Spider mites (Class Arachnida) are tiny sap-sucking plant pests. They attack the underside of leaves and suck the vigor from the plant, spider mite is one of the most difficult pests to control in greenhouses, conservatories, polytunnels, on pot plants and in the garden. Large infestations can even kill a plant. As soon as you spot an infestation, it's time to take charge and get rid of them!