Red spider mites
Learn organic methods to control red spider mites, in our expert guide.
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What are the red spiders on my plants?
Tiny red 'spiders' on house plants and greenhouse crops are actually sap-sucking mites called red spider mites. There are many types of spider mite, but in the UK the red spider mite (Tetranychus urticae) is the species most likely to be found on indoor and greenhouse-grown plants. Red spider mites are around 0.5mm long and are also known as the two-spotted mite. They are usually considered a pest as they suck sap from plants, reducing their vigour. If left unchecked, red spider mites can multiply quickly and wreak havoc in a greenhouse or on house plants indoors.
For most of the year, red spider mites are a pale green colour with two dark spots on their back, but in autumn and winter they turn orange-red and may be spotted in cracks in windowsills and in our homes, as they look for places to hibernate. Red spider mites thrive in hot, dry conditions so life in the greenhouse or a centrally-heated home suits them perfectly. If the temperature stays above 12ºC, red spider mites can breed all year round.
Red spider mite symptoms
Red spider mites are easy to spot on green house plants and house plants. The easiest way to identify them is to look for fine, cobweb-like webbing on the leaves and stems of plants, while mottling is visible on the upper surface of leaves. Using a magnifying glass, look for small mites and eggs on the undersides of leaves. Affected plants will lack vigour and may eventually die.
Find red spider mite on...
Spider mites are typically found on house plants and greenhouse plants, including greenhouse tomatoes, cucumbers, aubergines and peppers, along with perennial crops like grapevines, peach and nectarine trees. Vulnerable house plants include poinsettias and orchids. If conditions remain warm and dry in summer, you may find red spider mite outside on fuchsias, runner beans, pelargoniums and other bedding plants.
How to kill red spider mite
From early summer, inspect greenhouse and house plants weekly and deal with the first signs of infestation straight away. Remove infested leaves, buds and stems as these will contain eggs, so you will immediately reduce the size of the infestation and the potential damage done to your plants.
Red spider mites thrive in warm, dry conditions, so the easiest and safest way to get rid of them is to increase humidity. This will stop them from being able to reproduce, which will quickly stop them feeding on your plants. In the greenhouse you can boost levels of humidity by misting plants with tepid water, standing bowls of water on the benches between plants, and damping down the floor of your greenhouse with water. In the home, stand house plants on a tray of pebbles and water, with the water sitting just beneath the pebbles so it evaporates into the room instead of being absorbed by the plant's roots. Misting the leaves or moving the plant into a more humid room, such as your bathroom or kitchen, can also help stop red spider mites from reproducing on your plants.
For large infestations, you can use predatory mites to control red spider mite. In summer, once daytime temperatures in the greenhouse hover steadily around 21ºC, you can try releasing the predatory mite, Phytoseiulus persimilis on to your plants. This is the most common biological pest control for red spider mite in greenhouses and the home. The predatory mites feed on spider mites at all stages of their lifecycle, including eggs. This quickly reduces the spider mite population and therefore plant damage.
If temperatures remain lower than 15ºC (the minimum temperature at which Phytoseiulus persimilis can survive), try using sprays containing fatty acids or plant oils, instead.
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