When you first see tiny black flies near your house plants and small white grains on the ground, you’ll think that it’s a trifle and that these bugs will fly for a while and then die. Make no such mistake; in reality things are much worse! Your plants have already been infected and will soon start to wither. The larvae inside the pot have already begun eating the entire root system of your favorite flowers, even though you can’t see this yet. Do you have time to save the plants?
Fungus gnats don’t appear out of nowhere, they areattracted to moisture.In this article we will try to figure out what are the possible causes of fungus gnats indoor and outdoor infestation and will tell you what signs indicate fungus gnat plant infection, how to eliminate all stages of fungus gnats infestation in 5 steps and make sure that they do not reappear anywhere near your plants.
Are they dangerous to humans? Who could be mistaken for fungus gnats? Will the sticky traps help kill these insects? Find the answers below!
What Is a Fungus Gnat? Everything You Need to Know About This Pest
Fungus gnats are small brownish gnats, ⅛ - ¼ inches long and belong to Bolitophilidae, Diadocidiidae, Ditomyiidae, Keroplatidae, and Mycetophilidae from Diptera order. They have a strongly convex chest and long bristly legs. They love moisture, mostly live in damp dark forests and feed on mushrooms, hence the name. Those that you have encountered at home are often also called winter gnats since they are frequently found indoors not only in summer, but also in winter. It would seem that in winter almost all flying insects hibernate, but this is not the case for fungus gnats as they can withstand temperatures down to -25 degrees F (to say nothing of the comfortable environment in your house in winter).
What Causes Fungus Gnats?
Out in the wild fungus gnat larvae feed on the roots of other plants and fungi, and rotting wood and other organics. Thus, they have an important nursing role as without them the process of decomposition of organic matter and the formation of humus in the forest would occur much more slowly. That’s all when it comes to the use of fungus gnats.
Here is the most common adult fungus gnats’ habitat in your house and nearby:
- Overwatered home plants in the pots
- Wet and warm places, such as the greenhouses;
- Mold and mildew spots
Also, they are attracted to light. Michael Waldvogel of the Department of Entomology College of Agriculture and Life Sciences of NC State University is in favor of turning the street light off: "... Adults are attracted to lights and are often first noticed at windows or light fixtures". So make sure you extinguish the beacon lights, else a "ship" of fungus gnats will not take long to arrive!
Fungus Gnats Signs
Let’s start by inspecting your plants and soil. The first of fungus gnats is of course having some bugs fly over your plant pots. As for the less obvious signs, they are:
- White larvae on the ground;
- Sudden withering of the plants;
- Poor growth rate of young plants;
- Yellowing of leaves or massive defoliation.
How Dangerous Are the Fungus Gnats for Plants and Humans?
Fact #1: Fungus gnats neither bite nor suck your blood, as attested by the Colorado State University Extension specialists. Furthermore, unlike other gnats, they would never swarm near your face, buzz in your ear or get into your eyes. Long story short, you, our dear readers, are not at all interesting for them, and that is the only good news we have. The rest is much worse.
Fact #2: Fungus gnats transmit fungal diseases that are dangerous not only for plants but also for humans. This has been proven by Raymond A. Cloyd, Entomologist from the Kansas State University. People who have close contact with infected plants and inhale mold spores, can end up having respiratory diseases.
Fact #3: Fungus gnats feed on the juices of the plant root system. It’s the larvae that are most dangerous! According to Raymond A. Cloyd, “Today fungus gnats are recognized as major insect pests in greenhouses and nurseries and are one of the few insect pests in which the damaging life stage — the larva in this case — is located within the growing medium. They are especially a problem under excessively moist conditions during propagation,when plant cuttings or plugs are initiating root systems. Adults cause minimal plant damage, but females lay eggs that hatch into larvae that damage plants by root feeding. Both adults and larvae may spread plant pathogens”.
Fact #4: The larvae are most dangerous for young plants, seedlings and grafts. Such pot plants as the following are especially at risk:
All plants which have their root system on the surface are also in danger as their roots are not deep in the soil, so if you overwater them, the water will stagnate in the top layer of the soil, where the conditions for fungus gnat breeding are perfect!
Fact #5: The plants, which are well cared for and watered regularly, do not attract fungus gnats! However, the ones that infected at the moment of purchase or are constantly flooded with water (especially in winter) usually begin to rot ... Overly moist soil and rotting root system of the plant create the most favorable environment for the gnats to appear. Therefore, they are often found, for example, in greenhouses where it is warm and humid.
Helpful tip: When buying seedlings or pot plants, inspect the upper layer of soil for fungus gnats! If you see any white larvae or egg or the plant looks like it is withering, do not buy it!
How to Distinguish Fungus Gnats from Fruit Flies?
“Who knows what this thing flying over the plant pot is”, you would say and be right as it often difficult to understand what insect you are dealing with at home, especially when it is not clear where it came from, or when the flowers are in the kitchen. How do you know that this is the fungus gnats, rather than the fruit flies, with which they are sometimes confused? To help you we’ve drawn up a small comparative table below:
A tiny mosquito-like insect with long antennae
A tiny yellow and brownish fly
Habitat and breeding
Pot plants and greenhouses
Rotting fruit and vegetables, sweets and liquor (found in the kitchen)
All year around indoors, and mostly in winter and early spring outdoors
Summer and beginning of the fall
Do they bite?
As you can see, these small flying insects are not too difficult to tell apart as long as you’re attentive and find its origin, be it kitchen fruit or a plant pot.
If you’ve figured out that it is the fruit flies that bother you, read our review of “The Best Fruit Fly Traps”.
5 Important Steps for Getting Rid Of Fungus Gnats
Now that you have noticed the small bugs flying over your houseplants and found out that they are fungus gnats, what should your course of action be?
Step 1: Locate and eliminate the source of excess moisture!
As mentioned above, as a rule the plants which grow in overwatered soil, such as in the pots, flower beds or greenhouses are the source of moisture. Gnats can also inhabit an overly mulched soil which retains moisture for a long time after the rain. The gnats are not passionate flyers, so they usually settle near water. Have you found the source? If it’s rotting organic waste, dispose of it, should it be soil, pick through it and you’ll definitely find some white larvae.
Step 2: Don’t water the soil!
You can just leave an infected pot without watering for some time, the gnats’ eggs will not be able to turn into larvae, the existing larvae will die and fungus gnats won’t be able to reproduce in dry soil... But this will come at great cost: as a serious fungus gnat infection can put an end to the poor flower over these days of not watering the plant. And if your entire territory has been flooded, the waiting will take even more time! In such cases, do not wait for a miracle to happen and use Bti (see the next paragraph.).
Step 3: Treat the soil and get rid of the fungus gnats larvae with B.t.i
NB: According to the information from IPM Program, “Most of the fungus gnat’s life is spent as a larva and pupa in organic matter or soil, so the most effective control methods target these immature stages rather than attempting to directly control the mobile, short-lived adults.”
It’s not necessary to look for special “fungus gnat insecticides” in order to treat the soil or other wet environment nurturing fungus gnats, you can do with the well-known mosquito control methods. The NC entomologist Michael Waldvogel recommends using B.t.i. to get rid of gnats’ larvae. B.t.i. is a set of special live bacteria, which kill insect larvae before they become adults.
Summit 20-Pack Mosquito Dunk
This affordable (~$18) and safe product has proven to be a means for kill mosquito larvae, and, judging by the positive customer reviews (and there are over 1200 of them on Amazon.com), it also helps to effectivelyeliminate fungus gnats. To treat the pot soil, dip the dunk in a watering can to dissolve the product in water, and then spill the solution over the soil. If the infection occurs outside, break dunk into pieces and sprinkle the overwatered soil with it or dissolve them in water and spill as well. Follow the instructions and repeat the procedure if necessary (until new larvae stop appearing). Usually a fungus gnat problem is solved within 2-3 weeks.
Price: ~$18 Check the current price
If you don’t feel like crumbling the dunks, use granule B.t.i., Mosquito Bits for ~$16.36.
Step 4: Kill adult gnats using sticky traps
Kill adult females to prevent them from laying eggs along with getting rid of moisture and larvae. Sticky traps, which can then be left in the ground for permanent monitoring of the gnats, will be perfect for the task. They are not much different from each other as they all are bright yellow with an adhesive agent, and their principle of functioning is the same: a fungus gnat sticks to it and dies, without laying ant eggs. We have found the following 2 effective products that will help you eliminate fungus gnats:
For moderate infestation: Safer Brand 5025 Houseplant Sticky Stakes Insect Trap (7 traps) (~$5.66)
For severe infestation: Hafer 30-Pack (15ea.5"*3"yellow Dual sticky trap and 15pcs wire tie) set (~$6.99)
Step 5: Make a plant-watering schedule
Once you’ve finally got rid of fungus gnats, the main thing is to learn from the situation and prevent its relapse, i.e. not to overwater the soil! Make a watering schedule comprising these important aspects:
- Various plants require different frequencies of watering and lighting levels;
- Pick the right pots for indoor plants. The choice depends on the type of the root system;
- In winter all the plants consume less water, so make watering more moderate (it is no wonder that the fungus gnats called winter gnats!);
- Carry out routine inspection of potted and outdoor plants for parasites.
Fungus gnats are a slow-action bomb; they can cause great damage to your street and potted flowers and to your future crop, especially to the young seedlings. It’s in your power to prevent the formation of mold and overwatered areas and to timely eliminate organic waste in order to avoid rotting. In short, now you know what to do to prevent the appearance of fungus gnats. Keep the yard clean and don’t overwater the crops, my friends!