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The warm, humid, sheltered environment that makes a greenhouse perfect for growing is also what makes it a pest magnet. Outside, wind and predators keep populations in check. Inside, a single aphid colony can double in 48 hours and wipe out a bed of seedlings in two weeks.
The good news: greenhouse pests are manageable if you catch them early. Every one of the five pests below has a clear, organic treatment that works. The trick is knowing what you're looking at before the infestation takes hold.
I've dealt with all five in my 10×14ft greenhouse. Here's what actually works — and what doesn't.
Aphids
Aphids are the most common greenhouse pest by a wide margin. They cluster on new growth and the undersides of leaves, sucking sap and excreting sticky "honeydew" that promotes black sooty mould. A single aphid can produce 80 offspring per week without mating — infestations escalate fast.
How to identify
- Soft-bodied, pear-shaped insects 1–3mm long
- Green, black, white, or grey depending on species
- Clustered on shoot tips and leaf undersides
- Sticky honeydew residue on leaves and stems
- Black sooty mould on older infestations
- Leaves curl, pucker, or yellow in affected areas
How to treat
- Blast off with a strong jet of water first
- Apply neem oil spray to all leaf surfaces
- Insecticidal soap spray every 5–7 days
- Introduce ladybirds or lacewing larvae
- Yellow sticky traps to catch winged adults
- Remove heavily infested stems immediately
Kristian's note: Neem oil stops a light infestation within 10 days if you're consistent. Miss a spray cycle and they bounce back. For a bad infestation, physically removing aphids with water first makes everything else work better.
Whiteflies
Whiteflies are tiny, moth-like insects that cloud up in a white swarm when you disturb a plant. They feed on the underside of leaves, like aphids, and are particularly damaging to tomatoes, cucumbers, and peppers. Their eggs hatch into scale-like nymphs that are harder to kill than adults.
How to identify
- Tiny (1–2mm) white moth-like flying insects
- White cloud erupts when plant is disturbed
- White oval eggs and flat nymphs on leaf undersides
- Yellowing, mottled leaves on tomatoes/cucumbers
- Honeydew deposits and sooty mould
- Yellow sticky traps catch adults quickly
How to treat
- Yellow sticky traps as first line of control
- Insecticidal soap spray (undersides of leaves)
- Neem oil every 5 days for 3 cycles minimum
- Introduce Encarsia formosa parasitic wasp
- Reflective mulch disorients adults
- Avoid nitrogen-heavy fertiliser (lush growth = more feeding)
Kristian's note: Whiteflies are tougher than aphids because the egg and nymph stages are resistant to most sprays. You need consistent treatment over 3–4 weeks to break the lifecycle. The parasitic wasp Encarsia formosa is worth it for large greenhouses — one release and they self-sustain.
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Spider Mites
Spider mites are not insects — they're arachnids, about 0.5mm long and almost invisible to the naked eye. They pierce leaf cells and extract chlorophyll, leaving distinctive bronze stippling. In hot, dry greenhouse conditions, their lifecycle from egg to adult can be as short as 7 days. They build resistance to pesticides quickly — organic controls are essential.
How to identify
- Fine webbing on leaf undersides and between stems
- Tiny pale yellow or bronze stippling on leaves
- Leaves turn dry, bronzed, and papery
- Shake leaf over white paper — look for moving specks
- Worst in hot (25°C+), dry, poorly ventilated greenhouses
- Start from older lower leaves and work upward
How to treat
- Increase humidity above 60% (mites hate moisture)
- Strong water spray to dislodge from undersides
- Neem oil or insecticidal soap every 3–4 days
- Introduce Phytoseiulus persimilis predatory mite
- Remove heavily infested leaves in sealed bags
- Rotate treatments to prevent resistance
Kristian's note: Spider mites nearly destroyed my tomato crop in summer 2024 — a two-week heatwave with the vents closed was all they needed. Now I spray the undersides of leaves weekly as routine in summer, not as a reaction. The predatory mite Phytoseiulus persimilis is the nuclear option and it works.
Fungus Gnats
Adult fungus gnats are more annoying than harmful — they're the tiny black flies hovering around your pots. The damage is done by their larvae, which live in the top 2–3cm of soil and feed on fungal matter, root hairs, and sometimes young roots directly. Seedlings and young plants are most vulnerable to larval feeding.
How to identify
- Tiny (2–3mm) black flies hovering near soil surface
- Adults attracted to light and yellow sticky traps
- White larvae (5mm, black head) in moist growing medium
- Seedlings wilt or fail despite adequate watering
- Root inspection shows chewed or missing root tips
- Persistent in overwatered peat-based composts
How to treat
- Let soil dry out completely between waterings
- Yellow sticky traps catch adults before egg-laying
- Bti (Bacillus thuringiensis israelensis) soil drench
- Top-dress pots with coarse sand or grit
- Nematode drench (Steinernema feltiae) for larvae
- 3% hydrogen peroxide watered into soil kills larvae
Kristian's note: The single most effective fix is letting your growing medium dry between waterings. Fungus gnat larvae cannot survive in dry soil. Most growers who have a gnat problem are overwatering. Fix the watering habit first — then treat.
Scale Insects
Scale insects are easy to miss — they look like brown bumps or waxy spots on stems and leaves rather than insects. They don't move once established. Soft scale excretes honeydew; armoured scale dries the plant out by sheer volume of feeding. Difficult to shift once entrenched, so early detection is key.
How to identify
- Brown, tan, or white lumps on stems and mid-ribs
- Bumps don't move and scrape off with a fingernail
- Sticky honeydew on lower leaves and surfaces below
- Yellow patches or leaf drop on heavily infested plants
- Waxy white crawlers (juveniles) visible with magnification
- More common on woody plants: bay, citrus, figs
How to treat
- Scrape off adults manually with a soft toothbrush
- Dab isopropyl alcohol (70%) directly on scale with cotton bud
- Neem oil spray to smother crawlers (juveniles)
- Insecticidal soap for crawlers — adults are shell-protected
- Systemic neem oil soil drench for persistent infestations
- Introduce Metaphycus helvolus parasitic wasp
Kristian's note: Scale spreads slowly but is harder to eradicate than aphids because the shell protects adults from sprays. The alcohol-on-cotton-bud method is tedious but effective for isolated patches. Catch it early and you can clear it in a few days of spot treatment.
3 Products Worth Buying
These handle 90% of greenhouse pest situations. All organic, all effective, all available on Amazon.
Cold-Pressed Neem Oil
The single most versatile pest control product for any greenhouse. Disrupts insect growth, smothers mites, deters future feeding. Use on aphids, whiteflies, mites, and fungus gnat larvae. Mix at 2% with water and a few drops of dish soap. Apply weekly as prevention, every 4 days as treatment.
See on Amazon →Insecticidal Soap Spray
Kills soft-bodied insects on contact by disrupting their cell membranes. Fast-acting against aphids, whiteflies, and spider mite crawlers. Safe for beneficial insects once dry. Ready-to-use formulas remove the dilution step — good for beginners. Degrades quickly so there's no residue on edibles.
See on Amazon →Yellow Sticky Traps
Non-toxic, physical pest monitoring and light control. Essential for catching whitefly adults, fungus gnat adults, and winged aphids. Hang at plant canopy height — one per 10 sq ft. Change weekly. They don't eliminate infestations but catch outbreaks early and significantly reduce adult populations between spray cycles.
See on Amazon →Some links are Amazon affiliate links. GrowHaus earns a small commission at no extra cost to you. I only recommend products I've used myself.
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Prevention at a Glance
Stop pests before they start. These habits eliminate most infestations before they begin.
| Habit | What it prevents | How often |
|---|---|---|
| Inspect new plants before bringing in | All pests — most infestations start with an infected plant | Every time |
| Check leaf undersides weekly | Aphids, whiteflies, spider mites, scale | Weekly |
| Let soil dry between waterings | Fungus gnats — larvae can't survive dry medium | Every watering |
| Maintain airflow (open vents daily) | Spider mites — hate humidity and airflow | Daily in summer |
| Remove dead plant material promptly | Fungus gnats, scale, aphids — all use decaying matter | As needed |
| Hang yellow sticky traps | Whiteflies, fungus gnats, winged aphids | Ongoing |
| Quarantine new plants 7–10 days | All pests — gives time to spot problems before they spread | Every new plant |
| Weekly neem oil spray (summer) | Spider mites, aphids, whiteflies — suppresses populations | Weekly (June–Sept) |