Spider Mites

Spider mites are tiny arachnids that pierce leaf cells from the underside and drink the contents. They are the most destructive pest a home grower is likely to meet, because a population doubles every few days in a warm, dry tent.

What you are seeing#

The first sign is stippling: a fine scatter of pale pinprick dots on the top surface of the leaf, like a very light airbrush. Early on it is easy to mistake for dust. As the population grows the dots merge and whole leaves go pale, then bronze or yellow, starting in the lower and middle canopy and moving up.

Turn the leaf over. This is where they live and where you confirm. At 30x with a jeweler's loupe or a phone macro lens you will see slow moving oval specks the size of a grain of salt, pale tan or yellow with two dark spots on the back, plus clear round eggs stuck along the veins.

Webbing is a late sign, not an early one. Silk strung between leaf tips and blanketing whole colas means the infestation is weeks old and severe.

Why it happens#

They come in on a clone, a used pot, clothes, a pet, or from outdoor plants nearby. They do not appear from nothing. What lets them explode is the environment: a tent at 85F and 35 percent humidity is close to ideal for them. Heavy nitrogen feeding makes leaves softer, and a dense untrimmed canopy hides them for the two weeks it takes to notice.

How to fix it#

1. Confirm with a loupe first. Do not treat a plant for mites you have not seen.
2. Today, drop the temperature to 72 to 78F and raise humidity to 55 to 60 percent in veg. Mite reproduction slows sharply in cooler, damper air.
3. Remove the worst leaves. Any leaf more than half stippled is not producing anymore. Bag it, do not shake it, carry it out.
4. Spray the undersides, lights off, until dripping. In veg, insecticidal soap or 0.5 to 1 percent horticultural oil. Coverage matters more than product: a mite you did not wet survived.
5. Repeat every 3 days for at least 3 weeks. This is the part people get wrong. Soap and oil do not kill eggs, and eggs hatch in 3 to 5 days at tent temperatures, so one application kills the adults and hands the tent to the next generation. You are timing sprays to catch each hatch before it can lay. Rotate products between rounds.
6. In mid flower, use predatory mites instead. Phytoseiulus persimilis or Neoseiulus californicus, weekly for 3 weeks. Safe on buds, no residue, no pre-harvest interval.
7. Clean the room and quarantine new clones for 2 weeks going forward.

How to tell it apart#

Versus thrips. Both scar leaves, and this is the confusion that matters. Mite damage is a fine even stipple of round dots. Thrip damage is larger irregular silvery patches with a metallic sheen, plus black frass specks. Under a loupe a mite is oval, eight legged and slow. A thrip is narrow and torpedo-shaped and darts when disturbed. Webbing means mites, never thrips.

Versus low humidity stress. Dry air gives crispy edges, upward curling and even paleness across the whole leaf, not discrete dots, and it leaves nothing on the underside. If a loupe on five undersides shows no moving specks and no eggs, it is not mites.

Versus calcium deficiency. Calcium spots are small rusty brown patches with defined edges, usually on newer upper growth, and they do not multiply day to day. Mite stippling is pale rather than brown, starts lower, and visibly spreads within a week.

How long until it looks better#

The population should stop growing after the second spray and be visibly knocked down by 10 to 14 days. Getting to zero takes the full 3 weeks, and you should keep checking undersides weekly for a month after.

Stippled leaves never recover. Judge success on new growth coming in clean, and on finding no live mites when you check.

If you find mites in the last 3 weeks of flowering, you will not beat them before harvest. Hold the environment cool and humid enough to slow them, do not spray, harvest on time, and clean the room out before the next run.