The red spider mite is tiny and loves warm, dry conditions. Despite being so small, this annoying pest can badly damage the plants you keep in your conservatory, greenhouse or on a sunny window sill. March is the time to take early action against this infuriating pest.

What are red spider mites?

These are not the easily visible, bright red mites you sometimes see running around on walls and paths. The red spider mite (Tetranychus urticae) is tiny, up to 1mm (less than 1/16in) long, and very difficult to see without a magnifying glass. Rather surprisingly, it is not red, except during the autumn and winter resting period when it turns orange-red. For most of the year the mite is yellow-green with two dark spots. It is also known as the two-spotted spider mite.

Red spider mite

Red spider mite aka two-spotted spider mite (Tetranychus urticae). © J Holopainen and reused under http://www.gnu.org/copyleft/fdl.html

During the winter months the orange-red, mature females rest in cracks and crevices in places like brick walls, glasshouse frames, stakes, canes, soil and plant debris. They move out from late March and start feeding by sucking sap from plants and laying their round eggs on the underside of leaves. This can go on until October. They like all kinds of plants from houseplants to cucumbers and tomatoes.

It’s very difficult to see that you’ve got red spider mite until it's done a fair bit of damage to your plants. If your plants are not thriving, though, use a magnifying glass to look closely at the underside of leaves.

At first there is a very fine yellow speckling or mottling of the leaf surface. Then leaves look scorched or bleached, become brittle and die. In advanced cases fine webbing appears on the leaves and stems and, by this time, the plant is probably not worth saving.

Cucumber attacked by red spider mite

Cucumber attacked by red spider mite. © Rasbak and reused under https://creativecommons.org/licenses/by-sa/3.0/

Start early in the year to control red spider mite

Clean conservatories, greenhouses and window sills thoroughly to eliminate any adult females overwintering in nooks and crannies. Even if you did this in the autumn it's worth giving the place another going over.

Continue throughout the year

You could spray with an insecticidal soap based on fatty acids or use biological controls which can be bought easily online.

Before red spider mite appears and when temperatures average 10C or more, introduce the predatory mite Amblyseius. This is a good preventative measure.

As soon as you see red spider mite and when the temperature is above 16C (including at night) and below 30C, introduce the predatory mite Phytoseiulus persimilis. This reproduces much faster than the red spider mite and will soon get an infestation under control.

Good humidity levels can help prevent infestations and if there is one, can keep numbers down. Stand plants on trays of pebbles and water, mist regularly, and damp down greenhouse floors.