Spider Mites
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Spider Mites
Red or Two-Spotted Spider-Mites feed by puncturing cells and draining the contents, producing a characteristic yellow speckling of the leaf surface. They also produce silk webbing which is clearly visible at high infestation levels. At very high infestation levels, reddish-brown masses of mites can be seen hanging from the tips of leaves. When populations of spider mite are this high, the pest can be transferred accidentally to clothing, and spread around the crop by workers.
Spider mites are common pests on many vegetable and ornamental crops, including tomatoes, cucumbers, peppers, strawberries, roses, Gerbera and many other species.
In genereal, mites enter an over-wintering state known as diapause in autumn, and seek hiding places in the glasshouse structure or on equipment. They will emerge from diapause in the spring and can re-infest the crop. Occassionally, however, spider mites may also enter diapause in February in greenhouses not just for winter. Over-wintering forms are usually bright red, but red forms also occur later in the year.
Certain strains produce more severe ‘hyper-toxic’ damage, leading to severe leaf damage and leaf loss at low population densities. The exact cause of this damage is unknown, but these ‘hyper-toxic’ mites are usually considered to belong to the species Tetranychus cinnabarinus, or Carmine Mite, and are generally red in colour. Recent research also shows that these more damaging mites are able to lay more eggs than standard forms. Green forms of T. cinnabarinus are now known to occur, but do not appear to cause the same type of damage. Distinguishing between the species is not easy, and some taxonomists still consider T. cinnabarinus to be a form of T. urticae.
Other species of Tetranychus, such as T. evansi, T. turkestani and T. pacificus also cause crop damage in some countries. Related mites such as the Fruit Tree Red Spider Mite, Panonychus ulmi, and Citrus Mite, Panonychus citri, are also important pests.
Broad Mite (Polyphagotarsonemus latus)
Cyclamen Mite (Phytonemus pallidus)
These tiny mites can cause immense damage to plants, and yet are difficult to see, and often not present on the leaves that show the damage symptoms. This is because they occur in the very middle of the growing point of the plant, where new leaves are being formed but have not yet expanded. They feed on this delicate leaf tissue, and as the leaf expands it becomes distorted. Some types of plants show less distortion, but affected leaves often appear glossy and discoloured.
Heavily infested plants can become severely distorted, and infestation can rapidly spread through a crop from an initial focus.
The pest can be controlled biologically with high doses of Amblyseius cucumeris applied onto and around the foci of infestation. Lower rates must be applied around the area of visible damage, as the pest is almost certainly present in lower numbers, even when no damage is yet apparent. Amblyseius californicus has also been shown to offer control, but this is not available for use on outdoor crops in some countries. It is also an expensive option compared to A. cucumeris.