Thrips, also known as thunder flies and thunder bugs, are very small insects, and although they look innocuous, can cause a lot of damage to plants.
There are numerous species of thrips, most are black, brown or yellow-black, very thin, cigar-shaped and about 1-2mm long. They are sap-feeding insects, but they can also happily feed on the surface of a leaf.
The typical symptoms are mottled and discoloured leaves, with signs of bleaching. Thrips can also be responsible for spreading plant virus diseases through their mouthparts.
Treatment and control
Thrips are so small that it can be difficult to control them by physical means, although you could try wiping them off the plant with a damp cloth or sponge.
The best method of control is to spray affected plants with an insecticide.
A contact, natural-based insecticide, such as pyrethrin (a natural plant extract) will control thrips and other plant pests. As their name suggests, a contact insecticide only kills the insect pests it comes into contact with. Systemic insecticides also kill on contact but are then also absorbed inside the plant, and move all around the plant, providing protection against further attack for up to 3 or more weeks. Always check that the insecticide is approved for use on edible crops, before treating aphids on fruit and vegetables. A biological control, based on a predatory bug, is available for control of thrips in greenhouses from some mail-order biological control suppliers.