TL;DR: Red spider mite (Tetranychus urticae) loves hot, dry conditions — exactly what greenhouses and polytunnels deliver in summer.1 Spot it early by checking leaf undersides, raise humidity, and use biological or gentle soap-based controls before the colony explodes.
Why Spider Mite Thrives Under Cover
Spider mites are not insects — they are arachnids, more closely related to spiders than to beetles or aphids.11 They have four pairs of legs, no antennae, and a single oval body.11
They thrive in hot, dry weather and damage many types of ornamental and vegetable plants.2 Greenhouses and polytunnels trap heat and reduce air movement, creating exactly those conditions.4
The mite hides on the undersides of leaves and feeds by sucking sap.4 This causes upper leaf surfaces to turn pale and mottled — a sign many gardeners mistake for a nutrient problem.4
How to Identify Spider Mite Damage
There are three clear signs to look for. The first is pale, whitish or yellowish speckling on leaf surfaces — called stippling — where the mite has removed the green pigment.2 Heavy feeding can cause leaves to lose almost all green colour.2
The second sign is fine silk webbing on and under leaves.10 Once you see webbing, the colony is already large.4
The third sign is the mites themselves. They are less than 1 mm long and easy to miss.8 Look closely at the underside of a damaged leaf — what looks like tiny moving dust particles may be the mites.8
Damage typically starts in the lower growth of a plant and expands upward as the population grows.2 Check lower leaves first, since damage starts there.2
Despite the name, red spider mite is yellow or pale green for most of the growing season.4 Mites only turn red as autumn approaches.4 During summer, look for pale or greenish specks rather than bright red dots.
Plants Most at Risk
Two-spotted spider mite feeds on a wide range of plants.2 Under cover, aubergines are particularly favoured.8 Cucumbers, tomatoes, peppers, and many ornamentals are also common targets.1
Drought-stressed plants are especially vulnerable.3 Keeping your plants well-watered reduces stress and makes them less attractive to mites.
Step 1 — Scout Weekly
Walk your greenhouse or polytunnel regularly during warm spells. Turn leaves over and look at their undersides. Check lower leaves first, since damage starts there.2
If you see pale mottling, look for webbing and tiny moving specks.8 Catching an infestation early, before colonies spread, gives you more options for control.
Step 2 — Raise Humidity
Spider mites thrive in hot, dry weather, so raising humidity is one of the simplest preventive steps.2 Mist the floor and staging in your greenhouse on hot days. Damp air slows mite reproduction.
Drought-stressed plants are easily damaged by spider mites.3 Water consistently rather than letting compost dry out between waterings during warm periods.
Step 3 — Wash Mites Off by Hand
For mild infestations, a strong spray of water on leaf undersides dislodges mites and disrupts webbing.9 This is a chemical-free first response that dislodges mites without harming the plant.
Remove any badly affected leaves cleanly and dispose of them away from the growing area. Do not compost heavily infested material.
Step 4 — Use an Insecticidal Soap Spray
Insecticidal soap is a recognised control for spider mites.9 It works on contact, so thorough coverage of leaf undersides is essential.
Check that the product is approved for use on edible crops if treating vegetables, and keep treated plants away from children and pets until the spray has dried.
Step 5 — Try Biological Control
Broad-spectrum pesticides used for other insect pests can contribute to large spider mite populations by killing the natural enemies that keep mite numbers in check.3 Biological control avoids this problem entirely.
Maintaining a diverse garden habitat that includes many perennial plants can help support natural predator populations.3 Under glass, you can also purchase and introduce predatory insects.
Green lacewings are one example of a beneficial predator that feeds on spider mites.9 These are available from specialist suppliers and can be introduced into a greenhouse or polytunnel.
Introduce biological controls as early in an infestation as possible. Introduce biological controls as early as possible, before the infestation becomes heavy.
Step 6 — Chemical Controls as a Last Resort
If other methods have not controlled a heavy infestation, a proprietary miticide may be an option. Always follow the manufacturer's label precisely.1
Follow all label instructions carefully, including any restrictions near water or wildlife.
Note that using particular broad-spectrum chemicals for other insect pests can make spider mite problems worse, not better, by removing natural predators.3 Targeted, low-impact products are preferable.
Preventing the Next Outbreak
Good growing hygiene reduces the chance of mites returning. Clear out plant debris at the end of the season — spider mites can overwinter in sheltered spots.5 The pest is most active from March to October.5
Isolating new plants before introducing them to your greenhouse is a practical habit.9 A two-week quarantine lets you spot any hidden pest problem before it spreads to established plants.
Keeping plants well-watered all season matters too. Drought-stressed plants are far more susceptible to mite damage.3
Quick Reference Checklist
- Scout leaf undersides weekly, starting with lower growth.2
- Look for pale stippling, fine webbing, and tiny moving specks.8
- Mist floors and staging on hot days to raise humidity.2
- Water consistently — drought stress makes plants vulnerable.3
- Knock mites off with a water spray as a first response.9
- Apply insecticidal soap, following the label carefully.9
- Consider biological predators, introduced early.9
- Avoid broad-spectrum pesticides that kill natural enemies.3
- Clear debris at season's end to reduce overwintering sites.5
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