The main species of mites infesting greenhouse hydroponic crops are the two-spotted mite (T. urticae) and, to a lesser extent, the tomato russet mite (A. lycopersici). While the tomato russet mite only infests tomato crops, causing significant fruit damage (Fig. 10.7), the two-spotted mite can become a significant pest of a wide range of greenhouse crops, particularly under warm temperatures and low humidity. The tomato russet mite causes bronzing of tomato leaves and deformed fruit with a coarse browned skin; considerable damage can occur at high temperatures where population growth is rapid and the life cycle less than 7 days. Adult russet mites are small, cone shaped and orange brown in colour; a hand lens is usually required to see these mites and damage is often misdiagnosed as other conditions. The two-spotted mite, which is common on a wide range of greenhouse fruit, vegetable and ornamental plants, is also an extremely small insect pest which requires use of magnification for identification. Two-spotted mites lay eggs in webbing, these develop into six-legged larvae which then develop through two nymph stages to become adults. The adults have four pairs of legs and vary in colour from different stages of red through to brown and green. Adults feature a pair of large spots on the rear of the body.

Damage caused by the tomato russet mite.

Damage caused by the two-spotted mite includes leaf necrosis, interveinal chlorosis and distortion of leaf and petiole. This occurs through feeding on the epidermal cells by puncturing the cell wall and removing the contents. Plants eventually lose vigour, becoming stunted and discoloured, and in severe cases may die back. Fine webbing is produced in established populations which cover leaf surfaces; this provides protection for the mite pests and also hinders control spray penetration. Mites can be dispersed via air currents, being transported by threads of fine webbing; however, in cropping situations, mites are also easily spread on workers’ clothing, on equipment and plant debris and along crop wires and other structures.

Mites are often a seasonal pest in temperate-zone climates, with rapidly growing infestations occurring in late spring, through summer and autumn when warmer temperatures accelerate population growth. Under winter conditions, pest numbers are slower to establish and build. As with many insect pests, mites have developed widespread resistance to a number of chemical spray options and require a rotational insecticide programme to achieve control which should be applied early in the infestation stage. Predatory mites may be used either alone or after initial control with spray compounds and include the use of Phytoseiulus persimilis and Amblyseius californicus (Parrella and Lewis, 2017) (Fig. 10.8). As with all biological control, a balance must be maintained between the released predator and the two-spotted mite population for control to be effective. T his includes regular monitoring of the mite pest and biological control species’ populations, continual release of predators and use of other control methods when required.

Mites and biological control predators.

Source: A Practical Guide Hydroponics and Protected Cultivation by Lynette Morgan

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