You Cannot See Them. You Cannot See Them Working, Either.

Nothing about this treatment is visible at any stage, and that is not a flaw in how you did it.

The organism is half a millimetre long and colorless. It goes into the soil. It kills inside the body of a larva you were never going to see. And then — this is the part that makes the question so hard to answer — the evidence removes itself. University of Connecticut, writing for growers: infected fungus gnat larvae “rapidly disappear, so they may be difficult to locate.”

So there is no moment where anything looks different. No dead things on the surface, no cloudiness in the water, nothing on the soil. People reasonably conclude they have watered their plant with an expensive bottle of nothing.

There is no check you can run at home — that is dealt with below, because it is the first thing everyone reaches for. What there is instead is one measurement that answers the real question.

Whether they arrived alive

This is the first question everyone reaches for, and the honest answer is that you cannot settle it at home.

You will find advice — including from extension services — to filter the mix through a coffee filter and look for movement under a hand lens. We do not repeat that advice, and the reason is the thing those instructions leave out: an infective juvenile is half a millimetre of colorless thread and it is slow. The S-shaped movement that is supposed to tell you it is alive happens over seconds, not in front of your eye. At 20× you are looking at a speck that may or may not have shifted since you last blinked, next to identical specks of debris.

So treat the arrived-alive question as ours rather than yours. If you have a reason to doubt a batch — the box was crushed, the pack was open, the date has passed, the mix smelled sour — send us the order number and we will sort it out. That is a faster and far more reliable answer than a magnifier.

The measurement that answers the question

The real question is not “are the nematodes alive” but “did the larvae stop.” That is answerable, just not by looking at the soil.

Count adults. Yellow sticky cards are a measuring instrument here rather than a control: put one in, date it, replace it weekly, and keep the old ones. What you are reading is a sequence of four numbers, not any single card.

The reason a single card tells you nothing is timing. The adults flying in week one were already adults when you treated, and a soil product never acts on them. The card that first reflects the treatment is the second or third one — the generation that was larvae in the soil on the day you watered, failing to arrive.

A falling count across three or four weekly cards is the treatment working. A flat or rising count across the same period is the thing worth investigating, and the next section is the list of usual causes.

If you want to look at the larvae themselves, UConn's monitoring method is potato disks — a slice of raw potato face-down on the mix, lifted after a day or two to see what has gathered underneath. That counts larvae directly, and it is more work than a card for the same answer.

There is a longer piece on why the gnats you can see are not the ones you treated, which is the other half of this question.

The four ways they die before they find anything

When a nematode application genuinely does nothing, it is almost always one of four conditions rather than a bad batch. All four are published and all four are avoidable.

Light

Oregon State's handbook: “protect nematodes from excessive exposure to ultraviolet (UV) rays which can inactivate and kill them.” Mixing a batch in a clear jug on a sunny windowsill and leaving it while you do something else is a real way to lose an application.

Drying out

They swim. They need a film of water between soil particles to move at all — the same handbook calls for “a moist, not saturated, soil environment so they can move around and locate their host.” Florida's guidance is to keep the treated area wet for “at least 8 hours post application”; Oregon State's is to maintain soil moisture “for at least 7 days following nematode application.”

This is the one people miss, because the instinct after watering something in is to let the pot dry out. For the week after an application, do not.

Temperature

Temperature matters, but not as the wall it is usually described as. The Pacific Northwest handbook's application guideline is that soil “should be above 55 °F and less than 90 °F,” which is a reasonable default — but it is written for every species at once. The species differ enormously, and University of Florida groups them plainly: S. feltiae is among those “adapted to cooler temperatures,” while “more heat tolerant species include S. ribrave, S. glaseri, and H. indica.”

For a fungus gnat pot that cuts one specific way. Warm soil does not switch Sf off — in Flores et al.'s trial it still killed every host at 30 °C — but it makes it much less efficient: invading juveniles per larva fell from 26.4 at 20 °C to 3.1 at 30 °C, and at 30 °C the nematodes produced no offspring at all. Above that nobody has measured it, so we will not give you a ceiling.

A pot on a cold floor in winter, or against a hot window in summer, sits somewhere the room is not — worth noticing, and not on its own a reason to conclude the application failed.

Age and storage

They are alive in the packet and they do expire, so check the date on the pack. What they do not need is to go straight into the fridge. Storage is more forgiving than most people expect: a sealed pack sits at room temperature for about a month without trouble — what buys that shelf life is the moist carrier they ship in rather than the cold — and refrigeration is how you extend that window for longer-term storage rather than how you rescue a pack that spent a night on the counter.

The hazards are at the ends rather than in the middle. Freezing kills them. So does a closed car or a sunlit porch in summer, in a way that a warm hallway does not.

Whether to apply again

Usually yes, and on a schedule rather than in response to seeing gnats. UConn's guidance for growers is to “repeat applications every two weeks or as recommended by your supplier.”

The reason is the life cycle rather than the nematodes wearing off. A single application meets the larvae that are in the soil on that day. Eggs laid the day before hatch after it, and a second application two weeks later meets those.

What is not a reason to re-apply is still seeing adults four days later. That is the stage no soil treatment reaches, and re-treating for it is paying twice for the same larvae.

For what to put in the pot, Steinernema feltiae is the species for this job — Oregon State names it directly as “the best choice against fungus gnat larvae, often found on the soil surface of potted plants.” The comparison with the other three options covers when it is not.

Questions people actually ask

Should I be able to see the nematodes in the water?

No, and there is no home test that changes that. They are about half a millimetre, colorless, and slow — even under a hand lens you are looking at a speck that may or may not have moved, beside identical specks of debris. If you have a reason to doubt a batch, send us the order number rather than reaching for a magnifier.

Do they have to go in the fridge as soon as they arrive?

No. A sealed pack is fine at room temperature for about a month — the moist carrier they ship in is what buys that, not the cold. The fridge is how you keep them longer if you are not going to use them soon. Freezing is the thing to avoid.

How quickly do they kill?

Once a nematode has entered a host, the bacteria it carries multiply and the host usually dies within 24 to 48 hours. Finding the host takes longer and is not something you can observe.

Will I find dead larvae in the soil?

Probably not. Infected fungus gnat larvae are often opaque-white to light yellow, and UConn notes they disappear quickly enough to be difficult to locate. Absence of corpses is not evidence of anything.

Are nematodes safe around pets and children?

These are insect-specific parasites used routinely in greenhouse production. They are applied as a soil drench and are not a contact hazard in the way a pesticide is. Follow the label on the pack you bought — it is the instruction that governs.

I watered them in and it rained through the pot. Did I lose them?

Possibly some. They need moist rather than saturated soil to move, so a pot flushed hard enough to run out the bottom may have carried some with it. That is a reason to water gently after applying, not a reason to re-apply immediately.

It has been two weeks and I still see a few gnats. Did it work?

Look at the cards, not the air. A low background number is normal and adults arrive from outside, from other pots and from fresh mix. A count that is falling week over week is the answer you want.