10 Common Pests and How to Control Them in Greenhouses and Gardens (UK Edition)
Gardening and greenhouse cultivation bring joy, fresh produce, and blooming beauty to homes and allotments across the UK. However, nothing dampens a gardener’s spirits faster than discovering that prized plants have been gnawed, wilted, or discoloured due to pest infestations.
Whether you’re nurturing tomatoes in a heated greenhouse or tending to perennials in a cottage garden, pest control is an inevitable part of the process. But fear not—armed with the right knowledge and strategies, you can manage and prevent most infestations effectively and sustainably.
Here are 10 of the most common pests in UK gardens and greenhouses, along with practical tips on how to control them.
1. Aphids (Greenfly and Blackfly)
Description:
Tiny, soft-bodied insects that feed on plant sap. They cluster on new shoots, leaves, and flower buds, excreting sticky honeydew that can lead to sooty mould.
Control Methods:
- Biological Control: Introduce natural predators like ladybirds and lacewings.
- Manual Removal: Wipe or spray them off with a strong blast of water.
- Organic Sprays: Use a neem oil solution or insecticidal soap.
- Preventive Measures: Encourage companion plants such as nasturtiums, which attract aphids away from crops.
2. Whitefly
Description:
Small white insects that resemble tiny moths. They feed on the undersides of leaves, causing yellowing and plant weakening. They’re a common problem in greenhouses.
Control Methods:
- Sticky Traps: Yellow sticky traps can reduce populations.
- Biological Control: Encarsia formosa (a parasitic wasp) is effective in greenhouses.
- Ventilation: Improve air circulation to discourage establishment.
- Organic Treatments: Neem oil or soap sprays work well on early infestations.
3. Slugs and Snails
Description:
These gastropods thrive in damp, shady environments. They chew holes in leaves, flowers, and fruit, leaving slimy trails behind.
Control Methods:
- Hand Picking: Search and remove during evening hours.
- Barriers: Use copper tape, eggshells, or coarse sand around plant bases.
- Biological Control: Apply nematodes (Phasmarhabditis hermaphrodita) to soil.
- Beer Traps: Sink a container filled with beer to attract and drown them.
4. Spider Mites
Description:
Almost microscopic pests that thrive in hot, dry conditions. They suck sap from leaves, causing stippling, discolouration, and webbing.
Control Methods:
- Humidity: Increase moisture levels in the greenhouse to deter them.
- Biological Control: Use predatory mites like Phytoseiulus persimilis.
- Misting: Regularly mist plants to maintain humidity.
- Soap Sprays: Insecticidal soap can reduce mild infestations.
5. Fungus Gnats
Description:
Small, black, fly-like insects that lay eggs in damp compost. Larvae feed on roots and organic material, damaging seedlings and young plants.
Control Methods:
- Drying Out: Let the compost dry between waterings to disrupt their lifecycle.
- Biological Control: Introduce nematodes (Steinernema feltiae) into the soil.
- Sticky Traps: Yellow traps can capture adult gnats.
- Pot Hygiene: Keep greenhouse pots and trays clean and well-drained.
6. Cabbage White Caterpillars
Description:
The larvae of the cabbage white butterfly, these greenish-yellow caterpillars devour brassica leaves rapidly, often leaving plants stripped.
Control Methods:
- Netting: Use fine mesh netting to prevent butterflies laying eggs.
- Hand Removal: Regularly inspect and pick off caterpillars.
- Decoy Plants: Grow sacrificial plants like nasturtiums nearby.
- Biological Control: Introduce Bacillus thuringiensis (Bt), a naturally occurring bacteria harmful to caterpillars.
7. Vine Weevils
Description:
Adult weevils eat leaves at night, creating notched edges. However, the real threat comes from larvae in the soil, which feast on roots and can kill plants.
Control Methods:
- Nematodes: Apply Heterorhabditis bacteriophora to soil to kill larvae.
- Night Patrol: Hunt adults with a torch and collect by hand.
- Barrier Tapes: Place sticky barriers around pots and greenhouse benches.
- Container Vigilance: Avoid reusing compost where infestations have occurred.
8. Leaf Miners
Description:
These are the larvae of various flies, moths, or beetles that burrow into leaves, creating visible white or brown trails.
Control Methods:
- Remove Affected Leaves: Prune and destroy infested foliage early.
- Floating Row Covers: Prevent adult insects from laying eggs.
- Biological Control: Parasitic wasps like Diglyphus isaea can target leaf miners.
- Encourage Natural Enemies: Birds and predatory beetles help reduce numbers.
9. Thrips (Thunderbugs)
Description:
Tiny, slender insects that feed on plant tissue, causing silvering or mottling on leaves and flower deformation.
Control Methods:
- Blue Sticky Traps: More attractive to thrips than yellow ones.
- Biological Control: Introduce Orius laevigatus or predatory mites.
- Neem Oil Spray: Effective and safe for most greenhouse crops.
- Weed Control: Remove weeds that serve as alternative hosts.
10. Mealybugs
Description:
White, waxy insects that cluster on leaf joints and stems. They excrete honeydew, encouraging mould and deterring plant growth.
Control Methods:
- Cotton Swabs: Dab with alcohol or diluted methylated spirits.
- Quarantine: Isolate new or infected plants.
- Neem Oil or Soap Spray: Use regularly until infestation clears.
- Biological Control: Ladybirds and lacewings will feed on them.
General Tips for Preventing Pest Problems
While individual pests have their own preferred habitats and vulnerabilities, there are general practices that greatly reduce the risk of infestation:
1. Regular Monitoring
Inspect your plants weekly for signs of pests or disease. Early detection is crucial for effective control.
2. Good Hygiene
Remove dead leaves and plant debris, clean pots, and disinfect greenhouse surfaces regularly.
3. Healthy Soil
Healthy, well-fed plants are more resilient. Use compost and feed appropriately to ensure strong growth.
4. Crop Rotation
Rotate crops annually in your vegetable plot to break pest and disease cycles.
5. Companion Planting
Use plants like marigolds, nasturtiums, and garlic to attract beneficial insects or repel harmful ones.
6. Ventilation and Moisture Control
In greenhouses, regulate temperature and humidity to create an environment less hospitable to pests like spider mites and fungus gnats.
Embracing Nature’s Balance
Not all insects are pests. Many are allies in the garden—pollinators, decomposers, or predators of the real troublemakers. The key to successful pest control is balance. Over-reliance on chemicals can upset this balance and lead to more severe problems down the line.
Opt for integrated pest management (IPM), a strategy that combines cultural, biological, and mechanical methods to keep pests in check with minimal impact on the environment.
Final Thoughts
Pests may be a fact of life for UK gardeners and greenhouse growers, but they needn’t spell disaster. With vigilance, eco-friendly practices, and a bit of patience, you can protect your green haven and ensure your plants flourish throughout the seasons.
Do you have a go-to natural remedy or a curious pest you’ve battled recently? Share your experiences in the comments below—we’d love to hear your stories from the soil!
Happy gardening—and may your lettuces be hole-free and your roses unchewed.