Fungus Gnats, Spider Mites, and Root Aphids

Prevention is the Best Remedy


It’s easier to stay pest free than to have to fight off an infestation. Here is how to prevent pests in your garden.

  • Encourage beneficial insects: While pollinators are great to have in the garden, beneficial insects are also those that eat pest insects. Ladybugs, lacewings, minute pirate bugs, parasitic wasps, damsel bugs, and other beneficials naturally help keep pest numbers down by eating pests or using them to house and feed their developing young.
  • Choose your plants wisely: Some plants and plant varieties are more prone to pest issues than others. Preventing pests in your garden is sometimes as simple as choosing pest-resistant vegetables.
  • Employ physical barriers: One of the most useful methods of preventing pests in your garden is to put a physical barrier between the plant and the insect. Cover pest-susceptible plants with floating row cover, a lightweight, spun-bound fabric that rests on top of the plants or on wire hoops.
  • Utilize intercropping: Pest prevention in your garden can also be promoted  by increasing the diversity of your vegetable patch. By inter-planting different vegetable crops with each other – and with flowering herbs and annuals – pests may have a more difficult time locating their host plants.
  • Grow healthy plants: The healthier your plants are, the better able they are to fight off pests all on their own.

If prevention does not work, and you become plagued by some of these, here are some ways to help fight these the war on pests.

Fungus Gnats


Fungus gnats are black in color, with long legs and wings, and between 3 to 5 millimeters in length and resemble tiny mosquitoes. They hatch larvae that have black heads and a clear body. They are found down at the bottom portions of the cannabis plant, and even in the soil. The larvae that the females lay burrow down below the soil level about 1 to 3 inches. The adult fungus gnat does not eat at all, and instead live only to breed. The little baby gnats feed on fungus and decaying matter on the soil and roots and can over time cause a weakening of the strength of the plant.  A severe infestation will lead to pale foliage and a general loss of vigor that leaves plants susceptible to disease, especially root-rot. And while adult gnats do not directly harm the plant, they are vectors for disease, and also easily become trapped in resinous flowers. They are also very damaging to cannabis plants because they affect the drainage properties of the soil.gnat-larvae-pbase

Fungus gnat hatchlings can often be seen drifting in the run-off from the water that is fed to cannabis plants and gathers in the tray underneath the pot. Larvae hatch in 5 to 10 days and take about a month before they are considered adult fungus gnats.

Outdoor fungus gnats can be found in the soil. If moving outdoor plants to an indoor grow it is recommended to check for fungus gnats before introducing the plant to its new environment.

How To Prevent and Control Fungus Gnats

  • Keep the screens on the windows down all the time.
  • Keep the soil as dry as can be tolerated, meaning do not water unless it is really needed.
  • Put a piece of cloth over the soil so the gnats have nowhere to lay their eggs.
  • Place a potato down in the soil of the garden to check for larvae. If you see a bunch of little maggots on the potato after a half a day it is telling you that you have gnats larvae in there.

How to Get Rid of Fungus Gnats

Keeping the humidity low and making sure not to overwater are key in preventing fungus gnat infestations. The most effective way to prevent or end an infestation is to place a physical barrier over your grow medium. A two-inch layer of sand, perlite, or other similar product (Gnat Nix works wonders) on top of your grow medium prevents adult gnats from laying their eggs in the medium, effectively ending their life cycle. You can also get have fan blowing air over the top of the growing medium. This helps dry out the top layer of your soil, and also helps prevent the fungus gnats from being able to fly around and lay more eggs. Yellow sticky traps can also be used to catch adults, hopefully before they lay hundreds of eggs in your grow medium.

The naturally occurring soil bacteria Bacillus thuringiensis, subspecies Israelensis, kills fungus gnat larvae and is available in many inexpensive forms like Summit Mosquito Bits, Gnatrol, and Microbe-Lift BMC. As a last resort, insecticides like pyrethrum and neem oil can be effective when used as a soil drench, but must be applied several times and can have negative effects on plant roots.

Spider Mites


Spider mites are the most common pest associated with cannabis (especially when growing in soil), and also one of the most potentially devastating. These are tiny (.4 mm) arachnids that generally live on the underside of leaves, making them very hard to spot with the naked eye. They have tiny sharp mouths that pierce individual plant cells and suck out the contents, resulting in tiny yellow, orange or white speckles (that some people initially confuse as mold or mildew) on your plant leaves. Spider mites also spin little webs, so if you see anything that looks like miniature cobwebs on your plants, you can be sure you have a significant infestation.


Why are spider mites so hard to get rid of?

  • Rapid reproduction – a single mature female spider mite can produce a million mites in less than a month
  • Deceiving disappearance– spider mites often appear to be gone/killed, then they come back with a vengeance days or weeks later.
  • Hungry Herbivores – spider mites can eat through your plants incredibly quickly; serious infestations have been known to kill plants overnight.
  • Web weavers – spider mites will cover leaves and flowers with a fine mesh of silk webbing, ruining whole garden even if you manage get rid of the spider mites
  • Radical resistance – spider mites quickly become immune to whatever you do to try to kill them; if you don’t take care of your spider mite problem by eradicating them completely from your grow room, you may soon find you have a population of ‘Super-mites’.

How to Get Rid of Spider Mites

Spider mites thrive and reproduce quickly in warm, dry environments. To slow an infestation, lower temperatures and raise the humidity in your room. Since spider mites like the heat, they hate a windy environment. A cool breezy grow space won’t get rid of spider mites, but it makes it harder for them to reproduce so your other control methods are more effective. While there are many commercial miticides available, most of them are quite toxic and should never be used on cannabis.

Biological controls such as predatory mites and ladybugs are also viable options. You can buy 1,500 live ladybugs on Amazon for about $20.

As a last resort, things like insecticidal soap, horticultural oil, and neem oil are safe for use on flowering plants and are generally quite effective. Concentrated neem products like Azamax and Azatrol are very popular with cannabis growers and are safe for use throughout the grow cycle, but are expensive.

Root Aphids


Perhaps the most difficult pest to eliminate are grape phylloxera, or root aphids — tiny aphid-like insects that feed off the roots of all types of plants. The root aphid is asexual, meaning they always reproduce female offspring. The aphid can produce up to 60 -100 nymphs in 20 to 30 days. About 1 mm long and ranging in color from yellow to green to brownish-orange, root aphids are often confused with fungus gnats as some adults will grow wings once a population has reached critical mass, but root aphids are short and stocky and usually cannot fly far. Plant roots turn yellow, swell, and then harden as the root aphids feed on them, leading to secondary fungal infections and dead spots. Eventually, plants will become stunted and flowering will be greatly diminished.


Cause of Root Aphids on Plants

  • Over-watering your plants can lead to a more suitable environment for the root aphids to lay their eggs.
  • Soil purchased that is mass produced can possible harbor some the aphids from the beginning, and you may never know until it is too late.
  • By going unchecked, they can all of a sudden spring out of nowhere

How to Get Rid of Root Aphids

Sand is a good preventive technique to use if you are using soil or coco coir as a growing medium. Sand is like glass and stops them from going into your roots. Place about 1-2 inches on the top and bottom of your grow pots. Yellow sticky trap paper is a great first defense it starts to kill the adult and nymphs, hopefully before they can reproduce. Hydrogen Peroxide is also a great first step before you invest any money in a solution. Hydrogen peroxide works great a killing aphid larvae. (Let the soil dry out the soil for a couple of days, then mix 1 part hydrogen peroxide to 4 parts water and soak your infected cannabis plants. When the growing medium starts to foam, it means the larvae a now being killed.)

Once infestation occurs, it can be nearly impossible to eliminate root aphids from your plants. The number one treatment is BotaniGard, an insecticide composed of the living fungus Beauveria bassiana, which infects and kills the aphid and then releases spores to infect the next victim. It can be used in conjunction with pyrethrumneem, and citrus oils, but care should be taken with these to avoid harming the plant. Apply as a soil drench every other day for a minimum of six treatments. Predatory nematodes can also be quite effective in controlling root aphids.

General Pest Control

Though each bug is different and should be treated specifically, most bugs can be handled with just a few techniques. Using any of the practices listed below, you will see less pests and bugs in your garden. You can also overlap different methods to obtain better prevention. Avoid sprays that use abamectn or indane which are harmful to mammals (and toxic for humans if consumed). When using chemical solutions, never let the solution puddle around the plant. Puddling can cause chemical burn as well as stomas (tiny holes responsible for intaking nutrients) to be clogged. Spray your plant right before you put it on its night cycle or else the light can make the chemicals burn through the plant.

  1. Diatomaceous powder is fossil dust you sprinkle on top of soil or anywhere that is razor sharp to microscopic life, but harmless to us. This is an easy way to keep tiny bugs under control that you really can’t see that well. This is not effective to get rid of a strong infection and is better as a preventative method.
  2. Yellow sticky traps draw bugs over because of their color and trap them, which eliminates most flying bugs and also gives you a good idea of how bad your infection is.
  3. Bug Blaster is a special water hose that removes bugs from your plant by just using sprayed water.
  4. Many pests can be eradicated with SM-90 .
  5. Neem oil is great for eradicating pests too, but the smell is unpleasant and can leave an unpleasant taste when used during flowering. Also, it is harmful to humans.
  6. Spinosad is organic and does not harm pets or plants even if heavily doused.
  7. Fine minced garlic and chili powder mixed with water can be sprayed on plants to repel bugs that hate the smell and taste of these.
  8. You can constantly keep azamax in your water supply to prevent bugs.
  9. Good bugs that eat bad bugs include:lady bugsphytoseiulus persimilislacewings and more which you can attract using attractants or planting herbs such as: clovermintdillfennelcatnipyarrow or oregano.
  10. Try attracting the pests away from your garden by planting something better than your marijuana plant as far from it as possible such as: zinniasdahliascosmos or asters.
  11. Provide food and housing for birds like small trees that provide nesting room such as hydrangeasabeliaboxwoodsarborvitae and privet.
  12. No-pest strips emit a powerful vapor and should only be used if you are not breathing in the air around them.
  13. Floramite is strong with harsh chemicals and should be used as a last resort and is fairly cheap.
  14. Bleach solution one tablespoon per gallon of 95F pH balanced water should be used on the plant and surrounding grow area. You can also use alcohol with 30% more water.