Spider mites: pale speckles, then webbing

Damage can outlast the mites that caused it, so confirm they’re still there before you spray.

What it looks like

The twospotted spider mite feeds on the undersides of leaves. Each feeding spot leaves a white mark like a pin prick, and many together look like fine pale speckling. You may also see black specks, which are mite droppings (Pacific Northwest handbook).

As numbers climb, leaves dry out, turn yellow or bronze, and can drop early. Webbing means the infestation is already severe, and it can cover whole plants, which then look dirty or rusty (Colorado State).

Webbing on buds is a problem of its own, and mites can keep spinning it on buds after harvest while they dry (Purdue Extension).

Why it happens

Mites usually arrive on plants or cuttings brought in from elsewhere, and they can travel on people who have handled infested plants. Mother plants and the clones taken from them can carry serious infestations (Cranshaw et al. 2019).

They’re mainly an indoor problem. Under lights they stay active all year, while outdoors natural enemies usually keep them down (Colorado State). Damage is usually worst in hot, dusty conditions and on water-stressed plants (UC IPM).

They also breed quickly. In controlled environments a generation takes about 12 to 15 days, and as few as 5 to 10 in ideal conditions (Purdue Extension). That’s why a few speckles can become webbing so fast.

How to confirm it

1. Look under a leaf with a 10x to 15x lens. You’re looking for slow-moving oval mites about 0.4 mm long, round eggs and fine silk (Purdue Extension).
2. Shake a stem over white paper. Specks that walk away are mites.
3. Make sure mites are still there. They’re sometimes gone by the time you notice the damage, and plants often recover on their own (UC IPM).

What else it could be: thrips also leave pale marks and black specks, but the marks tend to look silvery and you’ll find slender insects rather than round mites. Hemp russet mites cause speckling too, but they make no webbing and you can’t see them with an ordinary lens.

What to do

1. Separate the plant. Move infested plants away from clean ones. Mites can also travel on people who have handled them (Purdue Extension).
2. Knock numbers down early. Sprays of water, insecticidal soaps and oils are the first options (UC IPM). Soaps and oils only kill what they touch, so cover the undersides of leaves and expect to repeat. Don’t use them on water-stressed plants or above 90°F, and test a small area first.
3. Consider predatory mites. Phytoseiulus persimilis is sold commercially and eats only web-spinning mites (Purdue Extension). A common guideline is one predator for every 10 spider mites, so reduce heavy infestations first (UC IPM). It needs prey to survive, so it isn’t a preventive.
4. Don’t reach for broad-spectrum insecticides. They frequently cause mite outbreaks by killing what eats mites (UC IPM), and misuse of miticides breeds resistance (Colorado State).
5. Don’t mix sulfur and oil. Leave at least 30 days between an oil spray and sulfur, and skip sulfur above 90°F (UC IPM).
6. Clean up afterward. Remove plant debris and clean the space between harvests, and check drying buds (Purdue Extension).

Keep sprays off buds you plan to smoke. One study of contaminated flower found residues of four synthetic chemicals carried over into smoke, as much as 69.5% depending on the chemical and the device (Sullivan et al. 2013). We found no similar study for soaps, oils or biological products, so we can’t tell you those are safe to smoke. Few products are registered for cannabis at all (Lemay et al. 2022), and which ones are allowed differs by state, so check the label and your state’s rules first.

How much to trust this

Most of what’s here comes from extension guides for hemp and from general spider mite biology. We didn’t find a controlled trial of these treatments on smokable cannabis in a home grow.

Results may also fall short of what suppliers describe. A 2022 review reports that many biological controls work less well on cannabis than on other crops, and research on managing pests in indoor cannabis is limited (Lemay et al. 2022). Trichomes may be part of the reason, but that’s still being worked out. Treat predatory mites as a tool that may need repeating.