Most greenhouse trouble in a heatwave gets blamed on the watering can, so the fix feels obvious: hold off, dry things out, let the air move. Red spider mite turns that instinct on its head. The pest you are squinting at loves a hot, dry house, and one of the cheapest ways to slow it down is to raise the humidity, which is the exact reverse of how you fight blight.
If the upper surface of your greenhouse leaves has gone pale and finely freckled in a warm dry spell, and a few of them carry the faintest webbing in the leaf joints, you are almost certainly looking at red spider mite. The greenhouse glasshouse mite, properly Tetranychus urticae, is the classic August headache on cucumbers, tomatoes, aubergines and French beans. This guide covers the humidity lever the gadget guides skip, the biocontrol that actually clears an outbreak, and how to tell mite stipple from a feeding fault that looks similar at a glance.
What red spider mite looks like in a UK greenhouse
Glasshouse red spider mite is tiny, up to 1 mm long, so you spot the damage long before the animal. Through the warmer half of the year the mites are yellowish-green with two darker spots, which is why the pest is also called the two spotted spider mite. They turn orange-red in autumn as they head into winter dormancy, and that autumn colour is where the common name comes from.
The tell is on the leaf. According to the RHS, a fed-on leaf shows fine pale mottling on the upper surface, a dusting of tiny dots that reads almost silvery in low sun. Tip a suspect leaf and check the underside, where the colony lives. In a bad case you get fine silk webbing across the leaf joints and growing tips, then the leaves bronze and drop. Once webbing shows, the count has already run away from you.
Where they hide and why they come back
Sanitation matters because the females overwinter inside the building. The RHS notes that the orange-red autumn females spend the winter months resting in cracks and crevices of the glasshouse, in plant debris and in the soil. Leave the house dirty over winter and you have booked in next year's outbreak. An autumn clear-out of old crops, a wipe of the staging joints and a sweep of fallen leaves removes a good share of the resting population before spring.
Why hot dry air is the mite's best friend
Red spider mite is a warm-weather pest with a short fuse. It is usually a problem from March to October, breeding without pause through that window, and it can run all year in a heated house. The reason warm spells trigger an outbreak is speed: the RHS puts the life cycle at roughly 55 days at 10°C but only 12 days at 21°C. Push the glass into the high twenties and a fresh generation can arrive in about a week. A few mites in June become a webbed mess by August because each turn of the cycle is faster than the last.
Numbers tell the rest. Each female lays around 15 to 20 eggs a day and close to 100 across her life, so a small colony compounds the way money does. The mite also prefers air on the dry side, with extension data placing its favourite zone in the heat with relative humidity under 50 per cent. Heat, dry air and a fast cycle make the recipe, and a sealed-up greenhouse in a UK heatwave supplies all three.
The growing advice here works whatever greenhouse you have. It helps if the house lets you move air and damp down without cooking the plants, which is partly down to glazing and venting. The NORDIC Swedish pine greenhouse uses twin-wall polycarbonate that steadies the swing between scorching afternoons and cold nights, and a rear opening window opposite the door gives cross-ventilation as standard, so you can keep humidity up at the floor while leaf surfaces still dry. None of that is required to beat red spider mite. It just makes the humidity balance below easier to hold.
The humidity lever: damp down to slow the mite
Raising humidity is the move most quick guides leave out, and it is the opposite of blight management. The RHS is clear that the mite thrives in warm, dry conditions, and that syringing and spraying plants with water, plus keeping humidity high, reduces the risk. Damping down works because the mite struggles in moist air while the leaves it feeds on stay turgid and harder to drain.
The catch is fungal disease. Wet, still, humid air is exactly what tomato and potato blight and botrytis want, so you cannot simply hose the foliage every evening and walk away. The way through is to separate humidity from wet leaves.
- Damp the floor and paths rather than the leaves. Wetting the concrete or gravel at the start of a hot day lifts ambient humidity around the plants without leaving water sitting on foliage overnight.
- Keep the air moving. Open vents and the door so leaf surfaces dry within the hour, which gives you mite-hostile humidity without a botrytis penalty.
- Do it in the morning. Humidity raised early has the day to settle, while foliage wetted late at night sits damp and invites disease.
Honesty about the number matters. No single humidity reading kills mites, and the RHS is plain that water and humidity reduce risk but will not control an established outbreak on their own. A house held in a humid 60 to 80 per cent band holds the mite back and keeps its predator hunting, while leaving enough airflow that mildew never gets comfortable. If you are juggling all this with the heat as well, our guide on keeping a greenhouse cool pairs neatly with damping down.
Phytoseiulus persimilis: the biocontrol that clears an outbreak
The reliable cure for a real infestation is a living one. Phytoseiulus persimilis, a 0.5 mm orange-red pear-shaped predatory mite, eats spider mites and nothing else. Koppert data put its appetite at up to 5 adult mites or 20 larvae and eggs a day at around 20°C, and it develops about twice as fast as its prey. Once released in enough numbers, the predator out-breeds the colony and burns clean through it.
When to release, and at what temperature
Timing is where most home releases fail. Phytoseiulus works best between 15 and 25°C, stays effective from 13 to 27°C, and stops working above 30°C. Release once the house is reliably warm, around 21°C in the day, because the predator stalls below roughly 13 to 15°C. For a cool early-season start, the RHS suggests Amblyseius (also called Neoseiulus) californicus, another 0.5 mm predator that tolerates lower temperatures.
The same humidity lever runs through the biology. Phytoseiulus falls off below 70 per cent relative humidity, so the damping down that hampers the mite is also what keeps your predator alive. A dry house starves the cure as well as feeding the disease.
How many, and how often
Introduction rates run from 2 to 50 predators per square metre per release, repeated once or twice at weekly intervals, with the heavier end for an outbreak already showing webbing. Release at the first webbing rather than waiting for the leaves to brown. A few other allies are sold for the job too: the predatory midge Feltiella acarisuga, and Atheta coriaria, a 3 to 4 mm rove beetle that mops up soil-stage pests. Whichever you choose, drop chemical sprays first, since most will wipe out the predators along with the pest.
Is it red spider mite or a feeding fault?
Fine yellow speckling on greenhouse leaves has two common causes that look alike from a metre away: mite stipple and a nutrient deficiency. Get your eye right down to the leaf. Mite damage is very fine, evenly scattered silvery speckling that starts on leaf undersides and comes with tiny moving dots, then webbing. A deficiency shows colour change with no speckling and no mites at all, so get in close before you decide.
Mite stipple versus magnesium and nitrogen
- Red spider mite: very fine pale or silvery dots, evenly spread, worst on leaves near hot dry glass. Look on the underside for moving specks and, later, webbing. The whole leaf bronzes, dries and drops.
- Magnesium deficiency: the RHS describes purple or yellow areas between the veins of older lower leaves, with the veins themselves staying green. No speckling, no webbing, no mites. Mottling of older leaves usually points to magnesium.
- Nitrogen deficiency: uniform pale-green to yellow across the whole older leaf rather than a dotted pattern, with the plant generally pale and weak.
Reach for a hand lens and give the leaf a tap. Mites move, deficiencies do not. If the yellowing sits on the lower leaves and follows the veins, reach for feed before you reach for predators. Our piece on cucumber leaves turning yellow walks through the feeding side in detail, and the wider tomato problems diagnostic helps when more than one thing is going wrong at once.
Spider mite on cucumbers and a season-long plan
Cucumbers are the canary. Their thin, fast-growing leaves show stipple early and webbing fast, so a cucumber house is often where you first meet the pest each summer. Tomatoes and aubergines follow once the same hot dry conditions take hold, with French beans not far behind. Catching it on the cucumbers buys you time to act before the whole house is involved.
A workable season runs in three moves. Through spring and early summer, keep the house humid by damping the floor on warm mornings and check leaf undersides weekly, especially near the glass. At the first sign of stipple or webbing, hold the humidity in that 60 to 80 per cent band and introduce Phytoseiulus once daytime temperatures sit around 21°C, repeating the release a week later if webbing persists. In autumn, strip out old crops and clean the staging joints so the orange-red females have nowhere to overwinter.
The same warm dry spell that breeds mites also stops fruit setting and stresses tomatoes, so the jobs overlap and the morning damp-down you do for the mites helps the flowers hold too. If your plants are dropping blossom in the heat, why greenhouse fruit is not setting covers the pollination side, and the general guide to growing greenhouse tomatoes ties the watering and feeding together.
Quick answers on red spider mite
The fastest route through an outbreak is one decision repeated: raise humidity into the 60 to 80 per cent band, release Phytoseiulus persimilis at the first webbing once the house holds around 21°C, and clean the building in autumn so the resting females have nowhere to wait out winter. Do those three and a hot summer stops being the pest's ally.
Does raising humidity really fight red spider mite?
Yes, within limits. The RHS confirms the mite thrives in warm dry air and that high humidity plus syringing reduces the risk. Moist air hampers the mite and supports its predator, but it will not clear an established colony alone, so pair damping down with Phytoseiulus persimilis.
How long until the predator clears the mites?
It depends on temperature and numbers. At about 20°C Phytoseiulus eats up to 5 adult mites a day and breeds roughly twice as fast as its prey, so a release of 2 to 50 per square metre, repeated once or twice at weekly intervals, brings most home outbreaks under control over a few weeks rather than days.
One last number to act on: check leaf undersides weekly from March to October, because at 21°C a new generation of mites lands every 12 days and the colony you ignore for a fortnight is the one that webs the house.
How to spot red spider mite
Red spider mite gives itself away as a fine, pale mottling or stippling on the upper leaf surface, as if the green has been dusted out of it. Turn a leaf over and you may see tiny moving specks, and as numbers build, fine silk webbing strung across the leaf joints and shoot tips. It thrives in hot, dry, still greenhouse air.
| What you see | What it means |
|---|---|
| Fine pale speckling or stippling on upper leaves | Early feeding damage from the mites |
| Tiny moving dots on the leaf underside (a hand lens helps) | The mites themselves |
| Fine silk webbing in leaf joints and shoot tips | A heavier, established infestation |
| Worst in a hot, dry, unventilated greenhouse | The conditions that let them multiply fast |
