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Description and biology of thrips
Thrips take their name from a Greek word that has been translated into French as "wooden louse", given their small size. They belong to the order of Thysanoptera (in Greek: fringed wings). Barely visible to the naked eye, around one millimeter in length, they have an elongated body and four narrow, long-ciliated wings.
Adult thrips, from an incomplete metamorphosis, somewhat resemble their larvae, which are yellowish white in color, but they are darker. Their populations are mainly represented by females, which reproduce by parthenogenesis (reproduction without male insect).
Thrips generally live hidden under the leaves, or in the flowers, which makes them difficult to detect. Larvae and adults are phytophagous. They also play a significant role in the pollination of flowers.
Despite their useful role as pollinators, some thrips become harmful, when they are in large numbers, by their salivary action of lysis and suction of plant cells. They cause whitish spots on the leaves, which eventually dry out and fall off, significantly weakening the parasitic plant.
However, the main harmful role of thrips is that of a virus vector for some of them, in particular those of the genera Frankliniella.
In France, the Frankliniella occidentalis or Californian thrips insect is responsible for the transmission of bronzed tomato disease, a virus found in more than a thousand plants, including around a hundred cultivated plants. There are at least two other well-identified viruses, that of the impatiens necrotic spots and that of the tobacco streak (necrosis), which can be transmitted by these insects.
Thrips are also very polyphagous: they can live at the expense of many plants.
Example of virus transmitted by thrips, TSWV, and the species concerned
These viruses, which we just discussed, are large particles in a spherical capsule. They are of the genus Tospovirus which belongs to the family of Bunyaviridae.
Of all the viruses inoculated by thrips, the best known and best identified is that of Tomato Spotted Wilt Virus. Those are thrips larvae who first ingest the virus on an infected plant, serving as a "reservoir". A few days later, the virus has multiplied in the larval organism, and it will soon be inoculated by the adult insect into a new plant host. The more viral the epidemic, the greater the number of insects.
The TSWV virus, which disappeared in France in the 1940s, reappeared in the 1980s, inoculated by F. occidentalis. He is considered to be excessively polyphagous. Its action is very worrying for at least a hundred dicotyledonous plants (tomato, melon, flax, salad, various vegetables and ornamental plants) and ten monocotyledons (barley, wheat, palm ...).
The symptoms of virosis should not be confused with physiological disorders. Often the leaves have whitish spots and are distorted. The internodes are shortened on the young shoots. The flower petals are smudged and part of the flower buds are aborted ... The TSWV virus is found worldwide, in various climates, continental, Mediterranean or tropical. Its development is favored by a high temperature and low humidity.
How to fight viruses transmitted by thrips?
Direct fight against viruses is impossible. So take common sense and fight the presence of thrips. First, eliminate the virus reservoirs by removing the weeds from the plots (weeding) and destroying the various plant waste, either by fire or by removing them from the cultivation areas.
When planting, use only healthy plants or virus resistant varieties. Practice crop rotation. Maintaining a high humidity level on plants, by successive close watering, is a limiting factor in the development of thrip populations. The chemical insecticide treatment may be considered and repeated if necessary, but alternating the nature of the molecules (consult the phytosanitary index), so as not to induce the appearance of resistance.
Take care to comply with the instructions of agricultural organizations, to treat usefully. Biological control is possible by introducing predatory bugs, like the genera Anthocoris and Orius, good regulators of these populations of thrips.