Damage. The piercing mouthparts of the mites inject poisonous secretions which cause localized death of leaf mesophyll cells. This results in a fine mottling symptom on the leaf (see Figure 14.1), not to be confused with the larger spots caused by thrips. In large numbers the mites can kill off leaves and eventually whole plants. Fine silk strands are produced in severe infestations, appearing as 'ropes' (see Figure 14.25) on which the mites move down the plant. On flowering crops such as chrysanthemums, these ropes make the plant unsaleable.
Life cycle. This pest is of tropical origin and thrives best in high greenhouse temperatures. Both species are 1 mm in length. The first species (T. urticae) is yellowish in colour, with two black spots (see Figure 14.25). The female lays about 100 tiny spherical eggs on the underside of the leaf, and after a period of three days the tiny six-legged larva moults to produce the nymph stage that resembles the adult. The life cycle length varies markedly from 62 days at 10°C, to 6 days at 35°C when the pest's multiplication potential is extremely high. In autumn, when the daylight period decreases to 14 h and temperatures fall, egg production ceases and the fertilized females, which are now red in colour, move into the greenhouse structures to hibernate (diapause), representing foci for the next spring's infestation.
Spread occurs when adults and nymphs crawl from plant to plant when leaves are touching. Wind currents can move mites attached to their silk strands. The small size and under-leaf habitat of the pest combine to keep its existence on introduced plants away from the gaze of growers, especially older growers.
The second species (T. cinnabarinus), which is dark reddish-brown, has a similar life cycle to T. urticae, but does not hibernate. The hibernation habit of T. urticae leads to it being a common pest on annual crops such
Glasshouse red spider mite (X100)
Figure 14.24 Glasshouse red spider mite. Note that the red spider mite may be light green or red in colour. Its extremely small size (0.8 mm) enables it to escape attention
Glasshouse red spider mite (X100)
Figure 14.24 Glasshouse red spider mite. Note that the red spider mite may be light green or red in colour. Its extremely small size (0.8 mm) enables it to escape attention as tomatoes, cucumbers and chrysanthemums, while T. cinnabarinus is found more commonly on the perennial crops such as carnations, arums and hothouse pot plants. The two species often occur together on summer hosts.
Control may be achieved in several ways. Amateur gardeners and professional growers should carefully check incoming plants for the presence of the mite, using a hand lens if necessary. A predatory mite, Phytoseiulus persimilis is commonly introduced into cucumber, chrysanthemum and tomato crops in spring. For the amateur there are products containing fatty acids that control the mite. For the professional, winter fumigation of greenhouse structures with chemicals such as formalin or burning sulphur kills off many of the hibernating females. A pesticide containing abamectin is commonly used.
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