Root Aphids, Fungus Gnats, and Spider Mites: Eliminate These Marijuana Pests Completely

How to avoid having your marijuana crop ruined by pests


Marijuana has been grown outdoors for thousands of years and during that time, cultivators have been forced to face a myriad of threats in the form of disease and pests. While humans don’t usually eat raw cannabis, it seems to fit the palette of many pests. If you are unable to spot an infestation in time, there’s a chance that your entire crop gets 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.

Root Aphids

Grape phylloxera, better known as root aphids, are soft-bodied insects that are hard to spot. Their range of colors include white, green, yellow, brown, red, and black; this depends on how old the aphids are and where you are in the world. A common mistake is to assume that greenflies or black flies are on your crop when, in fact, they are aphids with wings.

Unfortunately, it seems as if green aphids enjoy attacking cannabis, so the natural camouflage of the pest means it can remain undetected for a considerable period. You will normally find aphids beneath the stems and leaves, and they arrive in colonies, eagerly sucking on the plant’s juices.

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 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 weaken the outside shell of aphids, and don’t leave a great deal of residue. As soaps don’t stay on plants for long, we recommend several follow-up applications.
    • Neem oil: This oil is very effective against most pests, but damages buds and is potentially harmful to humans, so spray carefully.
    • Spinosad: Also known as safe and organic, these products are harmless to humans, plants, and pets. They kill aphids on contact but aren’t very strong. As a result, you need to use them numerous times.
    • Introduce predators: Ladybugs, lacewings, and lady beetles feast on aphids. You can introduce them 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!

Fungus Gnats

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 who 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 becoming resistant. Products such as BT 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.

Spider Mites

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:

  • A single female mite can reproduce up to one million mites in under a month.
  • A large infestation can kill a plant in less than 24 hours.
  • They are very resilient. When you think you have removed an infestation, it returns weeks or even months later.
  • 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 have an irritating ability to become resistant to different methods of eradication.
  • Occasionally, you may find a two-spotted mite. If so, be very afraid because they can become immune to insecticides. Their durability has resulted in the nickname ‘the Borg’ from Star Trek-loving marijuana growers.

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:

  • Consider reducing your grow room temperature, as spider mites like the heat. Just be careful not to put your plants into shock while doing so.
  • Place a fan in a position where it blows over the plants and top of the soil.
  • Spinosad products such as Mighty Wash and Azamax work well when sprayed directly on the roots.
  • Insecticidal soaps work, but you must apply them practically every day.
  • There are also expensive yet effective options such as Floramite.
  • DIY remedies include creating a solution of 1 tablespoon bleach to 1 gallon of water at 95 degrees Fahrenheit. Spray all surfaces of your room, but NOT your plants. For your plants, a solution of 9 parts water to 1-part alcohol works and doesn’t hurt plants.
  • Natural predators such as predatory mites and ladybugs kill spider mites.
  • Neem oil is effective, but as you know, you have to keep it away from buds.

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.
    • Find out your pest’s enemies, and purchase their urine! If pests smell this urine, they will be afraid and stay away. Believe it or not, some stores specialize in this product!
    • 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 host 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.