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Now known as the Two-spotted spider mite (Tetranychus urticae)
Although serious problems are uncommon on herb crops, the damage caused by their feeding can make fresh cut and potted herbs unmarketable.

Identification

All stages of two-spotted spider mite  are usually found on leaf undersides. The young mites and summer adults are up to 0.5 mm long, light green in colour, with two darker lateral marks on their backs. In September or October, in response to shortening day length and cooler temperatures, and also earlier in the year in heavy infestations when plants are senescing, adult females turn a brick-red colour. The brick-red females that appear in the autumn find a sheltered place in the structure of the glasshouse or tunnel, or in plant debris, to hibernate. Over-wintered adults become active again in the spring, in response to increasing temperature and daylength. They move onto host plants , where they feed and lay small, clear, round eggs on leaf undersides. The eggs hatch into 6-legged larval mites, that feed on the leaves and develop through two 8-legged nymphal stages into adults.

Symptoms

Spider mites feed by extracting cell contents using their needle-like mouthparts. This damage causes fine yellow speckling to be visible on the leaves, which later develops into yellow or necrotic patches, making the plants unmarketable (Fig.3). In severe attacks, leaves or plants can senesce and the mites can produce extensive webbing.

Sources of spider mite infestation and favourable conditions

The source of the pest is usually overwintered females that hibernate in the glasshouse or tunnel structure and migrate onto susceptible plants in the spring. The pest can also be brought in on infested plant material. Spider mites have no wings so they cannot fly. However, they can walk from plant to plant, or along glasshouse, tunnel structures or inside grow tents. They can also spin fine threads of silk which allow them to be carried on air currents. Spider mites can also be spread on people or clothing. Hot, dry conditions favour the pest and allow it to breed rapidly. Many generations per season can occur, and on nurseries growing all-year-round herbs with heat and light during the winter, the pest can continue to breed throughout the year.

How to avoid spider mites and other infestation

• Check all incoming plant material for damage symptoms, particularly highly susceptible herbs.
• Use a thorough clean-up procedure at the end of each crop. Dispose of unwanted and heavily-infested plants and plant debris promptly and carefully, and clean bench, tent or floor coverings.
• Avoid moving people or equipment from infested plants to ‘clean’ plants on the nursery, and wash hands after handling infested plants.
• Maintain good ventilation
• Misting infested plants with water, if practical, can reduce spider mite population growth during hot, dry weather.

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