Glasshouse red spider mite

Glasshouse red spider mite is a common sap-feeding pest causing mottled leaves and early leaf loss on greenhouse and garden plants. It is also known as the two-spotted spider mite.

Glasshouse red spider mites on webbing.

Quick facts

Common name Glasshouse red spider mite or two-spotted spider mite
Scientific name Tetranychus urticae
Plants affected Many greenhouse and garden plants, and houseplants
Main symptoms Mottled foliage and early leaf fall
Most active March to October

What is glasshouse red spider mite?

Glasshouse red spider mite is one of the most troublesome pests of greenhouse plants, houseplants. It can also attack garden plants in the summer. It is a sap-sucking mite that attacks the foliage of plants, causing a mottled appearance, and in severe cases, leaf loss and even plant death.

It attacks a wide range of houseplants and greenhouse plants, both ornamentals and edibles, including: vines, peach, nectarines, cucumbers, tomatoes, aubergines, peppers, Fuchsia, Pelargonium, poinsettias, orchids and Impatiens.
The host range of this pest is so wide that few plants are completely immune.

Glasshouse red spider mite thrives in warm, dry conditions, so is usually only a problem from March to October, but damage can occur at other times in a heated greenhouse. It will also cause problems outdoors in summer, especially in hot, dry weather.

Symptoms

You may see the following symptoms:

  • On leaves: Plants infested with glasshouse red spider mite show a fine pale mottling on the upper leaf surface. The underside of the leaves have many tiny yellowish green mites and white cast skins and egg shells. These are more easily seen with the aid of a x10 hand lens
  • On plants: In heavy infestations, you may see fine silk webbing on the plants, and the leaves lose most of their green colour and dry up or fall off. Heavily infested plants are severely weakened and may die

    Plants infested with glasshouse red spider mite show a pale mottling and may dry up and fall off.

    Control

    Glasshouse red spider mite can be difficult to control as it breeds rapidly in warm conditions and some strains of the mite are resistant to some pesticides. Biological control is a viable alternative to using pesticides, it can give good control and as it avoids resistance problems and the risk of spray damage to plants.

    Non-chemical control

    Cultural. Remove severely infested plants from glasshouses in late summer before lower temperatures and shorter days induce the females to seek sheltered places where they will remain dormant for the winter period.  To reduce overwintering mites to a minimum, clear out plant debris, old canes, stakes and plant-ties before the spring.  Empty glasshouses can be washed down thoroughly with a glasshouse disinfectant.  Weeds in and around the glasshouse should be kept down as these can act as hosts for the pest. Plants grown at high temperatures in dry, overcrowded glasshouses are more liable to severe infestation.  Regular syringing of plants with clear water and maintaining a high humidity reduce the danger of severe attacks, but will not on its own control this pest.

    Biological. A predatory mite (Phytoseiulus persimilis) feeds on the eggs and active stages of glasshouse red spider mite. It needs good light and daytime temperatures of 21ºC (70ºF) or more if it is to breed faster than the pest. Its effective period of use is normally April to October in greenhouses; June to September outdoors.

    As the predator is susceptible to pesticides, biological control cannot be used in conjunction with most chemical controls. The exceptions are those with very short persistence, such as plant oils or extracts (e.g. Vitax Organic 2 in 1 Pest and Disease Control, Bug Clear for Fruit & Veg) or fatty acids (e.g. Bayer Organic Pest Free, Doff Greenfly and Blackfly Killer, Bayer Natria Bug Control) or urea/mineral lattice (SB Plant Invigorator), which can be used to keep mite numbers down before it is time to introduce the predator.

    Note that Phytoseiulus persimilis will not control other species of red spider mite, such as fruit tree red spider mite, citrus red spider mite, box red spider mite and conifer red spider mite.

    Phytoseiulus and compatible biological controls for most other greenhouse pests can be obtained by mail order from specialist suppliers.

    Chemical control

    • Pesticides containing  acetamiprid (e.g. Bug Clear Ultra) or thiocloprid and methiocarb (e.g. Bayer Provado Ultimate Bug Killer, Aerosol)  are available for use on ornamental plants only
    • Edible plants can be sprayed with plant oils, plant extracts or fatty acids. These pesticides may require frequent applications to control the mite.

    Download

    Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

    Biological control suppliers (Adobe Acrobat pdf document)

    Biology

    Despite their common name, during the spring and summer these sap-sucking mites are yellowish-green with a pair of darker markings. Because of this, they are sometimes called the glasshouse two-spotted spider mite. They only become orange-red during the autumn and winter resting period. The mites are small, up to 1mm (less than 1/16in) long, and are just visible to the naked eye, when they are present in large numbers. They are usually found on the lower leaf surface, along with their spherical eggs.

    Orange-red, mature female mites spend the winter months resting in cracks and crevices of brick walls, glasshouse frames, stakes, canes, soil and plant debris.  From late March onwards they leave their resting places and start feeding and laying eggs.  The length of the life cycle depends on the temperature, but breeding is continuous from March to October.  At 10°C (50°F) the life cycle takes about 55 days, but this is reduced with increasing temperatures and at 21°C (70°F) development is completed in 12 days.  Both young and adult mites pierce plant tissues with their mouthparts and feed on the cell contents.  Severe damage can soon develop in warm dry conditions as this favours rapid reproduction by the mites.

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    • Julia99gf avatar

      By Julia99gf on 20/05/2014

      Hi I have a hydrangea in my garden which has red spider mite (pretty confident). Many of the leaves are brown, disintegrating and there are very small red bugs on the leaf underside as well as webs. I have red the advice on glasshouse red spider mite but still have the following questions: 1) if the female spiders (the red ones) are already active is it too late to save the plant (which looks pretty bad at the moment)? I suspect it may not flower as the flower buds look damaged and brown too. 2) Should I just get rid of it to stop the mite spreading to the cosmos and salvia that are next to it (only put them in recently)? 3) If I can save it and should treat it, I am not keen to use chemicals. The RHS website recommends "plant oils or extracts (e.g. Vitax Organic 2 in 1 Pest and Disease Control, Bug Clear for Fruit & Veg) or fatty acids (e.g. Bayer Organic Pest Free, Doff Greenfly and Blackfly Killer, Bayer Natria Bug Control) or urea/mineral lattice (SB Plant Invigorator), which can be used to keep mite numbers down before it is time to introduce the predator." Any advice on where to get these//what is best/what to do with them/if I have to introduce a predator too?!?! Thank you. I am a new but keen gardener and have never treated pests on plants before.

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