Sounds like fungus gnats - a pest but nothing unusual. If you want to get rid of them you will have to concentrate on killing the eggs and larvae, not just the adults (that are flying about) - break the cycle in other words. One way is to keep your soil surface dry so the females won't lay their eggs (easier said than done) - another way is to put a barrier down like gravel or even sand since female gnats don't have a long ovipositor (I hope I spelled that correctly). A drench to kill the larvae (some say mosquito dunks works) or predatory nematodes. You can kill adults all day long but if you don't break the cycle, then you will probably continue having new outbreaks of fungus gnats.

I don't worry about them - they won't kill a healthy plant - more obnoxious than a real threat. If you are growing seeds then it is another matter as the larvae may feed on the young roots and kill the seedling. A soapy mix or even nicotine drench will bring all the larvae to the surface but sometimes the cure is worse than the disease (or pest in this case). Choose your weapons accordingly...

Here is one site that gives some good information on fungus gnats.

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7448.html

And an image search for fungus gnat

http://www.google.com/search?q=fungus+gnat+barrier&biw=1053&bih=511&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwi6h4SYoYrNAhVI5yYKHY37ALYQsAQIMg&dpr=1.5

Fungus gnats (Sciarid flies) a sign of wet soil

Why is my iredesent? Rex Begonia Foliage Indoor begonia getting dry and brownish on some of the edges even though the soil is moist? Help, I'm new to them and love them!

Hi Lynne, You should start a new thread unless you are adding to this one about gnats.

As for your rex drying out and browning it could be an aging leaf or it could be low humidity. Post some photos of it (in a new thread) so we might make more informed "guesses" as to what may be a problem (or not).