Young and old, thrips are a threat to plant health
Thrips are commonly known as Thunder Flies. They are the tiny, tiny flies that you sometimes see in houses trapped behind picture frames. In summer they fly around in the garden, and feed by burrowing into plant tissue or developing leaves and buds. In the open garden they tend to cause most damage from June to September, but in a glasshouse they can often start in April. If you get close up to them and use a magnifying glass you can see that they are cigar-shaped insects, long and thin, in various colors of yellow, brown and green. Thrips are very tiny insects, only about 1-2mm long, so difficult to spot.
There are different thrips found on different plants - rose thrips and gladioli thrips have been with us for a long time, but the major threat to commercial crops at the moment is the more recent "Western Flower Thrips". The adults lay their eggs in a pocket in the leaf, and the larvae emerge to feed on the plant - growing buds and flower buds are particular targets. Young thrips (larvae) are similar to the adult, but without wings. The very young ones are white, and the older ones yellow-brown. The larvae live on the plant, but when they pupate the pupa often drops to the soil. The pupae may hide on the soil compost for several days or several months before the adults emerge, so remember that soil can be a source of infection.
Thrips suck juices out of leaves and emerging flowers, leaving them with a 'rasped' look. They do not generally kill plants, but make them look tired and unsightly. If they attack young emerging shoots then leaves may be crooked and misshapen. The photo shows a distorted mishapen pepper leaf which is a typical sign.
There are three main biological controls for thrips..