How To Kill Small Black Flies On Indoor Plants – Fungus Gnats

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Summary: Fungus gnats – those small little black flies on plants – can be a real nuisance on plants and the home, where they fly all over the place. They enjoy the perfect environment of houseplant soil and moisture that indoor plants grow in. Learn how to get rid of them.

Question: I recently bought a great looking potted house plant at a garage sale… mistake. I put the plant indoors in a nice decorative pot and after a few days, there were fungus gnats all over the house. How can I get rid of these small irritating black flies in my house? Glenn, Kicking myself!

Fungus gnats live in pot plants

Here’s how to get rid of little black flies in potted plants:

Answer: Glenn, sorry to hear about the fungus gnats in house… they can be irritating and a big nuisance, especially in large numbers, which it sounds like you have. Why is it fungus gnats and fruit flies always fly around your face?

If it makes you feel any better… You’re not alone with the fungus gnat problem.

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What Are These Gnats

These gnats are small black flies with long antennae and legs. Inside they are most commonly found at home at the top layer of soil of indoor house plants.

The adult gnats are about 1/8″ inch long or about the size of a mosquito but do not bite… they are just pests, and always seems to enjoy flying around your face.

The fungus gnat larva develops in the potting soil feeding on fungi (hence the name) algae, plant roots and decaying organic matter. In general, they are not considered a major plant pest.

Where Fungus Gnats Live

They live in the top of the soil, where the gnats infest the growing media, lay their eggs, and larvae feed on the decaying organic material.

I’ll bet if you look closely at the potting soil used in the houseplant you purchased you’ll notice it has some type of wood product. This decomposing organic material makes for a wonderful fungus gnat buffet and infestation!

One way you can offer some control is by watering the plant from below using sub-irrigation. The reason this can work is that the larvae are usually located in the top 2″-3″ inches of soil. They need moisture to feed.

A dry-growing medium will decrease egg survival. So keeping the soil dry does offer some degree of control.

Personally, I would chalk it up to lesson learned and get the plant outside and deal with knocking them out outdoors if possible.

The Fungus Gnats Life Cycle

The life-span of this gnat is about 7 to 10 days. During that time a female can lay about 200 eggs. The larvae develop fast reaching maturity on 2 -3 weeks, with continuous reproduction year round.

Many people notice the gnat problem indoors during the fall and winter. Why?

  • Some people move plants outdoors for the summer which allows the insect colony to grow during the warm months.
  • People are indoors more during the cooler weather making the gnats more noticeable
  • In a dry environment, the moisture level of the potting soil is a great home and breeding ground

NOTE: Always use a sterilized potting mix for potting house plants.

Damage Indoors and Outdoors

Plants can become stunted as the larvae feed on plant roots. Larvae and adults can spread other plant pathogens helping to promote disease.

Outdoors in the garden or landscape larvae may feed on plant roots. However, the damage is usually minor compared with their beneficial role in helping in the decomposition process converting dead vegetation into nutrients for plant growth.

How To Get Rid Of Fungus Gnats

There are several things you can do to gain some control and get rid of gnats – these pesky, irritating, flying insects.

Here’s how to kill gnats in plants.

The No Chemical Route

  • Allow the top 2″-3″ inches of soil to dry between waterings
  • To keep soil moist water plants from below
  • Replant into a new pot, after removing all the soil with a sterilized potting mix
  • Repot with a sterilized potting mix (for example: African Violet mix) when the growing medium breaks down and holding excess moisture.
  • Get rid of containers holding rotting plant material, decayed bulbs, roots or other food sources.

Some people use a yellow sticky trap. I’ve used these gnat traps in the greenhouse but indoors they look tacky.

Make A Simple Gnat Trap

Here are the instructions for making a DIY simple fruit fly trap, you can use for fungus gnats as well. The items are probably already hiding in your kitchen.

  • Apple cider vinegar
  • Dish soap or vegetable oil
  • Jar
  • Plastic wrap

Follow these steps:

  • Pour enough cider vinegar into the jar to cover the bottom
  • Add one drop of dish soap
  • Cover the jar with plastic wrap and poke a few holes in the plastic

The smell of the cider vinegar attracts the fruit flies and gnats in plants. When they land vinegar they sink.

The Chemical Route

Controlling the gnat is usually not a one shot deal. Excessive populations require multiple treatments of chemicals. Read: What’s the battle result – fungus gnats or neem oil?

What kills adults usually does not control the larvae. The key is to getting rid of the egg laying adults to reduce the larvae population.

Some insecticides and biological control agents can be used to control.

There are even some gnaticides – one called gnatrol – but have not used it. They gnaticides usually contain some type of pyrethrins.

I prefer to use a natural solution if possible, but the homeowner does not have many choices.

For the homeowner the best solution is probably to apply an insecticide neem oil soil drench. Using a spray bottle will “kick up” the gnats and they will start flying.

Personally, I start with organic neem oil. If neem does not work I’ll use diatomaceous earth. Next, I would move on to using something along the line of Malathion as a soil drench. Always read the pesticide label before applying any chemicals and follow the labeled directions on use.

I’ve heard of people using hydrogen peroxide for fungus gnats control to kill the fungi the gnats feed on.

There is no use spraying the plant. The eggs are laid in the soil.

Beware multiple applications may be required.

There is a biological control of a nematode which feeds on the fungus gnat larvae, but not available to most homeowners. You can also try Bacillus thuringiensis.

Another option is to use yellow sticky traps or tape. You’ll find these more in the greenhouse. They are not very attractive in the home!

NOTE: Some people use Dawn Dish soap to get rid of gnats, others like Mosquito Dunks to get rid of fungus gnats. Some use ladybugs to control fungus gnats.

Wrap Up

The first key in the control of fungus gnats for the homeowner is to purchase houseplants from professionals. They use soils which have been treated to kill these kind of insects so you do not have to deal with them.

However, that does not mean you’ll never get them buying at a garden center.

But, knowing how to control or get rid of them keeps the pests from being so annoying! Because these are true pests which no one wants in their home.

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