The spider mites feeding causes white to yellow speckles to form on the upper surface of the leaves. The eggs stick on the leaves undersides. The spider mite itself is located there, nesting in a cocoon resembling webbing. As infestation becomes more severe, leaves appear bronzed or silvery first and then become brittle, rip open between the leaf veins, and finally fall off. The mites spin a web that can cover the surface of the plant. Shoot tips can become bald and side shoots start to grow. In cases of heavy damage, the quantity, as well as quality of fruits, is reduced.
Damage is caused by spider mites from the genus Tetranychus, mainly T. urticae and T. cinnabarinus. The adult female is 0.6 mm long, pale green with two darker patches on its oval body, and long hairs on the back. Overwintering females are reddish. In spring, the females lay globular and translucent eggs on the underside of the leaves. The nymphs are pale green with darker markings on the dorsal side. The mites protect themselves with a cocoon on the underside of the leaf blades. The spider mite thrives in dry and hot climates and will produce up to 7 generations in one year in these conditions. There is a wide range of alternative hosts, including weeds.
In case of minor infestation, simply wash off the mites and remove the affected leaves. Use preparations based on rapeseed, basil, soybean and neem oils to spray leaves thoroughly and reduce populations of T. urticae. Also try garlic tea, nettle slurry or insecticidal soap solutions to control the population. In fields, employ host-specific biological control with predatory mites (for example Phytoseiulus persimilis) or the biological pesticide Bacillus thuringiensis. A second spray treatment application 2 to 3 days after the initial treatment is necessary.
Always consider an integrated approach with preventive measures together with biological treatments if available. The spider mite is very difficult to control with acaricides because most populations develop resistance to different chemicals after a few years of use. Choose chemical control agents carefully so that they do not disrupt the population of predators. Fungicides based on wettable sulphur (3g/l), spiromesifen (1 ml/l), dicofol (5 ml/l) or abamectin can be used for example (dilution in water). A second spray treatment application 2 to 3 days after the initial treatment is necessary.
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