I’ve dealt with fungus gnats in my indoor and outdoor garden. While they didn’t do a lot of damage indoors, they decimated my outdoor sprouts during a particularly rainy spring season. They may not have the strength in numbers, but they damage plants quickly.
Controlling fungus gnat populations is key. They go for potted plants and feed on organic material (plant roots, and root hairs too). Overly moist soil is one of the attractants for these creepers. But there are other things that bring them in.
Adult gnats need to be controlled to keep larvae at bay because their feeding isn’t the end of the problems. They can also infect plants as they spread fungal diseases.
Fungus gnats are a common nuisance for gardeners. These tiny flies seem to appear out of nowhere, swarming around plants and laying eggs that damage roots. While completely eliminating them is difficult, there are effective methods to control fungus gnat populations and minimize their impact.
What Are Fungus Gnats?
Fungus gnats are dark mosquito-like flies about 1/8 inch long. They are weak fliers and usually remain low walking over soil and plants. The larvae are thin, legless maggots up to 1⁄4 inch long. There can be multiple generations per year.
Two main species affect gardens:
- Sciarid fungus gnats: Larvae feed on fungi and plant roots.
- Drosophilid fruit flies: Larvae directly feed more on living roots.
Where Do Fungus Gnats Come From?
Female fungus gnats are attracted to damp organic material to lay 100-300 eggs Ideal breeding habitats include
- Wet potting soil
- Rich organic mulches
- Compost piles
- Overwatered beds
- Decaying plants
After hatching, larvae begin feeding for 2-3 weeks before pupating into adults that restart the cycle.
Problems Caused by Fungus Gnats
Though small, fungus gnat larvae can cause significant damage:
- Tunneling into roots as larvae, weakening or killing plants.
- Spreading fungal diseases like damping off.
- Causing stunted growth and reduced yields.
- Allowing pathogens to enter damaged roots.
- Annoying presence of adults swarming plants.
Effective Methods to Control Fungus Gnats
Controlling fungus gnats requires reducing breeding habitats and using techniques to kill both larvae and adults. Key methods include:
Adjust Watering Habits
Excess moisture breeds fungus gnats. Allow soil to dry out between waterings. Larvae die out when the top few inches become dry.
Remove Excess Organic Matter
Eliminate rotting leaves, fruits, vegetables, and other decaying matter larvae feed on.
Apply Beneficial Nematodes
Beneficial nematodes kill larvae by infecting them. Apply after watering so they actively move through soil seeking larvae.
Use Yellow Sticky Traps
Traps catch adult fungus gnats emerging from soil. Place just above soil level and replace regularly.
Create Homemade Vinegar Traps
Small bowls of apple cider vinegar attract and drown adults. Add dish soap to trap them.
Apply Diatomaceous Earth
Sprinkle this abrasive powder onto dry soil to scratch larvae and cause dehydration.
Use Insecticidal Soap or Neem Oil
Natural insecticides sprayed on plants can help kill larvae and repel adults.
Utilize Mosquito Bits
Contains Bacillus thuringiensis israelensis bacteria deadly to gnat larvae but not plants.
Drench Soil with Hydrogen Peroxide
Kills larvae and adds oxygen to the soil. Use 3% solution diluted 4:1 with water.
Preventing Fungus Gnat Infestations
Prevention is critical to avoid recurring issues. Key practices include:
- Letting soil dry adequately between waterings.
- Eliminating excess organic material.
- Allowing proper drainage and aeration.
- Using sterile seed starting mix.
- Covering seedlings with row cover fabric.
- Mixing sand or perlite into potting soil.
- Bottom watering plants when possible.
Identifying Fungus Gnat Damage on Plants
Fungus gnat larvae feeding on roots cause foliage to wilt, turn yellow, and experience stunted growth. Damaged roots become prone to rot. Look for slime trails similar to slugs on soil surface. Try organic methods first before considering pesticide use, which can harm plants.
10 Ways to Kill Fungus Gnats in Houseplants
Fungus gnats frequently affect indoor plants too. Recommended organic control methods include:
- Letting soil dry out between waterings.
- Using hydrogen peroxide drenches.
- Applying diatomaceous earth.
- Creating vinegar traps.
- Deploying beneficial nematodes.
- Spraying insecticidal soap or neem oil.
- Eliminating excess moisture.
- Placing yellow sticky traps.
- Using mosquito bits.
- Repotting severely infested plants.
Diligence and persistence are needed, but combining multiple techniques can eliminate fungus gnats. Prevention through proper watering and cleanliness is most critical for avoiding recurring issues with these annoying pests.
Fungus Gnats as Disease Vectors
Yes, fungus gnats damage plants, but it’s important to kill fungus gnat larvae and adults because they may carry diseases. Those plants in the process of recovering from fungus gnat problems still face the risk of fungal infiltration.
The larvae spread fungus spores dropped by adult gnats to your plant’s roots, potentially causing a number of common plant diseases. The most common ones include black root rot, Pythium blight, Verticillium wilt, Botrytis blight, and Fusarium wilt.
As a precaution, treat any affected plant and those in the area with a copper fungicide a day or two after pesticide application. This ensures that if your plant had contact with potentially dangerous disease fungi spores, they won’t contract the disease.
A root drench is more effective than spraying the plant’s foliage, as that’s where damage would be done.
What Are Fungus Gnats?
If you live in a temperate or tropical area, you’ve most likely seen insect pests called fungus gnats before. They’re small flies that infest soil potting soil, container media, and other organically decomposed materials.
Fungus gnat larvae feed on fungi and plant roots in organic matter. They’re a problem in commercial greenhouses, indoor houseplants, and potted plants. Adult fungus gnats emerge when warmer temperatures occur in spring and summer. I’ve personally dealt with the indoor version of fungus gnats.
Adult fungus gnats are weak fliers, giving them a similar appearance to that of mosquitos. They have long legs and antennae longer than their head. These antennae distinguish them from one of their look-alikes, robust short flies. However, short flies are strong fliers.
There are a few species that grow up to a ½ inch long, while most adult fungus gnats are 1/16 to ⅛ of an inch long. Their wings are clear and grayish, and the Bradysia species has a characteristic y-shaped wing vein. The larvae grow a bit larger than the adults.
Fungus gnat larvae have a black, shiny head with a white to clear body that can reach almost a quarter inch in length. They’re hard to locate because they tend to remain under the soil’s surface, out of view. They may, however, make a shiny trail on the soil surface of heavily infested plants.
The life cycle of the fungus gnat is made up of four stages: egg, larvae, pupae, and adult. Adult fungus gnats lay about three hundred eggs in the moist, rich soil at the base of the plant during their short lifespan. They live for about a week as adults, making the most of their time!
About four to six days after the eggs are laid, the gnat larvae emerge. These larvae are extremely tiny at first. During the roughly two-week period in which fungus gnat larvae grow, they rapidly increase in size by eating their way through organic matter in the soil.
Once they’ve achieved maximum growth, they undergo a three to four-day pupal phase. The pupae are hidden under the soil’s surface. When the fungus gnat adult emerges, they start laying eggs immediately.
Because of this quick life cycle, multiple generations are born in quick succession as larvae feed. Subsequently, overlapping generations of adults emerge from the soil surface.
Many stages of the life cycle may be present at any given time. Not only do you need to get rid of the annoying gnats themselves, but you need to deal with the larvae and find a way to sterilize the eggs before they hatch.
Fungus gnat adults don’t do much direct damage to plants. It’s the fungus gnat larvae that are most damaging. An infestation causes damaged roots that stunt growth, especially in young plants.
In houseplants that have too much moisture in their potting soil, high populations of adults and larvae kill plants. Before that, it’s difficult to tell from symptoms whether or not the issue is feeding larvae, overwatering, fungi infections, or generally unhealthy roots.
These don’t cause much damage outdoors – though they did for me in that one freak spring when it wouldn’t stop raining. More often, they infest houseplants or potted plants kept indoors or in a greenhouse.
Mixes that contain a lot of coco coir or peat moss are prime environments. Sod farms are another area where fungus gnat adults can easily proliferate.
Kill Fungus Gnats in Your Seed Starts, Transplants, Greenhouse, and House Plants: All the Steps!
FAQ
Why are there gnats in my raised garden bed?
Water and Soil Management
Because fungus gnats thrive in moist conditions, especially where there is an abundance of decaying vegetation and fungi, avoid overwatering and provide good drainage. Allow the surface of container soil to dry between waterings.
Are fungus gnats harmful to vegetable plants?
Yes, fungus gnats can be harmful to vegetable plants, particularly seedlings.
Do coffee grounds repel fungus gnats?
The professionals at U.K.-based lawn company Moowy recommend sprinkling a layer of fully-dried used coffee grounds over the fully-dried top layer of soil in an infested houseplant. Water the plant from the bottom of the pot, making sure to keep the top layers of soil and grounds dry.