How to recognise it
Thrips are cylindrical shaped, sap sucking insects which are 3 to 4mm long and can be coloured white, yellow, brown or black. Adults have a pair of narrow wings fringed with hairs, the larvae are wingless and usually lighter in colour than the adults. They can be found on plants from spring to autumn and overwintering in the soil or leaf litter.
The females, which live for about a month, can bear up to 100 eggs in that time, secreting them within plant tissues or onto the plant surface. The eggs hatch, the larvae feed, they then become non-feeding pupae and about a week later adults emerge. The adults develop their wings after the pupal stage. The whole process from egg to adult can take only a month in the summer, but much longer in lower temperatures. The adult and immature thrips overwinter in the soil or in the litter layer on top of the soil.
Why it’s a problem
A large thrips infestation can lead to the plant’s flowers, buds and leaves becoming mottled and silvery in colour as the thrips pierce the tissue to feed on the contents of the plant’s cells. Distortion of the flower, bud or leaf can occur and the insects can act as vectors for diseases such as the tomato spotted wilt virus. Plants aren’t usually killed, but they can look unsightly as a result of an attack.
The winged adults can travel long distances on thermal currents, so infestations can easily travel from plant to plant or garden to garden.
Where you are likely to find it
There are various species of thrips which attack the following plants in particular: onions, leeks, peas, broad beans, gladioli, roses, azaleas, citrus plants, ferns, fuchsias, orchids, zantedeschias, chrysanthemums, carnations, privet, lilac, palms and Ficus plants. However most plants are potential victims to thrips.
Thrips are more prevalent in hot, dry conditions (both indoors and outside).
How to deter it
Regular watering and maintaining a cool, humid environment can prevent an infestation taking hold of indoor plants (house plants or in a glasshouse).
Use sticky traps to monitor thrips levels within glasshouses, blue coloured traps are thought to be most effective at capturing them.
How to get rid of it
If the thrips infestation is severe enough then contact or systemic insecticides can usually deal with it effectively. Ideally select an insecticide which contains bifenthrin, imidacloprid with methiocarb, or thiacloprid. They should be applied as soon as the thrips or symptoms are seen and the application repeated 2 to 3 weeks later if the damage continues. It should be noted that some species, including the Western flower thrips, is immune to chemical controls.
The predatory mite Amblyseius cucumeris and/or the bug Orius laevigatus can be used as biological controls within glasshouses or for house plants.
Remove and destroy infested leaves, particularly any with eggs on them.
Is it good for anything?!
Thrips can also feed on other garden pests, such as aphids and mites.
Other useful information
Thrips belong to the biological order of Thysanoptera, a name derived from the Greek words for ‘fringe’ and ‘wing’, referring to the fine hairs lining thrips’ wings.
Thrips are also known as thunder flies because they sometimes take to the air in large numbers during thundery weather.
The name ‘thrips’ is both the singular and the plural of the word, you don’t say ‘thrip’ if there’s just one of them (but then, there’s never just one of them!!).
Information on specific species of thrips: