If you see tiny red spiders in the UK, there’s a good chance you’re looking at red spider mites. These damaging plant pests can be very difficult to see until they build up in large numbers and start producing their characteristic webbing. And, confusingly, they are rarely red!
Belonging to the same group of creatures as spiders, red spider mites have eight legs and their full name is ‘glasshouse red spider mite’ or GRSM (Tetranychus urticae). Red spider mites, like many insect plant pests, eat plant sap. They’re often confused with concrete mites, red velvet mites or clover mites, but it’s vital to identify and eradicate them as soon as possible, before these tiny red spiders do too much damage to your plants.
What do spider mites look like?
Red spider mites are perhaps the smallest of the common sap-feeding insects, being less than 1mm in size. They are so small you need a magnifying glass to identify them accurately and they are often easier to identify by examining the damage done to plants (see below). Despite the size of these tiny red spiders, the damage they can cause can be extensive.
For most of the year, red spider mites are not red at all – they are yellowish-green with two dark blotches towards the head. They turn red in the autumn and winter resting period. They aren’t to be confused with the larger, fast moving, bright red mites you sometimes see in the garden: these are actually red velvet mites and are not harmful to plants at all.
Red spider mites like it warm and dry, which is why they can commonly be found indoors, on house plants or in conservatories or greenhouses. If the summer is dry and warm red spider mites in the UK can also be found outside from March to September where they can become a pest on beans, strawberries, raspberries, currants, roses and other ornamental plants. As they suck the sap from the plants, their leaves begin to look mottled, and will eventually die off as their vigour reduces over time.
Red spider mite symptoms and damage
Leaves first develop a fine, pale mottling and become dull green. As the infestation progresses leaves turn yellowish white which can be mistaken for a mineral deficiency but look underneath the leaf with a magnifying glass and you will see the tiny red spider mites and their round eggs. Large colonies produce a characteristic fine silk webbing between leaves and stems. Plants shed their leaves, leaving just young leaves at the shoot tips.
Red spider mites treatment: removal and prevention
Removing these tiny red spiders is only one part of keeping your plants healthy — you’ll need to stop them coming back, too. There are several methods of spider mites treatment open to you, some of which are simple and manual, while others require biological or chemical assistance:
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Look for mites regularly: check susceptible plants frequently from March onwards so you can act before a red spider mite infestation takes hold.
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Clear out debris: remove any items that the mite can use as a host to multiply. Clear out plant debris, old canes, stakes and plant-ties from the greenhouse before the spring and clean the empty greenhouse. You should also remove any weeds growing in or around the greenhouse.
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Keep your greenhouse humid: maintain high humidity because plants grown at high temperatures in dry, overcrowded conditions are more liable to develop severe infestations of red spider mite.
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Remove infested plants: take any plants with bad infestations out of the greenhouse or away from the windowsill. This is especially important in late summer before the females start to look for sheltered places to spend the winter once the temperature drops.
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Introduce a predator: other species of mite, such as Phytoseiulus persimilis, eat red spider mites and so can naturally control the spread. This predator needs a high daytime temperature to be effective. You can also try using it on outdoor plants in high summer. Avoid using an insecticide in these circumstances. as it will also kill the biological control.
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Use a broad spectrum insecticide: spray affected plants several times to break the red spider mite’s life cycle. Before you treat red spider mite on fruit and vegetables, always check that the insecticide is approved for use on edible crops. To protect bees and pollinating insects do not apply to plants when in flower. Do not use where bees are actively foraging. Do not apply when flowering weeds are present.
