Glasshouse red spider mite

What is glasshouse red spider mite?

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

It attacks many common houseplants and greenhouse plants, both ornamentals and edibles, including: vines, peach, nectarines, cucumbers, tomatoes, aubergines, peppers, Fuchsia, Pelargonium, poinsettias, orchids and Impatiens.

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 late 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 easliy 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

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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 have developed resistance to some insecticides. Biological control is an attractive alternative to using insecticides as it avoids resistance problems and the risk of spray damage to the plants.

Non-chemical control

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 on a regular basis 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 insecticides, 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 (Vitax Organic 2 in 1 Pest and Disease Control, Growing Success Fruit & Veg Bug Killer, Scotts Bug Clear for Fruit & Veg) or fatty acids (Bayer Organic Pest Control, Doff Greenfly and Blackfly Killer or Greenfingers Organic Pest Spray) 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

• Insecticides containing bifenthrin (Bayer Sprayday Greenfly Killer Plus, Scotts Bug Clear Gun or Doff All-In-One Garden Pest Killer) will control red spider mite, provided it has not gained resistance to this substance. Bifenthrin can be used on some edible plants, but see the label instructions. This pesticide is being withdrawn from sale on 31 May 2010 but can continue to be used until 31 May 2011
• Alternatives are sprays containing thiamethoxam* and abamectin (Westland Plant Rescue Bug Killer Ornamental Plants Ready To Use) or acetamiprid (Scotts Bug Clear Ultra Gun), which are for use on ornamental plants only
• Edible plants can be sprayed with plant oils, plant extracts or fatty acids. The latter pesticides may require more frequent applications

* Please note this product is being withdrawn. The withdrawal (in effect a ban) comes into force on 30 September 2013, but there is to be a period of grace to use up these materials by 30 November 2013. After this time it will be illegal to use them, any remaining products should be taken to a manned local authority household waste site where they should be handed over to the staff. For information on waste disposal sites see www.pesticidedisposal.org. It remains legal to use other neonicotinoid-based products that are not affected by the withdrawal. This withdrawal follows concern over their effects on bees.

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

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.

They overwinter as non-feeding adult mites on plants or hide in crevices in greenhouse structures or the soil. They become active again during March in greenhouses but may not become a problem on outdoor plants until mid- to late summer.

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.