Pests don\u2019t give you any warning. One day your plants are perfectly healthy, and three days later the undersides of the leaves are covered in white spots, new leaves appear misshapen, or the potting mix starts to smell musty. The time it takes you to identify the problem is the time the pests have to multiply.<\/p>\n\n\n\n
This guide covers the most common pests affecting cannabis cultivation \u2014 both indoors and outdoors \u2014 with clear photographs of symptoms, life cycles, effective treatments and, above all, prevention. After all, the easiest pest to deal with is the one that never takes hold in the first place.<\/p>\n\n\n\n
\u2192 This guide is part of our Complete Guide to Growing Cannabis from Seed<\/a>.<\/p>\n\n\n\n Table of contents<\/strong><\/p>\n\n\n\n 80% of pest problems in indoor cultivation can be prevented by taking four basic steps. They aren\u2019t glamorous, but they work better than any treatment:<\/p>\n\n\n\n Spider mites thrive in dry, warm conditions. Fungi and soil-dwelling midges thrive in damp conditions. Most pests have preferred environmental conditions and become established when the environment is favourable to them. Maintaining the temperature between 20\u201326 \u00b0C and relative humidity at 50\u201365 % during the vegetative stage (dropping to 40\u201350 % during flowering) creates an inhospitable environment for most common pathogens and pests.<\/p>\n\n\n\n Insect pests \u2014 particularly thrips and aphids \u2014 move less and are less likely to establish themselves in areas with active air circulation. One or two fans that create movement amongst all the branches act as a real physical barrier, not just a means of strengthening the stems.<\/p>\n\n\n\n The most common cause of infestation in indoor cultivation is the introduction of contaminated plant material: cuttings from other plants, unsterilised potting compost or even clothing that has been in contact with outdoor plants. Any new material brought into the grow room should undergo at least a week of isolated observation before coming into contact with the main crop.<\/p>\n\n\n\n Set aside five minutes once a week to check the undersides of the leaves, the stem near the soil and the surface of the growing medium for each plant. Most pests can be spotted visually before their population explodes, provided you know where to look. An early-stage colony of red spider mites \u2014 ten or twenty individuals \u2014 can be eliminated in a single treatment. An established colony may take weeks to control.<\/p>\n\n\n\n The most feared pest in indoor cannabis cultivation. Small, quick to reproduce and capable of destroying a healthy plant within 10\u201314 days if left untreated. Under optimal conditions for the pest (dry heat, above 27 \u00b0C and humidity below 40 %), a female can lay 200 eggs in her lifetime, and the complete egg-to-adult cycle takes less than a week.<\/p>\n\n\n\n Mild infestation<\/strong> (few leaves affected, no cobwebs):<\/p>\n\n\n\n Moderate infestation<\/strong> (several affected plants, early signs of cobwebs):<\/p>\n\n\n\n Severe infestation<\/strong> (widespread cobweb-like patterns, yellow leaves):<\/p>\n\n\n\n Cleaning between cycles:<\/strong> Red spider mites spend the winter on the edges of the grow room, in the corners and in the ventilation systems. Cleaning all surfaces with a 3 % hydrogen peroxide solution or isopropyl alcohol between harvests and the start of a new cycle is essential if you have had an infestation.<\/p>\n\n\n\n The adult is a nuisance: a small mosquito (2\u20133 mm) that flies around plants and the growing medium. The real problem is the larvae, which live in the top layers of the growing medium and feed on fine roots and root hairs. A severe infestation can destroy the root system of a young plant in a matter of days.<\/p>\n\n\n\n Cultural measures (always first):<\/strong><\/p>\n\n\n\n Biological treatment:<\/strong><\/p>\n\n\n\n Chemical treatment (if biological treatment is not sufficient):<\/strong><\/p>\n\n\n\n Tiny insects (1\u20132 mm) that feed by scraping plant cells. They leave a distinctive trail that is very easy to identify. During flowering, as well as causing direct damage, they act as vectors for viruses such as TSWV (spotted wilt), which can ruin an entire crop.<\/p>\n\n\n\n Important during flowering:<\/strong> Thrips on buds are particularly problematic because contact pesticides do not penetrate the dense structure of the bud effectively. If you spot thrips during the late flowering stage, the safest option is to treat with a low dose of spinosad and harvest as soon as the trichomes are ready.<\/p>\n\n\n\n Aphids are colonies of soft-bodied insects (1\u20133 mm) that feed on sap by sucking it directly from the phloem. They weaken the plant, secrete honeydew (which encourages sooty mould) and, outdoors, can attract ants that protect them from their natural predators. They reproduce without the need for males \u2014 a single female can produce dozens of offspring a week \u2014 and in warm conditions, the population can explode within days.<\/p>\n\n\n\n Less common than spider mites or thrips, but particularly difficult to eradicate once established. Mealybugs protect themselves under a layer of white, cottony wax that acts as a physical barrier against contact insecticides. They congregate at growth points, in the leaf axils and, during flowering, inside the buds.<\/p>\n\n\n\n When growing outdoors, caterpillars pose a real threat, particularly during the summer months. The most common in Spain are the larvae of the cannabis moth (Helicoverpa armigera<\/em>) and several species of noctuid moths that attack at night. The most dangerous damage they cause is not to the outside of the plant: rather, they burrow into the buds to feed from within, creating damp tunnels where Botrytis fungus develops rapidly.<\/p>\n\n\n\n Golden rule for flowering:<\/strong> The closer you get to harvest, the more cautious you need to be with treatments. In the last two weeks before harvest, the only foliar treatment you can apply with complete confidence is plain water or very dilute potassium soap. Any product residue left on the bud will end up in the smoke or vapour.<\/p>\n\n\n\n Product rotation:<\/strong> Do not use the same insecticide for more than 2\u20133 consecutive cycles. Pests develop resistance quickly, particularly spider mites and thrips. Alternate between products with different modes of action (contact, systemic, hormonal, biological).<\/p>\n\n\n\n \u2192 Return to the Complete Cultivation Guide<\/a> for the next step: harvesting and curing.<\/p>\n\n\n\n\n
\n\n\n\nPrevention: the first line of defence<\/h2>\n\n\n\n
Environmental control<\/h3>\n\n\n\n
Air circulation<\/h3>\n\n\n\n
Quarantine of new material<\/h3>\n\n\n\n
Systematic weekly review<\/h3>\n\n\n\n
\n\n\n\nRed spider mite \u2014 Tetranychus urticae<\/em><\/h2>\n\n\n\n
How to identify it<\/h3>\n\n\n\n
What to see<\/th> Description<\/th><\/tr><\/thead> Dotted on the beam<\/td> Tiny yellow or whitish spots scattered across the upper surface of the leaf. The spider mites pierce the cells to feed.<\/td><\/tr> Spiders on the underside<\/td> They are 0.3\u20130.5 mm in size, and are red, yellow or green depending on the species and the season. You need a magnifying glass to see them clearly.<\/td><\/tr> Cobwebs<\/td> In advanced infestations, very fine silk threads can be seen between the leaves and buds. This is a sign that the colony is large and has been there for some time.<\/td><\/tr> Eggs<\/td> Translucent or whitish spherical spots attached to the underside, visible under a 30x magnifying glass.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n Treatment according to the level of infestation<\/h3>\n\n\n\n
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\n\n\n\nGround mosquito \u2014 Bradysia<\/em> spp. (fungus gnats)<\/h2>\n\n\n\n
How to identify it<\/h3>\n\n\n\n
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Treatment<\/h3>\n\n\n\n
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\n\n\n\nTrips \u2014 Frankliniella occidentalis<\/em> and other species<\/h2>\n\n\n\n
How to identify them<\/h3>\n\n\n\n
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Treatment<\/h3>\n\n\n\n
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\n\n\n\nAphid \u2014 Aphis gossypii<\/em> and other species<\/h2>\n\n\n\n
How to identify them<\/h3>\n\n\n\n
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Treatment<\/h3>\n\n\n\n
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\n\n\n\nMealy bug \u2014 Planococcus citri<\/em> and others<\/h2>\n\n\n\n
How to identify it<\/h3>\n\n\n\n
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Treatment<\/h3>\n\n\n\n
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\n\n\n\nCaterpillars and Lepidoptera<\/h2>\n\n\n\n
How to identify them<\/h3>\n\n\n\n
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Treatment<\/h3>\n\n\n\n
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\n\n\n\nSummary table: symptom \u2192 pest \u2192 treatment<\/h2>\n\n\n\n
Visual symptom<\/th> Possible pest<\/th> Priority treatment<\/th><\/tr><\/thead> Yellow spots on the leaves + insects on the underside + cobwebs<\/td> Spider mite<\/td> Potassium soap + neem; abamectin for severe infestations<\/td><\/tr> Small flying mosquitoes + unexplained poor growth<\/td> Fungus gnats<\/td> Allow to dry + Bacillus thuringiensis israelensis in irrigation water<\/td><\/tr> Silver streaks + clusters of black dots<\/td> Trips<\/td> Spinosad; blue traps; Amblyseius cucumeris<\/em><\/td><\/tr> Static colonies on the underside of leaves + molasses + ants<\/td> Aphid<\/td> Potassium soap; biological control using Aphidius<\/em><\/td><\/tr> White cotton in the armpits and knots<\/td> Mealy bug<\/td> Undiluted isopropyl alcohol; neem + foliar soap<\/td><\/tr> Dark excrement on leaves + holes + necrosis in the bud<\/td> Caterpillar \/ Lepidopteran<\/td> Bacillus thuringiensis kurstaki<\/em>; manual extraction<\/td><\/tr> White powder on the leaves (doesn\u2019t wipe off easily)<\/td> Powdery mildew (a fungus, not a pest)<\/td> Bicarbonate; wettable sulphur; improve ventilation<\/td><\/tr> Bud tips with grey mould inside, brown bracts<\/td> Botrytis (fungus, not a pest)<\/td> Remove affected parts; reduce humidity; improve circulation<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\nMost commonly used products: what they are, how they work and when to use them<\/h2>\n\n\n\n
Product<\/th> Origin<\/th> Target pests<\/th> Should it be used during flowering?<\/th><\/tr><\/thead> Potassium soap<\/td> Mineral<\/td> Mites, aphids, whiteflies<\/td> Yes, up to two weeks before harvest<\/td><\/tr> Neem oil<\/td> Plant-based<\/td> Mites, aphids, thrips, powdery mildew<\/td> Yes, during early flowering; no, during late flowering (affects flavour)<\/td><\/tr> Spinosad<\/td> Biological (bacteria)<\/td> Thrips, caterpillars, fungus gnats<\/td> Yes, lead time 3\u20135 days<\/td><\/tr> Bacillus thuringiensis<\/em> (Bt)<\/td> Biological (bacteria)<\/td> Caterpillars (kurstaki), fungus gnats (israelensis)<\/td> Yes, right up until harvest day<\/td><\/tr> Abamectin<\/td> Biological (fungus)<\/td> Spider mites, thrips<\/td> Not in flower. Safety period: 7\u201310 days minimum<\/td><\/tr> Natural pyrethrins<\/td> Plant (chrysanthemum)<\/td> Aphids, thrips, whitefly<\/td> Yes, during early flowering; use with caution during mid-flowering<\/td><\/tr> Wettable sulphur<\/td> Mineral<\/td> Powdery mildew, red spider mite (preventative)<\/td> Not at an advanced stage of flowering; not at temperatures > 30 \u00b0C<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
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