Gnats are annoying pests that tend to swarm houseplants. Depending on the severity of your problem, below are several tips you can try to eliminate gnats from your plants. While there are several species that are commonly referred to as gnats, it is important that you distinguish what type you have, as others may require different elimination methods. Also known fungus gnats, they are small flies that are drawn to moist plant soil and decaying plant matter. Although they share similarities, they are different from pests such as fruit flies and drain flies. Fruit flies are often found on fruits and are tan in color, resembling a common housefly. Fungus gnats are often found on overwatered plants due to the wide availability of decaying organic matter.
Gnats can be a real nuisance when trying to enjoy time in the garden These tiny flies seem to appear out of nowhere, swarming around plants and people, laying eggs, and damaging plant roots. If left unchecked, high populations of fungus gnats can severely inhibit plant growth and spread disease
While completely eliminating gnats may not be possible, gardeners can control them with some knowledge and effort. This article will explain what attracts gnats, the problems they cause, and most importantly provide tips to minimize their presence for healthier garden beds.
What Are Gnats?
There are many small fly species referred to as “gnats.” The primary garden gnats belong to either the Sciaridae fungus gnat family or Drosophilidae fruit fly family.
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Fungus gnats are about 1/8 inch long with long legs, antennae, transparent gray wings, and dark eyes. The larvae are thin, worm-like maggots
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Fruit flies are extremely small and round with red eyes Their maggots are whitish-clear
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Other gnat types like gall midges may also inhabit gardens. Despite differences, their habits and control are similar.
Where Do Garden Gnats Come From?
Gnats seek out damp organic matter to lay their eggs and breed. The female adults deposit eggs in soil, compost, mulch, or on decaying plant debris. The eggs then hatch into larvae that feed on fungi, plant roots, and rotting material.
Ideal gnat habitat includes:
- Wet potting mix
- Rich organic mulches
- Rotting fruits/vegetables
- Poorly draining beds
- Overwatered plants
- Decaying plants
- Unturned compost piles
After pupating, mature gnats emerge to start the breeding cycle again, generating multiple generations over summer.
Problems Caused by Gnats
Though small, gnats can create headaches for gardeners by:
- Tunneling into roots as larvae, damaging plants
- Spreading fungal diseases like damping off to seedlings
- Causing stunted growth and reduced yields
- Creating entry points for plant pathogens
- Annoying people with their presence
While fungus gnat larvae eat organic matter, fruit fly larvae feed more directly on live roots and can be very destructive.
How to Control Gnats in Garden Beds
Suppressing gnats takes diligent effort but these methods are effective:
Remove Excess Moisture
Gnats need moisture to breed. Allow beds to dry more between waterings. Their larvae die out when the top few inches of soil become dry.
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Check soil moisture before watering and only water when the first 3-4 inches are dry.
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Improve drainage by mixing in sand, perlite, or gravel.
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Avoid overwatering. Water deeply but less frequently.
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Fix any leaks from beds, hoses or irrigation.
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Discard excess water collecting in saucers under potted plants.
Eliminate Rotting Matter
Remove decomposing plant debris, dropped fruits/vegetables, and mulch that gnats may be breeding in.
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Clear out fallen leaves, dead plants, and rotten veggie waste.
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Rake mulch to expose larvae to dryness and predators.
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Turn and maintain compost piles regularly to kill larvae.
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Replace mulch annually to prevent heavy decomposition.
Apply Beneficial Nematodes
Beneficial nematodes are microscopic worms that prey on gnat larvae in soil. After watering them in, they actively seek out and infect larvae. Products like NemAttack and NemaSeek work well.
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Use early in season or at first gnat signs.
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Water beds thoroughly before and after applying.
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Apply monthly for severe infestations.
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Cover beds overnight with tarps after applying to increase humidity.
Use Yellow Sticky Traps
Traps lure in adult gnats with their color and stickiness. Position just above soil to catch emerging flies.
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Insert sticks into beds and pots around plants.
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Move traps to new spots every 2-4 weeks as they fill up.
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Combine with other methods targeting larvae in soil.
Apply Sand or Diatomaceous Earth
A thin layer of these abrasive minerals kills gnats through desiccation and abrasion. They also deter egg-laying.
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Apply 1/4 inch layer evenly across beds.
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Diatomaceous earth is very fine so doesn’t affect watering.
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Look for food-grade DE to avoid skin irritation.
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Reapply after rain/watering washes it away.
Repot Infested Plants
For severely infested potted plants, starting over fresh may be needed.
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Remove as much old soil as possible from roots.
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Discard old soil in sealed bags so larvae don’t escape.
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Sterilize pots before reusing.
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Repot in fresh pasteurized mix.
Preventing Future Gnat Issues
Good garden practices help avoid severe infestations:
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Let beds dry adequately between waterings.
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Eliminate weeds where gnats may breed.
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Remove spent plants immediately after harvest.
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Keep decomposing mulch/compost away from beds.
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Solarize soil in beds before planting to kill larvae.
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Cover seedlings with row cover fabric to exclude flies.
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Mix sand or perlite into potting mixes for better drainage.
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Use sterile seed starting mix and containers.
While some gnats will likely always be present, diligent monitoring and integrated control actions can significantly reduce their impact on garden plants. With persistence, gardeners can minimize gnats to just an occasional annoyance, rather than detrimental swarms.
Methods you should Avoid
Below are methods that you shouldn’t use, as they are not effective or may cause damage to plants.
A diluted hydrogen peroxide soak is one method that has been suggested. However, it will likely break down before it can get deeper into the soi. Even if you saturate it, the soil will be very wet, which encourages the proliferation of more larvae. Finally, it can destroy the soil ecosystem by killing the beneficial bacteria in your soil and damage plant roots.
While neem oil can be effective against a variety of plant pests, it is ineffective in killing gnats. The active ingredient in neem oil is absorbed by the plant, requiring the larvae to eat the plant for it to work, which is not guaranteed.
Since gnats are averse to the scent of cinnamon, it has been suggested as a deterrent. However, gardeners have reported that it did not deter gnat activity when it is sprinkled into the soil. Since cinnamon can inhibit the fungi in the soil, it can also decrease soil productivity.
Bottom watering is a method of watering your plants from the bottom. While this can be effective for larger planters, where the top of the soil is dry, it does not work for plants in smaller pots. Gnats can congregate near the drainage holes, which will create a gnat problem at the bottom of your plant.
How to Identify Gnats on Plants
Gnats are fairly harmless but irritating. While adults do not pose much of a problem to plants, their larvae can feed on plant roots or fungi in the soil, causing the plant to wilt. The larvae are about ¼ inches long and have transparent, whitish bodies with shiny black heads. If you see a slime trail similar to that of slugs or snails, then it’s likely that there are gnats. Since their population can increase rapidly, you may need more than one method to get rid of them. You should try natural methods before using pesticides, which can be harmful for plants.
- Make a trap using apple cider vinegar
In a small bowl, mix a half cup of water, two tablespoons of apple cider vinegar, and several drops of dish soap. Gnats are attracted to the sugary substance, and will become trapped by the dish soap. To prevent them from going out, you can cover the bowl with plastic wrap and poke holes in it, large enough for the gnats to enter. Set the bowl in areas where gnats are prevalent, and wait for the mixture to attract them. Do not add the mixture to the soil, as the acetic acid in vinegar will kill the plant structure as well as acidify the soil.
- Use an indoor bug zapper
Although it works better with larger bugs, an indoor fly catching device can be helpful if you live in a humid area or deal with a lot of bugs. These devices are usually USB powered with LED lights and siphon bugs though a vortex and trap them.
- Use sticky fly traps
If you do not want to make your DIY trap, you can buy fly traps online. They can be hung on the points of plants, or placed in the soil. Check the traps and replace them once they have filled with insects. It is recommended that you purchase traps that are non-toxic and double sided for maximum efficiency. Don’t use them outdoors, as it can trap beneficial insects as well.
- Cover the soil with a layer of sand
If you are experiencing persistent infestations, you can cover the soil with a layer of sand, which is known as topdressing. This traps the larvae in the soil and prevents adults from laying eggs. The layer should be about ¼ to ½ inch thick. You can also apply Food-Grade Diatomaceous Earth (DE) as a preventative measure. DE is an organic, abrasive powder that will trap gnats until they die of dehydration. Make sure the soil is dry before applying or it won’t be effective.
- Use Beneficial Nematodes
Beneficial nematodes are microscopic roundworms that will seek and destroy the larvae. They are effective as long as the soil stays moist. As they do not harm people or plants, they are safe to use. Keep in mind that they can be expensive and may perish during shipping due to their susceptibility to warm weather and dry humidity. It is not recommended you use this method if you have many plants.
- Use a BTI Drench
Bacillus thuringiensis israelensis (BTI) is a biological control that is designed to kill certain larvae but has no effects on insects such as bees and butterflies. This method will not work on adult gnats as they do not eat. To kill the larvae, follow the instructions on the product and use it to water your plants, making sure it is evenly distributed. Once the gnats are rid of, let the soil dry out slightly before watering to prevent them from returning.