Thrips on Plants: How to Spot the Damage and Get Rid of Them

Silvery streaks on the leaves and tiny black dots? That's the calling card of thrips: small, fast-breeding insects that feed on leaves and flowers. Here's how to spot them, why they're hard to shift, and how to get rid of them.

What thrips look like

Adults: very small (1-2 mm), long and thin, pale yellow to brown or black, with narrow fringed wings folded along the back.
Larvae: even smaller, pale yellow or cream, and wingless.

They're easy to miss. Hold a sheet of white paper under a leaf and tap it: thrips show up as tiny moving slivers. Sticky traps hung among the plants catch the adults and show you how bad things are.

Thrips damage

Silvery, bronze or bleached patches on the leaves, where thrips have sucked out the contents of the cells.
Tiny black specks on those patches: their droppings.
Twisted or stunted new growth and marked flowers when numbers get high.

Thrips breed fastest in warm, dry conditions, so a hot grow room in summer is where they do best.

Why thrips are hard to get rid of

Thrips lay their eggs inside the leaf, where sprays can't reach them. The larvae feed on the leaves, then drop into the growing medium to pupate before coming back up as adults. A spray that only hits the leaves misses the eggs and the pupae, so the population bounces back unless you repeat treatments and deal with the growing medium as well.

How to get rid of thrips

Trap and monitor. Sticky traps catch flying adults and tell you whether numbers are falling.
Treat the leaves. Athena IPM is a foliar spray its maker says kills many soft-bodied insects; the label allows it from seedling to week 5 of flowering, sprayed with the lights off. Desi Dust is a diatomaceous earth powder, which its maker says works against thrips by damaging their outer shell. Use a pump sprayer and cover the undersides of the leaves too.
Treat the growing medium. CX Tanlin is a biological control applied to the medium or in your watering, and thrips are one of the pests it's made for. That's where the pupae are.
Repeat. Keep treating every few days, as the label says, for at least a couple of weeks so you catch each new batch as it hatches.
Between grows. Once the room is empty, Insecto Fumers fill the space with smoke that kills adult thrips and spider mites. Never use them with plants in the room.

Biological control works well against thrips too: predatory mites that feed on thrips larvae are sold by specialist suppliers. Check any spray is safe for them before using the two together.

Keeping thrips out

Check new plants and cuttings before they go into your room, and keep them apart for a week or two if you can.
Clean the room and equipment thoroughly between grows - see our cleaning products.
Keep a sticky trap up all the time so you spot the first arrivals.

Whatever you use, read the label first. It gives the dose, how often to repeat it, whether it can go on plants you'll eat, and how close to harvest you can use it.

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