They make sticky traps but the best solution is backing off on the water if you're talking about fungus gnats.

I have read this, but not tried yet myself:

Monitoring: Yellow sticky cards, placed standing up on the soil surface, attract and hold the adult fungus gnats and give you an idea of the size of the population.

Placing a slice of potato on the soil surface sometimes attracts the feeding larva. The potato slices can be used to collect and dispose of larva and to gauge when the larva are actively feeding, for timing of pesticide applications. Make sure the potato slices do not dry out.

Don't know if they are fungus gnats, but sounds about right. Where do they sell the sticky yellow cards to trap em? Will also give the potato a try, but does it work as good as the sticky cards?

They are available at Lee Valley - check out the link.

Here is a link that might be useful: Yellow Sticky Traps @ Lee Valley

The fungus gnats themselves are harmless, except they mate and lay eggs on your growing medium. Those eggs hatch into small maggots that feed on the roots of your seedlings, severely stunting them or even causing them to die. You need to kill the maggots as soon as they hatch or before.

I powder up Mosquito Dunks and mix the powder with water to produce "Seedling Water" that I moisten my germinating medium with before inserting the seeds. When the seedlings emerge, they are already in a maggot-proof medium, which breaks the life cycle of the fungus gnats and protects those little root hairs from becoming maggot food. There are other forms of BT that you can use for this purpose, but I find the Mosquito Dunks to be effective and readily available.

ZM

Don't use dunks on tomatoes. They are very susceptible to a virus called tobacco mosaic and dunks contain imodacloprid which is a nicotinoid. It's much easier to let the top half of the soil dry between waterings. Fungus gnats lay eggs in wet soil.

If you've already got them, products like Knock-Out Gnats or Gnatrol, which contain bacteria that infect and kill the gnat larvae (without hurting humans), are effective. It will take a few applications to get them under control.

Meanwhile, and afterwards, as others have said, break the cycle by letting the top inch or so of soil dry out completely between waterings. This will be better for your plants' roots, too, and will help prevent fungus, damping-off, and rot.

taz,

"...and dunks contain imodacloprid..."

That is absolutely wrong. Mosquito Dunks do not contain imidacloprid or any other chemical insecticide. They contain a biological agent, Bacillus thuringiensis subspecies israelensis solids, spores, and such, which are the same ingredients in the products that nygardener linked to above.

ZM

The dunks they used to sell at my local Lowes contain imidacloprid or they did when I last read the package. I know what I read ZM. That may have changed since then. Sorry if I have mislead anyone.

I use a product called azamax on all my seedlings. It is a natural product and you can get it at an hydro shop.
I have excellent result with my plants, no more gnats.
It is also verry effective for spidermites.
Good luck.

The Mosquito Dunks I have came from Lowes or Home Depot, not sure which? But the website on the package is www.summitchemical.com (in Baltimore, MD)You could check w/them if any questions.

Depends on the species. If it's a variety that likes fruit, you can actually catch them with a cup full of ethanol, since, to these guys, the smell of alcohol in the air usually indicates the presence of an overly ripe fruit.

I agree with Donna. Azamax works great, but is kinda expensive. Another great organic alternative is beneficial nematodes. They're microscopic worm like critters that live in the soil and eat all soil born insects.

I've heard a small layer of sand on top of the dirt prevents the gnat from laying it's eggs, and those that do get laid get punctured or shredded by the coarse sand.

giving less water = plant stress, and I still have gnats.

I found out the best way by accident:

I placed a "bouquet" of fresh bay leaf cuttings next to my kitchen counter for cooking, within a few days I noticed there were NO gnats ANYWHERE. I thought it was just their off season, but I realized, they have no off season, lol!

so i looked up on the internet, and some people do use bay leaves (or other strongly scented herbs) to repel these insects. It worked extremely well for me. Next time I will just crumble dry bay leaves directly on to the soil.

Available at lee valley..check oout there...