What are the most common pests and diseases in cannabis?
In cannabis cultivation — whether indoor, greenhouse, or outdoor — a recurring group of threats shows up again and again. Knowing them all lets you identify them quickly and act before the problem escalates.
- Spider mite (Tetranychus urticae): a hot-and-dry mite, yellow pinprick dots on leaves, fine webbing on the underside. Explosive under high VPD and elevated temperatures.
- Thrips (Frankliniella occidentalis / Thrips tabaci): cell rasping leaves silvery streaks and black fecal specks. Penetrates buds during flowering, reducing yield and THC.
- Whitefly (Trialeurodes vaporariorum / Bemisia tabaci): white cloud when the plant is shaken, sticky honeydew, sooty mold. Thrives at low-to-medium VPD with temperatures of 24–28 °C.
- Aphids (Phorodon cannabis, Aphis gossypii, Myzus persicae): dense colonies on tender shoots and leaf undersides, honeydew, attendant ants. Peak reproduction at 20–25 °C.
- Powdery mildew (Golovinomyces ambrosiae / G. spadiceus): white powder on the upper leaf surface. Does not require free water — thrives at 50–70 % RH and low VPD.
- Botrytis / bud rot (Botrytis cinerea): grey mold inside buds, devastating during flowering under very low VPD and high humidity.
- Fungus gnats / sciarids (Bradysia spp.): small black flies, white larvae in the substrate feeding on root hairs. The direct cause is overwatering.
How does VPD influence pest and disease risk?
VPD (Vapor Pressure Deficit) does not only regulate plant transpiration — it also determines which pathogen or pest has the ideal conditions to thrive. Each VPD range has its associated threat:
- VPD < 0.4 kPa (very low · RH > 70–90 % · free water on surfaces): high risk of Botrytis cinerea (bud rot) and downy mildew. Free moisture is the trigger.
- VPD 0.4–0.8 kPa (low · RH 50–70 % · no free water): powdery mildew zone. The fungus does not need condensation; it germinates at moderate-to-high relative humidity.
- VPD 0.8–1.5 kPa: the crop's sanitary band — low pressure from fungal pathogens. The management target during late vegetative and flowering stages.
- VPD 1.5–2.0 kPa (high · elevated temperature · RH < 50 %): spider mites begin accelerating their reproductive cycle.
- VPD > 2.0 kPa (very high · T > 27 °C · RH < 40 %): spider mite proliferation — egg-to-adult cycle compressed to just 5–7 days.
- Sucking insects (aphids, thrips, whitefly): thrive at 20–26 °C and 50–70 % RH, exactly the vegetative crop's green zone. Pest risk is highest precisely when the environment is ideal for the plant.
- Fungus gnats (sciarids): the trigger is not aerial VPD but substrate moisture — a consistently wet surface caused by overwatering is the number-one risk factor.
How do you recognize each problem? Quick diagnostic signs
Before you act, identify correctly. Each agent leaves a distinct visual signature:
- Yellow or grey pinprick dots on leaves + fine webbing on the underside → spider mites. Inspect with a 10x magnifier.
- Silvery stippling + black fecal specks on young leaves and bracts → thrips.
- White cloud when the plant is shaken + sticky honeydew on leaves → whitefly. Eggs arranged in a semicircle on the underside of tender leaves.
- Dense colonies on shoots and meristems + honeydew + ants patrolling → aphids. Under a magnifier: soft pear-shaped bodies, visible cornicles.
- White-grey powder on the UPPER leaf surface that wipes off with a finger but returns → powdery mildew. Do not confuse with downy mildew, which appears on the underside.
- Brown-grey patches inside buds, dry mold texture, musty smell → botrytis. Cut the bud open: if there is grey inside, it is lost.
- Small black flies near the substrate + slow growth with no clear nutritional cause → sciarids. Press a piece of raw potato 2 cm into the substrate and check after 48 hours: if there are white larvae, there is an active infestation.
What are the best prevention practices?
Prevention is the most efficient rung of integrated pest management (IPM). Applied systematically, it cuts risk in half before any problem appears.
- Quarantine all new plant material: at least 7–10 days in a separate area before entering the grow space. Uninspected cuttings are the most common source of pest introduction.
- Weekly magnifier inspection (10–20x): check the underside of young leaves, the apices, and the interior of flower buds. Early detection is the difference between a manageable issue and a disaster.
- Sticky traps from day one: yellow for whitefly, aphids, and fungus gnats; blue for thrips. These are monitoring tools, not full control — they warn you before a colony explodes.
- Active VPD and ventilation control: keep VPD in the sanitary band (0.8–1.5 kPa) and ensure airflow throughout the canopy. Stagnant microclimates with high RH are the preferred breeding ground for fungi and insects.
- Cleanup between cycles: remove plant debris, disinfect surfaces with 3 % hydrogen peroxide, and inspect ventilation filter screens. A single thrips pupa in the substrate can restart the cycle.
- Avoid overwatering: letting the top 3–5 cm of substrate dry between waterings is the most effective and cost-free prevention against fungus gnats.
- Clothing and controlled access: change clothes before entering the grow space if you have been around other plants. Powdery mildew conidia travel on fabric.
What is IPM and how is it applied in cannabis?
Integrated Pest Management (IPM) is a stepped approach that prioritizes lower-impact methods and escalates to more disruptive ones only when earlier steps fall short. In cannabis, the order is always the same:
- 1. Prevention and quarantine: never introduce uninspected plant material. Monitoring traps from day one.
- 2. Cultural measures: remove heavily infested leaves, improve air circulation, adjust irrigation, open the canopy with selective pruning.
- 3. Biological control: specific predators and parasitoids — Amblyseius cucumeris or swirskii for thrips, Encarsia formosa for whitefly, Aphidius colemani for aphids, Steinernema feltiae and Stratiolaelaps scimitus for substrate sciarids. Bacillus subtilis (QST 713) and entomopathogenic fungi such as Beauveria bassiana are compatible through harvest.
- 4. Potassium soap and natural oils: potassium soap at 1–2 % on contact; neem oil (azadirachtin) as a hormonal disruptor. In advanced flowering: apply only to leaves, never directly onto buds, and stop the last 4–6 weeks before harvest to avoid residues and terpene alteration.
- 5. Synthetic chemistry — last resort and only in vegetative: for cannabis consumed by humans, there are no officially registered synthetic pesticides in Argentina for this species. Neonicotinoids are systemic and accumulate in buds. In advanced flowering the only realistic path is biological + cultural control.
Continue with the specific guide for each pest or disease
This article is the overview map. For the full diagnosis, detailed IPM, and control options by growth stage, go to the guide for each agent:
- Spider mites — heat-and-dry mite (high VPD)
- Thrips — silvery rasping, black specks
- Whitefly — white cloud when shaken, honeydew
- Aphids — colonies on tender shoots, honeydew
- Fungus gnats — overwatering-driven, larvae in the substrate
- Mealybugs — white cottony masses
- Powdery mildew — white powder on leaves (low VPD)
- Botrytis (bud rot) — rot inside the buds (very low VPD)
