How do I get rid of whitefly and red spider mite in a greenhouse?
Telling the two apart, why the conditions that suit each are opposite, and the biological controls that actually work under glass.
- Difficulty
- intermediate
- Time
- 45 min
- Read
- 4 min
Short answer
Whitefly are tiny white moths that fly up in a cloud when you touch a plant. Red spider mite are almost invisible and show as fine pale speckling and webbing. Both explode in a hot dry greenhouse — damping down and biological controls introduced early are far more effective than spraying.
A greenhouse is a closed, warm, predator-free environment, which is exactly what these pests need to go from a few individuals to an infestation in weeks. Catching them early is the whole game; both are very hard to clear once established.
What you'll need
- Yellow sticky traps
- Biological control — Encarsia for whitefly, Phytoseiulus for spider mite (optional)
- Hand sprayer
Step by step
- Identify which you have.Whitefly: small white insects on leaf undersides that fly when disturbed, with sticky honeydew and sooty mould below. Red spider mite: fine pale stippling on leaves, then bronzing, then fine webbing at the shoot tips. The mites themselves need a hand lens.
- Hang yellow sticky traps early.They catch adult whitefly and, more usefully, tell you the pest has arrived weeks before you would see it. They do not control an infestation on their own.
- Check leaf undersides weekly.Both live on the underside. By the time damage is visible from above, the population is large.
- Raise the humidity for spider mite.Red spider mite thrives in hot dry air. Damping down the floor daily in summer, misting plants and improving ventilation makes conditions actively hostile to it.
- Introduce biological control at the right time.Encarsia formosa for whitefly and Phytoseiulus persimilis for red spider mite both need warmth to work, roughly from May in an unheated UK greenhouse, and both need to be introduced when numbers are still low.
- Do not spray insecticide alongside biological control.Most sprays kill the predators as effectively as the pest, and the pest recovers faster. Choose one approach.
- Use soap or oil sprays if you are not using predators.Insecticidal soaps and plant oils work by contact and need thorough coverage of leaf undersides, repeated every few days. They do not persist, so they are hard work but low impact.
- Remove badly affected leaves.Bagging heavily infested lower leaves removes a large part of the population in one action. Do not compost them in an open heap next to the greenhouse.
- Clear out and clean in autumn.Both overwinter in frame joints, under staging and in debris. An autumn clear-out and wash down of the structure resets the following year.
- Isolate new plants.Almost every greenhouse infestation arrives on a bought plant. Keep new arrivals apart and inspect them for a fortnight.
Tips
- Sticky traps are an early warning system, not a control.
- Red spider mite hates humidity — damping down twice a day in summer is a genuine control measure.
- Introduce predators while you can still count the pests, not after.
Common mistakes
- Spraying insecticide and then releasing predators — Residues kill the predators for days or weeks afterwards, so the introduction fails and the pest returns to a garden with nothing eating it. Decide which route you are taking and commit.
- Waiting until damage is obvious — Both pests reproduce in days under glass. A population you can see from the doorway is one that biological control will struggle to catch up with.
If it doesn't work
Cloud of white insects when plants are touched
Cause: Whitefly — Fix: Sticky traps to monitor, Encarsia from late spring, remove worst leaves, and avoid broad-spectrum sprays.
Fine pale speckling then bronze leaves and webbing
Cause: Red spider mite — Fix: Damp down and raise humidity, introduce Phytoseiulus, remove worst leaves and improve ventilation.
Sticky leaves and black sooty deposit
Cause: Honeydew from whitefly or aphids, with mould growing on it — Fix: Control the insect. The sooty mould washes off with soapy water once the source has gone.
Infestation returns every year at the same time
Cause: Overwintering in the structure — Fix: Full autumn clear-out and wash down, and isolate new plants.
Questions people ask
Which is worse, whitefly or red spider mite?
Red spider mite does more damage and is harder to control, partly because it is nearly invisible until the plant is bronzed. Whitefly is more obvious and more of a nuisance than a killer.
Do biological controls really work in an ordinary greenhouse?
Yes, provided they go in early and it is warm enough — roughly from May onwards unheated. They work far better than sprays under glass, because the closed environment keeps the predators where you put them.
Can I just spray?
Soap and oil sprays work by contact and need repeat applications to leaf undersides. They are viable but laborious, and they rule out using predators at the same time.