How to identify and control an infestation of glasshouse red spider mite

Learn how to eradicate red (or two-spotted) spider mite in your greenhouse.

Scientific name for red spider mite: Tetranychus urticae.

Red or “two-spotted” spider mite are a common pest in the greenhouse. They cause problems for glasshouse and garden plants, both ornamental and edible.
As with most gardeners, I have experience in dealing with this minuscule mite. They can cause leaves to drop off and if not dealt with in time, a badly infested plant may die completely.

Below, I am going to tell you the symptoms to look for when spotting these tiny mites, and especially teach you the ways to control them, to ensure they do not damage your plants.

Facts about red or two spotted spider mite:

• They are the smallest, common, sap-sucking mite - only really visible to the naked eye when present in large numbers. You may need a magnifying glass to see them!
• They thrive in warm, dry conditions which is why they are most often found in conservatories and greenhouses. However during particularly hot, dry summers, you will also find them outdoors.
• They are most active in March through to October, but in a heated greenhouse, they can be found all year round.
• Despite their common name, “red spider mite”, they are not red all year round. During the spring and summer they are a yellowy- green with two darker spots (giving them their other name, “two spot spider mite”). It isn’t until autumn through to winter that they become orangey-red, during their resting period. 
• They are often confused with velvet or blood mites (larger bright red mites) which do not cause any damage to plants.
• They are related to their eight-legged spider friends.

Plants most affected by red spider mite:

• Spider mite affect many greenhouse, garden and house plants. Unfortunately their plant-host range is so vast that there are very few plants that are completely immune to them. 
• They affect both ornamental and edible crops including tomatoes, peppers, Pelargoniums, Impatiens and Orchids.

Signs that you may have an infestation of red spider mite:

• Mottled leaves - Leaves develop a pale mottling on the upper leaf surface. This is caused by the mites sucking up the plant’s juices and injecting digestive secretions - this destroys the cells and creates this mottled appearance (which will become increasingly yellowy-white).
• Mites on underside of leaves - Yellowy-green mites, white cast-skins and globe-shaped eggs will be visible on the underneath of the leaf. You may need a hand glass to see them, as they’re so tiny.
• Webbing on leaves - When the infestation is quite bad, you may be able to spot a fine silk-like webbing on the leaves. This webbing provides the mites with some protection from predators.
• Leaf drop - When leaves lose most of their green colour, they may dry up or fall off. Eventually, if the plant becomes severely weakened it will die.

• Biological control - There are several biological control options which can be used instead of using pesticides. These include a predatory mite (Pytoseiulus persimilis), a predatory midge (Feltiella acrarsuga) and a rove beetle (Atheta coriaria). Make sure you don’t use any biological controls in conjunction with insecticides because they will also be affected and killed by the chemicals.
• Physical control - Try physically removing infested plants from the greenhouse/environment in late summer, before the temperatures drop. This minimises the chances of the mites overwintering. As the weather cools down, the red spider mite seeks shelter and will remain dormant for the cold months - then “wake up”.
• Cultural controls - These are all really good habits to get into for generally keeping your indoor environment free from plant pests and diseases. Keep everything tidy and clean. Remove plant debris, old stakes/canes, plant ties etc. from your glasshouse/conservatory to reduce places for overwintering mites to shelter.
• Regularly empty your greenhouse so that you can properly clean and disinfect it with a suitable glasshouse disinfectant.
• Remove weeds from in and around the greenhouse, as weeds can act as hosts for red spider mite.
• Spray/mist the plants and environment regularly to raise the humidity - the mites thrive in dry conditions so a humid environment will help control them.
• Don’t overcrowd the space to allow for good air-flow and natural ventilation (which in turn will keep the temperature down a bit).
• Chemical control - Use a general insecticide to kill off this pest - they are easily controlled this way as it kills them on contact, but of course, this may not be the most eco-friendly way of destroying them. If you are spraying edible crops/plants, make sure you use an appropriate spray and always read the labels. 

If you have any questions about a pest affecting your greenhouse plants, or have a query about something I have said above, please comment below or get in touch.