One of the most challenging aspects of cannabis growing is preventing, detecting, and controlling harmful insect infestations or fungal diseases. Any grower worth their salt has surely suffered a pest in their marijuana crop at one point or another, so prevention is key to stop these annoying guests from settling in. Here we explain how to deal with these voracious enemies, which are as numerous as they are varied.
There are dozens of pests of various shapes and sizes that can attack cannabis. Your marijuana plants are full of nutrients and are therefore a great habitat for these intruders, so you should be prepared to fight them. Since some of the most common organisms have already been thoroughly researched for other commercial crops, we know that each one of them requires a different prevention and treatment strategy.
Cannabis pests are parasitic and benefit from their symbiotic relationship at the expense of the host plant, usually by directly damaging the tissue structure or by acquiring its resources, such as its precious sap. For this reason, pests can be found in all parts of the plant, either in the branches and leaves (phylosphere) or in its root system (rhizosphere).
Due to the large variety of hungry insects that have a craving for marijuana, it is good practice to learn to identify the presence of a pest as soon as possible. Each type creates different symptoms which require different solutions.
Some pests are almost invisible to the naked eye, so a magnifying glass or a pocket microscope might be useful for identifying small insects, eggs, and larvae. Knowing what you’re up against allows you to stay ahead of the game even when you can’t see your enemies at first sight.
Sticky traps or strips are cards that you can place in your grow room to attract (and catch) many species of insects. The traps won’t be effective for all pests and sometimes only a few insects will stick, but this method can be a good time saver for identification purposes.
Many times, the first thing you notice aren’t the actual insects but rather the signs of their presence, because the insects themselves are too small or are hiding away. Always check the leaves on both sides and look for white or dark spots, blurry patches, or anything that doesn’t look ‘normal’. Most of these pests feed on the plant itself, so they always leave signs of their feeding: bitten leaves, discoloured stains from the bites, yellow leaves due to the lack of sap, etc.
Most insects also leave a trace of their passage: things that shouldn’t be there, such as droppings, honeydew, cobwebs, or white cocoons that they use to protect their eggs. Look out for any unusual symptoms beyond the intense greenery of the leaves and stems of your cannabis plants.
Although there is a huge list of insects that can affect cannabis, only a handful seem to cause serious problems to growers. Even if the pest you detect is not on this list, fortunately many of the steps you can take after an infestation are similar. Here are the most common ones:
Red spiders: They damage the plants by sucking the sap from the leaves, killing the crop very quickly. Look for small white or yellow dots on the leaves.
Caterpillars: They can ruin your grow in a jiffy because they eat at the speed of light, leaving irregular bite holes on the leaves.
Thrips: These insects are both tiny and fast, so they can be hard to detect. Watch out for brittle leaves with silver marks on the surface.
White flies: Another old acquaintance of growers. These insects tend to congregate on the underside of the leaves. If they’re there, you’ll see them flying away as you shake the plants.
Aphids: These creatures spread really quickly. If you spot dry leaves, honeydew, or actual colonies under the stems and leaves, then you have a problem.
Cochineal: Another nemesis of growers that reproduces at a dizzying pace. They cause sooty mould, a black fungus that covers the leaves. Look for white fluff and you’ll find them.
Nematodes: They include 25,000 different species, half of which are parasites that are harmful to plants, while the other half are actually beneficial.
Slugs and snails: These need no introduction. Detecting them is relatively easy, as they leave bright traces of transparent slime.
Mites: These are tiny creatures, and typical symptoms mimic herbicide damage, including leaves that become small and atrophied, and sometimes curl inward.
Leaf mining flies: They lay their eggs in the leaf tissue. The larvae hatch inside the leaves, feeding and creating distinctive winding tunnels in their path.
Substrate flies: Their larvae live in the substrate, which then metamorphose and emerge to the surface as the most annoying winged flies.
Just like insects, fungi are one of the main plant pathogens that pose a threat to cannabis. They usually appear when there is too much moisture, in places where the light doesn’t reach, or when the plants are very close together. These are rather specialised microorganisms, with several fungal species specific to the host. The following are the most common in marijuana:
Powdery mildew: Spots of a white/grey powder appear on the surface of the leaves, which keep growing, getting closer to the buds as the infection progresses.
Mildew: Very similar to powdery mildew, but this one produces yellow spots on the upper side and whitish spots on the underside of the leaves. Mildew can remain dormant for years in plant debris.
Botrytis or grey mould: It appears during the flowering phase, causing severe damage to the buds, where it usually develops as a cottony substance that dries the plant tissue.
Fusarium: These are filamentous fungi that are normally found in the substrate. They start affecting the roots before then moving on to other parts of the plant through the vascular bundles.
Pythium: Another group of root fungi that can infect both stems and seeds, using the plant cells to reproduce. It can cause wilting (i.e. ‘damping off’) at the base of the stem of the seedlings.
Alternaria: These are plant pathogens that can cause serious damage. They develop speckled spots with areas of light and dark green which soon show signs of necrosis.
With pests, prevention is better than cure. Therefore, the first step to avoid any infestation is to take appropriate preventive measures. And the best way is to start with a safe environment for your crop to thrive, so you’ll need to adopt proper cleaning and disinfection practices. Here are our 10 top tips to achieve this:
But if prevention doesn’t work and a plague has already settled in your crop, do not despair: there are also methods to eradicate it. Oddly enough, pesticides and other chemical solutions are not as effective for these cases, as they are so frequently used that some insects are immunised against some of their chemicals.
Those who don’t wish to handle or risk ingesting or inhaling those dangerous chemical pesticides can opt for organic solutions, which can be equally effective but much safer. For instance, products derived from neem tree oil. Neem oil is often sold in combination with other essential oils such as rosemary, cinnamon, eucalyptus, or cloves, due to the systemic action of these components.
Potassium soap, also known as soft soap, works wonders too. This is defined as a biological insecticide of plant origin which is selective against pests, is harmless to people, and beneficial to the environment.
Another environmentally-friendly alternative is biological control, which consists of using insects to attack and eat other insects. Effective insects include ladybugs, parasitic wasps, predatory mites, Anthocoridae (or flower bugs), Chrysopidae (they are particularly voracious), Syrphidae, and spiders. Once you have identified which insect is damaging the plants, you can choose which one you are going to attack it with.
Finally, let’s address one of the most common issues amongst rookie growers. The most common reason why the leaves turn yellow is stress. Whether it’s due to inadequate irrigation, nutrient excess or deficiency, pH imbalance, excessive heat, light deficiency, or pest infestation, yellow leaves are often a sign of unhealthy cannabis plants.
If your yellow leaves have spots or bite marks, they are probably infested by a pest. Substrate flies, for instance, can trigger chlorosis, because the larvae feed on the roots. Mildew can also cause this, as this fungus produces yellow spots on the surface that can eventually dry the leaves.
But sometimes yellow leaves are simply a natural part of the growth cycle, as cannabis plants often shed the oldest fan leaves (especially at the end of their life), which turn yellow before falling to the ground.
Therefore, this could also be a symptom that tells you that the moment of the precious harvest is approaching; something that will definitely have been well-earned after so many months of hard work. Even more so if you have emerged victorious after a long fight against any of the many pests of cannabis!