I feel your pain... once did a research project with a couple other students, we had a greenhouse with 200 flats of river bank type seedlings growing... kind of a nightmare.

What works-

First, conditions. Fungus gnats (the particular gnat in question here) need moist soil and are often a sign of overwatering. Definitely something to consider.

Second, control-

We used diatamaceous earth to kill larvae, slices of potato to trap larvae (gross, needs daily or every other day changes, but effective, bowls of apple cider vinegar to trap adults.

I have also heard from more growers than I can count that a product called, "mosquito dunks" is highly effective.

I do not get these too often in my house, but my prefered solution was to complete the circle of life as it where, and now grow several carnivorous plants. Sundews, like Cape sundew are easy to grow if you can provide enough light and don't mind collecting rainwater or buying distilled.

I am sure others will chime in too with other options, and best of luck!

Does the systemic help much at keeping other pests at bay too? Been considering trying it.

i just repotted a hoya that had gnats. i would say if any of your plants, hoya or otherwise, are indoors, don't use an organic/natural fertilizer.

i used fish emulsion and then got the bright idea to toss in a little bit of a dr. earth dry fertilizer. i soon had many gnats, and some other tiny whitish crawly bugs. they seem to like the decaying organic matter, even if it's not soil mixture - i am using better gro orchid bark.

i also read a comment somewhere that said don't use peat.

When I give the systemic, I don't see gnats. I assume it keeps them away/kills their larva.

Be careful about fungus gnats. They're not just annoying they can do damage as they will start feeding on the roots of the plant. Gnats are a sign of over watering and usually a first sign of root rot. I would repot asap and rinse the roots really well.

I hate those things! I get them really bad in my African violets. It got much worse when I began wick-watering the violets. I read that letting the plants dry out should take care of them, but it never did. I've taken most of my violets off wick now, but still have occasional trouble with them. I'll have to find that systemic.

And yes, I agree that if there are bunches of them, they can be harmful to root systems.

Well, my car is currently in the hospital and I have some Bayer Rose and Flower pest killer (systemic as well) so I decided to try it instead of getting to the store for the tree and shrub mix. I have to say, 7 days after misting all my Hoyas foliage as well as the pots I'm seeing almost NO gnats! *knocks on wood*

I forgot how well this stuff really works! Just wish it prevented against spider mites too :/ I don't have any *knocks on wood* (that I've found yet) but it would be nice to have the peace of mind :p