Tetranychus  spp.

Red spiders

General

• Also called spider mites, yellow spiders depending on the species and stage, revealing other colors (orange, greenish) (figures 1, 2, 18, 20). They are also called weaving spider mites because they weave thin webs.
  
• Adults and larvae have four pairs of legs with the exception of the 1st instar larvae which have only three pairs.
• Many species of Tetranychus present in the world. In tropical regions, mention may be made of T. urticae (= T. cinnabarinus ), T. neocaledonicus , T. glover i, T. evansi . These species look very similar and can only be identified by specialists.
• Very polyphagous, they develop on several hundred hosts, whether cultivated or not, including most vegetable crops, in particular tomatoes, eggplants, cucurbits, and beans.
• Observed in the open field as well as under shelters, where they can sometimes proliferate.

• Symptoms :
  • Tiny, more or less dispersed punctiform * chlorotic spots appearing on and under the leaf blade (Figures 3, 6 and 7, 10 and 11, 13 to 15). The latter gradually turns yellow and the leaf eventually scorches, even drying out during severe attacks (Figures 8 and 9, 17) .
  • Reduction or even cessation of plant growth.
  • Comparable lesions on stems (figure 4), petioles, but also on some fruits (figures 5 and 12).
  • Presence of delicate silky webs within the plant cover, on the apices, on the leaves and fruits (figures 19 and 20).

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• Signs  : Presence of tiny spiders on the affected organs (figures).
• Possible confusion :

* The larvae, nymphs and adults, often present on the underside of the leaf blade, feed by biting and then sucking the contents of plant cells and are therefore responsible for these symptoms.

Biology

• Development cycle :
  • Spider mites go through 5 stages of development: egg, hexapod larva, protonymph, deutonymph and adult. The duration of the cycle (Figure 21) varies with temperature and is approximately 9 days in tropical conditions. Their movement is quite slow.
  • The eggs are laid mainly on the underside of the leaves. They are round, tiny (0.14 mm in diameter), translucent at first, then become opaque, and eventually turn yellow as they approach hatching.
  • Males are smaller than females which reach a length of 0.6mm.
• Dispersion : by movement on the ground, by means of people, equipment and tools, or by plants.
• Favorable conditions : growing plants, excessive nitrogen fertilization, hot and dry periods, elimination of auxiliaries by insecticide applications.

Protection

• Weed the crop and its surroundings.
• Disinfect greenhouses and equipment used
• Check the sanitary quality of the plants before and during their introduction into the crop or shelter.
• Produce the plants in an shelter insect-proof .
• Favor predators (phytoseides, certain bugs and ladybirds, etc.) by avoiding insecticides.
• Reason for  chemical protection, in particular if you use auxiliaries or biopesticides.
• Treat plants before uprooting in the presence of high pest populations.