Description
The body of the adult is 0.8 to 1.2 mm (0.031-0.047 inch) long and yellowish-gray to dark-gray in color. When unfolded, the wings show the unique structure typical of all thysanopterans: they comprise a single longitudinal vein with a fringe of hairs
Damage
When infestation begins, some adults position themsleves deep between leaf sheaths and in other places where they can find protection. Later, they spread preferentially on the lower surfaces of young leaves. Both nymphs and adults suck out epidermal cells, and sometimes mesophyll cells. The empty, air-filled cells reflect the light, which leads to the many typical small, metallic-looking marks. Small specks of black frass are usually found in the same places. During hot, dry weather or in the glasshouse, the population density will increase rapidly and damage can be severe. Foliage may be disfigured to such an extent that vegetable crops become unmarketable. Flower petals will become discolored and deformed, so ornamentals are particularly at risk.
In some vegetables, e.g. cabbage, the lesions will lead to cell proliferation and suberisation; the latter can impair machine-processing for preserves. Lasting infestations will cause leaves to curl, turn brown and drop. Shoots may be stunted, especially in young plants, occasionally to the point of a complete suspension of growth. Thrips is a vector of the Tomato spotted wilt virus; moreover, the lesions it causes in plant tissues facilitate infection by Botrytis spp. and Aspergillus spp..
Lifecycle
Thrips tabaci reproduces entirely parthenogenetically; males are very rare. The female lays about 30-80 whitish eggs into small gashes in plant tissue that it has cut with its ovipositor. The eggs are at first kidney-shaped, but after a short time within the plant tissue they swell, become elliptical, and protrude slightly. There are two nymphal stages that last up to 14 days, after which the larva will enter quiescence in a sheltered place, mostly on the ground. This pre-pupal stage is soon (1-2 days) followed by the pupal one.
All development times are very variable. The whole generation cycle can be completed in 12 days in warm climates. Adult females live only one or two weeks in warm weather, but can survive for many weeks at cool temperatures, which is of importance during winter. Thrips tabaci has no diapause, only a cold quiescence. The overwintering females seek shelter in the soil or in cracks in bark. While mortality during this time is high, the remaining females can rapidly spread and produce new offspring in the spring, as soon as the weather warms up.
The number of generations produced within a year is determined by climatic conditions. In warm temperate areas, there are usually three to five generations per year, but more than twice that many are possible in warmer regions, still more in a heated greenhouses. Onion thrips can be spread rather rapidly by passive wind transport. Active flying is limited to short distances and occurs only in warm weather. Thrips tabaci is assumed to be of Mediterranean origin, but is nowadays found worldwide.
