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Pests aren’t usually too much of a problem on outdoor crops, but in greenhouses commons pests include aphids, glasshouse whitefly and red spider mite (properly known as two-spotted spider mite). Commercial growers use biological control agents to manage these pests and these are available to gardeners: where to buy. Soft soap is useful for reducing the numbers of pests – use distilled, rain or soft tap water to dilute it. You can also use yellow sticky traps.
There is more guidance here: Pesticides for Home Gardeners RHS Advisory Service February 2015
Did You Know?
British tomato production amounts to about 75,000 metric tonnes per year - about a fifth of the total volume of tomatoes sold in the country through the year, and up to a half in the summer.