Adult thrips are very small, 0.8-1.2 mm in length, with 2 pairs of wings which when in the resting position extend beyond the end of the elongated body. Under close inspection a longitudinal hairy fringe can be seen on the wing margins. Onion thrips vary in colour from yellowish-grey to dark-grey. The younger nymphs are smaller, paler in colour and without wings. Both the adults and the larvae can be seen crawling across the plants and small specks of black frass can be seen in the feeding areas.
Thrips feed by scraping and piercing the cells on the leaf surface and sucking up the exuded contents. They target the young developing leaves. The first signs of attack are the leaves turning a dull green colour with the leaf surface displaying small whitish flecks (empty air-filled cells reflecting the light) which give an overall streaked silvery appearance to the leaf. This can not only adversely affect photosynthesis but can also help serve as an entry point for pathogens.
Thrips tabaci reproduce entirely parthenogenetically, males are very rare. Using its ovipositor the female lays 30-80 tiny white eggs (turning a paler yellow as they mature) into small cuts in the host plant tissue. After passing through two larval instars a pupa is formed in the soil from which the adult emerges. Air currents and wind disperse adults passively to new locations. Active flying only takes place over short distances in favourable weather. In warm conditions the entire life-cycle can be completed in 4 weeks (shorter in tropical environments). In field situations there are usually 2 generations a year but in warmer protected environments further generations are possible. Whilst adults only live for 2 weeks in warm conditions they are able to survive for longer in colder weather, this enables both the adults and larvae to overwinter in soil or sheltered/protected locations.
Despite the scientific and common name onion thrips will feed on a variety of different host plants including brassicas, herbs (chives and mint), leeks, tomatoes and many other species (including bananas and pineapples). Onions suffering from severe thrips pest pressure can produce earlier maturing, smaller bulbs.
Onion crops or leeks should be sprayed with a suitable insecticide where populations are starting to build. Onion thrips sampled from leeks and salad onions have recently shown resistance to pyrethroids.