Red Spider Mite
Overview:
Although the glasshouse Spider Mite tetranychus urticae is known as the Red Spider Mite they are actually better described as the Two Spotted Mite.
The reason being is when they are active they are pale greeny-brown in colour, and can be recognised by the two two dark dots on their backs - they are only red in winter.
The Mites are less than 1mm in length and difficult to see without the aid of a magnifying glass.
They are generally found on the underside of leaves, and are only active when it is warm.
In winter they hibernate either as eggs, or as inactive red-orange spotted young mites.
These small mites are serious pests of plants grown in greenhouses and in the home.
Sometimes they also attack shrubs and herbaceous plants outdoors.
Fuchsias, Chrysanthemums, Roses, Vines, Aubergines, Tomatoes and Cucumbers grown in the greenhouse are particularly susceptible.
Their presence is more often noticed by the damage they do, rather than visual sitings of the mite itself.
The adult and immature mites feed on the undersides of leaves where they suck the plant juices,this can cause a fine light mottling of the upper surfaces.
When the infestation becomes advanced, fine webbing appears on new growth.
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Spider mites are arachnids (spiders) and can spin very fine webs.
This webbing acts to shield the colony like a blanket, and it also provides a series of ladders which allow the mites to crawl from stem to stem.
Where webbing is widespread, the leaves will typically turn brown and drop off because of the extent of the cell damage, or in the worst cases, cause the death of the plant.
On plants with tough leaves, the mite eats young growth and flower buds, and may be even more difficult to spot.
The mites cease breeding in the autumn, and female mites over-winter in crevices in brickwork and woodwork,bamboo canes, stakes and plant ties.
Control:
Vigilance is paramount!
In the early summer, inspect vulnerable plants every week and deal with the first signs of infestation immediately.
If practicle, remove infested leaves, buds and stems, and this will reduce numbers.
Similarly, reduce the numbers of over-wintering female mites in late autumn and early winter,by clearing away plant debris that otherwise would act as over-wintering sites.
The mites prefer dry environments, so spraying the infested areas of a plant with water and damping the greenhouse floor areas will raise the humidity and slow the infestation rate.
Plants grown at high temperatures in dry, overcrowded conditions are often particularly susceptible to mite infestations regular spraying of infested plants with water may help to check the pests.
Thorough spraying with a suitable pesticide, will protect plants under glass and outdoors.
In the greenhouse, regular fumigation will prevent serious increases in mite populations.
The mites are likely to develop resistance to a given chemical if it is used too regularly, and control measures should be varied as much as possible.
Pesticides approved for use by amateur gardeners against red spider are limited to organic pesticides such as Rotenone (Derris), Fatty acids (Soft Soaps) and Vegetable oils.
The latter two work by blocking the mites breathing tubes, preventing respiration.
The use of the biological control, i.e. the use of predatory mites, is proving to be quite a successful and is now widely used in preference to pesticides.