Thrips

Thr. tabaci, Frankliniella Occidentalis, Heliothrips Haemorrhoidalis

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Adult Thr. Tabaci

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Adult Frankliniella Occidentalis

Thrips are insects of tiny dimensions, whose females have a morphological thread by which they depose the eggs inside vegetable tissues. The Frankliniella Occidentalis or California Thr., found in Europe in Italy for the first time in 1987 on carnations, has become in a few years, for all European Greenhouses and in the moderate climates of other countries worldwide, one of the most dangerous phytophagouses of the Gerbera and many other floral cultivation.

Present in many parts of the world, similar to a midge of a pale yellow colour, it is also unfortunately famous for the ability to transmit the dangerous virus of the Tomato Spotted Wilt Virus (TSWV), able to infest more than 300 species of vegetables.

The male is on average 0,9-1,0 mm long, while the female is slightly bigger (1,2mm). The eggs are invisible to the naked eye and the life cycle is divided into six phases of development: Egg – 1.larval stage – 2. larval stage – Prenymph – Nymph-Adult.

The egg is laid in the parenchymal cells of the flowers and of the leaves, and after a few days these open and a larvae immediately beginning to feed on the parenchymal cells and on the flower pollen. During the nymph 2 phase, they grow and their mobility increases.

The Prenymph and the Nymph complete the cycle. They spend the whole winter in shallow soil or residual vegetation.

The adults live on average 30-40 days and the females-male ratio is generally 3:1 or 4:1.

The Thrips, a few days after the eggs open become very active managing even to make jumps of3 cm.

Because of their small dimensions, they can also be spread by the wind. The females begins to lay eggs within the first 3 days of her life, the unfertilized eggs produce males, the fertilized eggs produce mainly females. Generally 1-2 eggs are laid per day, reaching a total of about 40-50 eggs during the whole life cycle.

In good conditions, like heated greenhouses or the right natural climate an insect can have up to 12-14 generations in a single year.

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Nymphs of I + II stage
LNymph of II stage

The damage to the plants is caused by the I and II age Nymphs, and by the adult Thrips.

These have rasping mouth parts that abrade the surface of flower petals and leaves that release plant sap, which is then sucked up. This rasping injures the plant tissue, leaving brownish streaks an light coloured flower petals, or whitish or silvery streaks on foliage or dark coloured flower petals. To this damage the action of the terebra can also be added, which causes great damage especially to the small buds. In case of large outbreaks, the younger damaged leaves are deformed and of silvery colour. The silvery colour appears due of the emptying of the epidermal cells by the sucking action of the parasite. As well as such directly caused plant damage, the insect is also able to transmit and spread dangerous viruses from a single infected Gerbers plant, and also from wild plants situated near the Gerbera plants.

Thrips Tabaci, on the other hand is native to the Mediterranean areas, and often found on the Gerbera together with the Frankliniella O. Compared to the latter, the Thrips T. Lives more on the plant surface, thus is more vulnerable.

An other common species, which affects the Gerbera is the Heliothrips Haemorrhoidalis. If present in great quantities, it can be recognized because it lays its eggs on the underside of the leaf, close to the main vein. In addition, this egg laying is unusual to this insect, since females cover the eggs with blackish drops of excrement.

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Thrips attack on flowers
Different degrees of Thrips attack

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