suck plant sap, spreading plant viruses and depositing sticky honeydew which attracts black sooty moulds that are hard to wash off leaves and fruits.  They lay eggs on the undersides of the leaves which develop into greenish scales, each with a grub inside.

The colour yellow attracts whiteflies, so you can hang up sticky yellow cards to catch them and detect their arrival or reduce their numbers. You can shake plants and catch them with a hand-held vacuum cleaner when they take off: avoiimageedit_2_6272177033d damaging the leaves.  Planting French marigolds among crops may help.  The parasitic wasp Encarsia Formosa can work well in large greenhouses when conditions are right.

Check under the leaves of new plants carefully before putting them in your greenhouse: plunging their tops into a bucket of water will kill many of the adults but not their scales.  Burn or bury dead or badly infested leaves.  Keep the greenhouse and its surrounds clear of plants over the winter to remove host plants and starve them out.  Sprays based on pyrethrum, neem oil or insecticidal soap have been used formerly, but with changes in pesticide laws may not now be available.  Whiteflies can quickly develop resistance to a chemical, so if spraying change the chemical often.