Tetranychus spp.

Red spiders

General

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

• Susceptible botanical family(s)

• Production areas affected :

• Organs attacked

Symptoms, damage

• Symptoms :
  • Tiny, more or less scattered chlorotic punctiform* spots appearing on and under the leaf blades (figures 3, 6 and 7, 10 and 11, 13 to 15). The latter gradually yellows and the leaf ends up scorching, 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 certain fruits (figures 5 and 12).
  • Presence of delicate silky webs within the plant cover, on the apices, on the leaves and the fruits (figures 19 and 20).

• 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 according to the temperature and is about 9 days in tropical conditions. Their movement is quite slow.
  • Eggs are laid mainly on the underside of leaves. They are round, tiny (0.14 mm in diameter), translucent at first then become opaque, and eventually turn yellow as they approach hatching.
  • The males are smaller than the females which reach a length of 0.6 mm.
• Dispersion : by movement on the ground, through people, equipment and tools, or even by plants.
• Favorable conditions : growing plants, excessive nitrogen fertilization, hot and dry periods, elimination of auxiliaries by application of insecticides.

Protection

• Weed the crop and its surroundings.
• Disinfect the greenhouses and the equipment used
• Control the sanitary quality of the plants before and during their introduction into the culture or shelter.
• shelter insect-proof
• Favor predators (phytoseids, certain bugs and ladybugs, etc.) by avoiding insecticides.
• Consider  chemical protection ( e-phy )**, in particular if you use auxiliaries or biopesticides.
• Treat plants before uprooting in the presence of high pest populations.