Red Spider Mite in Greenhouses – by our blogging horticulturalist Peter Whyte

A few years ago our cucumber plant wasn’t looking good. The leaves were mottled yellow and the plant had completely stopped growing. I watered and fed it, thinking it might be hungry. It got worse. That was my first encounter with red spider mites.

These tiny pests are only red in autumn. In summer adults are yellow or pale green, with two dark spots on their backs. They stay on the undersides of leaves, and you may need a hand lens to see them. Females lay their tiny spherical eggs in clusters and the larvae that emerge suck sap, sometimes leaving clusters of tiny yellow spots on the upper surface of the leaf. Leaves later become mottled and dry, and may fall early. Really severe infestations create a fine web under and between leaves, and may kill plants. The adults overwinter from about October to March on weeds, under soil clods and in crevices in the greenhouse or its equipment. Almost any plant can be attacked.

These mites thrive in warm, dry conditions. Mist or syringe the plants with water, and wet the ground to dampen and cool the air. Destroy (don’t compost) any unwanted infected plants. Clear all weeds and unused equipment and materials from the greenhouse to remove hiding-places. Wash down empty greenhouses with disinfectant. Give plants plenty of light, water and space. Chemicals are not very reliable because most don’t kill all stages of the mites and they quickly build up resistance. Frequent spraying with plant oils, plant extracts or fatty acids may be more effective. The predatory mite Phytoseiulus persimilis gives good control.