Pests are harmful organisms that have been a major pain in the ass since the dawn of agriculture. They are coming for your greenhouse, and you need to be ready. Pests are classified organisms that do not cause disease but rather inflict serious damage on your plants. Knowing how to spot and deal with them is essential to every grow operation.
Here’s the issue
Plants are nutrient-rich, and pests are uniquely adapted to seek out and feast on those nutrients. Your next pest battle is likely right around the corner, so we’ve developed pest management protocols to help you win your war against pests.
In an indoor grow, pests don’t naturally have predators. There are roughly 300 insects that have been identified as potential pests for cannabis plants, and will thrive if left unchecked. They all want to work as little as possible in order to survive, survive so that they can reproduce as much as possible, and reproduce like crazy so that their offspring have a chance to successfully pass along their genes.
Cannabis has evolved an elaborate defense mechanism to ward off pests. Cannabis plants naturally produce THC, the psychoactive cannabinoid, which works as an insect repellent. The predominant pests that feed on cannabis are insects with piercing-sucking mouthparts, so that they can pierce the plant deep enough to bypass its surface-based cannabinoid defense system, and extract the energizing sap from the plant.
How aphids sucks sap from your plants. Image Source
A small outbreak can quickly turn into an infestation that can devastate your crop. It is essential that everyone on your team knows how to identify pests and is trained on pest management issues before they arise. Is your team prepared for pests? Test your knowledge against our handy cheat sheet below.
Cannabis greenhouse pest management cheat sheet
Major Pest Alert: The 5 most common pests, and what you can do about them.
Spider Mites
The most destructive pest of indoor grown cannabis tends to be spider mites.
Mites are not insects but rather closer to the Arachnid family. There are over 60 mite species that have been identified as havoc makers for cannabis. The most frequent offender is the two spotted spider mite, Tetranychus urticae.
What they do
Spider mites feed on plant sap, lay their eggs, and set up camp by casting shrouds of web for protection. They tend to love hot, dry conditions, though different species are specialized for cooler climates. The females lay nearly 20 eggs each day for their 2 to 4 week lifespan which hatch in under 3 days, meaning it’s important to act fast in response.
How to spot them
Look for graying or yellowing of the leaves. Tiny yellow spots are often the first visible symptom.
Mite damage to the open flower causes leaves to brown and wither.
An advanced-stage spider mite infestation.
They are really small (1mm), and are generally found on the bottom of leaves. Grabbing a 15x magnifying glass can help you get a good look at them. Search leaves for mites, webbing, and cast skins. An easy option is to shake your plant over a plain piece of white paper to check for spider mites by looking for any black dots that appear on the paper. Later stages of damage leads to complete defoliation, sap removal, cell death.
How to solve the problem
Due to their accelerated reproductive cycle, Spider Mites can quickly develop resistances to pesticides, so repeat applications within a few days of each other is necessary to wipe them out. Having predatory mites and other beneficial organisms in your greenhouse consistently helps keep the pest population low. Oils and soaps can be used to clean the plants. Feeding your cannabis plants nutritional support to help activate their natural defenses is also a strong option. For prevention, make sure that you always treat or quarantine clones before transferring them into another grow room. Additionally, you can discourage mites by reducing dust build-up on your plants.
Aphids
Six species of aphids cause major cannabis problems. Some specifically target parts of your plant, and others are more general grazers.
What they do
Aphids will kick you while you’re down. They devour plant sap and as they feed drop a sticky honeydew mixture which invites sooty mold and ants. They can also transmit plant diseases, so getting rid of aphids is not only about dealing with the immediate sap-draining problem, but also dealing with the potential snowball effects of having an infestation.
In your greenhouse, aphids can continuously reproduce without having to take a break for the Winter. A single female can produce 1000s of miniature versions of herself over its 20-40 day lifespan. Some species have an adaptation where they can produce wings, meaning that the infestation can spread quickly throughout a room and potentially into other grow rooms. The situation can quickly spiral out of control.
How to spot them
Aphids generally group on the underside of leaves, though there are root-specific aphids as well. Cannabis loving aphids generally start as narrow and white, growing into their green winged or wingless adult forms. You may find white/silver exoskeletons that have been shed off aphids as they mature. As the colony grows, leaves will look distorted, wilting, and yellow. Some aphids will infest flowers. By the time you notice a significant reduction in plant vigor you have a serious problem on your hands.
Check for sooty mold (a black fungus) as an indication that aphids were feeding above. If you have an ant problem, check for aphids, since ants will actively farm aphids for the honeydew they produce.
How to solve the problem
Organic insecticides, fatty acid soaps, and beneficial insects are your best bet. Getting rid of ants can help make sure that aphid colonies are not being supported by external sources.
Fungus Gnats
Fungus Gnats are common indoor pests that can damage seedlings, carry diseases, have root-damaging larvae, and seem to aim for your open mouth when they fly near you in the greenhouse.
What they do
Fungus gnats spawn their larvae in the top few inches of wet soil to dine on decaying organic matter. The larvae hunt for fungus in the soil and will also bite at your cannabis roots. These gnats can indirectly cause root rot by spreading diseases.
How to spot them
Put out yellow fly traps to attract passing gnats, and check the cards on a weekly basis. The adults look like winged black flies, appearing similar to mosquitos. If you have a wet growing medium, be sure to keep an eye out since that is their ideal environment. You may see the miniscule larvae as white dots with black heads wriggling in your topsoil. Younger plants will show more visible signs of damage, but adults will display issues when the infestation is large enough. The big signs to look out for are plants appearing weak for no discernable reason, meaning they have lower yields, grow slower, lack vigor, or display nutrient deficiencies.
How to solve the problem
Let your top inch of growing medium dry out, or add another layer on top. A thin layer of sand and/or increasing airflow can be particularly helpful to jumpstart drainage. Mosquito Dunks are a simple product which contains bacteria to fight off gnats. A sprinkling of diatomaceous over your soil will shred the gnats’ exoskeletons. Revisit your watering policies to make an infestation less likely in the future, making sure that you allow your growing medium to properly drain.
Thrips
As the fastest growing group of invasive species in the world, Thrips are a major concern. In a cannabis greenhouse the most common thrips species is Frankliniella Occidentalis.
What they do
They live on the underside of leaves and can cause a host of problems. They potentially carry disease, and can destroy the health and harvest of your plant. With a fast reproductive cycle these pests can quickly swarm a greenhouse. They bite leaves and stems, lay their eggs either inside the plant, on the underside of leaves, or in flowers.
How to spot them
Thrips come in all shapes and colors. The juvenile thrips look like a small white worm, and the adults have dark colored or golden coloration, with or without wings. Look for signs of feeding on your plants which shows as silvery or brown stains on the leaves and stems. There may also be tiny visible black spots which are the thrips droppings. Focus on checking your plants’ younger leaves and buds for “burnt” or browning edges since that’s where thrips are likely to start their attack.
How to solve the problem
First and foremost, shaking your plant will throw the thrips off of it. Then applications of beneficial insects, insecticidal soaps, and biocides can work well. There are several specialized sprays and options available on the market, and the soil fungus Entomophthora thripidum can manage thrips passively. Thrips can quickly develop resistances, and subsets of their population hide in small crevices that may not be affected by your treatment. To really eliminate the infestation the applications need to be extremely thorough.
Whiteflies
The whitefly is similar to the spider mite in the way that it feeds and acts, but they secrete honeydew like aphids. They potentially spread disease, and unattended will mess up your grow fast.
What they do
They feed on plant sap, and do mechanical damage to your plant while feeding. Whiteflies inject harmful saliva while feeding, weaking the plant, while at the same time excreting honeydew that they attracts sooty mold. When whiteflies swarm a plant, they can quickly overwhelm it to the point of no return.
How to spot them
The mature whitefly looks like a 2mm sized white moth. On the underside of leaves whiteflies congregate and lay eggs that look like small grains of rice. The effect of their feeding shows as white spots or necrosis on the top of the leaf. For easy inspection, shake your plants so whiteflies are forced to fly around the grow room.
How to solve the problem
Yellow fly traps attract whiteflies which can help a little with population control but largely helps with identification. You can use a variety of companion plants to ward off whiteflies, and there are plenty of beneficial insects which would enjoy taking the pest problem off of your hands. There are insecticidal soaps and oils that effectively handle whiteflies as well.
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