Thrips

Thrips are narrow, fast moving insects that rasp open the surface of a leaf and drink what comes out. The scars they leave have a metallic silver look you will not mistake for anything else once you have seen it.

What you are seeing#

Irregular silvery or bronze patches on the upper leaf surface, often following the veins, with a dry metallic shine when you tilt the leaf toward the light. They are patches and streaks, not round dots, and they can cover a third of a leaf.

Scattered across the damage are tiny black specks like ground pepper. That is frass, and it is one of the most reliable confirmations you can get.

Check the undersides. At 30x, adult thrips are elongated and torpedo-shaped, about a sixteenth of an inch, pale yellow, tan or black depending on stage, with six legs and narrow fringed wings. Larvae are smaller, pale and wingless. Both dart when disturbed, which separates them from the slow crawl of a spider mite.

New growth comes in twisted or crinkled where the growing tip was fed on. On flowering plants they get into the buds, which is why you do not let this run. Blue sticky cards at canopy height are the best early warning you can set up.

Why it happens#

They arrive on clones, on soil, on cut flowers brought into the house, or through an intake vent from an outdoor garden. Adults fly and move between plants on their own.

They persist because of their life cycle, which is what makes them harder to clear than mites. Eggs are inserted inside leaf tissue where no spray reaches. Larvae feed, then drop into the top layer of the medium to pupate, which is where most of a population sits at any moment. Treat only the leaves and you are ignoring most of the infestation.

How to fix it#

1. Confirm with a loupe on leaf undersides and look for black frass specks. Silvery scarring without insects may be old damage from a population already gone.
2. Hang blue sticky cards and lay some flat on the medium. They catch adults and pupating larvae both.
3. Spray the undersides with spinosad, lights off, to the point of dripping. It is the most effective thrips control available to a home grower and it is approved for food crops.
4. Repeat every 5 to 7 days for at least 3 rounds, 2 to 3 weeks total. Eggs sit protected inside leaf tissue and hatch on their own schedule, typically 3 to 5 days, so one application never clears an infestation. You are spraying to kill each wave of larvae as it emerges, before it can pupate and lay again.
5. Treat the medium too. Half an inch of sand or diatomaceous earth on the surface disrupts pupation, or drench with beneficial nematodes (Steinernema feltiae).
6. Remove the worst leaves and bag them on the way out of the room.
7. Rotate to insecticidal soap for one round mid-cycle, and screen your intake with fine mesh going forward.

How to tell it apart#

Versus spider mites. This is the one that matters. Thrips leave large irregular silver patches with a metallic sheen and black frass specks. Mites leave a fine scatter of pale pinprick dots and, eventually, silk webbing. Thrips never produce webbing. Under magnification a thrip is long, narrow and fast; a mite is round and slow with two dark spots on its back.

Versus whiteflies. Whiteflies are white moths that fly up in a cloud when you shake a plant, and they leave sticky honeydew and sooty film, not dry silver scars. Thrips leave no stickiness at all.

Versus leaf septoria. Septoria makes round yellow spots with tan centers, starting on the lowest leaves, and it is a fungus, so there are no insects underneath. Thrips damage is silvery, irregular, has frass, and comes with visible bugs.

How long until it looks better#

New damage should stop appearing after the second spray, around day 10 to 14, and the cards should go quieter over the same period. Plan on the full 3 weeks and keep monitoring for a month after.

Scarred leaves do not heal. The silver stays until the leaf drops, so do not judge progress on the same leaf. Judge it on new growth coming in clean and flat, and on empty sticky cards.

If thrips show up in the last 3 weeks of flowering, stop spraying, use cards and manual removal only, and harvest on time.