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.
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.
Methods you should Avoid
Below are methods that you shouldn’t use, as they are not effective or may cause damage to plants.
- Hydrogen peroxide
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.
- Neem oil
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
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 Prevent Gnats in Indoor Plants
You can prevent gnats from appearing near your plants by targeting them at the larval stage. Here are some measures that help you prevent gnats in indoor plants.
- Don’t overwater your plants: Allow the soil to dry before watering. If the soil is continually moist, larvae can spawn in the soil and consume the decaying organic matter present.
- Choose a pot with good drainage: A pot with good drainage will prevent the soil from accumulating moisture, which can lead to fungal diseases and root rot. Make sure to empty any excess water that has gathered at the bottom of drainage trays to minimize moisture.
- Examine plants: When purchasing new plants or bringing them inside, examine them to make sure there are no infestations, especially if they have been purchased at a greenhouse. Gnat issues are common at greenhouses due to the high humidity and frequent watering. Use a sterile potting mix when potting or replanting.
How to Prevent Gnats Outdoors
As an alternative to a BTI drench, you can drop a mosquito dunk into a watering can or bucket. Once the solution has dissolved and released the helpful bacteria into the water, remove the dunk and pour the water over the soil of your plants.
Do not use electric traps or sticky traps, as they may kill beneficial insects along with pests. You can use traps with ultraviolet light, keeping in mind that they will only work for adult insects. If you see a large number swarming near your porch or outdoor lights, you can switch to yellow bug lights or keep the lights turned off to discourage them.