Tetranychus urticae Koch Twospotted spider mite

Infestations of this generally abundant mite occur on a wide range of plants, including glasshouse ornamentals (e.g. carnation, chrysanthemum and rose) and vegetables (e.g. cucumber, sweet pepper and tomato), indoor and outdoor fruit crops (especially wall-trained fruit trees, bush fruits, cane fruits and strawberry) and hops. Serious attacks may also occur on hardy ornamentals and outdoor vegetable crops (including French bean and runner bean), especially in hot, dry summers; occasionally, infestations also develop on potato and sugar beet. The mites feed mainly on the underside of leaves; they withdraw sap from the plant cells and this leads to a pale spotting of leaves and, later, distinct speckling, silvering or bronzing; infested parts of plants may also become coated in webbing. Heavy infestations often lead to withering of leaves and premature leaf-fall; on fruit crops, such as strawberry, this will have an adverse effect on plant vigour and cropping, and will reduce both fruit size and quality.

BIOLOGY

Adult female mites hibernate in cracks in glasshouse staging, posts, supporting stakes, walls and dry soil; they also find winter shelter in straw mulches and hollow bamboo canes, and amongst dead leaves and other plant debris. Activity is resumed in the early spring, usually when day length exceeds 14 hours. The mites then invade host plants and eventually lay eggs. Colonies soon develop on the underside of leaves, where the mites pass through egg, larval and two nymphal stages before maturing. The rate of development varies considerably, taking approximately 2-4 weeks during the summer but becoming greatly protracted at temperatures below 12°C. There are up to seven or more overlapping generations each year. All stages of the mite occur together, often protected by fine strands of silk (cf. bryobia mites, Bryobia spp., p. 265 et seq.; fruit tree red spider mite, Panonychus ulmi, p. 266). In September, as days become shorter (day length less than 14 hours), 'winter' females are produced. Colonies on host plants then decline as males and 'summer' females (the former usually forming no more than 20% of summer populations) die and breeding ceases.

DESCRIPTION

Adult female 0.5-0.6 mm long, oval, pale greenish to yellowish, with a pair of distinct dark lateral patches on the body (Fig. 345); 'winter' form orange to brick-red. Adult male 0.30.4 mm long, narrow-bodied and slightly pointed anteriorly and posteriorly. Egg 0.13 mm in diameter, globular, pearly-white and translucent, without a stipe (cf. fruit tree red spider mite, Panonychus ulmi, p. 266). Active immature stages pale green with darker markings.

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