Katchy vs DynaTrap (indoor)

The mechanism is nearly identical

Both of these are small mains- or USB-powered units that do the same three things: emit ultraviolet light to attract flying insects, run a fan to pull them in, and hold them so they cannot leave.

The fan is the important part, and it is why both of these beat a light-only trap on gnats. Fungus gnats and fruit flies are weak fliers — they are physically unable to escape even a modest airflow once they are inside the intake. A sticky card with no fan relies on the insect landing on it; a fan does not wait to be chosen.

The difference between the two products is only what happens after the fan.

Katchy pulls the insect onto a replaceable adhesive board at the base. The catch is visible, flat, and goes in the bin with the board.

DynaTrap indoor pulls the insect into a retaining cage where it dehydrates. There is no adhesive and no refill; you slide the cage out and tip it into a bin.

Everything below follows from that one difference.

Side by side

Where Katchy wins

Fungus gnats specifically. The combination of a fan and an adhesive surface is the right tool for an insect that flies badly and lands often. The gnat does not need to be killed by the mechanism — it only has to touch the glue once.

It is a diagnostic as well as a trap. A board that fills in three days tells you the source is active and nearby, which is information a sealed cage does not give you as readily. If you are not sure whether you have fungus gnats, fruit flies or drain flies, that reading is genuinely useful — and gnats vs fruit flies vs drain flies tells you what to do with each answer.

USB power. It will sit on a plant shelf with no socket nearby, which is exactly where a gnat trap needs to be.

Small insects do not escape. Anything stuck is stuck. A retaining cage depends on the insect being unable to fly back out against the airflow, and the very smallest ones sometimes manage it.

Where DynaTrap wins

No refills, ever. Over a couple of years that is a real cost difference, and it is the reason a lot of people prefer it.

Nothing sticky to handle. Emptying a cage is cleaner than peeling a full glue board, and if the idea of a visible pad of trapped insects bothers you, the cage version is the one to buy.

Slightly quieter in practice, which matters if the unit lives in a bedroom.

Better on a mixed problem. For general small flies rather than gnats specifically, the cage design handles a wider size range without filling up.

When neither is the right product

House flies. Both of these are built around small, weak-flying insects. A house fly is large, strong and fast, and it will mostly ignore a desktop fan trap. A plug-in UV light over a glue card — the commercial fly-light design — is the correct product for house flies indoors, which is why one is in the picks above. How to get rid of house flies and best plug-in fly traps cover that category.

Fruit flies on a kitchen counter. A liquid lure trap next to the fruit bowl outperforms either of these, because fermentation odour is a far stronger cue for a fruit fly than UV light is. Best fruit fly traps and Terro vs Zevo are the pages for that.

Mosquitoes in a whole house. A 300-square-foot desktop unit is a room, not a house, and a mosquito has far better reasons to come to you than to the trap — you are exhaling CO2 and it is not. Best indoor mosquito traps is honest about the limits, and DynaTrap vs Flowtron covers the outdoor units.

A room with a bright window or a lamp on. Both units compete with every other light source in the room. They work best in a dim room overnight, and badly in a sunlit kitchen at midday.

The part that actually ends it

Both traps catch adults. Neither touches the larvae, and the larvae are the population.

Fungus gnats breed in the top few centimetres of consistently wet potting soil. The adults you are trapping emerged from your pots and will keep emerging for as long as the soil stays wet. Two fixes, both necessary:

1. Let the top of the soil dry between waterings. Water from below if you can. Dry soil stops the cycle outright.
2. Treat the soil with BTI granules, which kill gnat larvae and are harmless to the plant, pets and people. One of the BTI products in the picks is the correct treatment for exactly this, and that dual use is the reason it is listed on a mosquito page.

Add the trap as the third step to clear the adults already flying. Fungus gnats in houseplants is the full method, and how to get rid of gnats covers the other species people call gnats.

Fruit flies breed on fermenting organic matter — an overripe banana, the film inside a bin, a splash of wine behind the bottles, a mop bucket. Fix the source and the flies stop arriving. What attracts fruit flies lists the places people miss.

Drain flies breed in the organic film inside a trap or a seldom-used drain, and no flying trap will touch them. Drain flies in the sink is the fix.

The verdict

• Fungus gnats around houseplants: Katchy, plus BTI granules in the soil and less water. The trap alone will catch adults forever.
• You never want to buy a refill: indoor DynaTrap.
• House flies indoors: neither — a plug-in fly light over a glue card.
• Fruit flies in a kitchen: neither — a liquid lure trap by the fruit bowl.
• You want to know what you are dealing with: Katchy, and read the board after three days.