Hi
I've got to say I've seen a lot of postings on fungus gnat I'm going out on a limb and saying I don't really thing they are a big deal.
I have a huge resident population of gnats in the orchids and think of them more of a annoyance rather than a serious pests.  Mealy bugs, thrips scale now those are pests but gnats?
I've rooted over a 100 cuttings with the gnats and have seen no issues.  Now if something gets over watered and rots then I see them move in but otherwise they have never seemed to cause any issues.  Once outside they seem to go away entirely so again no problem.  Just my observation.

They do cause damage mostly during rooting stage of cuttings under some condition; i.e. when enough moisture in the soil for continued life cycle of fungus gnats, and having fewer roots on cuttings when the life cycle is at hungry larvae stage and when there are leaves that need moisture and nourishment.
Once roots are well established, there is less chance of fatal damage to the plants and then it just bcomes a nuisance.

Hi Chaplin,
If you read you post from bottom to top -so to say- , you'll get a theory that I'm trialing : Give those beasts something else to feast on. They get to the fig trees because that's the only thing they have.
You give them orchids, and I give them lemon treelings . I seeded lemons from fruits that I bought for juice, and that seems to have calmed the gnats.
Are they feeding on the lemon treelings the time for me to let the figtrees get dry a bit more ... I think so .
So members with gnat problems, could you do that test for me ?  Put 3 lemon seeds in a pot -make 3 or 4 pots depending on the number of fig pots that you have - and let them grow . See if that helps .
Thanks a lot .
Keep us posted :) .
Hurry, lemons will probably be on high demand :) .

Munchausen Fig Proxy. The people who have the most trouble seem to have the least experience and give the most advice!

Without a doubt they aren't good, but how bad they are is difficult to quantify.  I had a huge outbreak for awhile, but my big question is:  what was actually killing my plants?  Was it overwatering?  The gnats?  Some combination?  I had some cuttings that I definitely killed by overwatering, and I had a couple that I feel like I definitely did NOT kill by overwatering, but they were dying anyways...this correlated with my outbreak...so take it as you will.

If you've got a few gnats on a well established plant (e.g. my hibiscus plant that I brought in for the winter, which definitely has gnats, and definitely does not care), its not a big deal.  Freshly rooted cuttings, however...I would make a concerted effort to get rid of them.

Personally I believe the gnats don't do much damage, I believe it's the overwatering that does the cutting in.

Fungus gnats are like vultures, fungi and bacteria are the lions and hyenas (except they are microscopic/invisible to the naked eye), and cuttings are like an impala. If nobody ever saw the predators (invisible) then vultures would be identified as the killers! Even though they are not equipped to actually kill a healthy impala, and it is the same with fungus gnats. The larvae are only large enough to eat fine root hairs and nibble the callous. They need a pathogen to attack the cutting before they can eat under the bark, or larger roots.

They can spread pathogens so maybe it does not matter what is actually doing the killing (vultures carrying invisible lions and hyenas around now)? Altering the conditions away from wet, stale and stagnant will reduce their numbers and also help prevent bacteria and fungi. Some cuttings are just duds, they are weak or already infected with a pathogen, maybe they hosted many fig mites over the winter and the buds are now depleted and choked with FMV. Maybe the fine particles in the potting mix have formed layers which prevent fresh air from getting in? Maybe there were too many cuttings in a sealed container and they used all the available oxygen?

Viewing the problem of failed cuttings though the fungus gnat lens really depletes overall understanding of other problems, they are a convenient scapegoat, not the insurmountable obstacle they are portrayed to be. 

If you have a bad enough infestation they are going to do plenty of damage , 2 years ago I got a bag of Miracle Grow that was infested with gnat eggs I watched my cuttings die one after another, trying everything to get rid of them when I would lift a dead plant out of the cup all the roots were gone and the bottom of the cuttings were full of gnat larvae , check out out this video from Iowa State it is very informative 

Honestly I think too wet is the real issue.  The orchids grow with high humidity but with good air circulation.  The gnat live off the fungus that is in the bark.  I suspect since they are always there the nursery also supports a healthy population of predators and the bark grows enough fungus to keep them happy.

Don't fool yourselves.  Fungus gnats do plenty of damage to young and newly rooted plants.

Fungus gnats and overwatering go hand in hand and they'll have done their dirty work long before you overwater the plant to the point of no return.  As fungus gnat larvae feast on young roots, the injured plant takes up less and less water which quickly leads to the moist environment that fungus gnats thrive in, thus furthering their life cycle.  But fungus gnats will also multiply (not as quickly) in a not so moist environment.  The difference (to me) is that they are more easily controlled in a not-so-moist environment.

When I started watering my containers by weight, it became easy to pick out the plants that were having issues.  Went from losing dozens to losing none just by using a scale.