Onion thrips is a common pest of onions, leeks, brassicas and tomatoes. This pest is more of a problem in hot, dry summers when heavy infestations can occur.
A light flecking and silvering appears on the leaves where the sap sucking insect has fed. The yellow or brown larvae and the 2mm long adults can often be found on the plants at the same time.
Light infestations cause little damage to the plant. If a large area of the leaves are affected this can check the plants growth and they should be sprayed with a suitable insecticide that can be used on vegetables.