There is ! But I can't remember !! It might be cinnamon -- it's not a pesticide, but I think ants won't cross it . . . or try Cayenne Red Pepper. They might not like the smell of it and might not cross it--and maybe they'd stay IN the fireplace. Another other option is to light a fire in the fireplace !! :-) (but if you do, be prepared to stomp on the inferno escapees )
For the gnatlike things ? It's ugly, but it catches bugs -- fly paper. Trouble is, it might have an odor to it. If there's a way to smell it before you buy, do so, cuz smells can be obnoxious when pregnant. I suppose if it smells, you could always hang up a strip of duct tape or packing trape. It's not quite so sticky, but it might catch some of those buggers. :-)
Good luck ! Sure hope there are brighter people than I with better answers for you !!