What is glasshouse red spider mite?

Mites are a group of aracnids belonging to the Acari. Most mites are small (< 1mm), they are important parts of most ecosystems, many species being predatory or feeding on decomposing organic matter. Some feed on plants. 

Glasshouse red spider mite can be one of the most troublesome problems of greenhouse plants and houseplants. It can also feed on garden plants during summer. It is a sap-sucking mite that causes a mottled appearance, and in severe cases, leaf loss and plant death.

The mite can feed on a very 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 mite is so wide that few plants are completely immune.

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

Symptoms

You may see the following symptoms:

• On leaves: Plants 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
• Despite the common name, red spider mites are usually yellowish-green with two dark patches. They may be entirely dark, or in the autumn they may become reddish-orange
• On plants: In heavy attacks, a fine silk webbing may be seen on the plants, and the leaves can lose most of their green colour and dry up or fall off. Heavily damaged plants can be severely weakened and may die

Control

Check susceptible plants frequently from spring onwards so action can be taken before a damaging infestation has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.
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.

Non-pesticide controls

Cultural control
Plants grown at high temperatures in dry, overcrowded glasshouses are more liable to damage and severe infestation. Regular syringing and spraying of plants with water and maintaining a high humidity reduce the risk, but will not, on its own control this mite.
Remove severely infested plants from glasshouses, especially 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 cleaned thoroughly with a glasshouse disinfectant. Weeds in and around the glasshouse can be kept in check as these may act as hosts for the mite.
Biological control
There are several biological controls available for glasshouse red spider mite, which are now widely used in preference to pesticides. These include a predatory mites Phytoseiulus persimilis and Amblyseius, a predatory midge (Feltiella acrarsuga) and a rove beetle, Atheta coriaria. These biological controls are available via mail order - download a list of biological control suppliers

Biological control and pesticides
All the biological controls are susceptible to pesticides and so cannot be used in conjunction with most pesticidal controls. The exceptions are those with very short persistence, which can be used with care up to a day before introduction of biological control predators. These include plant oils or extracts or fatty acids and plant invigorators (see under Pesticide Control for example brand names), which can be used to keep mite numbers in check before it is time to introduce biological control.

Predatory mite Phytoseiulus persimilis

• Phytoseiulus persimilis is the most commonly used biological control for red spider mite and is now widely used in preference to pesticides
• Phytoseiulus mites are about the same size as glasshouse red spider mite (0.5 mm), they can be distinguished if examined with a hand lens
• Phytoseiulus mites have orange-red pear-shaped bodies, are more active than the red spider mite, and can often be seen tapping the leaf surface with their front legs in search of prey
• Glasshouse red spider mites are less active than Phytoseiulus and their body shape is rectangular
• Phytoseiulus feed on all life stages of glasshouse red spider mite
• Phytoseiulus is dispatched as nymphs and adults which should be released in sheltered positions on infested plants
• Phytoseiulus does not control fruit tree red spider mite on apple and plum, but can be successful against glasshouse red spider mite on outdoor plants in the summer

• Amblyseius californicus is the most commonly supplied Amblyseius species for red spider mite control
• Amblyseius mites are not as an efficient predator of glasshouse red spider mite as Phytoseiulus and is less likely to control heavy infestations but can be used as a preventative out of season control
• Amblyseius mites about the same size as glasshouse red spider mite (0.5 mm), but they can be distinguished if examined with a hand lens, they are more elongate than red spider mite variable in colour but always lack two black spots
• Unlike Phytoseiulus, Amblyseius mites are a predator of various species of spider mite, and can also feed and reproduce on other prey and even pollen
• Amblyseius mites survive when glasshouse red spider mite activity is low and is more likely to survive winter in glasshouse than Phytoseiulus

• Feltiella acarisuga is native to Britain. The adults are a small black fly, the larvae (maggots) are predatory and will eat red spider mite as well as other mites and insects on plants
• Feltiella acarisuga can be used in spring before heavy red spider mite infestations develop, but should only be introduced if it is suspected that red spider mite has overwintered
• Feltiella acarisuga is less likely to work in hot dry conditions during the summer

• Atheta coriaria is a 3-4mm long dark brown rove beetle, like all rove beetles it is a predatory insect as an adult and larvae
• Atheta coriaria will feed on a wide range of invertebrate problems including fungus gnats, thrips and is sometimes sold for control of red spider mite

Pesticide control

• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give some control of glasshouse red spider mite. These pesticides have a very short persistence and so may require reapplication to keep glasshouse red spider mite numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults. 
• Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action against aphids (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These are not considered organic.
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available and labelled for spider mite control

Downloads

Pesticides for gardeners (pdf document)
Biological control suppliers (pdf document)
 
 
 

Biology

Despite the common name, during the spring and summer glasshouse red spider mites are yellowish-green with a pair of darker markings. Because of this, they are sometimes called the glasshouse two-spotted spider mite. They typically only become orange-red during the autumn and winter resting period, or when a plant has become inhabitable due to their feeding activities. The mites are small, up to 1mm (less than 1/16in) long, and are just visible to the naked eye, when 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 for example in 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 can be 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 immature and adult mites pierce plant tissues with their mouthparts and feed on cell contents. Severe damage can quickly develop in warm dry conditions which favours rapid reproduction by the mites.