I checked yesterday and found that a white oak and a sweetgum had actually grown together at the base; the spot that draws all the critters is on the white oak side. I realized it isn't necessarily sap that's attracting them, though. The bark in that section is somewhat rotten-looking and always wet, so maybe it's some kind of fungus they like. Or it could be the moisture. In any case, I have found tons of cool insects on this tree, including: hackberry emperors and a question mark butterfly, green June beetle, giant hornets, and several unidentified larvae.

I can see five small flies and a smaller beetle in the vicinity as well.