Two-spotted spider mite (Tetranychus urticae): identification and control

Pale, fine stippling on the top of a fan leaf, the kind that is easy to write off as light stress under a bright fixture, is two-spotted spider mite announcing itself. By the time silk shows at the leaf axils or cola tips, the population has been doubling every three to five days for two or three weeks, and no predatory mite will cross onto flower once it turns sticky ^[1][2]. Confirm it with a loupe while it is still just dots.

The identity in one look:

What it looks like on the plant, in the order you see it

The first sign is a scatter of pale, pin-prick dots on the upper surface of a fan leaf, usually mid-canopy first, where airflow is weakest. Nothing shows yet on the underside where the mites actually are. Left alone, the dots multiply and merge into a dull bronze or grey cast across the leaf, and the leaf starts to feel slightly gritty. Only once density is high does the population show itself openly: fine silk across the leaf axils, then across cola tips, with mites and cast skins visible on the strands. Heavily fed leaves yellow, curl and drop ^[3][4].

• StipplingFirst sighting: pale dots, upper surface only, undersides still clean to the eye
• StipplingAbout a week on: dots merge, leaf takes on a dull bronze cast
• StipplingHigh density: fine webbing at leaf axils and cola tips, not drawn here

Adults are pale yellow-green to amber, oval, under a millimetre long, with two dark patches visible through the translucent body wall. Those patches are gut contents showing through, not pigment, so a freshly moulted mite can look clear for a day ^[3][4]. Eggs are spherical, almost colourless, laid singly on the leaf underside near the midrib and turning creamier as they near hatching ^[3][4]. None of this is visible without magnification; a 10–15x loupe is the minimum useful tool.

1. 1Body, oval, translucent
2. 2Two dark patches (gut contents showing through the cuticle, not pigment)
3. 3Egg, spherical, near-colourless, creamier just before hatching

Adult female from above, on a leaf underside, as a 10–15× hand lens shows it. The scale bar is 0.5 mm, the middle of the 0.4–0.6 mm adult range. The patches come and go with feeding: a freshly moulted mite can look clear for a day.

Sources: UC Statewide IPM Program, Pest Notes: Spider Mites, Pub. 7405 (2023); Burrack & Baker, Twospotted spider mite, NC State Extension (2013); both accessed 26 Sep 2026. Schematic drawing: Horus.

Where to look, and how often

Check the underside of lower and mid-canopy leaves, not the youngest growth, since two-spotted spider mite generally colonises older leaves before new ones ^[3]. During vegetative growth, once a week is enough on a stable, quarantined room; during hot, dry stretches, or on mother plants that never leave the room, check twice. A five-minute route with a loupe, the same few plants each time, catches a rising trend before a single leaf shows colour. Waiting for visible damage means you are already weeks behind the population, not days.

Why it can go from clean to unmanageable in two weeks

Development from egg to reproducing adult is driven almost entirely by temperature. In one controlled study that reared the species through a full generation at seven constant temperatures, egg-to-adult time fell steadily as temperature rose from 17 °C to 27 °C, then rose again slightly above that:

That data comes from mites reared on peach, not cannabis; no cannabis-specific development study of this kind exists yet. The shape of the curve (a sharp fall to a minimum in the high twenties Celsius, then a rise again) is a well-established property of the species and a reasonable planning guide ^[1]. At 13 °C, no eggs hatched at all in that study; below roughly 14 °C, development effectively stops rather than merely slowing ^[1]. The same study measured population doubling time directly: about 3.2 days at 27 °C, against 4.7 days at 25 °C and 6.4 days at 30 °C ^[1]. A female kept at 25 °C survives around 13 days and lays roughly 40 eggs in that time; at 27 °C she lives only about 6 days and lays roughly 19, but because so many more of her offspring are themselves female and fast-developing, the population as a whole still grows faster at 27 °C ^[1]. Warmer is not simply worse in a straight line; the high twenties Celsius is where the maths turns against you fastest.

This is a simplified three-phase view built from the totals above, not a measurement of each of the five juvenile instars separately; real generation time varies with host plant, strain and humidity as well as temperature ^[1].

Bringing numbers down, in the order that actually works

Cultural controls first. Relative humidity above roughly 50% measurably slows mite reproduction; most extension sources agree on the direction even though the exact size of the effect on cannabis has not been tested ^[2][3]. More on running humidity at each growth stage is in the climate section. A forceful, plain-water spray to the underside of leaves knocks mites and eggs off mechanically and is cheap enough to repeat daily on a small grow ^[3]. Remove the worst-affected leaves and bin them rather than composting them on site. Quarantine every incoming clone or plant for at least a week with its own loupe check before it joins the rest of the room ^[5].

Biological control, started early, works better than any spray. Phytoseiulus persimilis is the standard response to an active outbreak: commercial suppliers recommend releasing it at roughly one predatory mite per 100 spider mites for control within about a week; for prevention before any sighting, Neoseiulus fallacis released at around 20 per m² (2 per sq ft) as soon as true leaves appear is the more common approach ^[2]. Both are compatible predators and do not interfere with each other, but neither will cross onto flower once resin makes the surface sticky, which is why the cultural and biological work has to happen in propagation and veg, not in flower ^[2]. This is grower and supplier practice rather than a controlled trial result: exact release rates vary by supplier and crop, so treat the ratios above as one commercial source's starting point, not a settled figure; the flower-avoidance behaviour is the more consistently reported part. The biological control for mites page covers release rates and timing for the whole mite cluster.

Chemical control last, and only what the label and your jurisdiction allow. Insecticidal soaps and horticultural oils reduce populations on contact but leave no residue, so repeat applications are needed ^[3][4]. Broad-spectrum insecticides (carbaryl, organophosphates and pyrethroids among them) kill the predatory mites and insects that would otherwise hold the population down and are a documented cause of mite population flares after use ^[3]. Whatever you spray, rotate between different modes of action if more than one application is needed, since resistance builds quickly in a pest with a two-week generation time ^[4].

Three look-alikes, and the feature that rules each out

Stippling alone is not a diagnosis; the right-hand column gives the one feature that separates each look-alike. The full three-way comparison goes into more depth on each pair.

If webbing is present, the differential is already settled: only spider mites make it.

What to change before the next cycle

A confirmed infestation this cycle is a quarantine failure, not bad luck, since two-spotted spider mite does not spontaneously appear in a clean, enclosed room. Trace it back: which clone, which vent, which piece of clothing. Add or tighten a quarantine bench for anything entering the space, and put the weekly loupe check on a calendar rather than relying on memory. If predatory mites were released too late to matter this time, order them for day one of the next propagation run instead of waiting for the first sighting.