Spider mites
Glasshouse red spider mite thrives in hot, dry air. Tiny mites feed under leaves, leaving a fine mottling that turns leaves yellow and bronze; heavy infestations spin webs over the shoot tips.
Pests
What it looks like
The damage is obvious long before the mites are.
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Fine speckling Tiny pale dots over the upper leaf surface. -
Bronzed leaves Leaves turn dull yellow-bronze and dry. -
Webbing Fine silk strung between leaves and stems. -
Specks that move Under a lens, tiny mites and eggs under the leaf.
How to confirm it
A white sheet of paper helps.
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Tap onto paper
Tap a leaf over white paper; moving specks are mites.
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Use a lens
Look under leaves for pale or reddish mites and round eggs.
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Check the air
Dry, warm air favours them; they are rare in humid conditions.
What to do
Raise humidity and bring in predators.
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Mist and damp down
Raise humidity in the greenhouse or around houseplants.
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Introduce Phytoseiulus
The predatory mite Phytoseiulus persimilis controls them under glass.
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Wash leaves
Hose or wipe leaf undersides regularly.
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Remove worst leaves
Take off badly webbed leaves and bin them.
Prevent it next time
- Keep greenhouses humid and ventilated in summer.
- Water plants well; stressed plants suffer more.
- Clean the greenhouse thoroughly in winter where mites overwinter.
What not to do
Do not let plants dry out in heat; drought and dry air are what mites need.
In the growing guides
Where the guides mention it, with plant-specific advice.
Sources
- Glasshouse red spider mite, Royal Horticultural Society