Here’s a quick summary of the guide:

• Fungus gnats keep coming back because the larvae live in damp potting mix, while adults are just the visible symptom.
• The quickest and easiest apartment-friendly way to “break the cycle” of fungus gnats is with a combination of: (1) sticky traps to capture the adults; (2) BTI drenches to kill the larvae; and (3) making changes in how you water so that it is less conducive to gnats living in the potting mix.
• Use the “14-day timeline” outlined below. You should be able to see a significant reduction in the number of adult gnats within a few days of implementing this plan, but if you still see many adult gnats at the end of the second week, you will need to continue BTI weekly for a total of three weeks (regardless of how much better things are).

Why Fungus Gnats Are Attracted to Houseplants, and Why Killing Just the Adults Will Not Solve the Problem.

Adult fungus gnats look like tiny mosquitoes and commonly occur in houseplant soil that has remained consistently damp. The adult fungus gnats are a nuisance next to windows, etc., while the main cause of the infestation is the eggs and larvae that are located in the top layer of soil.

The timing is a key component of eradicating fungus gnats. The eggs can hatch in approximately three days, the larvae will feed in the soil for about two weeks, and the adults can live for approximately one week. Therefore, if you only capture the adult gnats with your traps, there will be new adult gnats hatching from your soil and continuing the infestation. This is why the most effective approach is to utilize a multi-faceted approach to address all the different life stages of fungus gnats simultaneously.

Quick Identification: Are These Fungus Gnats or Something Else?

• Most likely fungus gnats–if you’ve seen little flies at your soil’s surface while watering or bumping the pot.
• Check the potting soil: Gently disturb/put your finger into the top inch of dry soil; fungus gnats have transparent or whitish worm-like larvae with darker heads (they can be very difficult to see in dim light).
• Not fungus gnats– If there is a bunch of flies on an item such as fruit, trash, recyclables, and near drains instead of around/on plants (more typical for fruit flies or drain flies).
• Simple home test: Place a raw slice of potato flat on top of the soil for 24 hours; often you will find fungus gnat larvae located under the slice of potato (this gives you a “peek” without digging into the soil).

The Three-Step Cycle Breaker (The Three Steps to Cycle Breaking)

Table: Why The Three Steps To Break The Cycle Work (And Why Each Part Is Important)

Step One: Use Sticky Traps For Adult Control & Track Progress

1. Place 1 or 2 small sticky traps (yellow) per pot or 1 large sticky trap for a group of smaller pots. Place sticky traps as close to the soil as possible (but not touching).
2. If you have many plants, you should target the “problem pots” (the pots that tend to stay wet for a long period of time) as well, as all the nearest plants to any window or other light.
3. Replace traps when they become full of insects or dust or when traps are no longer sticky. In heavy infestations, this may occur as frequently as every few days until the infestation has been brought under control.

Apartment tip: If you have pets (especially cats), you should place the insect traps in the foliage around the plants or place mini traps closer to the base of the plants. This will make it more difficult for them to reach the traps.

Step 2 – Applying BTI Soil Drenches

BTI (or Bti) is a naturally occurring bacterium used to control and eliminate certain types of fly larvae. When the fungus gnat larvae are in the potting media and come into contact with this bacterium while they are feeding, they will become infected and eventually die. An important thing to note is that BTI is only useful in regards to larvae stage life; therefore, if you are treating for larvae, it is best to couple the BTI product with sticky traps.

A practical “drench” method for using BTI

1. Prepare the BTI treated water according to the directions found on your product label. The timing and amounts are critical.
2. Use the treated water to ‘moisten’ the uppermost layer of your planting medium (which is where larvae will typically be found), giving them a consistent amount of moisture. Then, allow the planting medium to completely drain.

Treat every container located around this area (even those that do not appear to contain any larvae). If a single container is left untreated, that container could very well keep the cycle going.

Continue to repeat this treatment according to the frequency that is recommended on your label. (In many cases, weekly continue to apply BTI will turn from ”helped a little” to ”hey…they are actually gone”.)

Step #3 Transitioning Watering Practices (Removing the Breeding Conditions)

Often, fungus gnats are a result of a ‘watering pattern’ problem as opposed to being simply a ‘bug’ problem. Your goal should not be to let your plants’ root systems dry to, but rather to stop maintaining a continually moist upper layer of the planting medium which is ideal for egg and larval development.

• Allow the uppermost one to two (1-2) inches to dry out prior to re-watering (you may need to adjust for the type of plant you have – remember, you can still give drought intolerant plants the benefit of having a dry upper layer in their soil mix).
• Rosting your plants from the bottom as much as possible will allow the upper layers of your planting medium to remain dryer longer than using conventional watering methods. Simply put, fill a tray with water to supply your plant with water through the soil until it absorbs the majority (20-30 minutes) of water from the tray into the soils and allow your plant to drain.
• Remove all saucer and cachepot (especially after watering) from under/inside your planters. There should be no standing water present after watering your plants.
• Frequently remove any decaying leaves from the surface of your planting medium and remove any algae or mold from the rim or bottom of your plant’s cachepot/saucer (larval food source).
• If your planting medium consists of a substantial amount of peat and it remains wet for an extended period of time, you need to plan on (not panic) repotting your plants into a better draining planting medium in the future, however, you should NOT attempt to repot until the infestation has been eliminated (see options to escalate action below).

14-Day Timeline: Action Plan

Day 0 (Setup)
To accomplish the 14-day cycle break, on Day 0 you will place sticky traps in every pot and apply the first BTI soil drench as directed. Additionally, you should empty or not use any saucers under your pots and commit to allowing the top layer of soil in each pot to dry out between waterings.

• Immediate – adults captured on the traps will start immediately but may still be flying around as those were present prior to applying the traps.

Days 1-2 (Find Water Plan)
No significant changes should take place on Days 1-2 with respect to watering the pots as the objective is to keep the surface of the soil in each pot on the drier side (no “topping off” pots).

• Possible Observations – in heavy infestations, traps may get filled rapidly; therefore, if you see this occur frequently, it means you are trapping many adults.

Days 3-4 (Check Traps & Drainage)
On Days 3 and 4, if your traps become full or very dusty, you will want to replace them. While checking the pots, your only focus should be checking to see how many of the worst pots are showing any signs of soil drying out on the surface.

• Possible Observations – at this time, you should start to see adult gnats less frequently flying right above the soil surface; however, you may still see some of them flying around near windows and/or plants situated under light.

Days 5-6 (Remove Debris)
On Days 5-6, continue to reduce the watering of your plants by watering them from the bottom and ensuring there is no standing water in your saucers. Additionally, remove the dead/decomposing leaves or other organic materials that may be present on the surface of the soil in each pot. The gnat population should be declining as long as you have used the right amount of BTI to break their lifecycle. Using traps should result in lower numbers of adult gnats as compared to when you placed your traps for counting.

The plant should still be alive and looking more or less the same; gnat numbers should have decreased.

Grammatical/Word Errors

• Keep plants in weekly rotation.
• Gnat population should also have dropped.

Day 7 (reset Week 1)
Apply BTI soil drench in accordance with the product label directions; replace or rotate both types of traps (maintain at least one “tracking” trap in each georeference area).

Gnat numbers are expected to be dropping.

Day 8-10
Keep the surface dry. Any potted plants that do not dry out within 2-3 days of watering should be considered “high risk” (place them off to the side and assure adequate air circulation).

Gnats will be catching on traps that are used for Days 0-3.

Day 11-13
Replace traps only when you need to. Be diligent in your watering practices. Clean gnats off of the windowsills and drip trays.

You should be catching mainly “occasional” gnats rather than seeing gnat “clouds.”

Day 14 (evaluation)
Compare trap counts collected between Days 0-3 to those collected between Days 11-14. If you find you still have a gnat problem and your trap counts haven’t improved, additional weekly BTI treatments over the next 30 days may help. Apartments usually show an overall better reduction than at Day 0, however, full elimination of gnat problems may take longer.

Reason(s) fungus gnats remain a problem

Not monitoring populations of gnat adults there are numbers of gnat adult beetles and still uncovering new gnat eggs.

Only relying on gnat: adult sticky traps for counting gnat beetles and continuing with weekly BTI uses until reaching the 14-day cycle for eliminating fungus gnats.

1. Using BTI (Bacillus thuringiensis israelensis) to control fungus gnats should be done weekly until complete control is achieved. Many “failures” occur due to stopping too soon.
2. Identify and remedy any pots that stay excessively wet for long periods of time (oversized, poor drainage, compacted peat). Any one of these pots could be responsible for the gnat population.
3. If the potting mixture is broken down and soggy, repot into a fresh plant mix that is well-draining in a container with adequate drainage. Then, continue to use BTI as per schedule on the new potting medium.
4. If one plant is the obvious cause of the infestation, isolate it so that you can treat it without reinfesting the grow room.

Identifying and Eliminating Fungus Gnats

When trying to identify and eliminate fungus gnats, you first need to determine which insects are present before using traps to catch them. If you’re seeing gnats and your traps are mostly empty, you should check drains, trash, recycling containers, and pantries if you’re dealing with a different insect.

Preventing Gnat Infestations in Small Spaces

• Quarantine new plants for two to three weeks using a sticky trap in the pot.
• Don’t let the topsoil of the plant remain constantly wet. Use bottom watering whenever possible and empty the saucer.
• Do not leave open potting soils in the house where they can become or stay wet. Reseal and store them in a dry place.
• Remove dead or organic debris from the soil surface as often as you can.
• If you have a history of fungal gnat problems, use preventative BTI drench products during the warm months—if allowed by your product label.

Frequently Asked Questions (FAQs)