Small Flying Things: Which One Is It?
By IdentifyThese Editorial · Reviewed · 3 min read
The reflex is to spray whatever is flying. Three different pests fly, and the one causing the damage is often not the one in the air.
Where each one is
Fungus gnats come from the soil. Disturb the pot and they rise; they drift around the room and land on windows. They do not congregate on leaves.
Whiteflies live on leaf undersides. Brush the plant and a cloud lifts off, settles again within seconds, and always back onto the plant.
Thrips stay on the plant and mostly do not fly at all. They are found by looking, usually in flowers and along leaf veins, moving with a quick scuttle.
| Fungus gnat | Whitefly | Thrips | |
|---|---|---|---|
| Colour | Dark grey-black | White | Yellow, brown or black |
| Shape | Mosquito-like, long legs | Tiny moth | Narrow sliver |
| Source | Soil surface | Leaf undersides | Flowers, leaf veins |
| Behaviour | Drifts around the room | Cloud on disturbance | Scuttles, rarely flies |
| Damage | Roots, by larvae | Sap loss, honeydew | Silvery scars, black frass |
| Sticky leaves | No | Yes | No |
Fungus gnats
The larvae are the point. Adults live about a week, do no damage and are simply the visible symptom of larvae in wet soil eating organic matter and fine roots.
The fix is drying the surface. Let the top two centimetres dry properly between waterings and the life cycle breaks, because eggs need moisture to survive. Bottom watering does the same thing more reliably.
Yellow sticky traps catch adults, which reduces the nuisance and the next generation without addressing the larvae directly. Together the two measures clear an infestation in about three weeks, which is what tiny flies around houseplants covers in full.
Whiteflies
Sap feeders, and the honeydew they produce is what makes leaves sticky and lets sooty mould grow.
They are difficult because all life stages are on the plant at once and only the adults fly. Treating the visible cloud achieves nothing lasting; the scale nymphs on the leaf undersides continue.
Soap or oil sprays to leaf undersides, repeated every five days for three weeks, with sticky traps to reduce the adults. In a greenhouse, the parasitic wasp Encarsia formosa works well and is the standard commercial control.
Thrips
The most damaging of the three and the hardest to see. Adults rasp the leaf surface and drink what leaks out, producing silvery-white scarring with tiny black specks of excrement on it — a combination nothing else produces.
They also transmit viruses, which makes them worth taking seriously beyond the direct damage.
Blue sticky traps catch them better than yellow. Treatment needs to reach flowers and growing tips, and part of the life cycle is in the soil, so a soil drench alongside a foliar spray works better than either alone.
Deciding fast
Shake the plant. If things rise from the pot, gnats. If a white cloud lifts from the leaves, whiteflies. If nothing flies but the leaves have silvery patches and black dots, thrips.
That takes ten seconds and picks the right treatment, which matters more than the treatment itself — spraying leaves for a soil problem is the commonest wasted effort in houseplant care.
The fourth possibility
Fruit flies, which have nothing to do with the plants at all. They are stockier than fungus gnats, browner, and they hover near fruit bowls and drains rather than pots.
Worth eliminating before treating anything, because a plant owner who dries out their soil for a fruit fly problem gets no result and concludes the advice was wrong.
Prevention across all three
New plants isolated for a fortnight, since all three arrive this way far more often than they appear spontaneously.
Sticky traps left in pots permanently, which cost almost nothing and turn a problem that would be noticed in three weeks into one noticed in three days. Catching any of these early is the difference between a fortnight of treatment and a season of it.
Problems referenced
Check these two things first
- Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
- Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.
Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.
Keep reading
- Tiny Flies Around a Houseplant: Fungus Gnats, and the Two ImpostorsFungus gnats breed in the top few centimetres of wet soil. Which is why the fix is the watering, not the spray — and why sprays reliably fail.
- Sticky Leaves: What Honeydew Means and Who Is Making ItSticky residue is insect excretion, and it always comes from above. Four sap-feeders produce it, and each leaves a different sign on the plant.
- When Not to Spray: Most Plant Problems Fix ThemselvesFungicides do nothing against pests, nutrients or watering faults — and most garden damage is cosmetic. A short guide to when treatment helps and when it just costs money.