The gnats flying around your plant today were adults before you treated the soil. Nothing you put in the pot was ever going to touch them, and they will be there for about another week whatever you do.
That is not a treatment failing. It is the ordinary shape of a treatment working, and it catches almost everybody, because the stage of the insect you can see and the stage a soil product acts on are two different animals living two different lives in two different places.
Why you can still see them
- What you treated
- Larvae, in the top inch of soil
- What you can see
- Adults, in the air
- How long adults live
- About 7 to 10 days
- What adults do to a plant
- Nothing
- What larvae do to a plant
- Damage roots, in numbers
- When to judge it
- By the count falling over 2 to 4 weeks
Two insects, one name
A fungus gnat spends most of its life as a larva in the top inch of your potting mix, eating fungi and organic matter and, in numbers, root hairs. Then it becomes the small dark fly you keep waving away.
Those two things need completely different treatments, and only one of them is a problem. University of California is blunt about it: “adult fungus gnats don’t damage plants or bite people; their presence is primarily considered a nuisance. Larvae, however, when present in large numbers, can damage roots and stunt plant growth.”
Cloyd’s review of fungus gnats in greenhouse production says the same in one line: “fungus gnat adults are mainly a nuisance causing minimal direct plant damage.”
Which is the awkward part of the good news
The stage that does the damage is the stage you cannot see, and the stage you can see is the one that does not matter.
So the visible evidence and the actual problem run on separate clocks. For the first week or two after a soil treatment, the visible evidence is the worse guide of the two.
Ten days of a four-week life
University of California gives the timings, measured at 75 °F: “eggs hatch in about 3 days, the larvae take approximately 10 days to develop into pupae, and about 4 days later the adults emerge.” A whole generation, female to female, in about 17 days. Ohio State puts the full cycle at “about four weeks under normal spring and summer weather conditions,” with adults living “about 7 to 10 days.”
Read the line and the week you are having stops being mysterious. On the day you treated, the pot contained eggs, larvae at every age, pupae, and adults in the air. The treatment acts on the larval band. Everything already past it is already past it.
And there is a tail on the front end too, because the adults were not idle. Ohio State: “a female may lay up to 100 eggs … over her lifetime.” Some of those were laid the day before you treated.
How to tell whether it is working
You cannot judge this by looking at the air, for the first fortnight at least. What you can do is count.
Yellow sticky traps are the standard monitoring tool and University of California names them for exactly this — “yellow sticky traps can be used to trap adults.” They are not really a control; on a houseplant they are a measuring instrument.
Put one card in, and date it. Replace it weekly, and keep the old ones. You are not looking for zero — you are looking at four numbers in a row. A card catching fewer gnats each week is a treatment working, no matter how many are still flying at the end of week one.
The generation clock tells you how long to give it. Since a generation runs about 17 days at room temperature, the first card that reflects the treatment rather than the population you already had is the one from the second or third week. Judging it on day four is judging the previous generation’s adults.
What good looks like, week by week
- Week one — adults still flying, roughly as many as before. Expected. These are the ones that were already adults.
- Week two — noticeably fewer on the card. The larvae that would have become this week’s adults are not arriving.
- Weeks three and four — the count keeps falling. This is the generation that was treated as eggs and young larvae failing to appear.
- After that — a low background number rather than none. Adults can walk in from anywhere, and one on a card is not a re-infestation.
When it really has gone wrong
There is a version of this that is a genuine failure, and it has a different shape: the card count does not fall. Not “is still not zero” — does not fall, across three or four weekly cards.
The usual reasons are about where the product went rather than whether it works. Steinernema feltiae nematodes are, as Oregon State’s handbook puts it, “the best choice against fungus gnat larvae, often found on the soil surface of potted plants” — but they have to arrive alive, reach the larvae, and stay wet enough to move. Whether the nematodes themselves did anything is its own question, and a more answerable one than it sounds.
The other common shape is that the soil never stopped being ideal for them. Fungus gnat larvae live in the top inch, which is also the part of the pot that dries first — so letting that inch dry between waterings removes the nursery, and is the one cultural change that does more than any product.
If you want the whole picture of what eats them and what does not, we compared the four options.
Field notes
Date a yellow card and keep the old ones. Four cards in a row is the only honest measurement available to you. Memory is not — everyone remembers last week as worse.
Do not re-treat on day four. You would be treating a generation that is already dead and paying for it twice. Give it the 17 days the insect takes.
Let the top inch dry between waterings. That inch is where the larvae live. It is free, it works, and it is the difference between a treatment and a habit.
Count the card, not the air. Adults you disturb when you move a pot are the most visible and the least informative gnats in the room.
Expect a low background number forever. Adults fly in from other pots, from outside, from a bag of mix. Zero is not the target and chasing it is how people over-treat.
If nothing changes by week four, look at the soil, not the product. Wet mix, a deep layer of organic matter on top, or a pot that never dries will outrun any larval treatment.
Questions people actually ask
I treated three days ago and there are just as many. Did it fail?
Almost certainly not. Adults live about 7 to 10 days and a soil treatment does not act on them. Day three is too early for the treatment to have changed anything you can see.
Do the gnats I can see hurt my plant?
No. University of California is explicit that adults do not damage plants. The larvae in the soil are the ones that damage roots, in numbers, and those are what the treatment is for.
How long until they are gone?
Nobody can give you an honest number of days, because it depends on how many were already pupating when you treated. The useful answer is a shape rather than a date: fewer on the card each week, with the biggest drop between weeks one and three.
Should I spray the adults as well?
It will kill the ones you hit and change nothing about the pot. It can also make the next release worse, because most sprays are harder on beneficial insects than on the pest. If the adults are unbearable, sticky cards catch them without leaving a residue.
Can I apply nematodes again straight away?
Re-applying is normal practice — UConn's guidance for growers is to repeat every two weeks — but re-applying on day three because you still see adults is spending money on the wrong stage. Wait for a card count.
There are still a few after a month. Is that failure?
A low steady number is not the same as an infestation. Adults arrive from outside, from other pots and from fresh mix. What matters is whether the number is falling, flat or climbing.
References
- Bethke, J.A. & Dreistadt, S.H. (2013). Pest Notes: Fungus Gnats. UC ANR Publication 7448, University of California Statewide IPM Program. ipm.ucanr.edu
- Shetlar, D.J. & Andon, J.E. Fungus Gnats. Fact Sheet ENT-59, Ohio State University Extension. ohioline.osu.edu
- Cloyd, R.A. (2015). Ecology of fungus gnats (Bradysia spp.) in greenhouse production systems associated with disease-interactions and alternative management strategies. Insects 6(2):325–332. doi.org
- Lee, J. & Dreves, A.J. Entomopathogenic Nematodes. Pacific Northwest Insect Management Handbook, Oregon State University. pnwhandbooks.org
- Pundt, L. (2024). Beneficial Nematodes: An Easy Way to Begin Using Biological Controls in the Greenhouse. University of Connecticut Extension IPM. ipm.cahnr.uconn.edu
The larvae are in the top inch. That is where the treatment goes.
