This is the base of a white oak with some poison ivy, etc. For days i have been noticing all these bugs swarming noisily around this spot: yellow hornets plus bald-faced hornets, houseflies, horseflies, butterflies, and some smaller flies or something.
I have no idea what's going on there and i'm not about to go poking around. Hornets nest in hollow trees, and there could be a hollow spot in this healthy white oak that has about a 1' diameter trunk. White oaks are very rot-resistant, though, so it seems unlikely. And i have not seen insects actually entering or exiting the tree.
I never thought they had tasty sap. I don't know what the insects are after.
Hornets are also supposed to eat horseflies, but these seem more interested in something else. You can see a horsefly near the top at 0:05. I have seen several more there the last couple days.
Bald-faced hornets (those black-and-white guys) are actually a type of wasp that supposedly preys on houseflies, yet they co-exist with them here.