Fungus Gnats on Cannabis

Fungus Gnats on Cannabis

Fungus gnats on cannabis are the small dark flies that lift off a pot when you water. They run across the soil, hop into a short flight, and land on a leaf. The adult in that photo is the part you notice. The larva in the mix is the part that matters.

These are Bradysia-type flies. The body is slim, the legs are long, and the wings look a little smoky in a good light. Adults are weak. They do not chew leaves the way a caterpillar does. A few of them on a mature plant mean the surface has been staying wet. Larvae live in the top inch of constantly wet peat, coco, or soil, about a couple of centimeters down, where fungus and decaying bits collect. Fine roots sit in that same layer. Seedlings stall when the layer is crowded. A big plant usually shrugs off a light catch and still needs the wetness fixed before the next generation gets larger.

In a warm tent the life cycle often runs about three to four weeks. Eggs go into the wet film on top. Larvae feed, pupae form in that same shallow band, and adults lay again if the surface is still a bog. Eggs and young larvae fail once that film dries. Adults already in the air keep flying for about a week after the pots start to behave. Judging the job on day two sends you to a spray the soil never needed.

This page is the gnat itself: how to name it, where it breeds, and the order that actually clears a room. The weekly loop of scouting, cards, and matching a hunter to a pest lives on the integrated pest management page. The neem bottle, the mix, and the lights-off rule live on the neem oil page. Soil hunters live on the beneficial insects page and the beneficial nematodes page. Read those when you get to the step they own. The doses on a product label and on a supplier card stay on those labels. Nothing below replaces them.

In Canada the federal baseline is four cannabis plants per household, not four per adult. Quebec still prohibits recreational home growing. Manitoba, since 1 May 2025, allows adults 19 and older up to four plants, and only indoors.

How the fly on the leaf compares

Put one on a finger or on the lip of a tray and look before you name it. A fungus gnat is small, dark, and leggy. It prefers the pot to the middle of the room. You will find them on the soil, on the plastic rim, on a yellow card, and sometimes resting on a leaf the way the photos here show. White specks on a leaf are a separate question. Gnat eggs are laid in the mix, so a dusty leaf is dust, dried splash, or something else. Deal with the fly by looking down, at the soil.

Shore flies are the stockier cousins that hang around algae on trays, plugs, and reservoir lids. They breed in the same wet film. Drying that film and wiping the algae cuts both. You do not need a second product line because the fly looks a little thicker.

Rice root aphid is the mix-up that wastes a week. Slide the root ball out of the pot. If the insects are aphids stuck on the roots, pale and soft, you are looking at a root aphid, and the gnat routine will not clear them. Stop and use the IPM page for that pest. One glance at the root ball is enough to separate the two. This article stays with the flies that breed in the wet top.

A raw potato slice is an identification trick. Set a fresh slice on the soil in the evening. In the morning, larvae with dark heads on the cut face mean the pot is occupied. Throw the slice away the same morning. Left in place, it feeds the layer you are trying to calm down.

The wet top inch is the nursery

Females lay where the surface stays shiny. That is the seam between the mix and the pot wall, the low spot in a coco pot, the green film on a rockwool cube, and the scum in a saucer you forgot to dump. The gnat at the rim in that photo is standing on the real estate she wants. Constant moisture plus a little fungus is the whole requirement. A pot that never lightens between waterings is a nursery, even when the plant on top looks fine for now.

Peat and coco hold that film longer than a gritty mix that goes pale on top. A careful soil mix still gets gnats the week the watering can loves it too much. Switching the room to a new bag only moves the same habit.

Algae is a breeding film you can see. Green shoulders on a cube, a slick tray, a puddle under a net pot: wipe them, and let those surfaces dry. In a hydro or coco room, fungus gnats usually mean a wet plug, algae, or a soil pot sharing the space. Flies over a bubbler are a reason to check the plug and the other pots, and a reason to leave the nutrient bottle alone.

New plants and new media bring eggs in more often than people admit. A clone from a wet bench can carry larvae in the plug. A bag of peat or coco can sit damp at a supplier with a resident population. Open a new bale away from the flower room. If flies walk off the plastic when you cut it, that bag stays out until you have dealt with it. A new plant gets its own yellow card for the first week, off to the side, until the card stays nearly clean. A folding table and a card are the difference between one pot and a whole flower run.

Saucers left full keep the bottom wet and wick dampness back up. Empty them the same day you water. Bottom watering is useful when you still let the top inch dry before the next pour. The watering guide owns how hard and how often you pour. This page only cares that the surface gets a dry spell.

A living soil pot can run fungus gnats for a boring reason. Indoor mulch that stays thick and wet is a mat over the nursery. On the no-till page the mulch stays thin indoors for this exact reason. Outdoors, sun and air do some of that drying for you. In a tent, you are the weather. Keep the compost bin outside the flower room too. A working bin belongs with the compost setup, where the flies it attracts are a normal part of decay and stay out of the buds.

Seedlings stall and a big plant warns you

A seedling has a few fine roots in a small cup. Larvae in that cup chew the roots the plant cannot spare. New growth stops. The seed leaves hang on while the plant sits there, green enough that you blame the light or the feed. Lift the cup. If it is heavy every day, and a card beside it loads with flies, you have the cause. The seedling stage page owns how far a young plant can dry. Dry the surface. Leave the root ball with some moisture. Baking a ten-day sprout down to dust trades a gnat problem for a dead plant.

A weed plant in a one-gallon pot, with roots against the wall and a stem you can pinch, handles a light catch. You still empty the saucer and let the top dry, because a warning ignored becomes a cloud. Chewed root hairs also make a plant look hungry while the feed in the pot is fine. Pale new growth on a soaked plant is a reason to check the roots and the wetness before you raise the nutrient strength. Heavier feed on a damaged root burns what is left.

Yellow leaves have a lot of causes. A heavy, cold pot is a watering problem first. Larvae wound fine roots, so a plant that stalls after weeks of swampy media needs drainage and dry-back before a heavier feed. Adults walking on a flower are a different problem from mold inside a dense cola. If the cola is browning from the inside, that is bud rot, and a gnat treatment will not touch it.

During vegetative growth a plant can outgrow a light catch once the surface dries and the larvae are interrupted. Flower is a worse week to discover that every pot has been wet since transplant.

Dry the surface before you reach for a bottle

The order is short. Stop the swamp. Count the adults. Then treat larvae if the cards say you still have a nursery. People reverse it. They soak the soil with something from a forum, leave the saucer full, and watch a fresh hatch walk out of the same wet film a week later. The bottle was never going to beat a pot you water every morning out of habit.

Tonight, empty every saucer and every runoff tray. Lift each pot. A heavy pot is still wet inside even when the top looks pale. Write down which ones are heavy, because memory is bad at this in a room of twelve. Set a small fan so air moves across the soil, the way you would dry a bench, and keep it off a direct blast that lays a seedling on its side. Low and steady beats a gale. The top inch is what has to dry. The lower root ball on a plant in mid veg can stay evenly moist.

Then change the habit that made the film. Water when the pot asks, by weight, and follow the watering guide rather than a calendar that says every day. A peat pot that stays dark on top has not asked yet. A coco pot stays darker when it is ready than a soil pot does, so learn the weight of your own containers instead of copying a soil rule onto a coco block. In a hydro room, let plug shoulders and tray surfaces dry, and keep soil pots out of that space if you can.

Over the next few days, water only the pots that went light. Leave the heavy ones alone. Stick a yellow card just above the soil of the worst pot and one card for the room, and start the count described in the next section. If the surface is drying and the count falls through the following week, you may be done. Many light infestations end here, with no product at all. If the cards keep loading, the larvae already in the mix need a soil treatment, and you add that without going back to a swamp.

Steinernema feltiae has to move in a moist mix, which trips people up. Dry the surface first so new eggs fail. When the supplier card says to apply the nematodes, water them in so the top is damp enough for them to travel, then let that surface dry again. Moist for the application. Dry between waterings after that. A permanently wet cap keeps breeding gnats in the same pots.

Yellow cards are the scoreboard

Yellow cards catch adults. A card parked up in the canopy grabs the odd flyer and misses the population that lives at the pot. Stake or hang the card just above the soil, close enough that a weak flier hits it on the way up. The loaded card in the photo is finished. Replace a card when the sticky face is that crowded. A fresh card tells you what happened since yesterday.

Write the number down, or take a phone photo with the date. Six flies in a week on a four-pot tent is a watch-it situation. A card that fills in two days is an active nursery, and you look for the wet source the same hour. Larvae in the mix never touch the glue. A clean card on Monday and a loaded card on Thursday means adults emerged or a new source arrived. The card is how you know the dry-down worked. It is also how you know it did not.

When the count climbs after the tops look dry, hunt the second source before you buy anything. A saucer you missed. A runoff tray under the table. Algae on a cube. A mother plant in the corner on a different schedule. An open bag of mix on the floor. A friend who set a wet clone on your bench. One soaked pot will restock the air for the whole tent. Find that pot by lifting, not by spraying the ones that already dried.

Glance at what is stuck. Leggy dark gnats match this page. A heavier fly near algae is the shore fly. Thrips are slender and leave silver tracks on leaves, which belongs to the IPM routine. A gnat treatment will not clear thrips that happen to share the glue.

Larvae need a soil treatment

Adults on the tray will keep showing up after you have started on the larvae. That photo is the leftover generation. They already left the soil. Killing them one by one changes little if the mix is still hatching. Aim the treatment at the top inch, and give it enough repeats to meet larvae that hatch from eggs laid before the surface dried.

The larval bacteria that fits this job is Bacillus thuringiensis subsp. israelensis, called Bti. It is the mosquito-bit and mosquito-dunk family, and it is what you want when a product is labeled for fungus gnat larvae. The caterpillar product on the IPM page is Bacillus thuringiensis kurstaki. That one belongs on chewing worms. It is the wrong subspecies for a gnat larva. Read the label until you see israelensis, or a fungus-gnat claim that names it. Follow that label for the mix and the repeat interval. Bti breaks down in a wet pot, so one watering rarely covers a three-to-four-week cycle. Repeat it on the waterings you actually give through that cycle. Skip any dunk-per-litre recipe from a comment section. The label in your hand is the dose.

When cards stay busy after the surface has had real dry-back, Steinernema feltiae is the nematode that enters the larva. The beneficial insects page and the nematode page walk the release. The rules that keep the application alive are simple, and they are easy to break. Mix in room-temperature water. Skip hot water. Keep the mixed jug moving and apply it promptly, because a solution left sitting for an hour is a waste of the shipment. Leave the fine filter off the pump so you do not sift the nematodes out. Water them into a mix that is moist enough to travel in, and avoid a swamp. Adults already in the air are outside their reach, so flies continue for about a week. Judge the nematode by the next card, a week out, after that adult wave has died of age.

Stratiolaelaps scimitus, also sold as Hypoaspis, hunts larvae, pupae, and eggs in the mix. It is the mite you add when you want a hunter living in the pot rather than a single drench. The supplier card sets the amount. This page will not invent a count per square foot or a count per pot. Release what you bought the way the card says, keep the top moist enough for the mites to work that week, and then return to normal dry-back so you are not breeding a fresh round of gnats under a predator that cannot keep up with a bog.

Neem is optional support while the plant is still in veg. A soil drench can bother larvae. Bti is the cleaner larval tool, and it does not leave an oil film on the way to flower. If you still want neem, the neem page owns the bottle: cold-pressed oil, the emulsifier, lights off, and the point where you stop. Copy nothing from memory onto a measuring spoon. Once flowers are stacking, that option is gone. Oil on a bud is a quality problem you cannot fan away.

A thin cap of coarse grit can make egg-laying harder by covering the film females prefer. It also hides the color of the mix, so you lose the visual cue and have to lift the pot anyway. Fine play sand is a poor cap. It packs into a crust, sheds water to the gap at the pot wall, and can leave a dry shell over a wet core. Skip a cap entirely on a seedling in a small cup, where you need to see the surface. Diatomaceous earth cuts soft-bodied insects while it stays bone dry. The next watering wets it, and wet dust stops cutting. In a pot you must irrigate, that dust is a brief irritant between waterings, and it is a bad thing to count on as the fix.

What flower week allows

A fly will sit on a leaf in flower the same way it sat there in veg. That does not open a spray window. The flowering stage is when you stop putting oil, soap, or neem on the plant. Buds hold whatever you fog onto them. A flowering marijuana plant with a few adults and a drying soil surface is in better shape than a seedling in a swamp, and it stays that way if you keep the sprays in the cabinet.

Flower week for gnats is three moves. Let the top inch dry between waterings. Keep a yellow card at soil level and replace it when it loads. If larvae are still in the mix, use a Bti product labeled for fungus gnat larvae as a soil drench, on the schedule that label prints. That is the list. Foliar poison aimed at adults in the canopy misses the nursery and lands on the flowers you plan to dry.

Kitchen mixes stay in the kitchen. Vinegar, cinnamon, dish soap, and peroxide drenches are the suggestions that circulate because they are already under the sink. A sprinkle of cinnamon has a smell and no hold on a fly that lays underground. A soap film and a peroxide percentage copied from a comment can burn root hairs that were already chewed. If a product is labeled for fungus gnat larvae in soil, use that label. If it is labeled for mosquitoes in a pond and the fungus-gnat use is a forum story, leave it for the pond.

Keep the count going for two weeks after the cards calm down. A life cycle that runs three to four weeks can still hand you a late hatch from eggs that were in the mix before you changed anything. Two quiet cards, a week apart, with pots that actually lighten between waterings, is the point where you stop shopping for a stronger bottle. If a single pot stays heavy and its card keeps catching flies, treat that pot and check its drainage holes. A packed bottom that never dries will restock the tent no matter how careful the other plants are.

When the round is finished, saucers are empty and air moves over the soil. Cards at pot level catch a fly now and then. New clones sit apart until their own card stays quiet, and the compost bin stays outside. If flies walk out of a fresh bag, deal with the bag before it sits beside a plant in flower.

If the insects on the root ball are aphids, move to the IPM page. If adults are still thick a week after a real dry-back, add labeled Bti or Steinernema feltiae from the supplier card and read the card again seven days later. The flies you see today already made it out. The generation you can still stop is in the top inch, and it stops when that inch spends time dry.