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Pests, Bugs & Viruses
by Nebula Haze
Unfortunately, bugs and other garden pests can totally mess up your marijuana harvest!
This page aims to be a comprehensive resource on the different types of bugs / pests / mold that can affect your marijuana crop, along with tips for preventing and solving each problem.
Pests that can affect your marijuana plants include aphids, fungus gnats, thrips, green flies, black flies, mosaic virus, spider mites, caterpillars, inchworms, whiteflies, white powdery mildew / white powdery mold, stem rot, and even mammals such as deer or cats!
- Barnacles / Scales
- Bud Rot or Mold
- Caterpillars & Worms
- Fungus Gnats
- Leaf Septoria / Yellow Leaf Spot
- Root Rot
- Slugs / Snails
- Spider Mites
- Tobacco Mosaic Virus
- Whiteflies / White fly
- White Powdery Mold
It's time to fight back against cannabis bugs, mold and pests!
Quick Tip! Whenever you spray plants with anything, make sure to get the undersides of the leaves too, as this is where many pests like to hang out! A one-hand pressure sprayer / mister is also really helpful for spraying leaves.
Aphids live under leaves & have different forms depending on their stage of life
Barnacles / Scales
These bugs that look like barnacles and stick to the plant on stems and underneath leaves
Bud Rot or Mold
When bud rot strikes, certain buds may start looking sickly overnight, with leaves turning yellow and/or bud becoming discolored. When opened up the inside of the bud is dead or moldy.
Caterpillars, Inchworms & Cabbage Loopers
Caterpillars and worms eat holes in leaves and leave droppings that look like black specks
Fungus gnats look like tiny dark flies. They hang around soil that stays wet for long periods of time, and their worm-like larvae crawl around in the wet top soil. Plants start getting sick if a gnat infestation gets out of control.
Leaf Septoria / Yellow Leaf Spot
This fungus causes round yellow or brown spots, with symptoms often starting on lower parts of the plant
These tiny white bugs look "hairy" and are found crawling on leaves and buds
Root rot is a common problem in hydroponic systems though overwatered plants in containers often display similar symptoms. Plants with root rot wilt and leaves may become discolored. In the reservoir the roots turn brown, smelly and slimy.
Slugs / Snails
Slugs and snails usually come out at night, leaving holes in leaves which have scalloped edges from their individual bite marks. They also leave slime trails on leaves and on the ground.
Spider mites are often caught from another grow room, and their bites leave small white speckles all over your leaves. They're so small they can be hard to see, though the best place to look is underneath leaves. You may see webbing if there's enough of them living on the plant.
Thrips leave irregular bronze or silver marks that may look like "dried spit" or tiny snail trails. Their young look like fat, tiny worms.
Tobacco Mosaic Virus
It's not known whether mosaic virus has jumped from tobacco to cannabis plants, but in this article I'll share what I've learned so far...
Whiteflies/ White fly
Whiteflies look like tiny white moths and hang out under your cannabis leaves
White Powdery Mold
WPM leaves a white powdery substance that looks like flour or powder on leaves and stems
Don't see your bug here? Send us a pic!
Aphids are very small insects that can be many colors including green, yellow, black, brown or red. Young aphids are often white and narrow, while adults are usually colored and oval-shaped (adult aphids may or may not have wings). They colonize your plant and congregate underneath your leaves while munching on your poor plant.
The bigger yellow bugs are adult aphids, while the smaller white bugs are young aphids (nymphs)
What Causes an Aphid Infestation?
The most common way your marijuana can become infected is when "colonizer" aphids land on your plants and lay eggs.
Learn more about how aphids attack and colonize cannabis plants
(plus tons more pics and solutions to get rid of them!)
Winged aphids are sometimes called "blackfly" or "greenfly" bugs depending on the color (because they can be black or green, and they kind of look like tiny flies).
This is a "black fly" aphid. Winged aphids also come in green.
Immature aphids (nymphs) usually appear white and narrow. Note: If you see tiny white bugs but they look more round, fat and worm-like than these ones, you may actually have thrips.
Adult aphid on cannabis plant
These gross bugs hang out on your stems and under your leaves. They suck out the life of stems and leaves, which can start to weaken your plant if it gets too bad. They release a sweet sticky substance known as "honeydew" (very similar to an aphid) which can cause dark patches of mold known as sooty mold to start appearing on your plant. The honeydew can also attract ants.
In fact, the way to treat barncales or scale insects is almost exactly the same as what you would do for aphids.
Bud rot, the bane of outdoor cannabis growers. Although things may look normal on the surface at first, a bud attacked by the Botrytis cinerea fungus rots from the inside out.
With bud rot, your buds actually start rotting from the inside out :(
Bud rot thrives when there is lots of moisture and temperatures around 60°F (15°C). That means bud rot is more likely to attack buds in the later part of the flowering stage when buds are fat and (for many parts of the world) the weather becomes cool, rainy and/or humid.
Bud rot can affect indoor cannabis plants in humid environments, which is most common in poorly ventilated spaces with very leafy plants. It's also heartbreakingly common to see bud rot outside after plants are exposed to wet, cool weather, which can sometimes last for days or weeks.
The unfortunate thing is that bud rot can demolish a whole crop! After attacking one bud on the plant, it quickly starts spreading to the others if the conditions don't improve, since you're combining a mold-friendly environment with plenty of spores in the air.
The best way to prevent bud rot is to keep humidity low, maintain good air circulation, and inspect buds regularly. If you catch it in action, remove infected buds immediately and possibly harvest early, so it doesn't spread to other parts of the plant. If plants are bushy and have lots of leaves, you may also consider removing inner and lower leaves (especially any leaves that don't get access to the sun) to help a breeze move through the plant.
Often the first sign of caterpillars/worms/cabbage loopers is noticing chunks of your leaves missing. You may also see clumps of black/brown "dirt" on your leaves, which is caterpillar poop. Lastly, you may actually catch a caterpillar in the act munching on your leaves! Read the full tutorial on getting rid of caterpillars (with tons more pics and info!).
Caterpillars take chunks out of leaves. If your holes have scalloped edges, you may actually being seeing slug or snail damage. This is what caterpillar damage looks like.
Caterpillar Droppings on Leaves
Fungus gnats are tiny little bugs that buzz around your soil. While the adult gnats don't bother your cannabis plants much, their larvae can attack cannabis roots in the soil. This can cause all sort of unexpected problems for your plants, like nutrient deficiencies, spots, droopiness, and more. Young plants are most susceptible.
What causes fungus gnats?
Lack of air circulation
How to Get Rid of Fungus Gnats:
- Let the top inch of soil dry out before watering each time and they will go away on their own. In other words, make sure you're watering plants properly.
- Blow air over soil to help the top inch dry out if you're in the midst of an infestation.
- Need to get rid of fungus gnats fast? Water your plants with a pest treatment that contains BT bacillus thuringiensis bacteria (like Mosquito Dunks). It gets rid of fungus gnats almost immediately!
- Use yellow sticky cards to see how bad your fungus gnat infestation really is, and to know when they're gone.
Don't Let Fungus Gnats Get This Bad!
Learn more in our tutorial about getting rid of fungus gnats
Sometimes called "yellow leaf spot" or "leaf septoria," this condition is caused by a fungus that attacks cannabis plants (usually in warm, wet weather). The symptoms first appear on the bottom leaves of the plant and start spreading.
These small bugs look white and almost hairy. You'll find them crawling all over your leaves and buds. They enjoy warm temperatures and don't tolerate the cold. Like aphids and scales, mealybugs produce a sweet liquid known as honeydew which can cause sooty mold to grow on the leaves as well as attract ants. Read the full tutorial and see a video of these bugs on a cannabis plant!
The roots of an affected plant affected by root rot turn smelly, slimy and brown. If root rot gets bad it can quickly kill a plant! Root rot appears most often alongside hot temperatures, though it can strike any time plant roots are not able to get enough oxygen. While pot-bound plants can be affected by root rot when they're overwatered, root rot is usually a problem for hydroponic plants grown directly in water, such as deep water culture or bubbleponics.
Symptoms of Pythium Root Rot include: Roots are slimy, smelly, brown or dead. Plants stop growing and start looking droopy. Leaves begin to yellow and start getting what may appear to be nutrient deficiencies.
These pests attack in the night and will eat leaves and buds. A sure sign of snail or slug damage is a trail of slime on leaves or on the ground around your plant. The holes left behind can sometimes be confused with caterpillar damage, but one difference is snails or slugs leave "scalloped" edges, from these pests eating chunks out of your leaves one bite at a time.
Learn more about slugs and snails (with lots more pics!)
Spider mites are not insects but are more closely related to spiders. The most common way for a grower to get infested by these tiny bugs is by bringing in new clones from another grow room without treating or quarantining them. Learn How to Kill Spider Mites Quickly!
Spider mites are usually found under the leaves, but they are tiny and easy to miss
These tiny yellow spots are spider mite bite marks. If the spots are bigger and/or more irregular, you may actually have thrips.
If a spider mite infestation is left unchecked you'll also start seeing webbing on your leaves and buds.
Heavily infested plants may be discolored, stunted or even killed so don't let spider mites get out of control!
Thrips are small, fast-moving insects and can come in many forms, from pale wormy looking things to dark winged insects, depending on the stage of life and where you live. They leave shiny (sometimes people thing it looks "slimy"), silver or bronze spots wherever leaves are bitten. The spots are bigger and more irregularly shaped than the bites left from spider mites.
Learn more about thrips and cannabis
(plus see lots more pics!)
Example of thrips damage on marijuana leaves (irregular silver or bronze spots).
Here's a picture of an adult thrip on a finger for scale - they're tiny!
In their "nymph" (juvenile) form, thrips appear pale, fat and almost wormy from afar. A thrip nymph looks more tubular and worm-like than an aphid nymph.
The mosaic virus can attack a wide range of plants in addition to tobacco, including tomatos, peppers, eggplant, spinach, petunias and marigolds. Although it's found in many species of plants, at this point no one has confirmed for sure that mosaic virus actually has spread to marijuana plants.
However, certain cannabis plants exhibit symptoms that are eerily similar to what happens to other plants suffering from mosaic virus...
White flies behave just like spider mites, hanging out under the leaves and infesting your plants while sucking up essential nutrients from the leaves.
This results in white spots on the top side of the leaf, though you'll likely notice the white bugs flying around your plant before you notice the leaf damage.
White flies are easily spotted with the naked eye. If you shake the plant a little, they'll fly around. They look like little white moths, around 2 millimeters in size.
White Powdery Mold looks like someone spilled flour or powder on your plants. It is usually found on plants in an area with high humidity and/or not enough airflow. If plants are kept close together in an enclosed space without air circulation, this can be a breeding ground for white powdery mold, especially if plants are leafy or have lots of leaves toughing each other.