UK greenhouse pests guide

A five-minute inspection routine

Walk the greenhouse before watering so you can see wilting and leaf marks in their natural state. Look at growing tips, the undersides of representative leaves, stems near the compost, damaged fruit and the shaded corners of staging. Note whether insects fly up, crawl, remain fixed in place or leave webbing or sticky honeydew. Photograph a close-up and a whole plant before removing everything.

Keep newly bought plants apart where possible long enough to observe eggs and small pests. The RHS specifically recommends checking incoming plants for whitefly. One infested purchase placed beside a crop of tomatoes can become a house-wide problem while the gardener focuses on watering. Our greenhouse hygiene guide gives a clean-to-dirty work order.

Whitefly: small adults, persistent young stages

Glasshouse whitefly are tiny white-winged insects that rise when a plant is disturbed. Flat pale nymphs remain on leaf undersides. Honeydew on foliage can support black sooty mould. Warm greenhouse conditions let generations overlap, so trapping some adults will not remove the immature stages. See how to control greenhouse whitefly for identification, isolation and early biological-control choices.

The RHS says yellow sticky sheets chiefly help monitor whitefly, not deliver full control, and can capture parasitoid wasps. If using Encarsia formosa or another biological control, follow supplier instructions about sticky cards and other treatments. Do not present a card as a pesticide-free cure.

Red spider mite: look for fine webbing and speckling

Glasshouse red spider mites are difficult to see without magnification. Fine pale speckling on leaves and, in heavier infestations, webbing are common clues. They flourish in warm, dry conditions. Increase inspection of nearby plants, remove severely affected leaves where appropriate and consult the current RHS red spider mite advice before choosing a response.

Do not assume every pale leaf has mites; nutrient, watering and sun issues can also discolour foliage. A hand lens and examination of the leaf underside provide better evidence than spraying on the basis of colour alone. Biological predators need a compatible environment and should be introduced according to supplier guidance.

Aphids, thrips and other sap feeders

Aphids often cluster on soft growing tips and can distort new growth or leave honeydew. Thrips may cause silvery feeding marks and have different life stages that require correct identification. Glasshouse leafhoppers and other insects also occur. The treatment that suits one group may be ineffective or unnecessary for another.

Isolate a modestly affected pot where possible, remove small colonies by hand or a suitable water rinse if the plant tolerates it, and inspect neighbouring plants. Do not blast fragile seedlings with a hose or wet electrical equipment. Encourage ventilation and avoid overfeeding soft lush growth, which can be attractive to some pests.

Soil, compost and night-time visitors

Fungus gnats often fly around wet compost. Their presence can signal a medium kept too damp, though larvae may damage vulnerable young roots. Let suitable compost surfaces dry between waterings and remove decaying plant matter. Slugs and snails leave a different trail and may hide under pots, shelving or dense foliage; check at dusk and remove them by hand where practical.

Always identify the actual problem. A wilting seedling could have root disease, underwatering, waterlogging or a pest. It is possible to worsen the situation by applying a treatment to the wrong cause.

Make the greenhouse less favourable to outbreaks

Keep the central aisle and vents unobstructed, remove spent crops promptly and space plants so leaves can be inspected. Wash reusable pots and staging between batches. Avoid carrying pests from a suspect plant to clean seedlings on the same gloves or secateurs. Reduce volunteer weeds that provide alternate hosts.

These steps do not guarantee a pest-free greenhouse. They make early detection easier and reduce the shelter and residue that can support overwintering populations. The cleaning guide covers the structural work, while the hygiene guide covers everyday plant-handling habits.

Choose a proportionate response

For a few insects, monitoring and removal may be enough. If a population is increasing, identify the species and consult current RHS advice for cultural and biological approaches. Biological controls can take time and often work best before a severe infestation. A product that kills a pest indiscriminately may also undermine those beneficial organisms.

If considering a pesticide, check the HSE pesticide databases for the current UK authorisation and read the exact product label for crop, situation, dose, application, protective measures and harvest restrictions. An Amazon listing or generic article is not proof that a spray may legally be used on your tomatoes. This guide does not recommend a specific chemical treatment.

Record what happened

Note the first symptom, crop, location, weather and action taken. Return after a few days to see whether the population is growing, stable or declining. Replace monitoring cards when covered, but remember that fewer insects on a card may reflect location or season as much as successful control. Inspect the plant itself.

At season's end, remove infested crop residue and review the layout. A repeated whitefly problem in one crowded corner may suggest earlier introduction on new plants, poor access for inspection or surviving host plants—not a need to spray everything by default.

Sources and related reading

The RHS greenhouse-vegetable guide lists common under-glass pests and monitoring principles. The RHS glasshouse-whitefly guide details symptoms and biological controls. For chemical products, use the current HSE pesticide database and the product label.