Advice
RHS Help & Advice
Glasshouse red spider mite (Tetranychus
urticae)
Plants affected
A wide range of glasshouse plants, houseplants and garden plants.
Symptoms
A fine pale mottling develops on the upper leaf surface. In heavy infestations fine silk webbing can be seen on the plants, the leaves lose most of their green colour and dry up or fall off. Heavily infested plants are severely weakened and may die.
A very wide range of plants in glasshouses and homes can be attacked, and red spider will also cause problems in gardens in late summer, especially in hot dry summers. Large numbers of mites, up to 1mm long, and spherical eggs can be seen on the lower leaf surface (easier to see with a x10 hand lens).
Cause
Despite their name, during the spring and summer these sap-sucking mites are yellowish green with a pair of darker markings. They only become orange red during the autumn-winter resting period. Because of this they are sometimes called the glasshouse two-spotted mite.
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.
Biological 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 glasshouses or June to September out of doors.
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) or fatty acids (Bayer Organic Pest Control, Doff Greenfly and Blackfly Killer or Fito Get Off Insect) or mineral lattice (SB Plant Invigorator), which can be used to keep mite numbers down before it is time to introduce the predator.
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 or Doff All-In-One Garden Pest Killer) will control red spider mite, provided it has not gained resistance to this substance. Alternatives are sprays containing plant oils, plant extracts or fatty acids (see above). The latter pesticides may require more frequent applications.
Andrew Halstead