Mites, thrips, aphids or gnats — which one do I have?

The short answer

Most pests that matter in a grow tent are piercing-sucking insects and mites, several of them too small to see without a hand lens. So identification by sight of the animal usually fails. Identification by the shape of the damage and where it sits usually works, and that is the order to do it in.

• Fine webbing and pinprick stippling, hot dry tent → spider mites.
• New growth twisted, hardened or curled, no webbing → broad mite (curl down, growing points) or hemp russet mite (curl up, dull russet leaves) — both need a 10–20× lens to confirm.
• Silvery or bronze scraped patches with tiny black specks → thrips.
• Soft-bodied clusters on stems and new growth, sticky leaves below → aphids.
• A cloud of tiny white flyers when you shake the plant → whitefly.
• Small dark flies at the soil surface, plant limp → fungus gnats, and usually a watering problem first.
• Winged aphids on a hydro or coco plant that is underperforming → check the roots. That is a root aphid, a different pest with a different fix.
• Large ragged holes and dark pellets → a caterpillar, outdoors or through an unscreened vent.

Read the whole plant before you read one leaf

The single most useful habit is also the one most often skipped: look at every leaf on the plant before deciding anything. One pest makes one kind of mark, and it usually makes it in several places. A picture of one leaflet can support three different stories; the whole plant supports one. We say this because we have got it wrong — reading pale dots on the upper leaflets as mites when a lower leaflet, not in the crop, carried a winding tunnel that named a leaf miner outright. That mistake is first-party and it is why the rule is here.

A second habit, from the field’s reference chapter: scout with a hand lens and sticky traps, not a phone camera. The pests that do most damage in controlled environments are near-microscopic. The chapter puts a hand lens at 10–20× for the mites and says thrips species need a scope at 45× or better. A blue or yellow sticky card catches what the lens misses.

The damage, one pest at a time

Two-spotted spider mite. Fine webbing, especially at leaf axils and growing tips, and a scatter of pale pinprick stippling where individual cells have been emptied. Look at the underside. Under the lens the adult shows two dark spots on its back — that is its gut, not a marking. It explodes in hot, dry air. If your tent runs warm and low-humidity and you see specks moving, this is the first suspect.

Broad mite. Too small to see unaided — a tenth to a fifth of a millimetre. It feeds on growing points and buds and injects a toxin, so what you see is the plant’s reaction: new growth that curls downward, twists, hardens, corks or scabs, and buds that fail to open. There is no webbing. It likes warm and humid conditions — the opposite of spider mites. The chapter’s own warning is that this is routinely misread as herbicide damage or a virus, because the plant looks poisoned rather than eaten.

Hemp russet mite. Also near-microscopic, and cannabis-specific. It feeds on the upper leaf surface, so the tells are a slight upward curl of the leaf edge, petioles that go brittle, and a dulling or bronzing of the leaf colour — the “russet.” Left alone it moves into the flowers and the trichome stalks. It spreads on air currents and on plant material, which is why an infested cutting brings it into a clean room.

Thrips. They rasp the leaf surface rather than pierce it, leaving silvery or bronze patches that look almost metallic in raking light, dotted with tiny black specks of frass. The adult is a slender sliver that scuttles when you breathe on it. Sticky traps are the reliable monitor; naming the species takes a scope.

Foliar aphids. Soft-bodied, in dense, mostly stationary clusters on stems and the undersides of new growth, and they leave honeydew — sticky leaves below the colony, sometimes with a black sooty mould growing on it, sometimes with ants tending them. Several species turn up on cannabis and the chapter is blunt that they are hard to tell apart by eye; the identification that matters is aphid-versus-not, and whether the colony is still alive (touch one).

Whitefly. The one you cannot miss: disturb the plant and a cloud of tiny white insects lifts off and resettles on the undersides. They can carry viruses between plants.

Fungus gnats. Small dark flies walking on the medium and drifting up when you water. The adults are a nuisance; the larvae in constantly wet medium are the damage, chewing seedling roots and opening them to rot. The chapter’s framing is the useful one: gnats are usually an overwatering symptom before they are a pest. Let the top of the medium dry between waterings and much of the problem leaves with the moisture.

Root aphid. The one that sends growers to the wrong place. It lives on the roots — dark green to brown, rusty-red near the tail — and thrives in hydro, coco, rockwool and any recirculating system. Nothing shows on the leaves except that the plant underperforms. The tell is that its winged forms climb up to the foliage to disperse: winged aphids on a hydro plant with no foliar colony means look at the roots. It matters because the usual aphid answers do not work on it — the chapter records that no parasitoid controls it and the soil predators do not either — so treating it as a leaf aphid wastes weeks.

Caterpillars. Large, ragged holes in leaves or bored into buds, with dark fecal pellets left behind. Mostly an outdoor or greenhouse problem; the chapter notes that even basic vent screening excludes the moths indoors.

Leaf miners are not in the chapter’s list for controlled-environment cannabis, and we mention them only because of the mistake above: a winding, pale tunnel inside the leaf blade is the larva’s track, and the pale dots nearby are the adult’s punctures. One insect, two marks — which is the point of reading the whole plant.

Two look-alikes that are not pests

Stippling versus deficiency. Spider-mite stippling is pinprick-sized, scattered, and on the underside there are specks that move. Interveinal yellowing from a nutrient problem follows the veins and moves nothing. If in doubt, read the pattern.

Twisted new growth versus heat, light or a damaged growing tip. Broad and russet mite damage looks like the plant has been poisoned. So does a growing tip that was knocked, a tent that ran too hot at the top, or an overdose. The lens is the only thing that settles it; if 10–20× on the newest growth shows nothing moving after a careful look at several tips, think environment before mite.

How to check this yourself

• Walk the whole plant first. Top, middle, bottom, undersides. Note every distinct kind of mark before you name anything.
• Match the mark, then the conditions. Webbing + hot and dry = spider mite. Twisted tips + warm and humid = broad mite. Metallic patches + black specks = thrips. Sticky = aphids.
• Then the lens. A 10–20× hand lens on the underside of a stippled leaf, or on the newest growth for the invisible mites. Tap a leaf over white paper and watch for movement.
• Hang a sticky card at canopy height for two days. Whatever is stuck to it after two days tells you what is flying, which is often more than the leaves tell you.
• Hydro or coco and winged aphids? Lift the plant and look at the roots before anything else.

On what to do about it — deliberately not on this page

We are not going to tell you what to spray. What is permitted on a flowering cannabis plant depends on where you live and changes over time, and a stranger on the internet is the wrong source for that decision. The chapter discusses control in broad families — physical removal and exclusion, soap- and oil-based products, biological control with predatory mites and parasitoid wasps, and environmental levers such as humidity and temperature that favour the plant over the pest — and which family fits depends entirely on getting the identification right first, which is what this page is for.

What would show that we have this wrong

If the lens shows nothing on several tips and undersides, the sticky card comes back empty after two days, and the damage is not spreading to new leaves, you are probably looking at an environmental or nutritional problem wearing a pest’s clothes — read the pattern instead. And if the marks are old, dry and not multiplying, you may be looking at damage from a colony that has already gone: touch one.

Where our device fits, stated narrowly

Croplock does not identify pests, and we would be wary of anything that claimed to from a photograph of a two-millimetre animal. Where it helps is the same place it helps on everything else: it photographs the same canopy every hour, so “is this spreading?” has an answer over days rather than a memory, and it logs the temperature and humidity that decide which of these pests is even plausible in your tent.

Sources

• Lemay, J. & Scott-Dupree, C. “Management of Insect Pests on Cannabis in Controlled Environment Production.” Ch. 9 in Cannabis in Controlled Environment Production (CRC Press). DOI 10.1201/9781003150442 — every pest signature above, the magnification figures, the scouting guidance, and the control families. Restated in our own words.
• The whole-plant rule and the leaf-miner example are first-party and are given as such; leaf miners are not in the chapter’s cannabis list.

More of these at Answers. The device is at croplock.com.

Related: Are those bugs still alive? — the two-second check before you treat. From the build: Trained on close-ups, served whole plants.