Red spider mite is a very damaging plant pest. They are tiny and very difficult to see, until they build up in large numbers and start producing their characteristic webbing. And confusingly, they are rarely red!
Red spider mites are perhaps the smallest of the common sap-feeding insects, being less than 1mm in size. They are so small you may need a magnifying glass to see them – but despite their size, they’re very damaging pests.
They like it warm and dry, which is why they can commonly be found indoors, in conservatories, sun lounges or greenhouses. They can also be found outside if the summer is dry and warm.
For most of the year, they're not red at all – they are a yellowish-green colour. They only turn red as summer turns into autumn. The large red mites you sometimes see climbing up walls are actually velvet or blood mites, and are not harmful to plants at all.
Leaves first develop a pale mottling, but as the infestation progresses the leaves become increasingly yellowish white. Turn the leaf and look closely or with a magnifying glass to see the tiny mites and their globe-shaped eggs. Large colonies and infestations produce a characteristic fine webbing, which provides some protection from predators.
Look out for fine filamentous webbing on leaves
Most broad-spectrum insecticides will control red spider mite. For edible plants, only use an insecticide that is approved for use on the relevant crop. For ornamental plants, you will get much better control using a systemic insecticide. These kill on contact and also protects the plant for up to 3 weeks.