How to get rid of red spider mites

Red spider mites are tiny wingless insect-like creatures that have eight legs and a one-piece body. They are so small that a hand lens is needed to see them clearly. Greenhouse spider mites (Tetranychus urticae) are also known as two-spotted spider mite or red spider mite. Another spider mite found indoors is the carmine spider mite (Tetranychus cinnabarinus). 

Are red spider mites something to worry about?

Although they are extremely small, red spider mites can build up to enormous numbers in the right conditions, making them one of the most destructive pests in the garden. They can ruin foliage, spoiling ornamental plants and reducing yields of vegetables, fruit and salad crops. Heavy attacks can even kill plants.

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Which plants are most at risk from red spider mites?

Red spider mites have been found on more than 180 different species of plant. However, some are more susceptible than others. Ones likely to be affected include: begonia, brugmansia, carnation, chrysanthemum, crocosmia, cucumber, dahlia, datura, French and runner beans, fuchsias, fruit trees and bushes, houseplants, impatiens, melon, pelargoniums, primulas, spruce (picea), strawberries and vines.

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Are there any pesticides available for the control of red spider mites?

There are a range of insecticides for control of red spider mites. However, bear in mind that if you have used a pesticide and then want to use a biological control, you’ll need to wait at least a month, as residues from spraying may kill the predators you introduce. By this time the mites will probably be out of control.

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Is there a spray for red spider mites that's suitable for organic gardeners?

Yes, you can use ready-to-use pyrethrum-based sprays which are suitable for traditional or organic gardeners. Spray the leaf undersides every seven days. Biological control can be used safely three to four days after spraying.

Can you tell me more about biological control for red spider mites?

Introduced predatory mites (Phytoseiulus persimilis) can eliminate red spider mites indoors. They can eat up to five adults or 60 eggs per day. The drawback is that they should be introduced when there are only a few spider mites present. You have to examine your plants closely from April onwards, using a hand lens to spot the first few mites. When you see them, order predators from the suppliers listed here. You might have to make further introductions throughout the summer, but often one will be enough.

The effectiveness of the biologicial control is reduced if the conditions are too hot. If you use shading paint on glass and ventilate the greenhouse well, they should be all right.

You can keep an eye on the phytoseiulus by looking with a hand lens for bigger, rounder, redder mites than spider mites. Phytoseiulus won’t survive the winter, so you have to introduce it every year.

If spider mite is a problem every year, then try Amblyseius andersoni. It works more slowly than phytoseiulus, but it can be introduced before overwintering spider mites re-emerge in mid-late March. This is because it can survive for a few weeks without food and also eats pollen, thrips, springtails and other invertebrates. It's active at temperatures as low as 6°C and is also more tolerant of hot, dry conditions. It can be used outside as well as under protection, provided temperatures are suitable. While it can overwinter in the greenhouse, this probably won’t be in sufficient numbers to keep red spider mite in check.

Release all predators (according to the instructions) on to the problem plants.

Suppliers of red-spider-mite controls

• Nematodes Direct nematodesdirect.co.uk
• Dragonfli dragonfli.co.uk
• Ladybird Plantcare ladybirdplantcare.co.uk
• Green Gardener greengardener.co.uk

What causes red-spider-mite outbreaks?

Hot, dry, dusty conditions lead to red-spider-mite problems. They are often worst in greenhouses, conservatories and on houseplants. These conditions encourage the mites to feed often, causing damage to the plants every time they feed. Even plants that do not normally suffer from spider mites become at risk in these conditions. Unhealthy plants are especially vulnerable. Cool, moist conditions can help to reduce spider-mite populations, perhaps by limiting their breeding success.

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