The red spider mite, more accurately known as the two-spotted spider mite (Tetranychus urticae), is one of the most common and damaging pests in greenhouses. It is also a frequent problem on houseplants, garden plants and outdoor crops across the UK. In British usage, the name red spider mite is often used even though the mites are not always red.
This article explains where spider mites occur, which plants they attack, how to identify damage, and how to prevent and control infestations effectively under UK conditions.
Occurrence and Distribution
Where are spider mites found?
Spider mites are extremely widespread and adaptable. In the UK they are especially common in:
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Greenhouses and polytunnels
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Conservatories and indoor plant displays
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Gardens during warm, dry summers
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Outdoor crops in sheltered or sunny locations
In greenhouses they are often referred to as glasshouse spider mites, but they are the same species that infest houseplants and outdoor plants.
Spider mites thrive in warm, dry conditions and are most problematic from late spring through summer.
Host Plants and Feeding
What do spider mites feed on?
Spider mites attack more than 200 different plant species, making them a major problem for gardeners and growers.
Common host plants include:
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Vegetables such as cucumbers, tomatoes, beans, peas and lettuce
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Fruit plants and bushes such as strawberries, currants and blueberries
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Trees and shrubs, including fruit trees
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Ornamental plants and flowers
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Many common houseplants
They feed by piercing plant cells with needle-like mouthparts and sucking out the contents, which leads to visible leaf damage and weakened plants.
Identification
How can spider mites be identified?
Spider mites are extremely small and difficult to see with the naked eye.
Key identification features include:
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Very small size (about 0.4–0.6 mm)
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Colour varies: greenish, yellowish, orange, reddish or brown
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Two dark spots visible on the body when viewed closely
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Fine webbing on leaves and between stems in established infestations
To confirm their presence:
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Use a magnifying glass to inspect the undersides of leaves
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Look for mites, webbing and shed skins
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Hold a white sheet of paper under a leaf and tap or scrape the underside – mites will fall onto the paper and become visible as moving dots
Early Control Measures
Removing heavily infested leaves
To reduce mite numbers quickly, remove the most heavily infested leaves. These leaves are already severely damaged and contribute little to plant health.
Always destroy removed plant material immediately to prevent mites spreading to other plants.
Biological Control
Using natural predators
Biological control is the most effective and sustainable method of managing spider mites.
Predatory mites and insects feed on spider mites and keep populations under control without harming plants, people or beneficial insects.
Commonly used predators include:
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Predatory spider mite (Phytoseiulus persimilis) – the most effective and widely used predator
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Predatory mite (Amblyseius cucumeris)
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Minute pirate bug (Orius majusculus)
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Green lacewing (Chrysopa perla)
Phytoseiulus persimilis can consume up to 20 spider mite eggs or 5 adult spider mites per day, making it the primary choice for control in greenhouses.
Predatory mites can be distinguished from spider mites by their longer legs, faster movement and often red or orange colouring.
Environmental Management
Temperature and humidity
Spider mites reproduce fastest in hot, dry conditions. Adjusting the growing environment can significantly slow population growth.
Effective measures include:
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Lowering temperatures where possible
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Increasing humidity, especially in greenhouses
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Regular misting of plants (especially leaf undersides)
These measures also act as the main preventive strategy, although complete prevention is rarely possible.
Spider Mites in Greenhouses
Common greenhouse problems
In greenhouses, spider mites most commonly attack:
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Cucumbers
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Tomatoes
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Beans and peas
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Lettuce
Greenhouse infestations should be managed primarily through biological control and environmental adjustment. In severe cases, insecticidal soap may be used as a last resort.
Spider Mites on Houseplants
Control indoors
On houseplants, infestations are often easier to manage than in greenhouses.
Recommended steps include:
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Regularly spraying plants with water to dislodge mites
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Paying particular attention to the undersides of leaves
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Increasing humidity around plants
If water alone is not sufficient, insecticidal soap may be used carefully on affected plants.
Chemical Control
Insecticidal soap
In practice, insecticidal soap is the only widely effective chemical option against spider mites.
Important considerations:
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Use only on heavily infested plants
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Apply directly to mites, especially on leaf undersides
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Be aware that soap kills all mites, including beneficial predators
For this reason, biological control is usually preferred whenever possible.
Plant Damage Caused by Spider Mites
Typical symptoms
Spider mite damage develops progressively:
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Leaves turn pale, greyish or yellow
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Fine speckling appears on leaf surfaces
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Dead patches develop in advanced infestations
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Leaves may dry up and fall off
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Flowers may turn brown and wilt
All spider mites spin fine webbing, which protects eggs and helps mites move between plant parts.
Life Cycle
The spider mite life cycle
Spider mites go through four life stages:
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Egg
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Larva
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Nymph
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Adult
Under ideal conditions (high temperature and low humidity), development from egg to adult can take 5–7 days. At around 26 °C, development typically takes about 10 days. Under normal UK conditions, development usually takes 15–20 days, depending on weather.
A single female can lay 50–200 eggs during her lifetime. Unfertilised eggs develop into males, while fertilised eggs become females. Females usually outnumber males.
Adult mites overwinter in grass, weeds, leaf litter and tree bark. As temperatures rise in spring, they move onto host plants, mate and begin laying eggs.
Effect of Weather on Spider Mites
Why hot, dry weather increases infestations
Weather plays a major role in spider mite outbreaks.
Warm, dry conditions:
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Shorten development time, producing more generations
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Reduce naturally occurring fungal pathogens that kill mites
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Reduce populations of natural predators
As a result, spider mite populations can increase dramatically during dry summers.
Spider mite success is strongly linked to temperature and humidity, which is why careful environmental management combined with biological control is the most effective long-term strategy.