What are thrips, and how do I know if I have them?

Thrips are under-diagnosed because almost nobody sees them. They are small, thin, fast, and they drop or run into a crevice when a leaf is disturbed. What they leave behind is distinctive, and once you know the two signs you find them easily on gladioli, roses, onions, peas, houseplants and anything under glass.

Step by step

1. Look for silvering plus black specks.Pale, silvery, slightly metallic patches on leaves where the cell contents have been rasped out, sprinkled with minute black varnish-like drops of excrement. That combination is thrips and very little else produces it.
2. Look at the flowers.Pale flecks, streaks and browning on petals, buds that open lopsided or fail to open, and colour breaking on dark flowers. On gladioli, roses and chrysanthemums this is often the first thing noticed, and it is where the damage matters most.
3. Tap a shoot over white paper.Hold a sheet of white paper or card under a flower or shoot and tap sharply. Thrips fall onto it and are visible as slim moving slivers, pale yellow, brown or black depending on species and stage. Watch for a few seconds; they are hard to see until they move.
4. Use blue sticky traps.Thrips are strongly attracted to blue, more than to the yellow traps used for whitefly and gnats. A blue trap among plants confirms the presence and gives you a measure of numbers over time.
5. Understand where they breed.Most species pupate in the compost or soil beneath the plant, which is why treating only the foliage fails. That is also why a soil-applied biological control is effective where a foliar one is not.
6. Remove affected flowers and badly damaged leaves.Flowers concentrate the population. Deadheading promptly and binning the flowers removes a substantial proportion, and on gladioli and roses this alone makes a real difference.
7. Raise the humidity and ventilate.Thrips thrive in hot, dry, still conditions. Damping down a greenhouse floor, ventilating in hot weather, and misting or grouping houseplants all reduce them, and the same measures suppress red spider mite.
8. Use biological control in a greenhouse or conservatory.Predatory mites applied to the foliage and nematodes watered into the compost together cover both the feeding and the pupating stages. Introduce them early, before the population is large, and do not combine with a broad-spectrum spray.
9. Keep weeds and old plant material down.Thrips overwinter in plant debris, in weeds and in greenhouse structures. Clearing at the end of the season and washing down a greenhouse over winter reduces the starting population substantially.
10. Be aware of the specific named cases.Gladiolus thrips ruin gladioli and overwinter on stored corms, so dust or treat corms before storing and inspect them. Onion thrips silver the foliage of onions and leeks in hot dry summers. Western flower thrips is a greenhouse pest that also transmits plant viruses.

Tips

• Thrips damage to a flower is done inside the bud, before it opens. By the time you see the flecking, the treatment window for that bud has passed.
• Silvering on a whole branch of a plum or cherry is silver leaf disease, not thrips, and is a much more serious problem.
• Hot dry summers produce thrips years. Watering properly and keeping plants unstressed matters more than any treatment.
• Thrips on houseplants are frequently introduced on cut flowers brought into the same room.

Common mistakes

• Treating the leaves only — Most species pupate in the compost. A foliar treatment alone leaves the next generation intact and the problem returns in a fortnight.
• Assuming nothing is there because nothing is visible — Thrips are a millimetre long and hide in buds and crevices. The paper-tap test is the only reliable way to look.
• Spraying then introducing biological control — Residues kill the predators. Choose one approach and follow the label interval if you switch.

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