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.
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 of plants with clear water and maintaining a high humidity reduce the danger of severe attacks, but will not on its own control this pest.
Biological. 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 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 pesticides, 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 (e.g. Vitax Organic 2 in 1 Pest and Disease Control, Bug Clear for Fruit & Veg) or fatty acids (e.g. Bayer Organic Pest Free, Doff Greenfly and Blackfly Killer, Bayer Natria Bug Control) 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.
- Pesticides containing acetamiprid (e.g. Bug Clear Ultra) or thiocloprid and methiocarb (e.g. Bayer Provado Ultimate Bug Killer, Aerosol) are available for use on ornamental plants only
- Edible plants can be sprayed with plant oils, plant extracts or fatty acids. These pesticides may require frequent applications to control the mite.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Biological control suppliers (Adobe Acrobat pdf document)