Fungus gnats: do you have tiny black flies at home?

Small but very annoying black flies that fly out of planters are among the most common houseplant pests and among the most frequent nuisances at home. If you do not act in time, there will soon be so many of them that they fly around, settle on things and bother you absolutely EVERYWHERE. Do you know how to get rid of them effectively and as quickly as possible?

In this article, we will introduce proven and widely recommended methods that can make your fight against fungus gnats easier, as well as tips on how to prevent them.

 

Fungus gnats: what are they and how do they harm us and our plants?

Fungus gnats are small, blackish-gray flies that usually fly around close to plants or sit on the potting mix, so you may not notice them right away.

Adult flies are about 2 to 4 mm long, and a single adult female fungus gnat can lay up to 200 eggs in your plants’ potting mix. Apart from that, the adults do not actually do any harm, but they drive many of us up the wall simply by being in our home.

Moisture (most often from watering) then causes white larvae to hatch from the eggs. They are almost invisible to the naked eye (about 1 mm long). It is these larvae that can become a big problem for plants when they multiply, because besides dead organic matter they also feed on the roots and tissues, especially of young plants. They can also spread various plant diseases (molds and fungi).

You can usually tell that a plant is infested with fungus gnat larvae because it dries out and wilts.

The life cycle of fungus gnats (egg, larva, adult) takes about 3 weeks. If conditions in your home are more favorable (warmth, moisture), the cycle can shorten to 1 to 2 weeks.

That is why you need to act as soon as the first flying intruders appear.

Prevention: watch out for new potting mix and new members of your jungle

Fungus gnats most often get into your home in one of two ways. First, their eggs are in potting mix you buy (unfortunately, this happens often), or second, you bring them home in the potting mix together with a new plant from a store or from friends. In short, they sneak in hidden in the soil.

There are several ways to stop them from “waking up.”

Sterilizing the potting mix: the easiest way to make sure no fungus gnats hatch in newly bought soil is to clean it before you use it.

You can let the potting mix dry out for 1 to 3 days before use, but it is more effective (and more reliable) to heat the soil in the oven or microwave (spread the potting mix on a baking sheet and let it bake through in the oven, but be prepared for a possible unpleasant smell during the process). Remember that if you heat the soil, you need to moisten it once it has cooled down so that it can absorb water later when you water your plants.

Flushing: you can also rid the potting mix of intruders by pouring a solution of potassium permanganate or hydrogen peroxide through it.

Home remedies can also inspire you: push garlic cloves into the soil; use a nettle or onion brew for the first watering; smokers can use a tobacco infusion (let cigarette butts and tobacco steep in water for half a day), and if you look around, you will surely come across many more…

Testing for larvae: do you want to be sure you have not brought intruders home in your potting mix? Cut a raw potato in half and place it on the soil, cut side down. Check it after 48 hours. If there are fungus gnat larvae in the potting mix, you will find them on the potato.

Special fertilizers: you can also prevent fungus gnat problems with special types of fertilizer (from brands such as Floria, Vitality komplex and Substral Careo). They are available in liquid, granular or stick form.

A few black flies: act fast

As soon as you notice the first black fly, it is in your interest to act before the fungus gnats have had a chance to really multiply.

Letting the potting mix dry out: you can get rid of fungus gnats simply by not watering your plants for a while and letting the root ball dry out thoroughly. In dry conditions, these annoying flies very often do not survive and, above all, cannot reproduce.

If you have plants that could be harmed if their potting mix dries out, try a combination: control the adults (sticky strips or traps, see below) and get parasitic nematodes (see below).

Top-dressing the potting mix: if any of the larvae survive the dry spell, you can stop them from hatching by covering the top of the potting mix with sand or small pebbles, which keep the young flies from getting out. Surviving larvae can also suffocate this way.

Dealing with the adults (the flies): the most effective way to get rid of the flies is to place sticky strips or traps (the most commonly sold ones are yellow, and in our online store you will also find blue) near your plants to attract the adults. The fly gets stuck to the strip and so cannot lay any more eggs.

An infestation: what now?

Sometimes, however, the fungus gnats are faster and manage to hide from us until they have simply multiplied out of control. Do not lose heart: you can still get rid of them, and fairly quickly too.

It is important to keep catching the adults, so you will definitely need yellow sticky strips. Every fungus gnat that gets stuck means fewer eggs and potential larvae, and that counts.

Removing the top layer of potting mix: you can also outsmart fungus gnats and thwart their attempts to reproduce by removing the top part of the potting mix and throwing it away. Simply remove the top 1 to 2 cm of soil from your plants (that is where the eggs are laid) and then top up with fresh potting mix.

Parasitic nematodes (Steinernema feltiae): the gentlest and most effective way to wipe out a fungus gnat infestation is to call in their natural enemies: parasitic nematodes. When using them, follow the instructions included in the package. These little critters are applied by watering them into the soil, where they use their sense of smell to seek out and then destroy fungus gnat larvae.

Nematodes are completely harmless to people and plants. You do not even have to worry that instead of flies you will then have critters everywhere: once the nematodes have wiped out the fungus gnat larvae, they disappear on their own. In other words, no fungus gnats, no nematodes.

When buying parasitic nematodes, make sure you get the right ones for fungus gnats (there are several species, each specializing in specific pests).

Fighting pesky insects can sometimes be a long, demanding process, but never give up: after all, our plant jungle is worth it.

Jana Beránková