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PestLeavesFlowers

Thrips

Slender insects barely a millimetre long that rasp the leaf surface and drink what leaks out. The feeding empties the cells, so the leaf takes on a silvery or pewter sheen quite unlike ordinary yellowing — and the tiny black specks of frass scattered over that silvering are what separate thrips from spider mite.

Caused by: Thysanoptera spp. · also called thunderflies, thunderbugs

What brings it on

Warm, dry conditions, indoors or under glass. Populations build fastest on plants that are already stressed, and infestations often arrive on newly bought plants.

source

Sourced guidancehigh confidence

  • Royal Horticultural Society: Thrips

Checked 2026-08-13

Usually seen: May, June, July, August, September

Preventing it

Ordered by how much difference each measure actually makes, not by how often it is repeated.

  • Inspect new plants before bringing them in, paying attention to flowers, where thrips congregate. Isolating a new plant for a fortnight catches most introductions.
    source

    Sourced guidancemoderate confidence

    • Royal Horticultural Society: Thrips — prevention

    Checked 2026-08-13

Dealing with it once it is there

  • Remove the worst-affected leaves and flowers, and raise humidity — thrips do best in dry air. Sticky traps confirm whether the population is still active and give a crude measure of whether anything is working.
    source

    Sourced guidancemoderate confidence

    • Royal Horticultural Society: Thrips — control

    Checked 2026-08-13

  • Under glass, introduced predatory mites are the established biological control. Any plant-protection product must be approved for the crop and the situation, and used exactly as its label states.
    source

    Sourced guidancemoderate confidence

    • UC Statewide Integrated Pest Management Program — University of California Agriculture and Natural Resources: Thrips — management

    Checked 2026-08-13

Signs that lead here

Each of these lists the other things that produce the same sign, and how to tell them apart.

On sprays and treatments

GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.

Plant-specific guides

Where this is one of several causes that look alike on a particular plant, GrowingHQ publishes the differential rather than the general answer.