You really need to keep on top of pests like white fly and red mite. They breed so fast, especially in a greenhouse, that a small problem can become a big problem in just a week. As soon as you spot them, deal with them. For white fly, sticky yellow cards are fairly effective and marigolds help to keep them away from your crops in the first place.
The second line of defence is to spray with a contact killer. Unlike poison sprays, you can eat the crops safely but you need to be thorough when spraying.
I have a method that makes sure I get as many as possible. First spray over the plant to cover the topside of the leaves and any in flight, then under the leaves and finally the compost in which they’re planted.
I’ve used Bug Clear and Agralan in the past but on the advice of Medwyn Williams who is the expert’s expert of vegetable growing, changed to SB Invigorator. This not only kills white fly and spider mite but also controls powdery mildew whilst being a foliar feed to help the plant resist attack and repair damage.
If you’re losing the battle against a pest, then bring in more troops with a biological control. Encarsia Formosa will quickly reduce your white fly problem to manageable levels. They come as pupae on cards which you hang in the shade between the plants. Take down any sticky yellow cards though if using Encarsia – you don’t want to catch your own soldiers.