Spider Mites on Cannabis: Practical Guide to Identify, Eliminate, and Prevent Them

Key takeaways

• Yellow-silvery stippling on top of the leaves is the first sign of a spider mite infestation.
• These mites thrive in hot, dry conditions (23–30°C, humidity below 50%) — July–August is the critical window in Quebec.
• The full life cycle takes as little as 7 days under ideal conditions — treatments must be repeated for at least 3 weeks to break the cycle.
• Phytoseiulus persimilis, a predatory mite, remains the most effective biological solution once an infestation is established.
• Raising ambient humidity is often the fastest measure you can take.

In this guide

• What is a spider mite?
• What damage does it cause on cannabis?
• What are the 7 methods to eliminate spider mites?
• How do you prevent an infestation?
• FAQ — Spider mites on cannabis

Leaves speckled with tiny yellow or silvery dots. Fine webbing under the leaves. A plant that seems to be "burning" for no apparent reason. This clinical picture has one near-certain culprit: the spider mite — a microscopic mite capable of decimating a crop within weeks under the right conditions.

Here's how to identify it, fight it, and keep it from coming back.

What is a spider mite?

The spider mite (Tetranychus urticae) isn't actually a spider — it's a mite from the Tetranychidae family. It measures 0.3 to 0.5 mm and is almost invisible to the naked eye, but its damage is very visible.

How do you identify it?

• Size: 0.3 to 0.5 mm — a magnifying glass or pocket microscope is recommended.
• Color: yellowish-green in spring and summer, orange-red in fall.
• Location: underside of leaves, between the veins.
• Webbing: fine silky webs under the leaves and between stems, a sign of an advanced stage.
• Early sign: yellow-silvery stippling on the top surface of the leaves.

Spider mites multiply exponentially in hot, dry weather. Their development is optimal between 23 and 30°C, with relative humidity below 50% — under these conditions, the full cycle from egg to adult can be completed in as little as 7 days, for a total of 6 to 7 generations per growing season (Quebec's Integrated Pest Management program). In Quebec outdoor grows, July–August remains the highest-risk period.

What damage does it cause on cannabis?

Mites pierce leaf cells to suck out their contents — symptoms progress through three distinct stages, which help gauge how severe the infestation is.

Stage 1 — Early infestation

• Fine yellow or white stippling on the top surface of the leaves.
• Slight overall discoloration.
• Little to no visible webbing.

Stage 2 — Established infestation

• Bronzed, silvery, or uniformly yellowed leaves.
• Visible webbing under the leaves and between stems.
• Slowed growth.

Stage 3 — Severe infestation

• Leaves entirely covered in webbing.
• Defoliation, drying out, death of young shoots.
• Potential contamination of the flowers and resin.

A plant heavily infested during flowering has its trichome production disrupted, resulting in less resin and lower-quality flowers.

What are the 7 methods to eliminate spider mites?

As with most cannabis pests, combining several complementary approaches speeds up elimination far more than repeating a single method alone.

• Increase humidity — spider mites hate high humidity. Indoors, raise humidity to 60–70%; outdoors, mist the foliage with clean water every 2 days.
• Insecticidal soap (black soap) — a 2–3% solution sprayed over the entire leaf surface, especially underneath, acts on contact and dehydrates the mites. Repeat every 3 days for 3 weeks to break the cycle, and treat on cloudy days or in the evening, never in direct sunlight.
• Neem oil — the azadirachtin it contains inhibits reproduction and disrupts the mites' molting. Dilute to 5 ml/L with emulsifier and treat every 5 days; effective against adults, larvae, and eggs.
• Natural pyrethrin — this insecticide extracted from chrysanthemum flowers (Chrysanthemum cinerariifolium) acts quickly on contact against adults, but has no residual effect — combine it with neem oil to extend the effect.
• Phytoseiulus persimilis — this natural predator, a mite specific to Tetranychidae, remains the most effective biological solution against spider mites. Introduce 50 individuals per square meter at the first signs; it feeds exclusively on spider mites, posing no danger to the plant. For outdoor grows in Quebec, prefer Neoseiulus californicus, which is more tolerant of climate variations.
• Diatomaceous earth — these crushed fossilized algae tear the mites' exoskeletons on contact. Dust it under the leaves and on the soil surface, and reapply after rain; 100% natural, safe for humans and pollinators.
• Intensive rinsing — for light infestations, a strong lukewarm water spray directed under the leaves physically removes mites and their eggs. Repeat every 2 days for 2 weeks.

How do you prevent an infestation?

Spider mites thrive in dry environments and on already-weakened plants — correcting these two factors significantly reduces infestation risk.

• Maintain relative humidity above 50% in indoor grows.
• Avoid water stress — thirsty plants are more vulnerable to mites.
• Inspect the underside of leaves every week, before you even suspect a problem.
• Outdoors, encourage natural predators like lacewings and ladybugs.
• Systematically quarantine any cutting or outdoor plant newly introduced into your grow.