Thrips on Cannabis: Signs and Treatments

You have spotted silvery, winding scars on your leaves with tiny black specks. That is the calling card of thrips. They need treating on both the leaves and in the soil, since part of their life happens there.

Cannie showing advanced thrips — Advanced

“Someone has been drawing silver doodles on my leaves. Thrips, I see you.”

Where it shows

Upper leaf surfaces.

What it looks like

Silvery-white winding scars like dry snail trails etched across the leaf, with tiny black specks of frass in the damaged areas.

Canna GLAP illustration: Silvery streaks, black specks — Silvery streaks, black specks

How to confirm it

Look at the upper leaf surfaces.
Silvery-white winding scars, like dry snail trails.
Tiny black specks, their droppings, in the damaged areas.
Small slender insects that move quickly when disturbed.
Blue sticky traps catch the adults.

Why it happens

They rasp the leaf surface and drink what leaks out. Often arrive on new clones or in unsterilized media.

Infested clones or new plants.
Unsterilized soil or media carrying pupae.
Warm conditions that speed their life cycle.
Open doors and vents letting them in.

How to fix it

Cannie's quick fix, in order:

Sticky blue traps catch the adults and tell you whether it is still active.
Treat the soil as well as the leaves. Part of the life cycle happens in the medium.
The silvery streaking is distinctive; nothing else marks a leaf quite like it.

Put up blue sticky traps to catch adults and see if it is still active.
Treat the soil as well as the leaves. Part of the life cycle happens in the medium.
Remove badly damaged leaves.
Repeat treatments as labels direct, usually every few days.
Keep checking traps to confirm progress.

Organic options

Spinosad

Commonly OMRI listed and effective on thrips. Use in veg, follow the label, and wear gloves and eye protection. Do not spray buds in flower.

Orius or Amblyseius cucumeris

Predators that hunt thrips on the plant. Safe in flower.

Hypoaspis (Stratiolaelaps) mites

Live in the soil and eat pupae. Good for breaking the soil part of the cycle.

Beneficial nematodes

Water into the soil to target pupae. Follow storage and application directions closely.

Insecticidal soap

Kills on contact. Use in veg only, and spray leaf tops and undersides.

Read and follow every product label. Organic friendly does not mean certified organic; check the listing if that matters to you.

Veg or flower?

In veg, sprays plus soil treatment work well. In mid to late flower, avoid spraying buds and rely on beneficial insects and soil treatments.

Prevent it next time

Quarantine new clones and plants.
Use fresh or sterilized media.
Keep blue sticky traps up as an early warning.
Inspect leaves every few days, and daily once you have seen one.

Questions growers ask

What do thrips look like on cannabis?

Silvery, winding scars on leaves with tiny black specks. The adults are small, slender and quick.

Why do I need to treat the soil?

Thrips spend part of their life cycle in the medium. Treating only leaves lets the next wave hatch.

Are sticky traps enough?

No, but they help. Traps catch adults and show if the problem is still active, while other treatments handle the rest.

Where these figures come from

These points match published grow books, page by page. They are grower practice, not research: none of these books cites a study, so where science and the books differ, this page follows the science.

Often arrive on new clones or in unsterilized media. Danko (2018), e-book p. 78 to 79. Green (2001), p. 78; p. 326 to 327.
Silvery-white winding scars with black specks of frass. Danko (2018), e-book p. 130 to 131.
Orius or Amblyseius cucumeris; safe in flower. Danko (2018), e-book p. 130 to 131.

Books: Danko, Cannabis: A Beginner's Guide to Growing Marijuana (Hampton Roads, 2018); Green, The Cannabis Grow Bible (4th ed., 2001); Paull and Zetta, Foolproof Guide to Growing Cannabis (2020) and The Cannabis Grow Bible (2020).