Fine pale speckling on the upper side of tomato, cucumber or pelargonium leaves, followed by dry, dropping foliage, is a classic sign of a mite that many gardeners meet each summer. The Royal Horticultural Society (RHS) describes glasshouse red spider mite as one of the most troublesome problems of greenhouse plants and houseplants. This article summarises the RHS’s page on the mite. It relates to greenhouses, gardens and houseplants in the UK, it reports the RHS’s gardening advice and not any legal requirement, and it is general information.
What the mite is
The RHS identifies glasshouse red spider mite (also called two-spotted spider mite, Tetranychus urticae) as a sap-sucking mite. Mites belong to the arachnids, and most are under 1 mm long. This one pierces plant tissue and feeds on cell contents, causing a mottled appearance and, in severe cases, leaf loss and plant death.
Its host range is wide. The RHS names vines, peaches, nectarines, cucumbers, tomatoes, aubergines, peppers, fuchsias, pelargoniums, poinsettias, orchids and impatiens, and says the range is so wide that few plants are completely immune. It thrives in warm, dry conditions and is usually a problem from March to October, though damage can occur at other times in a heated greenhouse. It can also cause problems outdoors in summer, especially in hot, dry weather.
Recognising it
The RHS lists these signs:
- a fine pale mottling on the upper leaf surface;
- many tiny yellowish-green mites, white cast skins and egg shells on the underside of leaves, easier to see with a x10 hand lens;
- in heavy attacks, fine silk webbing on the plant, with leaves that lose most of their green colour and dry up or fall.
Despite the name, the mites are usually yellowish-green with two dark patches. They may be entirely dark, and they typically turn orange-red only in autumn and winter, or when a plant has become uninhabitable because of their feeding. The RHS says adults are up to 1 mm long, just visible to the naked eye in large numbers, and are usually on the lower leaf surface with their spherical eggs.
Why it takes off in warm, dry greenhouses
The RHS explains that mature orange-red females spend the winter resting in cracks and crevices such as glasshouse frames, stakes, canes, soil and plant debris. From late March they start feeding and laying eggs. Breeding can be continuous from March to October. The life cycle length depends on temperature: about 55 days at 10 degrees C, but complete in about 12 days at 21 degrees C. Severe damage can therefore develop quickly in warm, dry conditions, and the RHS says plants grown at high temperatures in dry, overcrowded glasshouses are more liable to serious problems.
Prevention and clearing out
For management, the RHS says regular syringing or spraying of plants with water and keeping humidity high reduce the risk, but will not control the mite on their own. Its other steps are to:
- remove severely affected plants from glasshouses, especially in late summer before cooler temperatures and shorter days send females to shelter for winter;
- clear plant debris, old canes, stakes and plant ties before spring to reduce overwintering mites;
- clean empty glasshouses thoroughly with a glasshouse disinfectant; and
- keep weeds in and around the glasshouse in check, since they may act as hosts.
The RHS refers gardeners with the mite on indoor plants to its separate page on sap-feeding insects on indoor plants. Its indoor-plant page says the RHS does not support pesticide use, recommends non-chemical options wherever needed, and notes that biological controls are susceptible to pesticides and so cannot be used with most of them.
Biological controls the RHS lists
The RHS says several biological controls are available by mail order:
- Phytoseiulus persimilis, a predatory mite. This is the most commonly used control. It is about the same size as the pest (0.5 mm) but has an orange-red pear-shaped body, is more active and taps the leaf surface with its front legs while hunting. It feeds on all life stages of the mite and is dispatched as nymphs and adults to be released in sheltered positions on infested plants. The RHS notes it does not control fruit tree red spider mite on apple and plum, but can succeed against glasshouse red spider mite on outdoor plants in summer.
- Amblyseius californicus, a predatory mite. The RHS says it is a less efficient predator and less likely to control heavy infestations, but can be used as a preventative control out of season. It can also feed and breed on other prey and even pollen, and is more likely than Phytoseiulus to survive winter in a glasshouse.
- Feltiella acarisuga, a predatory midge native to Britain. Adults are small black flies and the larvae eat the mites. The RHS says it can be used in spring before heavy infestations develop, only if it is suspected the mite has overwintered, and is less likely to work in hot, dry summer conditions.
- Atheta (Dalotia) coriaria, a 3 to 4 mm dark brown rove beetle. It feeds on a wide range of invertebrates, including fungus gnats and thrips, and is sometimes sold for red spider mite management.
The RHS also provides a downloadable list of biological control suppliers on its page.
The bottom line
According to the RHS, glasshouse red spider mite is a warm-and-dry problem: keep humidity up, clear out debris and weeds, remove badly affected plants late in the season and, if a predator is used, match the species to the timing and the conditions. It regards damping down as helpful but not a control by itself, and it does not support pesticides.
Sources
- Royal Horticultural Society, “Glasshouse Red Spider Mite: Prevention Tips” (rhs.org.uk)
- Royal Horticultural Society, “Indoor plants sap feeders: Identify” (rhs.org.uk)