Glasshouse red spider mite

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

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

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 and spraying of plants with water and maintaining a high humidity reduce the danger of severe attacks, but will not, on its own control this pest.

Biological
The most commonly used biological control for red spider mite is a predatory mite (Phytoseiulus persimilis), originally from Chile, is now widely used in preference to pesticides.

• The predator is about the same size as glasshouse red spider mite (0.5 mm), but they can be distinguished if examined with a hand lens
• The predators have orange-red pear-shaped bodies, are more active than the pest, and can often be seen tapping the leaf surface with their front legs in search of prey
• Glasshouse red spider mites are more lethargic than the predator and their body shape is rectangular. Despite its common name, they are usually yellowish-green with two dark patches. They may be entirely dark, or in the autumn they may become reddish-orange
• The predators feed on all life stages of glasshouse red spider mite
• The predators are dispatched as nymphs and adults which should be released in sheltered positions on infested plants
• The predator 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
• The predator is susceptible to pesticides and cannot be used in conjunction with most chemical controls. The exceptions are those with very short persistence, which can be used up to a day before introduction of the predator. These include plant oils or extracts (e.g. Vitax Organic Pest and Disease Control, BugClear for Fruit & Veg Gun!) or fatty acids (e.g. Bayer Bug Free, Doff Greenfly and Blackfly Killer) or a blend of surfactants and nutrients (e.g. SB Plant Invigorator), which can be used to keep mite numbers in check before it is time to introduce the predator

The predator is available via mail order - download a list of biological control suppliers.

Chemical control

• Pesticides containing  acetamiprid (e.g. Bug Clear Ultra) are available for use on ornamental plants and some listed edible plants. Check the label for details and ensure that instructions are followed regarding maximum number of applications and harvest interval.
• Edible plants can be sprayed with plant oils, plant extracts or fatty acids. These pesticides may require frequent applications to control the mite
• Plants in flower should not be sprayed due to the danger to pollinating insects.

Downloads

Pesticides for gardeners (Adobe Acrobat pdf document)

Biological control suppliers (Adobe Acrobat pdf document)

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 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 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 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.

Aphids
Glasshouse leafhopper
Glasshouse whitefly
Greenhouse cleaning
Mealybugs
Scale insects