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 are said to help keep them away from your crops in the first place.
The second line of defence is to spray with a contact smothering killer. Unlike poison sprays, you can eat the crops safely but you need to be quite 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 to good effect 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.
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