Marijuana has been grown outdoors for thousands of years and during that time, cultivators have been forced to face dozens of threats in the form of disease and pests. While humans don’t usually eat raw cannabis, it seems to fit the palate of many bugs, insects, and other animals. If you are unable to spot an infestation in time, there’s a chance that your entire crop will be ruined.
A simple and effective way to deter and kill pests is to use chemical insecticides. However, these products can damage the crop too, and ruin the taste. Think about it for a second: Would you like to smoke weed that has been heavily sprayed with commercial-grade chemicals?
In this guide, we provide you with natural methods of removing three of the most common cannabis pests: Root aphids, fungus gnats, and spider mites.
Rhopalosiphum rufiabdominale, the Rice Root Aphid, is a soft-bodied insect which infests the foliage and root systems of its host plants. These bugs can range from reddish-brown to bright green in coloration.
If a plant is heavily infested, the leaves become yellow and begin to wilt. Aphids also produce a large amount of honeydew, a sugary liquid waste. The honeydew emitted from the anus of the aphid often attracts sooty mold which accumulates on leaves and branches, turning them black.
Your cannabis garden can become infested when a winged aphid lands and lays eggs. It only takes a handful of aphids to cause an infestation, and the eggs quickly hatch to produce ‘nymphs’ which begin feeding on your crop. These young aphids mature in 7-10 days, and when they shed their skin, they leave behind silver-colored exoskeletons.
Removing Root Aphids
A root aphid colony can get out of control within a couple of weeks, so you have to act fast. Examine your plants at least once a week and look beneath new leaves for clusters. If you spot some, it is a sign that several colonies are well-established in your garden. Here is a quick list of things to do once you spot an infestation of aphids:
- Insecticidal soaps: These soaps suffocate or dissolve the exoskeleton of aphids. As soaps don’t stay on plants for long, we recommend several follow-up applications.
- Neem oil: Azadirachtin, the active ingredient in Neem oil, is effective against most pests. The chemical can however be damaging to buds (and is potentially harmful to humans), so spray carefully.
- Spinosad: They kill aphids on contact, but aren’t very strong. As a result, you need to use them numerous times.
- Introduce predators (like ladybugs and lacewing larvae): You can introduce these insects to your garden for the most natural of solutions. One issue with ladybugs is that they tend to fly away after a couple of days.
- Remove ants: Ants farm aphids to collect their honeydew. As a result, you have to get rid of ants in your garden because they keep the aphid population high!
These pests look similar to tiny mosquitoes and are just 2-4mm long. They produce larvae up to 6mm long which live in your growing medium. The larvae damage the roots of the marijuana plant, and a severe infestation reduces a plant’s strength and makes it susceptible to diseases such as root rot.
Fungus gnats thrive in moist conditions, which means they love it when growers overwater their soil. After fungus grows or overwatered matter decays into the soil, the gnat lays its eggs in the wet soil’s top layer.
How to Remove Fungus Gnats
The first step is to make sure humidity in your grow room is low to reduce the risk of a fungus gnat infestation. Signs of gnats include:
- Tiny black bugs crawling on the soil or flying around the plants.
- White/translucent larvae with black heads on the soil.
If left untreated, fungus gnats can result in a nutrient deficiency, a halt in plant growth, and reduced yields. If you find gnats, here’s how to get rid of them:
- Yellow sticky cards: These are special traps designed for gnats, which love the color yellow. The glue traps the gnats and severely reduces their numbers. You can monitor the cards to see if an infestation is being managed. If your cards become less covered in gnats, it is a sign that the population has been severely reduced.
- Use a fan: Place a fan in a position where it blows air out the top of the growing medium. It helps dry out the top layer and stops gnats laying more eggs.
- Dry out the soil: Avoid watering your plants for a few days to dry out the soil. This should kill a large percentage of larvae. Once the first few inches have become dry, the next step is to add a treatment.
- Kill the larvae: Spray the top layer of soil with neem oil. Make sure you don’t use the oil less than a week before harvest, and that it doesn’t touch the buds. Alternatively, sprinkle food-grade diatomaceous earth over exposed parts of the growing medium.
If there are still larvae remaining after your treatment, it is time to switch things up because they are resisting treatment. Products such as Bacillus thuringiensis (a species of bacteria), SM-90, and Essentria IC3 Insecticide should do the job if you don’t mind going down the chemical route. After a fungus gnat infestation, water your crop less often and keep using your sticky cards.
These mites are the most common cannabis pest, and at 0.4mm they are extremely hard to spot with the naked eye. In fact, you will probably need a magnifying glass to spot these insects. As small as they are, spider mites can ruin a crop. They have sharp mouths that pierce the individual plant cells and remove the contents.
You may spot orange, white, or yellow specks on your plant’s leaves and wonder what they are. We’re afraid to say; they are probably spider mites! Their diminutive nature means it can take days or even weeks to spot mites unless you are very careful when checking your crop. Here are a few reasons why growers dread spider mites:
- Spider mites are easily able to re-infest a crop if they are not totally eradicated, including in nearby areas.
- Spider mites produce silk webbing which covers buds and leaves. Even if you get rid of the mites, their webs can still ruin the quality of your crop.
- Spider mites become resistant to different methods of eradication.
Removing Spider Mites
Even when you destroy a spider mite infestation, it can quickly repopulate. Early detection is critical, but you can only do so by thoroughly inspecting both sides of the leaves of your plants. Once you spot the mites, you must assume your garden is infested for the duration of the marijuana plant’s growth cycle. Here is how to get rid of spider mites once you have found them:
- Because they are exotherms, consider increasing the temperature above 30 degrees Celsius. This is because spider mites cannot regulate their body temperature in excessive heat.
- Spinosad products such as Mighty Wash and Azamax work well when sprayed directly on the roots.
- Stethorus punctillum is the Spider Mite ladybeetle, and it feeds on spider mites voraciously. Phytoseiulus persimilis is another predatory mite that evolved specifically to feed on spider mites as well!
- Be sure to rotate treatments as part of an integrated pest management approach.
After your initial treatment, follow up in 2-3 days with a different method. Let’s say you used neem oil. This time, try the DIY alcohol spray remedy. You will need to repeat treatment at least once more and again; it must be different from the first two treatments.
In the future, prevent spider mites by keeping a clean grow room, checking new plants and clones for mites, ensuring good airflow, and maintaining a comfortable room temperature. It is also worth sprinkling diatomaceous earth on top of your soil and around the grow room.
Final Tips on Preventing and Removing Pests from Your Cannabis Garden
Growing cannabis can be challenging, and there is nothing worse than having your hard work ruined by unwelcome pests. Prevention is better than cure, but if you have root aphids, spider mites, or fungus gnats damaging your crop, you have to act fast or risk the ruination of your crop. Here are some quick tips to kill pests and keep them at bay:
- Don’t use anything other than sterilized soil or fertilizer. To sterilize soil, put it in the oven at a temperature of 160-180 degrees Fahrenheit for 30-45 minutes.
- Consider growing ‘companion’ plants such as basil, garlic, mint, or marigolds. Leaf eating pests tend to hate the smell of these plants and steer clear.
- For mammalian pests, purchase the urine of their enemies. Some stores specialize in these products.
- Make sure all of your cannabis growing equipment is sterile. You MUST wash your hands before touching plants, and remove any debris you find as soon as possible. A clean grow room is a safe one!
- Build a fence around your cannabis garden if it is outside. Animals carry a bevy of pathogens and bugs, so keeping them away from your crop is half the battle.
- If you are growing weed indoors, seal your grow room. Spray foam or caulk is useful to fill in gaps, and also seals windows and doors effectively.
- Change your clothes before entering your grow room if you were outside. Otherwise, you risk bringing in pests.